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CHAPTER FIVE |
No less important than the categories of foundations, ground improvement, and
soil behavior, a group of studies were conducted on individual topics that
didn't fit under the three major projects of the geotechnology research program.
For lack of an appropriate category title, these topics are herein referred to
as stand-alone studies.
Rock mechanics and rock slope engineering were at
one time considered for project status, but budget and staffing constraints
reduced the amount of effort that could be expended in this area. This is not to
take away from the importance of the research that was done, or the topics that
didn't get initiated, but merely goes to show that there were a number of
important issues to be addressed that didn't have an official home or umbrella
to be placed under. It was decided to include rock fall and shale research under
Task 5 of the Soil Behavior project and rename the project "Soil and Rock
Behavior."
5.1 Background
Two good examples of stand-alone studies are the computer-aided design system
for geotechnical engineering and the National Geotechnical Experimentation Sites
program, which transcend all the boundaries of geotechnical engineering research
and practice. As discussed in chapter one, these two current assets were missing
during most of the program, and can be directly blamed for the slow advancement
and difficulties that were faced in attempting to improve the state of the art
in geotechnology. Now that these assets are firmly in place, advancements should
come faster and easier.
The remaining topics also helped stretch the
upper bounds of the present technology limits in their own right. Some of them
contributed to more than one area of geotechnology, and a couple of them touched
all the bases, not to the same extent as the NGES or the geotechnical databases,
but important nonetheless.
5.2 Automated Geotechnical Information and Design System
FHWA recently initiated a major effort to develop an Automated Geotechnical Information and Design System (AGIDS) to integrate all of the FHWA research quality technical data bases, plus the information data bases in geotechnology into a comprehensive design aid system. AGIDS (figure 43) will allow geotechnical and structural engineers to quickly and economically obtain information and evaluate design alternatives from a centralized computer source of databases. These databases will be connected by developing commonality features and the design of a user interface application for performing cross queries, correlations, and engineering analyses.
FIGURE 43. Automated Geotechnical Information and Design System.
Several of the data bases already contain modules for performing
correlations, predictions, and analyses; however, they need to be linked through
a multi-user workstation that contains an interactive system for automatically
generating design solutions based on interactive user input. Such a system will
take most of the guesswork out of geotechnical design, and replace it with an
objective, quantitative system that supports sound management
decisions.
A wealth of research-quality geotechnical data have been
gathered from the literature, SHA files, other agencies, and foreign
organizations for placement in a series of FHWA geotechnical data bases to
increase the effectiveness of current and future research efforts to improve
design and prediction methods. These tools are also an effective way for
practitioners to improve their state of the practice on routine design
work.
Each data base can be utilized as a stand-alone information source
with its own information management and analysis modules, plus user interface
applications. Each has a statistically meaningful source of high-quality data
that can be used for the development and verification of new or improved
analysis methods or simple design checks by providing a fast, convenient, and
economical source of project specific or generic information for inclusion in
reports in minutes instead of days or weeks. The data base also allows users
to:
5.2.1 Deep Foundations Load Test Data Base – This data base
provides a centrally located source of technical information on piles and
drilled shafts, including soil data, load test results, instrumentation data,
and driving records that can be used to verify and refine deep foundation
theories. It can also be used to develop new theories or assist practitioners to
perform routine design projects. In addition to basic search and retrieval
functions, a correlations module and several static analysis programs have been
incorporated into the data base to facilitate the performance of a comparative
analysis.
Test data and well-defined soils information from thousands of
pile and drilled shaft load tests have been collected and evaluated for
inclusion in the data base. New data were also generated by making funds
available for installing instrumentation and conducting load tests on active
bridge construction projects at appropriate sites. The data base serves as a
"standard" against which new and existing design procedures can be compared.
Statistical correlations can be developed from the data base and used to develop
new design aids (charts, curves, and tables) for pile and drilled shaft design
procedures. The data base has been distributed for beta testing.
The data
base was developed using SYBASE System 10 tools and resides on a UNIX platform
on a RISC 6000 Geotech Server at FHWA's TFHRC. The data base operates in a
Windows environment, and utilizes Menu bars, drop down lists, icons, and message
boxes to make it more user-friendly. It also allows the user to directly print
results to a local printer or be downloaded through a file transfer protocol and
stored.
By entering the available choices in the menu, the user can
obtain information on soils, piles, drilled shafts, instrumentation setup,
construction method, driving records, and load test data. The user can also
perform design predictions, analysis, and correlations, and obtain frequency
distributions on the data available. From the statistics menu, the user can
obtain data and statistical graphs from a selection of relationships, such as
the number and types of deep foundations in a selected State, country, or in the
entire data base. Statistical data can also be obtained for load tests, soil
tests, and construction methods.
5.2.2 Shallow Foundations Data Base – The Shallow
Foundations Data Base (SHALDB) is divided into two main parts: the user
interface and the data base files. The user interface is a program that enables
the user to easily add, access, and modify the data files that describe the many
case histories stored in the data base. The data base files are grouped into
five categories: general information, footing data, footing behavior, soil data,
and settlement predictions.
The user-friendly program was written in
Visual Basic and runs under Windows 3.1. The program is in three parts:
maintenance, inquiry, and analysis. With the inquiry option, the user can select
and view the footings from the data base that satisfy a set of criteria chosen
by the user. The analysis option consists of making plots of one variable
against another, such as predicted vs. measured behavior; or it can predict the
settlement of footings on sand according to 13 different methods. There are more
than 150 spread footing case histories in the data base for which either a load
test was performed or the behavior was observed during and after construction.
The data base will also provide a standard format for the reporting of new
tests.
The SHALDB is a valuable tool that allows the user to observe the
performance of actual case histories for spread footings of various sizes on
sands with varying parameters under different loading conditions. This organized
data base can be used to evaluate existing prediction methods or to develop
and/or check a new method. It will also be useful to point out what kind of test
information is missing from the data, and therefore what kind of tests need to
be performed to fill the gaps. It will be useful to practitioners as well as
other researchers (28).
5.2.3 Ground Improvement Data Base – FHWA recently joined
forces with the International Center for Ground Improvement Technology in
Brooklyn, New York, to develop an International Knowledge Data Base for Ground
Improvement Technology (IKDGIT). The comprehensive, user-friendly data base
provides access to experiences from many parts of the world specific to a
selected technology, application, or location. It allows the user to retrieve
information on possible technologies for a project under design by viewing
similar case histories, problems encountered, possible remedial action schemes,
comparative cost data, specifications and codes, and QA/QC. The engineer is able
to supplement local experience with that of others with similar
projects.
As discussed in chapter 3, ground improvement technologies have
recently played a very important role in solving major geotechnical problems in
highway construction and in other civil engineering fields as well. While recent
research and development efforts have increased our knowledge base, many of the
concepts have not been made fully accessible to many in the professional world.
This requires a reliable, efficient, and interactive technology transfer process
and geographical expansion of locally based experiences. The IKDGIT is a good
tool that can be used to help expedite the technology transfer
process.
The data base was formed in three parts. Part One is a
compendium of national and international codes of practice. Part Two is a
collection of monitored case histories that includes site observations, design
methods, construction details, and performance monitoring data. Part Three
contains information on instrumented structures plus data and analysis records
on models and full-scale experimental studies.
The data base currently
contains more than 200 documented records of ground improvement case histories
from 15 countries. As with other technical data bases in the FHWA suite, the
work is never finished. There is much more input expected from the participating
countries in the near future that will increase the value of IKDGIT in its role
as part of AGIDS.
5.2.4 Supplemental Data Bases – In addition to the three main data bases previously discussed, AGIDS will have access to the NGES and FHWA publications data bases to provide necessary technical and administrative information to assist engineers in planning, design, and construction of highway projects. The NGES data base is described in section 5.3 of this chapter. The FHWA publications data base is not described in this report; however, most of the references contained in that data base are listed in this report.
5.3 National Geotechnical Experimentation Sites (NGES)
A major focus of the FHWA geotechnology research program was the development
of a designated system of national geotechnical experimentation sites to improve
our ability to find and evaluate new techniques for constructing safer and more
economical highways and bridges. With this objective in mind, FHWA teamed up
with the National Science Foundation to establish such a system with a national
management board and individual site managers. This section of the report
describes the system that was developed to help investigators accelerate
geotechnical research to solve many serious geotechnical engineering problems
facing the highway community.
During the last two decades the
geotechnical profession has witnessed major changes in the approach to site
characterization and quantification of soil behavior. New in-situ testing
methods and improved field instrumentation have provided valuable new tools to
complement and/or create testing alternatives to laboratory procedures. These
new techniques are leading to a better understanding of the static and dynamic
properties of soils.
Although the evolution of new techniques has been
relatively rapid, duplication of effort and lack of cooperative work among the
various research groups has made progress slower and more costly than might
otherwise have been possible. A lack of well-characterized, well-documented,
reference sites has impeded the development and evaluation of new in-situ
testing methods.
Such sites would allow ready comparison of new methods
against known soil conditions and past testing programs. In the past, most
researchers had to spend a considerable portion of their budget on creating a
well-characterized site at which to conduct their studies. Unfortunately, in
many cases, these previously studied sites are no longer available or are
unknown to other researchers. As a result, the originators of a new method must
perform their own extensive site investigation before reaching the initial
objectives of the research. This increases the total project cost and wastes
valuable time and effort.
Benefits from well-characterized and
well-documented sites are not solely restricted to evaluation of new in-situ
testing methods. A prime objective of geotechnical engineering is to predict the
performance of constructed facilities—with or without soil and site improvement.
The geotechnical profession needs to be able to evaluate its predictive
capabilities by making comparisons with records of actual field performance.
Thus, new geotechnical design and construction methods may be developed and
tested at these sites, addressing not only the more conventional earthwork and
design problems, but also environmental problems such as hazardous waste
containment.
To quantify ground response and ground failure potential,
geotechnical earthquake engineers badly needed sites that were
well-characterized and permanently instrumented to record earthquakes. The
development and verification of new tools to assess site-specific liquefaction
potential, for example, require access to cohesionless soil sites where
liquefaction has been observed during earthquakes and where soil characteristics
are well-documented. Instrumentation of such sites could provide field records
for the solution of several important problems, including the quantification of
pore pressure response and deformations that develop during liquefaction.
Analogous sites in clay deposits are also necessary to improve our understanding
of how such deposits amplify detrimental earthquake motions.
A workshop
was sponsored by NSF and FHWA at Orlando, Florida, in October 1991 to initiate
the implementation of the NGES. Participants selected a small number of sites
from a list of 40 candidate sites to form the core of the national system. The
group selected the original 40 sites because they had reasonably good
documentation of the soil conditions and previous experimentation results, a
reasonable probability of continued access for at least 10 years, and a soil
type of sufficient interest to geotechnical researchers. An initial screening
prior to the workshop identified the nine most promising candidates for the
designation of "national geotechnical experimentation site."
The
evaluators decided that none of the sites met all of the criteria for selection
and recommended establishing a national system of multiple sites according to a
hierarchy of graded levels that could fluctuate as conditions changed. Texas
A&M University and Treasure Island, California, the two sites that came
closest to meeting all of the selection criteria, were named as Level I sites.
Three sites—located at the University of Houston, Northwestern University, and
the University of Massachusetts—were found to have some limitation that dropped
them into Level II. The remaining four finalists were designated as Level III
sites, and all others were grouped in Level IV. Each site will be reviewed
periodically to determine if conditions warrant upgrading to a higher level.
Loss of access or other negative circumstances may also result in downgrading a
site.
The Orlando workshop participants also founded a System Management
Board to set policies for the use and operation of the sites and to ensure
continuity. They also established positions for a system director and for
individual site managers at each of the top five sites—Levels I and II—that form
the central core of the system. A draft plan and suggested budget for managing
the system and funding improvements to the core sites were prepared for
submission to FHWA and NSF. In 1997, it was decided to add a fourth Level II
site to the NGES system. The site is located near Opelika, Alabama, on property
owned by Auburn University. It has been officially designated as the Spring
Villa NGES test facility.
Following the workshop, FHWA awarded a contract
to develop a computerized central repository for all the data contained in the
NGES catalog, plus any future data generated at the individual test sites. The
cost of this project was shared by nine state departments of
transportation—Iowa, Louisiana, Massachusetts, Minnesota, Nebraska, New York,
Texas, Washington, and Wisconsin (62).
FHWA and NSF later awarded a large
system-support contract in 1992 to provide for the overall management of the
program and to operate and maintain a Central Data Repository (CDR). They
awarded subcontracts to each of the five site managers and a part-time system
director. The board approved improvements to each site on the basis of proposals
submitted by the site managers.
The data base of the CDR includes graphs
of representative profiles and typical plots of data for each site. Modem
hookups provide remote access to allow users to review the quality and numerical
details of the results. An electronic bulletin board provides late-breaking news
about various sites and programs available within the system. The CDR is a
user-friendly system shell with online computer search and data retrieval
capabilities that enable geotechnical researchers to select the most appropriate
site for their work. It can accommodate all essential information about each
site, such as generalized soil conditions, listing of all available test data,
site logistics and limitations, published references, and other site information
(63).
The availability of a national system of geotechnical test sites
that are already well-characterized and permanently instrumented will serve to
accelerate innovative research on soil behavior and foundation engineering.
Future research performed at these sites will be less individually oriented,
with greater documentation maintained for the benefit of other
investigators.
Researchers and practitioners can exchange information and
ideas through the NGES system to focus their thought processes into more
definable channels because they will be comparing theories and testing
procedures against the same reality. This, in turn, should lead to better
communication of the effects of geotechnical phenomena to the geotechnical
community, thereby reducing the misunderstandings, inconsistencies, empiricism,
and untested theories that pervade geotechnical practice today.
The NGES
program will foster more cooperation between public agencies, universities, and
private sector groups— something that has been missing from geotechnical
engineering. In addition to providing a standardized base upon which to judge
the results of new research, NGES will provide research sponsors like FHWA, NSF,
and SHA's with more accountability than in the past, because investigators will
know that others can come to the same site and repeat the experiment.
In
summary, the development of well-characterized sites that are readily available
to geotechnical engineering will encourage a variety of experimental activities,
which will lead to techniques for constructing safer and more economical
structures. As an additional benefit, these improvements will make U.S.
geotechnical design and construction firms more competitive in the international
arena. More information can be obtained from the references and/or the NGES web
site at "http://www.unh.edu/nges."
5.4 Evaluation and Improvement of Bridge Foundations
Various bridge components wear out or deteriorate faster than others. The
deck, in general, deteriorates faster than the superstructure and the latter in
turn deteriorates faster than the substructure, which includes the piers,
abutments, and foundations. It is therefore not surprising that, in the majority
of bridge rehabilitation jobs, the substructure usually can be salvaged with
relatively minor or cosmetic repairs. Since the cost of the substructure
represents a substantial portion of the overall cost of the bridge, evaluation
of the condition of the existing substructure must be considered in any bridge
rehabilitation or replacement project.
In certain instances, such as when
a major change in the alignment is required to upgrade the existing structure or
hydraulic requirements dictate the removal or relocation of the substructure
components within the waterway, replacement of the substructure may be
necessary. However, for widening, upgrading for increase in live loading, or
replacement with a different type deck and/or superstructure, a thorough
evaluation of the substructure (including the foundations) will be required.
Correcting a deficiency to restore the integrity of an existing substructure or
foundation may be many times more difficult and expensive than correcting a
deficiency either in the deck or in the superstructure. Maintenance costs for a
restored substructure may be higher than corresponding costs for a new
substructure on a bridge replacement project.
5.4.1 FHWA Research Study – Because of potential savings,
more than just cursory effort should be made to determine the feasibility of
reusing existing substructures and foundations. At the beginning of the major
emphasis period for the national bridge replacement program, it was discovered
that rational guidelines for evaluating existing foundations did not exist, nor
were there useful guides on how to make improvements to restore marginally
acceptable foundations to an acceptable level of performance. In recognition of
this need, FHWA initiated a contract research study to develop the engineering
guidelines for making these evaluations, and to provide guidance for improving
the soundness and bearing capacity of those units that needed upgrading to meet
current standards.
It was first determined that there are many evaluation
techniques and repair or construction methods that are applicable for the deck
and the superstructure of existing bridges. The same thing, however, could not
be said for the substructure, especially for those elements below the waterline
or the ground line. The decision process for the repair or replacement of a
bridge can be quite subjective. The purpose of the study, therefore, was to
develop recommended guidelines for: (1) techniques for evaluating, and (2)
design guides and construction methods for improving existing bridge
substructures for replacement or rehabilitated bridges.
Part I of the
guidelines report deals with deterioration of bridge substructures, effects of
loading and unloading on the foundations, time effects on soil properties, and
bearing capacity and settlement of foundations. Part II deals with current
methods of inspection, substructure analysis, new methods for evaluating
soundness and bearing capacity of foundations, and instrumenting foundations for
future performance. Repair methods and techniques to increase the capacity of
existing foundations by strengthening the foundation and/or soil and methods for
reducing loads on the substructure are covered in Part III. Case histories of
bridge substructures and recommendations for research in the subject area
comprise Part IV of the report (64).
Valuable contributions were made by
numerous materials experts in the concrete, steel, and timber industries
pertaining to reuse and repair of bridge substructure elements. Many bridge and
staff engineers from the departments of transportation of California, Illinois,
Massachusetts, New York, Pennsylvania, and Virginia also contributed valuable
information on current practices in this topic area in their respective States
that will help designers evaluate existing foundations for upgrade studies.
5.4.2 Unknown Bridge Foundations – During the 1980's
numerous bridge collapses occurred due to scour failures of the foundation
systems, causing significant injuries, loss of life, and property damage. These
events prompted the U.S. Congress to revise the National Bridge Inspection
Standards to include an item (#113) on Scour Critical Bridges, which requires
that all bridges be evaluated for their vulnerability to scour damage. The FHWA
Technical Advisory on "Evaluating Scour at Bridges," October 21, 1991, is the
implementing document. FHWA Hydraulic Engineering Circular #18 recommends a
process to perform the scour evaluations to determine the vulnerability of
existing bridges to scour-induced collapse.
The process described in
HEC-18 requires that specific knowledge of foundation type, size, and depth be
available to make the evaluations. SHA's have plans and records on their bridge
foundations for most bridges on the Federal-aid system, except some of the older
bridges and many of the off-system bridges. A survey was completed in 1990 that
included a preliminary assessment of all bridges in categories of low scour
risk, scour susceptible, or with unknown foundations. It was recommended that a
strategy be developed to help manage the risk of not knowing the type, size,
depth, configuration, or condition of a bridge foundation. It was also necessary
to develop procedures for SHA's to use in ascertaining these unknown
characteristics.
In 1991, FHWA contracted to develop the required
strategy and procedures. The study began by developing a statistical profile
from the FHWA and SHA data bases to define the extent and severity of the
problem. Next, a risk-based strategy was developed to assess and manage the risk
of not knowing the foundation particulars and help SHA's determine which bridges
most urgently need these data for scour evaluations. Finally, a method guide is
presented in the final report that describes measures that can be used to
determine foundation characteristics such as type, size, depth, and service
condition.
The guidelines cover deterioration of bridge substructures,
effects of loading and unloading on the foundations, time-effects on soil
properties, current methods of inspection, and substructure analysis. Repair
methods and techniques to increase the capacity of existing foundations by
strengthening the foundation and/or soil, methods for reducing loads on the
substructure, new methods for evaluating soundness and bearing capacity of
foundations, and instrumenting foundations for future performance are also
covered in the report (65).
5.5 Geotechnical Risk and Reliability
Most engineers design under the condition of uncertainty with regard to
material properties, service requirements, and engineering models to name just a
few. Geotechnical engineers have a very pronounced problem with uncertainty
because of the highly variable nature of soil and rock properties. In the past,
geotechnical engineers have dealt with the high level of uncertainty by
conservatively assigning or specifying much larger capacities than the projected
demand. This ratio of capacity to predicted demand is the classical safety
factor approach, which requires significant experience levels to be done
right.
Risk-based design can be used to reduce some of the conservatism
inherent in the factor of safety approach by attempting to quantify these
uncertainties and deal with them in a more rational manner. Uncertainties in the
data need to be identified and then quantified with statistical methods that are
easy to use. Mathematical modeling techniques can be used to estimate the effect
of these quantified uncertainties on performance predictions. This will result
in a quantified measure of confidence that the engineered structure will perform
adequately.
A reliability index can be calculated to give a measure of
the relative error contained in a prediction of performance behavior with
respect to the margin of safety desired in a particular structure. Although
geotechnical engineers routinely design for a "probability of failure," it is
much more prudent to use the term "reliability index" or "geotechnical
reliability assessment," especially in a court of law where an aggressive
attorney would have a career day in front of a lay jury.
To assist
highway engineers to make better geotechnical predictions of performance, FHWA
contracted to evaluate the state of the art and develop a Geotechnical Risk
Analysis User's Guide. A report was prepared that surveys the available
literature at that time and presents a large bibliography of references to
explain the information contained in the guidelines manual. The manual shows how
to quantify uncertainties and adjust design conservatism accordingly in a
simple-to-use approach. Design problems from engineering practice are presented
to illustrate the approach (66,67).
5.6 In-Situ Soil Testing
In-situ soil testing is an important method for determining geotechnical design parameters, especially for hard-to-sample soils needed for laboratory testing. Most of the current techniques in use were developed in the United States and Europe without FHWA funding; however, a few guidelines type manuals were developed by FHWA as user-friendly informational documents for U.S. highway engineer practitioners (68,69,70). In some other separate instances, FHWA spent considerable resources developing special in-situ tools to obtain soil design parameters. These efforts are described in the following sections.
5.6.1 Stepped Bladed Vane – Lateral soil pressure is an important element in soil mechanics theory and practice, but it is very sensitive to disturbance and difficult to measure. A step-tapered blade was developed to compensate for disturbance by measuring soil and pore-water pressures on three thicknesses of blades, and extrapolating for the hypothetical soil pressure at zero blade thickness. The three-bladed vane (figure 44) contains nine teflon-diaphragm pneumatic stress cells to measure soil pressure. The equipment has been shown to provide reproducible, reliable, and economic measurements of lateral stress in sands, silts, and clays.
FIGURE 44. Close-up view of step bladed vane.
The device measures quickly, accurately, and inexpensively the total and effective lateral stress in soils, which is a property that is fundamental to virtually all soil mechanics, including foundation-bearing capacity, pressure on retaining walls, and slope stability. The device and test procedures were used in a number of case histories where the design predictions were compared with measured performance. A detailed description of the equipment and sample design problems can be found in the final report (71).
5.6.2 Simplified Torsional Cylindrical Impulse Shear Test –
This device was developed in 1994 for FHWA to predict the behavior of soil
deposits during earthquakes. It provides detailed information for soil deposits
on in-situ nonlinear shear stress vs. strain characteristics that is needed for
commonly used computer analysis procedures for earthquake engineering.
Predictions can be made for large ground motions and soil liquefaction that may
occur during future earthquakes to prevent the source of immense losses such as
those of past earthquakes.
The impulse shear test device addresses the
major problem of obtaining the required information without disturbing in-situ
conditions excessively. Disturbances can create considerable uncertainty in
behavior predictions, which can lead to costly over-design or worse
unconservative designs that fail in seismically active areas. Improving our
ability to estimate the soil characteristics of interest will allow us to
realize more fully the potential of dynamic geotechnical computer analysis
procedures used, for example, by the California Department of Transportation.
This will lead to more effective earthquake engineering and, in turn, to greater
safety, economy, and reliability in the earthquake- resistant design of highway
structures.
The device
consists of a single cylinder with a diameter of 7 cm located at the end of a
probe that penetrates the soil below the base of a borehole. An impulsive torque
is applied to the cylinder by an excitation system to induce shear stresses and
strains in the surrounding soil. The applied torque and resulting rotation are
measured by sensors mounted in an instrumented head attached to the top of the
cylinder. Figure 45 shows the device being lowered into a soil borehole at a
seismic site on the Treasure Island NGES.
The device has been tested at several NGES facilities and at TFHRC to establish the operability of the testing system and to develop test procedures. It was found to be a promising technique from a technical and practical standpoint. Current efforts are attempting to improve the existing prototype with respect to efficiency, usability, economy, and reliability.
5.6.3 Controlled Source Spectral Analysis of Surface Waves
(CSSASW) – In addition to the two previous methods, which involve
the insertion of a probe into the underlying soil, FHWA also funded a study to
investigate the use of CSSASW techniques for nonintrusive shear wave velocity
profiling of soils with high liquefaction potential. The results were compared
with the results of direct measurements made at a test site with extensive
subsurface data and seismic instrumentation. FHWA has also evaluated this
technique for determining the thickness or depth of unknown bridge abutments and
foundation systems.
The CSSASW system is a rapid and cost-effective
technique that can accurately determine the apparent dispersion of Rayleigh
surface waves at a site, which can be correlated with shear velocity profiles to
produce essential data to analyze site response during earthquake loading. This
is especially important in hard-to-sample soils. The device (Figure 46) uses a
powerful electromagnetic vibration source powered by a generator and two data
receivers that pick up the soil vibrations generated by the exciter. The
Rayleigh waves are measured by the two vertically oriented accelerometers
positioned at a known distance from the exciter.
FIGURE 46. Controlled source spectral analysis
of
surface waves (CSSASW) testing at Treasure Island.
5.6.4 Multiple Deployment Model Pile – The installation of
full-scale test piles to obtain pile design parameters during the design phase
is sometimes used on large projects to save money on construction costs by
reducing the conservatism that generally occurs due to unknown or unsure
conditions. Conservatism leads to higher safety factors, which means higher
costs. Full-scale test piles are an expensive and inconvenient way to obtain the
desired information. Geotechnical engineers have long searched for an accurate
model pile test that can properly simulate the field behavior of full-scale
piles.
Many model piles have been developed during this quest. Examples
include the In-Situ Model Pile (IMP) developed at Oxford University, the MIT
Piezo Lateral Stress Cell (PLSC), the Norwegian Geotechnical Institute (NGI)
model pile, and many others that are available, but none of which are quite up
to par for highway bridge standards. These model piles are calibrated tools that
are equipped with instrumentation to monitor the pile-soil interaction during
pile driving, soil setup with time, and subsequent loading to failure. This
behavior is measured by strain gauges, load cells, and pore pressure transducers
attached to the model pile. Some also have accelerometers and displacement
transducers.
FHWA recently sponsored the development of the Multiple
Development Model Pile (MDMP) for use as an in-situ soil testing device for site
investigations to obtain essential pile design parameters. The main purpose of
the MDMP is to duplicate the driving and after driving conditions that
full-scale piles experience. The instrumentation must therefore be rugged enough
to withstand driving stresses and maintain the required standard of accuracy.
Restrike of the model piles in clay soils can be used to assess load transfer
during pore pressure dissipation with time; thus, the pile's capacity gain from
"setup or freeze" can be accurately quantified.
The MDMP was successfully
used at two sites in Newbury, Massachusetts, on a SHA bridge construction
project. The results compared very well with full-scale data from instrumented
pile load tests to failure. The model results show that the MDMP is capable of
providing very accurate soil-structure interaction relations during static load
testing. These findings were used to predict the time-dependent behavior of the
full-scale instrumented piles, and to reevaluate the pile capacity gain
phenomenon. These results helped to explain some unanswered questions and
facilitated the development of new procedures that incorporate pile capacity
gain in design and construction.
5.7 Seismic Design of Highway Bridge Foundations
In a recent letter from Mr. James Roberts, Chief Engineer of Caltrans, to the
FHWA, special emphasis was made on the need for additional research on the
seismic behavior of foundation soils during major earthquakes. An excerpt from
this letter follows:
FIGURE 47. Liquefaction damage due to Loma Prieta earthquake.
NCEER is presently conducting two separate FHWA-sponsored contracts. One contract is focused on the seismic vulnerability of new highway construction, and the other is concerned with the seismic vulnerability analysis and retrofitting of existing highway structures. Revised seismic design and construction guidelines will be developed through a synthesis of existing knowledge and the development of new information from analytical and experimental research activities being conducted under the NCEER and other FHWA projects.