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CHAPTER TWO |
Prior to the establishment of this research program, foundation engineering
guidelines were mostly empirical and barely sufficient for most highway
applications. The result was an inability to accurately predict the performance
of foundation systems, which, in turn, led to very conservative design
procedures as well as an occasional foundation failure. A comprehensive research
plan was initiated in the late 1970's to develop improved design and
construction guidelines for building safer and more cost-effective bridge
foundations. Engineering improvements were expected to reduce the cost of these
foundations and stretch the highway dollar to buy more bridges that will last
longer.
An analysis of future needs for highway bridge foundations was
completed under this project to provide essential planning information for
conducting effective research on bridge foundations. Estimates of the number of
new bridges to be constructed and those that would need to be replaced or
rehabilitated during the remainder of this century were made in 5-year
increments using data from FHWA bridge inventory and inspection reports. An
analysis of FHWA foundation management review reports was also made to determine
typical foundation types used in each State and likely to be used on future
construction. The analysis was also supplemented with personal interviews with
selected State and FHWA bridge engineers from various regions of the country
(1).
Results of these analyses indicated that more than 100,000 bridges
would be constructed, replaced, or rehabilitated in the United States during the
last 20 years of this century. Approximately 20,000 new bridges would be built
and more than 15,000 existing bridges had deficient foundations. Many of the
satisfactory foundations would also have to be replaced because of retrofit
problems caused by replacing the superstructure. A large number of reusable
foundations required special design and construction procedures that needed to
be developed. It was also noted that more than two-thirds of these bridges were
likely to be supported on piles, one-fourth of them on spread footings, and the
rest of them on drilled shafts or other types of foundations (1).
2.1 Background
Because the safety and cost-effectiveness of bridge foundation systems is of
major importance, an appropriate research program to develop better design tools
has long been advocated by both structural and geotechnical engineers.
Foundations represent about 30 percent of the cost of highway bridges in typical
applications; however, this cost can be even higher where bridges are built near
or on difficult soil conditions. The total annual expenditure of public funds
for bridge construction is conservatively estimated to be more than $2 billion,
which means that foundations are costing more than half a billion dollars per
year.
Because the predominant type of foundation system used in the
highway industry is piles, the first priority for this project was the efficient
design of piles. Research was also needed to develop new concepts for reducing
our excessive reliance on pile foundations. In situations where deep foundations
are required, the overall cost can be significantly reduced by using smaller
piles and fewer of them as a result of design economies developed under this
research project.
The indiscriminate use of pile foundations was partly
due to a lack of confidence in the safety and dependability of drilled shafts
and shallow foundations, but mainly due to unreasonably restrictive deformation
and movement constraints. Therefore, the establishment of rational criteria for
tolerable movements was a necessary first step toward improving foundation
design (see section 2.5).
Assuming a 25 percent reduction in the use of
piles can be accomplished by substituting cheaper drilled shaft or shallow
foundation units, the use of piles will still account for much more than half
the foundation systems used for highway structures. Therefore, the continued
investigation of pile-soil behavior will provide cost-effective improvements.
One of the most incongruous aspects of foundation design is the overwhelming use
of a system (friction piles) that is the least understood.
2.2 Objectives
The overall project objective was to develop improved foundation design and
construction methods for highway bridges. This objective was approached through
efforts directed toward accomplishment of the following four separate, but
related goals:
2.3 Scope
Research efforts in this project were directed toward developing new predictive techniques for foundation design, better criteria for tolerable movements, and permissible stress levels in piles and drilled shafts, as well as improved construction control techniques. Each component of the total foundation system was studied individually and each total system was evaluated as a whole. The behavior of the foundation systems was also examined under both working and ultimate stresses.
2.4 Project Description
The major research efforts were organized into five tasks:
The Foundations Project was the "flagship" project of the FHWA Geotechnology Research Program. It could and probably will be a separate report by itself; in fact, each of the five main tasks could also be a separate report. The following six sections of this chapter provide a summary of the major highlights of this project.
2.5 Structural Consequences of Foundation Movements
During the development of the FHWA research project on foundations, it was
determined that one of the major reasons that bridge foundations were
over-designed had its roots in the lack of rational criteria for tolerable
movements of the superstructure. The American Association of State Highway and
Transportation Officials (AASHTO) design code at that time was relatively silent
in this area, and basically promoted a "zero" settlement design procedure. This
resulted in a very high cost of obtaining little or no settlement in terms of
utilizing the soil load-carrying capacity because it is almost impossible to
achieve this objective. The cost was often extremely high and very
wasteful.
Foundation movements under highway bridges (figures 1 and 2)
had occasionally been predicted and measured; however, there had been very few
investigations of the effect these movements would have on the safety and
serviceability of the bridge structure. As a result, tolerable foundation
movement had not been established beyond the conceptual stage, and much
controversy among engineers was prevalent.
In addition to the
disagreement on the definition of tolerable settlement, there was also
disagreement on the accuracy of settlement predictions. As a result, many
engineers still prescribed strict settlement constraints to guard against
erroneous predictions. In many cases, the precautions were unrealistic and
underestimated the accuracy of the prediction techniques, which are normally
accurate within 10 to 20 percent on average and are rarely more than 50 percent
off the measured value.
It was noted in earlier studies that using piles
did not guarantee that there would be little or no settlement, unless the piles
were founded on hard rock. The nature of load transfer from the pile to the soil
surrounding (side friction) and beneath the pile (tip bearing) requires some
movement of the pile to mobilize the load-bearing capacity of the soil material.
The relative movement required to mobilize maximum frictional strength is
approximately 6.35 mm (0.25 in), while the displacement required to mobilize the
soil's shear strength under the pile tip is usually much larger than that value.
If the allowable settlement is restricted to a smaller value than what is
required to mobilize tip resistance, the load-carrying capacity of the pile is
artificially reduced to the level of side friction support. The support capacity
available at the pile tip then becomes an additional safety factor.
FIGURE 2. Bridge girder jammed against abutment and excessive rocker tilt.
Overly restrictive settlement constraints can reduce significantly the
supporting capacity of friction piles. To comply with the prevalent requirements
of zero or near zero settlement, the foundation engineer must reinforce his or
her design by adding more piles, using longer piles, or increasing the diameter
of the piles. The conservative effect of this design philosophy is evident by
examining any load-settlement curve from a pile load test. A slight increase in
the allowable settlement value can impact significantly the allowable design
load for a pile foundation, especially within the straight line portion of the
curve.
A relaxation of the stringent movement criteria was also expected
to impact significantly the use of spread footings, whose major drawback is the
risk of some settlement. Because the economic breakpoint in the normal design
procedure for spread footings centers around 25 mm (1 in) of settlement,
the majority of bridge engineers were reluctant to use this less expensive
method of foundation support. As a result, the highway industry has been accused
of having numerous "buried treasures" beneath bridge abutments and piers,
because piles were used instead of spread footings or because more and/or larger
piles were used than necessary.
Research performed under this project
indicated that bridge structures can withstand a reasonable amount of
settlement, and the amount varies according to the span arrangement (simple
versus continuous) and length of span as well as other design variables. This
indicates that the AASHTO blanket criterion approach was inappropriate and the
zero settlement design approach was too conservative.
2.5.1 Preliminary Studies – Prior to the initiation of a
comprehensive research study by FHWA, a series of smaller studies that were
conducted in Ohio, Washington, Connecticut, Canada, and by FHWA staff were
evaluated to help develop a detailed research plan. Studies done for the
building industry were also included in the evaluation. A study of the measured
movement data and corresponding damage assessments were very enlightening, but
insufficient to establish rational tolerable movement guidelines. It was also
noted that the building codes in many major U.S. cities actually permitted
larger settlements than AASHTO bridge specifications, even though building
elements, such as glass doors and windows, elevator shafts, utility lines, and
brittle wall panels, are more sensitive to foundation settlement than bridge
elements.
Although no well defined set of criteria was generally agreed
upon, all of the researchers agreed that the development of a rational set of
tolerable movement criteria for bridges was a high priority research need.
Everyone then turned to FHWA's Office of Research to conduct a major effort to
solve this problem.
2.5.2 FHWA Tolerable Movement Study – A comprehensive
analysis of the data from the earlier studies that evaluated field measurement
surveys was conducted as part of an overall analytical investigation of the
effect of different magnitudes of settlement on the potential level of distress
produced in a wide variety of steel and concrete bridge structures of different
span lengths and stiffnesses. A total of 314 bridges in the United States and
Canada was analyzed to determine if the measured foundation movements and
corresponding damage assessments could yield sufficient insight toward the
development of rational criteria for tolerance to movement. The data by
themselves proved to be insufficient, but when combined with the structural
analysis studies, and an appraisal of existing design specifications and
practice, a rational set of criteria was developed.
The researchers
determined that functional distress is more difficult to assess than structural
distress because of its subjectivity. Functional distress is defined under this
study as damage to the architectural elements or a reduction in ride quality.
Architectural damage is less severe than structural damage that affects the
integrity of a main supporting element of the bridge; however, architectural
damage is usually more visible and causes an annoying or insecure feeling on the
part of the motorist. It is also referred to as "cosmetic damage" and includes
cracking or misalignment of bridge railings, curbs, decks, abutment wing walls,
and damage to light poles and utility lines. The deterioration of ride quality
involves the "bump" at the end of the bridge and other roadway unevenness
associated with bridge foundation settlement.
The specific results of
this study have shown that, depending on type of spans, length and stiffness of
spans, and the type of construction material, many highway bridges can tolerate
significant magnitudes of total and differential vertical settlement without
becoming seriously over-stressed, sustaining serious structural damage, or
suffering impaired riding quality. In particular, it was found that a
longitudinal angular distortion (differential settlement/span length) of 0.004
would most likely be tolerable for continuous bridges of both steel and
concrete, while a value of angular distortion of 0.005 would be a more suitable
limit for simply supported bridges.
For continuous steel bridges,
differential settlements of 25 mm (1 in) or more would be intolerable for span
lengths up to 15.2 m (50 ft) because of the significant increase in stresses
caused by these settlements. However, for span lengths between 30.5 and
61.0 m (100 and 200 ft), the stress increases caused by differential
settlements up to 76 mm (3 in) were quite modest, and for spans longer than
61.0 m (200 ft), the stress increases caused by 76 mm (3 in) differential
settlements were negligible.
A basic design procedure was developed that
permits a systems approach for designing the superstructure and the foundation
system. This design procedure incorporates the tolerable movement guidelines
that are based on strength and serviceability criteria which, in turn, are based
on limiting longitudinal angular distortion, horizontal movements of abutments,
and deck cracking (2,3).
2.5.3 Publications and Implementation Items – Research and development activities for the establishment of tolerable movement criteria were very successful. The FHWA R& D reports documented the efforts that were made to develop the rational criteria and presented detailed recommendations on how the new criteria should be used in typical bridge and foundation engineering scenarios. A technology sharing report was developed to help practitioners implement the new criteria in standard design situations (4). Workshops and presentations were also held to aid the implementation process. A new article was developed for the AASHTO Bridge Specifications (see chapter 7).
2.6 Pile Foundations
The first difficult problem confronting the designer of a foundation is to
establish whether or not the site conditions are such that piles should be used.
The most common case is that in which the upper soil strata are too compressible
or generally too weak to support heavy vertical reactions transmitted by the
superstructure. In this instance, piles serve as extensions of columns or piers
to carry the loads to a deeper, more rigid stratum such as rock (point-bearing
piles). If such a rigid stratum does not exist within a reasonable depth, the
loads must be gradually transferred by friction along the pile shaft. Scour and
the relative inability of spread footings to transfer inclined, horizontal, or
uplift forces and overturning moments also require the use of piles in many
instances.
Another problem facing the pile designer is choosing from
among the 100 or more different kinds of piles. There are many variations in
materials, configurations, and installation techniques. Guidelines for selecting
the best pile for various situations can be found in one or more of FHWA's
guidance manuals (5, 6, 7) and the general literature.
After a particular
kind of pile has been selected for use, the designer then must determine the
number, length, and size of the piles required. Simple guidelines were not
available at the start of this research program to design and analyze piles for
the various situations that can occur in bridge foundations; however, improved
pile design guides are now available in various FHWA references listed at the
end of this report. The ultimate load that can be supported by a certain kind of
pile depends on the strength of the soil and/or pile material. Usually the
ultimate load is determined by soil failure; however, the pile itself may fail
if forced to penetrate difficult site conditions such as dense soil or
rock.
Calculating the ultimate load on the basis of soil failure is one
of the most difficult problems confronting the foundation designer. Because
available theories for determining the ultimate load of a pile and a group of
piles were not accurate enough to provide economical foundation designs,
research was conducted to define the complex mechanics of load transfer between
pile and soil and among the various piles within a pile group. Load transfer in
piles and pile groups was measured in the laboratory and field to develop and
refine analytical models of the pile behavior process to provide more economical
design procedures.
Assuming the pile material is not over-stressed, the
ultimate load capacity of a pile is equal to the sum of two major soil
components — point resistance and side friction. The amount of support
contributed by each component varies according to the soil properties and pile
dimensions. These resistances can be calculated through mathematical
relationships; however, the required input data are difficult and expensive to
obtain. As a result, less expensive index testing usually is performed to obtain
approximate values for estimating the resistances.
Pile capacity usually
is predicted using static and dynamic analysis procedures. On large, expensive
structures, pile load tests often are used to verify the design loads. Recently
completed FHWA experimental studies of instrumented piles load tested to failure
have increased our understanding of load-transfer behavior of single piles and
pile groups; however, more tests are needed to confirm the new prediction
methods.
A mathematical model of pile group behavior was deemed to be a
valuable analytical tool that systematically can convey engineering experiences
from one site to another. Several mathematical models had been developed,
including one by FHWA; however, none had been validated adequately because of
the lack of precise field data on pile group behavior. This was especially true
of new foundation design methods based on finite element analysis. To date FHWA
has performed most, if not all, of the pile group load tests to failure. These
results have been used to validate and refine the FHWA pile group design model
called PILGP (8).
2.6.1 Areas of Emphasis – Because of the high reliance on
driven piles, a large portion of FHWA funds for foundation research was directed
at improved design and construction procedures for driven piling. FHWA pile
research was divided into the following three major areas of emphasis:
2.6.2 Single Piles in Clay – Field load tests on single
piles in soft clays have provided experimental data to evaluate a promising pile
capacity predictive technique based on the general effective stress method. The
effective stress method for predicting pile capacity was validated and its
accuracy and general applicability were improved. Figure 3 shows a full-scale
pipe pile being instrumented in the laboratory prior to field
installation.
The accuracy of the method was limited by the assumptions
made to describe the changes in effective stress between pile driving and
subsequent loading. Major uncertainties arise in attempted modeling of the
effective stresses after the disturbed soil has reconsolidated. Current analysis
techniques were considered applicable to full displacement piles driven into
normally consolidated clays (9).
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FIGURE 3. Lab instrumentation of a pipe |
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FIGURE 4. Field instrumentation of a pipe |
2.6.3 Single Piles in Sand – Previous methods for predicting
the behavior of driven piles in sand gave widely different results because of
several sources of error. Variability of the soil and methods used to obtain the
design parameters also contributed to the different results as did some of the
simplifying assumptions made in developing the theoretical methods.
An
FHWA research study on the behavior of piles in cohensionless soils involved the
analysis of data on instrumented piles (figure 4) tested to failure under
vertical loads. The data, collected through an extensive literature search,
consisted of 35 pile load tests at 10 different sites. The piles were of various
kinds, lengths, and diameters and included steel pipe and H-piles and
prestressed concrete and timber piles. Average pile diameter was 0.4 m (1.27
ft), and average pile length was 15.1 m(49.6 ft ).
Statistical
analyses determined the vertical and horizontal variability of the soil at each
case history site. Load transfer characteristics were analyzed for each pile
without consideration of residual driving stresses. In those few cases where
residual stress data were available, the load transfer characteristics were
reanalyzed to learn the effect of residual driving stresses. It was found that
residual stresses play an important role in pile design and that proper
consideration of residual stresses can result in shorter pile lengths for driven
piles in sand (10).
2.6.4 Allowable Stresses in Piles – In addition to the
ability of soil or rock to carry the load transferred from a pile, the load
capacity of the pile also is important. The load capacity of a pile is governed
by its structural strength and, to a lesser extent, by the surrounding
environmental conditions. The structural strength is a function of the allowable
stress levels that apply to the particular pile material and the cross-sectional
area of the pile.
To provide a factor of safety against failure (figures
5 and 6), allowable stress levels normally are specified as a percentage of the
peak strength value of the pile material (for example, steel, concrete, or
timber). Allowable stress levels for piles vary significantly because of
different building codes in different jurisdictions. Significant controversy had
arisen concerning the allowable stress levels used in the highway industry at
the time of this research project (circa 1980) because the choice of allowable
stress values greatly affects the dimensional analysis and thus the economy of
the foundation system. Another factor to be considered was that competition
between materials producers was keen and significantly affected by changes in
the allowable stress codes.
FIGURE 5. Concrete piles damaged by difficult driving conditions.
FIGURE 6. Steel H-piles badly damaged by hard driving.
A comprehensive investigation of the design of piles as structural members
was performed to define and establish, through structural analysis and
supporting field data, a rational guideline or design methodology for
determining allowable stress levels for pile design codes used in highway bridge
foundations. These methods must take into account not only the material
properties of the pile itself, but also the individual effects of long-term
loads, driving stresses and driveability, imperfections in form or material, and
various environmental conditions that tend to reduce pile capacity.
As
part of the overall review, the investigators studied the codes and
specifications of an array of national and foreign organizations and code
bodies. The basis for each code was reviewed and corroborative data were
assembled to develop improved values of allowable stresses. The claims of
material suppliers for increased allowable stress levels also were evaluated and
documented. Some of the claims were found to be overstated, and all of them
ignored at least one or more of the important factors that govern the structural
strength of a pile.
In general, it was recommended that allowable stress
values in use at that time be decreased for concrete and timber piles and
increased for steel piles. The new procedures for determining appropriate
allowable stress levels on a case-by-case basis according to the major factors
governing structural strength of piles are a significant improvement over the
previously unsubstantiated blanket stress levels. Reasons for each suggested
change to then current methods for determining allowable stress values are
documented in the FHWA report (11).
2.6.5 Performance of Pile Driving Systems – Proper construction control of pile driving operations is as essential as good design practices. Construction control is more difficult for piles than for spread footings because the excavation and construction of footings can be observed. The construction control for pile driving involves checking the pile materials and the installation equipment. The pile inspector can visually check many requirements; however, some of the most important checks require instrumentation. The high cost of pile foundations prompted FHWA to seek more efficient pile driving systems as well as more efficient design methods.
Pile driving technology has evolved from an archaic system of pounding a
"stick" in the ground with a heavy mass to a sophisticated system of installing
long, slender structural elements in a well-defined soil mass. In the previous
era it didn't matter how efficient the pile driving system was because the only
objective was to pound the stick into the ground. However, the vertical advance
of a pile under a given hammer blow can be used as a measure of the pile's
bearing capacity. Thus the hammer takes on a second function, and doubles as a
piece of testing equipment.
A comprehensive
study of available pile driving systems and corresponding measurement techniques
was completed under this research program. In addition to the direct
measurements of output, researchers investigated the usefulness and practicality
of installing gauges (figure 7) and other instrumentation on the pile driving
system to determine performance rating, spot the cause of erratic or inadequate
operation, and evaluate the significance of erratic behavior in terms of
performance capability.
The researchers developed three devices to evaluate the field performance of
pile hammers. The ram velocity measurement (RVM) apparatus uses radar to record
ram velocity as a function of time for conversion to kinetic energy for driving.
The study also developed a system that uses piezoelectric accelerometers to
measure pile hammer impact velocity. This system is referred to as AMS (for
Acceleration Monitoring System). Both the RVM and AMS are expensive and
complicated devices for routine use on pile driving projects. A simpler device,
the Saximeter II, was developed for FHWA to aid the pile inspector in log
keeping and monitoring pile driving performance. It electronically measures
blows per minute and converts time duration to the corresponding fall height
(stroke) of the ram, and converts stroke to the potential energy.
An
inspection manual was also developed to aid construction engineers in evaluating
the performance of pile driving systems (12). It can be used to ascertain that
the pile hammer conforms to certain minimum standards, and to record
observations on hammer and driving system behavior. The manual explains the
theory of operation and inspection procedures for various hammer types,
describes all components of the driving systems, includes a glossary of pile
driving construction terms, and provides summary sheets and forms to aid the
inspector in recording the pile installation process. A slide-tape show was also
developed to provide a visual rendering of the inspection manual for
instructional purposes.
2.6.6 Pile Wave Equation Technology – For many years, pile
design used pile driving formulae that were established to relate dynamic
driving forces to available pile bearing capacity. The original dynamic formula
was developed more than a hundred years ago and was based on a simple energy
balance between the kinetic energy of the ram at impact and the resulting work
done on the soil. The concept assumed a pure Newtonian impact with no energy
loss.
A close examination of the pile driving process recently disclosed
that the concept of a Newtonian impact does not apply. It was also noted that
when the hammer strikes the pile, a force pulse is created that travels down the
pile in a wave shape. The amplitude of the pile wave decays before reaching the
pile tip because of system damping properties; however, a portion of the force
in the wave reaches the tip and forces the pile to penetrate the
soil.
Before the demise of the dynamic driving formulae and before the
wave equation analysis was fully developed, pile designers relied on static pile
analyses to predict pile capacity and determine pile sizes and lengths. The
success of this method depended greatly on the accuracy of boring data and the
engineer's ability to properly classify zones in the subsoil with regard to
relative pile support capability. Static analysis methods will always be very
useful because they are much more accurate than the old driving formulae;
however, the value of wave equation type methods has been steadily
increasing.
The wave equation method is a computer based analysis that
was developed from the classical wave theory, which models wave propagation in a
slender rod subjected to an applied force at one end. Modifications to the
classical theory are necessary to account for changes in the traveling wave form
due to pile and soil properties. If the wave is completely dissipated by the
pile material and soil properties before reaching the pile tip, no penetration
will occur. This can be due to either too small a hammer for the pile, or too
small a pile cross-sectional area specified for the length being driven. Wave
equation analysis can detect both of these scenarios before they happen in the
field, plus it can also predict pile damage if too large a hammer is
selected.
Soil data input requires both an understanding of site-specific
soil properties and the effects of pile driving on those properties. These
dynamic properties are known as damping and quake and are roughly correlatable
with soil type. These properties are best determined by experienced geotechnical
engineers. Research was needed to provide better methods to determine soil
damping and quake values from laboratory and/or in situ soil testing equipment
to better predict pile capacity.
2.6.6.1 Pile Capacity Prediction – In response to
this need, FHWA initiated a contract research study to perform an in-depth
assessment of current techniques and potential methods for determining soil
quake and damping input parameters to the wave equation computer analysis
program. The contractor was required to develop a data base of pile foundation
sites containing research-quality soils data, pile driving records, load test
information, and Pile Driving Analyzer (PDA) data. These data were to be used to
make correlations among quake and damping factors, Wave Equation Analysis
Program (WEAP) capacity predictions, and soil test data. It was also a
requirement to locate 6 to 10 actual projects where load testing and dynamic
measurements were made, plus good soil data had to be available to perform the
required correlations to evaluate the newly developed procedures and propose
additional modifications, if necessary.
The following is a reprinting of
the report's abstract:
"Research has been conducted on the potential
improvement of dynamic wave equation analysis methodology using in-situ soil
testing techniques. As a basis for this investigation, the literature was
reviewed and a summary was compiled of efforts made to date on the development
of models and associated parameters for pile driving analysis. Furthermore a
data base was developed containing more than 150 cases of test piles with
research quality data on static load tests, dynamic restrike tests, soil
information, driving system data and installation records. One hundred data base
cases were subjected to correlation studies using the CAse Pile Wave Analysis
Program (CAPWAP) and various static analysis methods. This work yielded dynamic
soil model parameters which did not indicate a specific relationship with soil
grain size" (13).
2.6.6.2
Simplified Capacity Predictions – Static load testing (figure
8) to failure is probably the best method available to determine the actual
static capacity of a pile; however, these tests are expensive and time
consuming. As a result, they are not routinely conducted. In many cases a load
test to twice the design load is conducted to verify safety factors and save
money on the cost to extend the loads to the failure range. These tests give
some measure of assurance to the project design team, but do little to advance
the knowledge of how to design a particular pile in a particular soil. The
simple act of striking a pile with a heavy hammer can also be thought of as an
instantaneous load test to failure; because in order for the pile to penetrate
further into the soil, the soil must fail under the driving forces. In other
words, pile driving is actually a very fast load test under each hammer blow.
With the right type of instrumentation, the engineer can take advantage of these
failure measurements and use the information to make a prediction of potential
static capacity in the field during pile driving operations.
This concept
was proposed to FHWA by the developer for further study. The idea to develop a
simplified method based on energy balance between the total energy delivered to
the pile and the work done by the pile/soil system was examined under a research
contract study in the early 1990's. This method, called the "Energy Approach",
uses the calculated transferred energy and maximum pile displacement values from
the measured data, together with the field blow counts as input parameters to
calculate the pile capacity.
To verify and refine the new method, two large data sets were retrieved from
the FHWA Deep Foundations Load Test Data Base (see section 5.2). One set
contains 208 dynamic measurement cases on 120 piles monitored during driving and
followed by a static load test to failure. These cases reflect various
combinations of soil-pile driving scenarios. The other set contains data on 403
piles monitored during driving, but without static load test data.
The
Energy Approach method was found to be very accurate in predicting the pile
capacities that were measured in the static load tests. These estimates were
also found to be more accurate than the sophisticated office methods commonly
used in engineering practice. This comparison was especially true for records
obtained at the end of initial driving. This approach can be used in place of or
as an independent check of the office methods (14).
2.6.7 Micropiles – Micropile technology was conceived in
Italy in the early 1950's to fill the need for an economical and versatile
foundation system that could be used to underpin, repair or retrofit badly
damaged infrastructure elements in war-torn Italian cities. It was introduced in
the United States about 20 years later; however, it did not catch on near as
much as it did in Europe, especially, in the late 1980's and early 1990's. In
addition to static applications, significant growth in the use of this
technology has occurred in seismic and slope stabilization
applications.
All of these uses are directly applicable to highway
projects, which caught the eye and interest of FHWA geotechnical engineers. Two
major earthquakes in California and one in Japan also stirred interest in the
use of micropiles for seismic retrofit of highway bridge foundations. At the
same time, the French Highway Administration began a major research project to
investigate the basic mechanisms and engineering characteristics of micropile
systems.
Micropiles are defined as small-diameter, drilled
piles composed of placed or injected grout with some form of steel reinforcement
in the center of the grout to resist the bulk of the design load. The central
reinforcing element is either a high-strength steel bar or tube that is secured
in the grout that is injected under high pressure to improve bonding with the
surrounding soil. Micropiles can be installed through virtually any ground
condition and at any inclination. Modern construction techniques keep noise and
vibration to a minimum, and they can be constructed under limited headroom
conditions to within a few millimeters of adjacent properties. Figure 9 shows an
early model micropile that was exhumed for inspection purposes.
In 1992
FHWA was invited to become a cooperative research partner in the French
micropile project.
The major objective of this cooperative research
project was to establish reliable engineering guidelines and safe design methods
for the use of micropiles in the reinforcement and stabilization of foundations
and slopes.
It was anticipated that the development of reliable engineering guidelines,
combined with site monitoring for field performance assessment of micropile
systems, would significantly increase the engineer's confidence in the technique
and, thereby, greatly enhance its expansion to new fields of
application.
The initial investigations included a comprehensive review
and critical assessment of available information on current state of the
practice, research case studies, site performance monitoring, quality control
issues, and any comparisons or analyses of current codes of practice. Several
ongoing micropile construction projects were also instrumented and monitored to
provide researchers with additional case histories to study and evaluate. The
major physical experimental tasks involved centrifuge modeling and related
analytical/numerical simulations, full-scale testing, and field monitoring,
which were designed to study the engineering behavior of individual micropiles
and micropile groups and/or systems under axial and transverse load response
modes (figure 10). Buckling, corrosion, and seismic aspects were also
investigated under actual working conditions as well as at failure under
ultimate loading.
FIGURE 10. Load testing of micropiles in California.
2.6.7.1 State-of-the-Practice Review – In 1993 FHWA initiated a contract to produce a comprehensive review document that would delineate all of the known information about micropile technology that could be gathered from various organizations around the world. The contractor put together an international group of experts to evaluate the information obtained in the surveys. The review report (15) provides a comprehensive introduction to micropile technology, and assisted FHWA in developing an engineering guidelines manual (7) containing reliable design guides, construction specifications, and quality-control procedures for the wide spectrum of micropile applications.
2.6.7.2 Seismic Behavior of Micropiles – In 1995, FHWA initiated a contract research study to develop improved seismic design methods for micropile systems used in new construction in earthquake zones, seismic retrofitting of bridge foundations, vertical excavation retainment systems, and slope stabilization. Laboratory centrifugal model tests were conducted to correlate with numerical model studies on various micropile systems (isolated piles, groups, and networks of piles) to evaluate their behavior under axial, lateral, and combined loadings in selected engineering applications. Shaking table tests were also conducted at Christchurch University in New Zealand as part of the overall study to improve seismic design methods. This study is scheduled to be completed after this report is printed.
2.6.8 Publications and Implementation Items – Research and development activities for pile foundations were very successful. Results were documented in numerous reports, and several technology sharing reports were issued to help implement the findings. Several workshops and symposia were held to promote the new guidelines and prediction methodologies that were developed under the research program. Several new articles were added to the AASHTO Bridge Specifications and many others were updated based on the FHWA pile foundation research results (see chapter 7).
2.7 Pile Groups
The state of the art for pile foundation design at the beginning of this
project did not include an accurate method for relating the ultimate bearing
capacity and settlement behavior of a single pile to the behavior of a group of
closely spaced piles. To develop such a method, it was necessary to conduct
field load tests to failure of full-scale pile groups to obtain field data that
were useful in interpreting fundamental phenomena that control the behavior of
groups of driven piles.
Older methods of pile group design treated the
group as a collection of individual piles requiring an adjustment factor.
Predicting the behavior of pile groups required correction of the load capacity
of the individual piles in the group for the interaction effects transmitted
through the soil mass. How this should be done was never certain, and it was
recognized that it would probably be different in cohesive as contrasted to
granular soils.
In 1980, FHWA initiated a research program to investigate
pile group behavior through carefully performed experiments. First, existing
mathematical models used to design pile groups were identified and evaluated.
From this evaluation, the "hybrid model" was selected and used to analyze a
proposed full-scale pile group to be load tested.
The hybrid model, a
load-deformation model, reasonably predicts load versus settlement, load
transfer patterns, and load distribution to pile heads. The model was especially
helpful in designing the instrumentation system for the full-scale pile group
load test. The field data acquired from the load tests were used to refine the
hybrid method, which led to the FHWA PILGP1 computer program, a modification and
refinement of the hybrid model (see section 2.7.3).
2.7.1 Pile Groups in Clay – To develop the field data
required to verify and refine PILGP1, 11 instrumented steel pipe piles were
driven into a very stiff, saturated, over-consolidated clay soil at the
University of Houston, Texas, campus. The outside diameter of the piles was 273
mm (10.75 in), with a wall thickness of 9.27 mm (0.365 in). The piles were
driven closed-ended 13.1 m (43 ft) deep. Nine piles were driven in a 3 x 3
square array on a spacing of three pile diameters. The two remaining piles were
driven apart from the group to serve as control piles.
Each of the 11
piles was instrumented with full bridge strain gauge transducers and mechanical
telltales to monitor load transfer from the piles to the surrounding soil. Four
of the group piles and one control pile were instrumented with piezometers and
lateral total pressure cells. The surrounding soil also was instrumented with
piezometers and vertical ground movement devices. Electronic load cells measured
applied loads, and settlements were measured at each pile head.
Three-dimensional translations were measured on the massive concrete pile cap
that was suspended off the ground.
The load testing program consisted of
11 compression and 6 uplift (tension) tests, all of which were carried to
ultimate failure loads. The control piles were load tested in compression at
three time intervals to study the effect of soil setup. Each control pile also
was tested in uplift. The nine-pile group (figure 11) was tested in compression
atthree time intervals to assess setup characteristics of the group. Four of the
group piles were tested in uplift; however, these tests were preceded by
compression tests on two smaller groups of four and five piles. The five-pile
subgroup was formed by cutting away the four corner piles, leaving the four edge
piles and the center pile. The four-pile subgroup was formed by removing the
center pile (8,9).
FIGURE 11. Pile group load test in clay.
2.7.2 Pile Groups in Sand – A second pile group load test
study was performed on a group of eight timber piles in sands at the Locks and
Dam No. 26 near Alton, Illinois, as part of an evaluation of several
rehabilitation schemes for the distressed locks and dam structures. The timber
piles were instrumented to measure load transfer and deformation up to and
including the failure load. The acquired field data were used to evaluate and
refine the PILGP1 program (16).
The third pile group study involved load
testing five single piles and a group of five piles in sand at a test site in
Hunters Point, California (figure 12). The load test results were also used to
evaluate PILGP1 and a simplified procedure developed under previous studies of
the behavior of single piles and pile groups. Data were obtained on the
comparative behavior of piles of different types with the same relative
geometry. Residual stresses, load transfer, and load-settlement characteristics
were measured and used to evaluate the effectiveness of the new pile design and
analysis procedures. An isolated control pile and three of the five group piles
were fully instrumented, to obtain the load-response data for the analysis. Soil
and ground water conditions were evaluated with standard and state-of-the-art
equipment, such as Standard Penetration Test, Dutch Cone, Stepped Bladed Vane,
Dilatometer, and Pressuremeter testing devices (17). Figure 13 shows a closeup
view of the test pile group and two soil instrumentation boreholes.
After
each of the first and third pile group load test studies, a pile group
prediction symposium was held to discuss the test results and evaluate the most
popular pile group design methods in use at that time (18,19,20).
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FIGURE 12. Reaction frame for pile group |
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FIGURE 13. Instrumentation bore holes |
2.7.3 Predictive Model for Pile Group Design and Analysis
(PILGP1) – A comprehensive computer-aided design method called
PILGP1 was developed for FHWA at the University of Houston for the design and
analysis of pile groups. It is a load deformation model that predicts load
versus settlement behavior, load transfer patterns, and load distribution to
pile heads. The model designs the pile group as an interactive element rather
than as a collection of individual piles requiring an adjustment factor. It was
developed from the results of the first full-scale field load test program on
single piles and pile groups. Two additional full-scale field load test programs
were later completed to further refine and validate the computer model. The
current version of the model (PILGP2) is being tested by several consulting
firms, States, and Federal agencies. Continued testing, evaluation, and
refinement of the model is currently under way.
The development and
verification of PILGP1 was based primarily on the full-scale field tests of pile
group behavior under both working loads and failure conditions. Because of the
many variables involved, numerous full-scale field tests needed to be conducted
to provide a statistically meaningful data base. However, the costs involved in
full-scale field testing significantly restricted the number of tests that could
be conducted. The alternatives to full-scale field testing are model field
testing, laboratory model studies, and centrifuge model testing.
2.7.4 In-Service Monitoring of Pile Groups – The three full-scale pile group load tests to failure produced very accurate and valuable data; however, they were very expensive to conduct. In addition to the pile group load tests to failure, four full-scale field projects were initiated with FHWA research funding to observe pile group behavior under working loads. The short- and long-term behavior of the in-service piles was compared with analytical predictions made by PILGP1.
One of the projects was located on the Natchez Trace Parkway (NTP) in
Mississippi, where a pile-supported bridge abutment was instrumented to obtain
load transfer data on a group of six steel piles (12 HP 53) in soft clay and
silt. Samples of load/settlement curve data are shown in figure 14. Another
project was located at Fort Belvoir, Virginia, where soil and pile conditions
were similar to those at the Mississippiproject. Instrument readings were taken
weekly during the first year at each site, monthly during the second year, and
quarterly for several years.
The third site, at the Mocks Bottom
over-crossing in Portland, Oregon, near Swan Island, is underlain by a thick
compressible clayey silt deposit. High down-drag loads were expected because of
the approach embankment loads on the compressible soil. A bitumen coating was
used to reduce down-drag loads by about 90 percent. Although the bitumen-coated
piles cost about 15 percent more than the uncoated piles, fewer piles were
required. Pile instrumentation included settlement and load transfer
monitoring.
The fourth site was located at the West Seattle Freeway in
Washington where a group of twelve 310-mm- (12-in-) diameter concrete piles
supports a pier in medium-dense sands. The bridge pier and pile cap were
instrumented to measure the amount of load transferred to the pile cap, and each
pile was instrumented to measure load transfer from the pile cap to the top of
the piles. Three piles were instrumented for load transfer along the entire pile
length of 30.5 m (100 ft).
2.7.5 Model Testing – The high cost, measured in both time
and money, of obtaining high quality datafrom full-scale load tests on single
piles and pile groups, led to the FHWA staff study to determine if accurate data
could be obtained by conducting model tests in simulated ground conditions.
Using carefully controlled large test pits located at FHWA's
Turner-Fairbank Highway Research Center (TFHRC), more than 200 model tests on
single piles and pile groups have been completed on several different types of
piles in sand and clay soils. These data are being compared with the carefully
controlled centrifugal and full-scale single pile and pile group load test data.
This work is continuing as more variables are studied, and results are entered
in the load test data base to be used for verifying and refining the PILGP1
model.
An investigation of scale effects is an important part of this
study to determine if the model test data can be productively correlated with
the load test data of the full-scale field tests. The establishment of
appropriate scale factors allowed more model tests to be substituted for
expensive full-scale tests. Small-scale models permit numerous parametric
studies at a reasonable cost, and allow soils and other conditions to be
carefully controlled.
The first series of laboratory model tests was
patterned after the timber pile field study at Alton, Illinois. The sandy soil
at the Alton test site was matched as closely as possible at TFHRC. Model load
tests were run on single piles and pile groups (figure 15) at 1/20, 1/15, 1/10,
and 1/3 full scale. As a minimum, three load tests were performed for each
scale. Each pile was instrumented with strain gauges to measure load
transfer.
FIGURE 15. Load test on small-scale model pile group in test tank.
The second series of laboratory model tests was patterned after the full-scale load test on steel pipe piles in clay. Model tests were run at 1/15, 1/10, 1/6, and 1/4 full scale. The laboratory model tests on the 1/20, 1/15, and 1/10 scales were performed in a steel tank 1.5 m (5 ft) in diameter and 1.5 m (5 ft) deep. Because the 1/6, 1/4, and 1/3 scale models were too large to be tested in the laboratory test mold, outdoor test pits were constructed at the TFHRC site (figure 16). The third series of tests was patterned after the full-scale pile group load test to failure at Hunters Point, California.
FIGURE 16. Load test on large-scale model pile group in test pit.
The scaling factors identified in this study were used to establish relationships between load deformation behavior of reduced-scale and full-scale piles and pile groups. These small-scale tests provided a significant amount of data to validate PILGP1 at less cost than full-scale field testing.
2.7.6 Centrifuge Model Testing – Small-scale models permit
parametric studies at reasonable cost and allow soils and other conditions to be
carefully controlled; however, it is difficult to achieve similitude between
corresponding stresses and strains in the model and prototype. The response to
load of a small pile and a large pile cannot be modeled by any simple, direct
relationship derived by ordinary dimensional analysis. The question of scale
effects must be resolved before any useful relationships for pile design can be
developed.
Models of large, heavy structures where gravity is a principal
loading factor are not effective indicators of prototype behavior because the
state of stress in the model caused by self-weight will be much lower than in
the prototype. If the model can be placed in an artificially high gravitational
field, the state of stress limitation can be counteracted almost entirely. A
centrifuge apparatus provides the necessary accelerated gravity rate to load
test the model under simulated gravitational forces. However, to accurately
measure stresses and strains, the centrifuge must be able to accommodate a model
that is large enough to handle the required instrumentation. The larger
centrifuge capacity provides more accuracy in direct modeling of large prototype
structures. A pilot study validated the feasibility of using centrifuge
techniques for corroborating the PILGP1 mathematical model.
In a larger
study, the centrifuge was used to test models of the full-scale pile groups that
were load tested to failure in the previously described field studies. The
combination of centrifuge model testing and small-scale laboratory testing of
conventional models at TFHRC provided valuable physical data to establish
relationships between pile groups of varying scales in the same
environment.
The larger, more comprehensive centrifuge testing program on
single piles and pile groups was performed in sand and clay soils. The results
were used to test and verify assumed similitude relationships between scaled
models and full-scale prototypes. The load deflection and load transfer data
were used to predict full-scale performance and compared favorably with both
measured results and predictions made by computer generated results. The
experimental procedures developed and the verification established in the
program has encouraged the investigation of the many factors that influence pile
performance under controlled laboratory settings. The centrifuge technique
should also be useful and very cost-effective in establishing the predicted
behavior and the sensitivity to design changes of pile foundations for large
projects (21).
2.7.7 Lateral Loads on Pile Groups – Several analysis and
design methods for pile groups under lateral loading have been proposed, but
none had been validated using carefully performed field experiments on
full-scale structures. The theory of elasticity and a number of "efficiency"
formulas often were used as analytical methods for lateral load design. One of
the most promising methods is called the Poulos-Focht-Koch (PFK) procedure. This
method was evaluated through field load tests. Data from the load tests also
were used to develop an improved method of analysis.
The main objectives
of the field load tests were to provide high-quality field data on the
performance of a full-scale pile group under lateral loading and to compare
measured response with that predicted by the PFK method. The load test was
performed at the FHWA pile group test site at the University of Houston, Texas,
campus using the same piles and some of the instrumentation used in the vertical
load tests previously described.
The results of these tests were also
evaluated under a cooperative study with the U.S. Army Corps of Engineers that
was performed at the University of Houston site. After the lateral load tests in
clay were completed, the researchers replaced the top 3 m (10 ft) of clay with
sand backfill and repeated the load tests. The test results were used to refine
and validate the predictive models (22).
2.7.8 Publications and Implementation Items – Research and development activities for pile group foundations were very successful. Numerous quality reports were generated from an unprecedented program of research into an area where few have ventured and none to the same extent. Textbooks and specification codes have incorporated the results of these tests, and the state of the art was advanced significantly.
2.8 Drilled Shafts
A drilled shaft is a machine-excavated circular hole in soil and/or rock that
is filled with concrete and reinforcing steel to support heavy structural loads
in single or multiple units. Vertical loads are resisted by both the base area
of the shaft and in side friction which can be very significant because the
concrete is cast wet and cures directly against the soil forming the walls of
the borehole. They are sometimes socketed in rock, and steel casing is sometimes
required for hole stabilization that may or may not be removed. Horizontal load
is resisted by the shaft in horizontal bearing against the surrounding soil or
rock.
Drilled shafts have many advantages that set them apart from piles
and spread footings. In fact each foundation element has certain pros and cons
that have to be weighed one against the other when deciding on which system to
use in a particular design situation. Each system also has several disadvantages
that have to be evaluated for each design situation. To give confidence as to
performance with respect to the intended task, any foundation element requires
both a reliable method of construction and a standard to define acceptance after
construction.
In this regard, drilled shafts were considered to be less
reliable than others because of the uncertainty of the effects of construction
on the actual service behavior, and the limited knowledge of either reliable
quality-control tests to locate and evaluate defects or inexpensive load test
procedures. Even if there are only occasional failures, they highlight the
variables and unknowns present when working underground, particularly in
water-bearing and potentially caving soils. This results in a lower risk
tolerance for a single or double shaft supported pier compared with multiple
pile supported foundations.
Because drilled shafts for many design
situations offer higher capacities with potentially better economics than driven
piles, the FHWA has spent considerable time and money in research and
development of improved design and construction guidelines for drilled shafts.
Other advantages include less noise and vibration during construction and the
ability to go very deep to avoid scour problems. A major research program was
designed to evaluate existing nondestructive testing techniques for identifying
defects and/or results of adverse downhole conditions that impact the load
transfer/settlement behavior of drilled shafts. It was also planned to develop
rational acceptance criteria for defective drilled shafts on the basis of
quality control during construction, plus a field test program to verify the
research findings that included a search for more economical methods to test
shaft capacity with and without defects.
2.8.1 Nondestructive Evaluation (NDE) of Drilled Shafts – In July 1988, FHWA initiated a contract research study to examine drilled shafts for the effect of defects on performance, and to develop acceptance criteria for use by construction engineers to accept, reject, or modify a newly constructed drilled shaft. The study included the construction of 20 drilled shafts with and without defects for different soil sites located in California and Texas (figure 17). The shafts were constructed using different techniques: dry construction, and wet construction using drilling water, controlled bentonite slurry, and controlled polymer slurry. Five instrumented shafts were statically load tested (figure 18) to determine the effect of the man-made defects on shaft performance, and all shafts were dynamically load tested to correlate with static results. All shafts were tested non-destructively using both surface reflection and direct transmission techniques to determine their effectiveness in identifying defects and/or the results of adverse down-hole conditions that impact the load/settlement behavior of the shafts; results were summarized and evaluated in the report. The allowable defect criteria developed consider the design basis, the ratio of design stress to a maximum code allowable, the type of stress, the level of quality control, and the risk tolerance.
FIGURE 18. Vertical load test of defective drilled shaft.
In addition to the establishment of acceptance criteria, a rating guideline for implementing special integrity testing and a decision tree were developed to guide the engineer or decision maker through construction of a drilled shaft project that includes nondestructive testing. The report also includes detailed information on four promising low-strain NDE techniques (sonic echo, impulse response, cross-hole sonic logging, and gamma-gamma logging), which can be used as part of the quality control procedure, plus information on three alternative load test procedures that show high potential for being economical substitutes for expensive static load tests (see section 2.10).
The sonic echo (SE) and impulse response (IR) test methods use impact energy
applied to the top of the shaft to generate energy waves down the shaft and
return to a receiver that measures the vibration response. These methods are not
able to locate deep defects and they aren't able to detect the size and location
of the defects. Smaller defects lying below a larger defect are easily masked
from above. Cross-hole sonic logging (CSL) and gamma-gamma logging (GGL)
overcome these problems by being downhole methods that pass ultrasonic or
radiation waves through the concrete between source and receiver probes in a
water-filled tube or hole pair as the probe cables are pulled from the bottom
back to the surface over a depth measurement wheel. These methods test the
quality of the concrete lying between a pair of tubes. Four tubes installed in a
shaft before concreting gives sufficient coverage to adequately inspect a shaft
for defects. Figure 19 shows an end view of a rebar cage with four NDE access
tubes.
Results clearly show that the CSL method is superior. The GGL
method requires a radiation source and requires PVC tubes because steel is not
compatible with the radioactive materials. This method is also sensitive to
defects close to the test tubes, and doesn't have the same range away from the
installed tubes that CSL has. In addition, CSL is generally much faster to
perform and does not use radioactive materials (23). Figure 20 shows a
technician performing a surface deflection test on one of the defective
shafts.
FIGURE 19. Instrumented drilled shaft showing man-made defect and NDE access tubes.
FIGURE 20. Low-strain impact test with instrumented hammer and geophone sensor.
2.8.2 Drilled Shafts in Intermediate Geomaterials – Current
design methods for drilled shafts in soil or competent rock are reasonably
well-founded; however, comparatively little effort has been expended to
determine design parameters for intermediate materials such as shales,
claystones, and marls. In September 1991, FHWA signed a second contract to
develop, test, and recommend criteria for determining appropriate exploration,
in-situ testing, and the necessary inputs to the load transfer function or other
design methods for drilled shafts in intermediate geomaterials.
The
contract consisted of essentially three tasks:
Data were collected from projects in Australia, Georgia, Texas,
Massachusetts, and Kentucky. These data have been used to develop the criteria
for exploration and sampling of soil materials to use for developing new load
transfer functions for the design of drilled shafts in shales, tills, and other
materials between soils and hard rock. A field test program was also designed to
validate the new design methods (24).
2.8.3 Free Fall of Concrete in Drilled Shafts – This study
developed information and data that were used to justify increased depths of
free-fall placement of concrete into properly constructed clean and dry shafts
without meaningful loss of strength or segregation of the concrete aggregate.
The report of findings has had a significant economic impact on the drilled
shaft industry.
Prior to this study, the question of whether the
free-fall of concrete over long distances adversely affects the concrete
strength and integrity in drilled shafts persisted in the minds of many
engineers and construction inspectors. This question persisted despite past
efforts to answer it and dispel the concern that concrete does not segregate
during free-fall at any height, and that free-fall placement can be accomplished
without adverse effect on the concrete.
To accomplish the research goals
in a cost-effective way, four 18-m- (60-ft-) long, 0.9-m- (3-ft-) diameter
shafts evenly spaced and tangent to a central 1.5-m- (5-ft-) diameter access
shaft were constructed. The four test shafts were divided into six 3-m (10-ft)
sections with one of four different concrete mixes placed in each section. The
slump, maximum aggregate size, and placement procedures were also varied. The
low slump mixes were also placed with and without super plasticizer. The three
placement procedures were free-fall central drop with careful control to be sure
that the concrete didn't strike the rebar cage, free-fall sloppy drop with
effort actually made to see that the free-falling concrete did hit the rebar
cage, and tremie placement with a tremie pipe extended all the way to the
concrete placement level.
The results of the research were very
conclusive and positive. Construction codes have been changed in many cities,
counties, and States, accordingly. More specifically, the results were as
follows:
None of the lifts placed by central drop free-fall procedures
within the research program exhibited any signs of aggregate segregation. The
design strengths of all centrally dropped lifts varied from 13 percent less to
20 percent more than the reference cylinder strengths. All of the strengths were
well above the intended design strength.
Due to the small variation in
the compressive core strength and lack of aggregate segregation, no definitive
effect of slump, aggregate size, height of drop, depth of fluid pressure, or
addition of super plasticizer was discerned.
Surprisingly, in six out of
seven direct comparisons made between sloppy drop and central drop placement
procedures, the sloppy drop methods actually resulted in higher average
compressive core strengths than equivalent central drop procedures. Also, no
segregation of aggregate was noted for any of the sloppy drop mixes placed.
Thus, on the basis of this research, it is concluded that striking the rebar
cage or the side of the shaft does not have a detrimental effect on the strength
or integrity of the concrete.
Due to the high strengths and lack of
segregation that were apparent in all of the sloppy drop lifts, the effects of
aggregate size, slump, height of drop, height of fluid pressure, and addition of
super plasticizer did not appear to affect the results in a meaningful way for
the well-designed concrete mixes. Even though sloppy drop procedures were not
found to affect the strength or segregation of the concrete, it is not intended
that contractors should begin to place concrete in a haphazard fashion. The
sloppy drop procedure adversely affected the placement of the rebar cage and
also caused additional concrete contamination as a result of traveling down the
soil sides of the shaft. In all cases, the shafts were fully formed and no
honeycombing, voiding or exposed rebar was evident (25).
2.8.4 Load and Resistance Factor Design of Drilled
Shafts – In early 1997, FHWA co-funded a research study on
"Resistance Factors for Drilled Shafts with Minor Defects" with the Association
of Drilled Shafts Contractors (ADSC) and a number of SHA's including California,
Florida, Illinois, Minnesota, Montana, North Carolina, Pennsylvania, and South
Dakota. The objective was to develop design techniques to account for
sub-detectable minor defects that can occur during construction. The analytical
study and most of the experimental work was done by the University of Houston at
its National Geotechnical Experimentation Sites (NGES) facility. Some of the
future testing will be done by FHWA staff at the TFHRC.
It is proposed to
derive resistance factors (or "workmanship factors") for drilled shafts through
analytical modeling that will be calibrated by performing field loading tests to
structural failure on drilled shafts with selected defects. The load testing
will primarily be lateral because the loss of moment resistance in the lateral
mode of loading is potentially more severe than loss of axial capacity for
drilled shafts with steel percentages normally used in highway bridge
construction.
A computer program called LPILE Plus was modified to
perform accurate and realistic modeling of the influence of minor defects on
drilled shaft behavior. The modified version allows for the development of an
independent stress-strain curve within the defect area. It also allows for a
reduction in the tensile strength of both the defective and nondefective
portions of the shaft section. A defect may be located in either the compressive
or tensile portion of the shaft. It is also possible to include more than one
defect in the same section. Six field lateral loading tests were conducted to
verify and calibrate the modified LPILE model.
Lateral load tests were
conducted by jacking a test shaft against a reaction frame consisting of a wide
flange steel girder supported by two reaction shafts. A manual jacking system
was used to jack the test shaft against the reaction frame. Loads were measured
with a load cell and dial gauges were used to monitor lateral deflections of the
test shafts. All test shafts were constructed under wet conditions using a
polymer slurry.
The test shafts were then extracted and examined
carefully to determine the location of the plastic hinges, and to caliper the
exact dimensions along the shaft axes. The results of the load tests will be
used to verify or to calibrate the modified version of LPILE. This modeling will
be eventually used to develop recommendations for resistance factors for drilled
shafts under combined axial and lateral loading. Preliminary recommendations
will be developed in this phase of the research, while more general and refined
recommendations will be developed in the following phases if the results of the
preliminary phase justify further study.
Because typical drilled shaft
defects are usually found at the boundaries between soil and concrete, they can
have effects on both the structural performance of the drilled shafts and the
soil-structure interaction. To isolate the effect of the structural defects from
the variation in the soil resistance, structural laboratory tests will be
conducted on slightly defective shafts out of the soil at the TFHRC, if the
preliminary field test results appear to justify the effort. The results of this
testing program will also be used to verify the method in the modified version
of LPILE that is used to model the changes in cross-section and stress-strain
behavior within the concrete drilled shaft.
Eight drilled shafts will be
tested in the TFHRC laboratory experiment. Both the diameter and steel
reinforcement of the laboratory experiment will be identical to those of the
field load tests. The laboratory test shafts will include defects essentially
similar to those included in the field tests. To investigate the effect of rebar
corrosion, the diameter of the rebars of two additional defective shafts will be
reduced to one-third of the original size within the defect. Six slightly
defective drilled shafts will be tested under pure bending moment conditions.
One of the other two shafts will be tested under combined shear and bending
moment, and the last shaft will have no defects (reference shaft) and will be
tested under pure bending moment conditions.
If the results of the load
tests at TFHRC and NGES-UH prove successful, additional field loading tests will
be conducted on slightly defective shafts over a larger range of soil conditions
and for a larger range of defect types sufficient to provide recommendations for
the AASHTO code committees. It is also possible that the testing will be
extended to include fixed-headed shafts because the preliminary work only looked
at free-headed shafts.
It is expected that these results will help to
solve the problem or source of conflict between some (usually owners) who
interpret any anomaly that shows up on NDE signatures as a defect and others
(usually contractors) who believe that all anomalies are not necessarily defects
that require attention. Serious defects require further probing, immediate
repair, or penalty charges assessed against the contractor; however, minor
defects should be accounted for in design through the derivation of appropriate
resistance factors.
2.8.5 Publications and Implementation Items – Research and development activities for drilled shafts were very successful. Several quality reports were generated and appropriate specifications and codes were revised (see chapter 7) to reflect the positive results and improved procedures developed under the research program. The FHWA design manual and instructional workshops were also updated to reflect the advancements and contributions of this program.
2.9 Spread Footings
Foundation engineering is one of the oldest professions of mankind. The major
decision in selecting a foundation system has always been whether to use shallow
or deep foundations to support a superstructure. Foundation support is achieved
by transferring the loads to the ground materials without incurring excessive
settlements or distortions. Shallow foundations (spread footings or mats)
normally are less expensive to construct than deep foundations (piles, drilled
shafts, or caissons). However, surface soils usually are less capable of
supporting heavy loads than deep soil deposits or bedrock. Deep foundations
provide extra security against bearing capacity failure or settlement. A greater
risk of unexpected engineering and contractual problems exists, however, during
deep foundation installation. Engineering and cost analyses are necessary to
determine the proper foundation system for major structures, such as bridges,
buildings, dams, and nuclear power plants.
The use of spread footings to
support highway bridges in the United States varies widely between States and
appears to be far below the optimum level. A recent survey by FHWA determined
that most States use pile foundations to support the majority of their bridges.
The extensive use of piles to support U.S. highway bridges is contrasted by the
extensive use of spread footings to support highway bridges in some foreign
countries. For example, highway bridges in England seldom are founded on piles
despite severe subsidence problems from coal mining. Another contrast can be
seen in the building industry where spread footings and mat foundations are used
quite extensively even though building elements (for example, doors, windows,
elevator shafts, and utilities) have less tolerance to settlement than
bridges.
In the United States, many tons (megagrams) of pile materials
and large sums of money have been devoted to using pile foundations where spread
footings might have been appropriate. The extensive use of piles in highway
bridge foundations may have been encouraged by the AASHTO Bridge Specifications,
which at one time stated that "piling shall be considered when footings cannot,
at reasonable expense, be founded on rock or other solid foundation material"
(circa 1990).
Some of the reasons that piles were preferred over spread
footings as a foundation for highway bridges include the following:
The advent of reinforced concrete in the early 1900's and recent improvements
in excavation technology have increased greatly the appeal of spread footings
for bridge foundations. Modern soil mechanics and improved methods of site
investigation and laboratory testing have improved the accuracy of settlement
and bearing capacity predictions. Also, compaction control and improved grading
procedures have minimized spread footings being founded on weak and/or
compressible soils. Finally, special ground improvement techniques such as soil
reinforcement, stone columns and dynamic compaction have increased the
attractiveness and applicability of spread footings.
Many highway
agencies have a policy not to use spread footings on cohesive soils because of
the concern for bearing capacity failure and/or excessive settlement. This is a
conservative approach because some cohesive soil deposits (especially
over-consolidated clays) can support heavy bridge loads without distress
resulting. Although bearing capacity on sands is not a problem, some highway
agencies do not use spread footings on cohesionless soils because of the concern
for excessive settlement. This also is a conservative policy because a spread
footing on sand usually will provide satis-factory support because consolidation
of sands usually is minimal and occurs rapidly. Most of the settlement occurs
before the sensitive superstructure elements are erected.
The use of
spread footings on compacted fill also is infrequent. Although a properly
compacted fill often is stronger and more stable than natural ground and easily
able to support a spread footing, designers often use spread footings on in-situ
soils and avoid their use on prepared fills. Large settlements from fills
usually can be traced to older, nonuniform fills that may have been constructed
of poor soils or uncompacted waste materials dumped on unprepared natural ground
surfaces. However, the use of random, uncontrolled fill in modern highway
construction, especially near bridges, is readily avoidable. The Connecticut and
Washington State transportation departments frequently support bridge abutments
on spread footings on compacted fill.
A foundation system must be
functional as well as safe. There is a wide degree of engineering performance
between an unyielding support system and one that fails. Persistent maintenance
problems and failures of noncritical elements (such as parapet walls and joints)
are expensive to correct and should be avoided if peculiar to certain systems,
situations, or methodologies. To improve the design process, engineers should
correlate functional distress (bumps, cracks, and misalignments) with system
characteristics (abutment type, soil type, superstructure type, and amount and
kind of movement) to determine where spread footings are and are not
appropriate.
2.9.1 Performance Evaluation Studies – To increase the
number of documented case studies of spread footing performance, FHWA staff, in
cooperation with the Washington State Department of Transportation (WASHDOT),
evaluated the performance of numerous highway bridge abutments sup-ported by
spread footings on compacted fill. During this review, the structural condition
of 148 highway bridges throughout Washington State was visually inspected. The
approach pavements and other bridge appurtenances also were inspected for damage
or distress that could be attributed to the use of spread footings on compacted
fill.
On the basis of this review and detailed investigations of the
foundation movement of 28 selected bridges, it was concluded that spread
footings can provide a satisfactory alternative to piles, especially when high
embankments of good-quality borrow materials are constructed over satisfactory
foundation soils. None of the bridges investigated in Washington displayed any
safety problems or serious functional distress; all bridges were in good
condition (26).
On the basis of the results of the Washington Department
of Transportation study, FHWA decided to expand the idea of spread footing
evaluations by initiating a comprehensive field investigation of 10 new bridges
being constructed in 5 northeastern states. Under this research contract, a
long-term study of the settlement performance of 24 spread footings supported on
sand was completed to provide a reliable data base for engineering evaluation.
Figure 21 illustrates the instrumentation monitoring plan for this
study.
The results of the northeastern bridge studies served to confirm
the performance results of the WASHDOT study and added to the value of the FHWA
data base. The researchers were also able to locate 10 good-quality case
histories in the literature for use in the data base. Many cases were identified
but most were found to be lacking sufficient settlement or soils data to be
included in the study. It was also noted that all of the case histories in the
data base represented actual projects where loads were limited to working stress
levels. A definite need existed to obtain settlement and soils data at failure
loads to provide a larger and more comprehensive statistical basis to judge the
effectiveness of settlement predictions and the satisfactory performance of
spread footings on sand (27).
FIGURE 21. Schematic of instrumentation plan for spread footing performance evaluation study.
A follow-on study was initiated to develop a rational prediction method and a user-friendly data base with prediction and correlations modules for use in designing spread footings on sand. As part of this study, the contractor built five large model footings on sand at the Texas A&M University's National Geotechnical Experimentation Site. Each footing was carefully instrumented and load tested (figure 22) to failure (28). A prediction symposium was also held to discuss the results of the load test program and to evaluate the most popular settlement prediction methods in use at that time (29).
FIGURE 22. Load testing of large model spread footing.
2.9.2 FHWA Staff Research on Spread Footings – A series of
smaller model load tests was concurrently conducted at TFHRC to evaluate the
effects of a high water table, depth of embedment, size and shape effects, and
the use of geosynthetic reinforcing elements within the sand beneath the
footings (see section 3.5.3 for complete details). A total of more than 100
model footings was instrumented and load tested to failure. The test data were
added to the data base (see section 5.2.2). All of the footing load tests were
performed by following the ASTM Guidelines (D1194-72-1993).
A minimum of
five nuclear density and moisture content readings were taken at each 0.3-m lift
of sand during placement operations to characterize the soil fill. After filling
the test pit, in-situ soil tests were done to complete the soil characterization
work.
Results show that embedment has a very significant positive
influence on the perfor-mance of footings of the same size, even when the water
table is close to the bottom of the footing. Placement of the footing base at a
depth equal to the width of the footing increases the ultimate bearing capacity
by three to five times the value of a footing resting on the ground
surface.
The depth of the water table also affects the ultimate bearing
capacity of the footing. Those footings resting on sand with a high water table
performed much worse than those where the water table was lowered some distance
below the base of the footing. Bearing capacity values increased by a factor of
two when the water table was lowered from the surface to a depth equal to the
width of the footing. Also, as expected, the bearing capacity increased with
increasing width of the footing and relative density. A comparison was made
between the measured results of each footing load test and the predicted values
for bearing capacity using the results of each in-situ test (30).
2.9.3 Dynamic Testing of Footings – At both the TFHRC and
Texas A&M University (TAMU) sites, a separate but coordinated study was
conducted to evaluate a novel idea to develop a dynamic measurement system that
can be used to quickly determine a soil stiffness modulus beneath a footing in
order to ascertain if the natural ground or man-made fill is of sufficient
quality to adequately support the imposed loads on the footing. An NDE test,
termed the Wave Activated Stiffness test, was proposed to be a more efficient,
less costly method of checking for bearing capacity and settlement adequacy than
conventional static load testing or performance observations. These are
"after-the-fact" methods as opposed to the "before-the-fact" dynamic methods
that are done during construction, just after the footing is built and before
loads are applied.
This nondestructive impact test is called the WAK test
for short, rather than WAS because of the common use of "K" to symbolize soil
stiffness in most design equations, coupled with the fact that the impact of the
instrumented hammer when it strikes the footing sounds like a loud "whack." The
instrumentation consists of a force transducer attached to the tip of the
sledgehammer and geophones placed at the diagonal corners of the footing to
record the footing-soil response to each impact blow. The force transducer
provides the input to the system (impact force) and the geophones yield its
response (velocity of the footing).
The impact of the hammer causes the
footing and a bulb of soil directly beneath the footing to vibrate. The velocity
of this vibration is measured by the geophones. The theory is based on a
simplified, linear system model of the soil-footing assembly. This model
consists of a mass, a spring, and a dashpot, which is frequently used to
describe the vertical vibration of soil-structure systems. The compression wave
that travels down a steel pile when struck by a heavy hammer is another example
of this modeling tool.
Twenty-two WAK tests were conducted at the FHWA
TFHRC site and 16 at the TAMU site. Conventional static load tests were also
performed on each footing where a WAK test was done. All results indicate good
agreement between static load test stiffness and the dynamic stiffness from the
WAK test. Each of the footings was tested four times with the WAK equipment;
twice before and twice after the static load tests were conducted.
A plot
of dynamic stiffness results against static stiffness results for the entire
series of footing tests yielded a graphical relationship that allows one value
to be picked off the plot if the other one is known or determined by testing.
Therefore, a dynamic result can be used to approximate a static stiffness
parameter K (which represents the slope of the initial part of the static load
settlement curve) without actually running an expensive static load
test.
The manufactured K value can be used to estimate the supporting
capacity during construction when there is still time to correct any problems.
This type of test can also provide a small measure of comfort to design
engineers who fear that construction operations may not actually provide a solid
foundation base beneath the footing. Such a test will help to identify poor
compaction problems and other possible construction deficiencies that could
occur when spread footings are used in lieu of piles (28).
Research and development activities for spread footings were very successful. The research reports documented the testing and evaluation results and presented recommendations and supporting information for using spread footings in lieu of deep foundation elements in situations where foundation soils could safely transmit bridge loadings without damaged settlements resulting. Technology transfer items were developed to inform practitioners about the research findings and new ideas being promoted. The AASHTO Bridge Specifications were revised as a direct result of this research project on spread footings (see chapter 7).
2.10 Foundation Load Testing
During the past 20 years of bridge foundation testing and evaluation, FHWA
researchers have learned a lot about what works and what doesn't work. Hundreds
of load tests have been conducted at TFHRC by staff personnel, and hundreds more
have been funded by FHWA in field and laboratory projects. Numerous innovative
improvements have resulted from these efforts, which, in turn, have led to
advancements in the testing techniques and in foundation engineering design and
analysis.
All of the early work and most of the later work was done by
static load testing, which is considered by most engineers to be the most
reliable way to determine the bearing capacity and load/settlement behavior of a
foundation element or groups of elements. It is, however, also very time
consuming, cumbersome, and expensive to perform, especially on large elements or
groups. Safety is also an important issue because of the sheer massiveness of
the required loading systems. As previously discussed, dynamic pile driving
methods have become a common alternative to predict the bearing capacity of
piles, but these methods are empirical, their accuracy varies considerably, and
the trend toward much larger and deeper elements reduces their applicability.
Also, they do not provide direct measurements, they induce high accelerations,
and the load/settlement behavior is controlled by the action of stress
waves.
In an effort to overcome the practical difficulties of both
conventional static and dynamic load testing, FHWA initiated a series of
research studies to evaluate several promising load test methods that were
developed by U.S. and European inventors. None of these methods came with
sufficient documentation for standardized test procedures or for the
interpretation of data produced by these tests. The following sections summarize
the efforts to evaluate and refine several of these innovative methods.
The term "Statnamic" is a combination of the two words — "static and dynamic"
— because it was neither one nor the other, but somewhere in between. Upon
closer study it is seen to be closer to being quasistatic than dynamic,
especially now in its latest version where there is no dynamic impact. In
Statnamic testing a high-energy, fast-burning solid fuel is ignited within a
pressure chamber to act as an upward propellant force. As the fuel pressure
increases, an upward force is exerted on a set of reaction weights while an
equal and opposite force pushes downward on the foundation element. Loading
increases until it reaches a maximum, and then it is vented to control the
unloading cycle. Figure 23 schematically illustrates the statnamic
concept.
In the original version the weights would come toppling down on
the pile or shaft after the fuel had completely burned off, and possibly damaged
the foundation element being tested. This problem was solved by placing sand in
an outer, concentric container that allowed the sand to flow downward (while the
weights were pushed up and off the foundation) and cushion the blow from the
falling weights (dynamic impact). The extra time to set up the sand container,
and cleanup for retesting created a different kind of problem that was later
solved by the development of a catch mechanism and the pressure venting system.
A calibrated load cell located between the piston and the pile top is used to
measure the applied load and a photovoltaic laser sensor records displacement
during
FIGURE 23. Schematic of statnamic load test method.
the test. The assembly and take down is very straightforward. All components
are easily handled with a small hoisting rig. This system is capable of
producing a given force or load using only 5 percent of the mass required in an
equivalent static test. The original development in 1988 began with a 0.1 MN
test device; however, current test devices are capable of producing loads up to
30 MN. Furthermore, because the direction of force is along the cylinder
assembly, loading is perfectly axial or horizontal if the apparatus is shifted
90 degrees to perform a lateral load test. A batter pile test configuration can
also be adopted if desired. The relatively slow application and release of
compressive forces eliminates negative tensile forces, compressing the pile and
soil as a single unit.
Throughout loading, load and displacement signals
are digitized and sent to a raw voltage data file. After the event, the raw
signal voltages are converted to load and displacement values using factory
calibration values. Load-displacement graphs are presented immediately to
on-site engineers for quick evaluations. Supplementary graphs of velocity and
acceleration versus time can also be generated with simple post-processing
commands. All data are stored for future analysis and reference.
During
the 1990's FHWA has performed or funded many Statnamic tests and correlation
studies with conventional static load tests to develop standardized testing
procedures and improved data interpretation methods. Many SHA's and other
domestic and foreign agencies are now conducting their own evaluation programs
to further expand the data base of case histories and performance studies.
Measurements made with the Statnamic device owned by FHWA compared very well
with static load tests performed by TFHRC researchers on spread footings and
pile groups in the research test pits (see appendix A). Measurements on
full-scale piles and drilled shafts on other FHWA sponsored research projects
have also compared well with static load test results. Figure 24 shows a test at
FHWA's foundation load test facility.
FIGURE 24. Vertical statnamic load test on model pile group at TFHRC.
In late 1996, FHWA participated in a research program with the Utah DOT on a major rebuilding project on I-15 in Salt Lake City. The research involved the lateral behavior of a nine-pile group that was tested in both a free and fixed head condition. A 14 MN device was used on the fully instrumented group. FHWA also funded a similar test program at the Auburn University NGES during the same time period (see chapter 5).
The Osterberg Cell is another innovative technique that has been evaluated by
FHWA during the 1990's to determine its applicability and potential for reducing
time and costs of performing foundation load testing, especially for drilled
shafts. The drilled shaft designer must face both the problems of predicting
subsurface soil and rock strength and compressibility characteristics, and the
difficulty of estimating the impact of construction installation technique on
the completed shaft. Model testing and laboratory investigations don't provide
sufficient insight to assess the complex interaction between soil (especially
intermediate geomaterials) and the concrete shaft. The Osterberg Cell, commonly
called the "O-cell", is well suited for this problem and has provided an
attractive short duration alternative for testing drilled shafts
(31).
The O-cell is a hydraulically driven, high-capacity sacrificial
jack-like device that is installed at the bottom of the reinforcement cage
(figure 25) of a drilled shaft or at the tip of a driven pile. Unlike the
conventional static and dynamic load tests that apply a
FIGURE 25. Osterberg load cell ready for placement in drilled shaft.
compressive force at the top of the pile or shaft, the O-cell loads the unit
in compression, but from its bottom end. It requires no overhead reaction frame,
dead load, or other external system. As the O-cell is pressurized and expands,
the shaft and the soil provide reaction to the applied loads. The end bearing
support provides reaction for the side friction along the shaft, and vice versa,
until reaching the capacity of the cell or either of the two support components.
O-cell tests automatically separate the end bearing support component from side
friction values. Thus, an O-cell test load placed at the bottom of the shaft has
twice the testing effectiveness of that same load placed at the top of the
shaft. Production shafts can also be tested, if the device is grouted after
testing is completed.
Movements of the foundation element during an
O-cell test are measured by electronic gauges connected to a computerized data
acquisition system. Linear vibrating wire displacement transducers (LVWDT's) are
attached to the bottom plate of the O-cell. Telltales are used to measure both
the compression of the test pile and the upward movement of the top of the
O-cell. The downward movement of the bottom plate is obtained by subtracting the
upward movement of the top of the O-cell from the total extension of the O-cell
as determined by the LVWDT's. The upward movement of the top of the test pile or
shaft is measured by digital gauges mounted on a reference beam. The loading
mechanism has evolved from the original bellows-type expansion cell to the
current piston-type jack. However, the piston extends downward instead of upward
like that of a conventional load test.
The O-cell test offers a number of
obvious advantages over conventional load testing, including economy, high load
capacity, safety, reduced work area, and the ability to separate the end-bearing
and side-friction components. Disadvantages include the need for advance
installation, sacrificial expendability, unsuitability for certain types of
piles, and the balanced component limitation, i.e., the test capacity is limited
to twice the capacity of the support component reaching ultimate first. It has
been noted that the majority of load tests on drilled shafts are now being done
with the O-cell.
Boston Engineers were the first to use the O-cell in a
practical application in 1987 on a bridge site near Boston, Massachusetts and
later that same year at Rochester, New York. After testing, they recovered the
cell and used it on another test site. In 1988, two more tests were performed
with FHWA research funds at a bridge in Port Orange, Florida. More than 200
additional tests have been performed with O-cells in the United States on piles
and drilled shafts during the 1990's, including some lateral load
tests.
The Minnesota DOT recently completed a major load-testing research
program featuring the use of O-cell devices in both axial and lateral
load-testing modes. The shafts were 58 m deep and 1.3 m in diameter. One of the
test shafts was instrumented with Sister Bar strain gauges to develop a better
understanding of the load transfer distribution. The first-ever embedded O-cell
lateral load test was on a second test shaft on this same project.
Other
interesting applications in FHWA-sponsored projects include two previous record-
setting performances in Massachusetts and Kentucky on highway bridge projects.
The Owensboro, Kentucky record of 53.4 MN (6000 tons) was soon broken by the
Boston Central Artery project record of 55.9 MN (6,280 tons), which was easily
broken by the Florida project. The site conditions on Boston's Southeast
Expressway (I-93) were treacherous because the test shaft was drilled between
the two fast lanes inside two jersey barriers only 2 m apart. The testing was
completed without obstructing or disrupting traffic flow.
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The use of a heavy weight (90 KN) falling from various heights (0.3 to 5 m) is another method to predict the load capacity of a pile or drilled shaft (figure 26). The foundation element is instru-mented with strain gauges and accelerometers to measure the force and impact velocity of the stress wave generated by striking the element. The measured data are correlated to driving resistance to predict load capacity. An electric theodolite is used to measure the settlement associated with each hammer blow. The friction in the hammer leads |
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and the cushion assembly cause the actual energy delivered to the pile or shaft to be much less than the free-fall energy of the ram. This is accounted for by the instrumentation gauges similar to the Pile Driving Analyzer equipment used during pile driving operations to monitor resistance to driving. A service crane is required on site to perform the tests. |
Three different methods (one U.S. and two European ) were evaluated on several FHWA research test sites to determine the applicability of this type of testing to check pile or shaft capacity. The GRL method was developed at Case Western Reserve University in Cleveland, Ohio, by the same people who developed the PDA, WEAP and CAPWAP. The "SIMBAT" method was developed by the French government's Center for Experimental Research on Buildings and Public Works (CEBTP). The TNO method was developed by the Dutch government at their national research center in the Netherlands. The Europeans have amassed a large data base of static/dynamic test result correlations. As a result, these methods have become established practice in several European countries, but have been slow to catch on in the United States because of the greater success of the O-cell and statnamic methods. Also, test results on FHWA projects have not measured up to expectations thus far, usually over-predicting the measured capacities. Further investigation is planned to complete the evaluations.
No job
appears to be too big or too small for the use of an O-cell. Several world
records for load testing have been set recently, including the current
world record for total load of 135 MN (more than 15,000 tons) set in
Florida in 1997. Whether inside a building or under a bridge with limited
access or low headroom, or within a cofferdam in the middle of a river,
O-cell testing can perform well. It can be used to isolate portions of a
pile or shaft by installing multiple cells, and is fast becoming a
favorite of engi-neers and contractors because of its speed of
installation and cost advantages.