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CHAPTER FOUR |
Highway bridges and pavements are usually supported on whatever ground
materials are located directly beneath or within easy access of the roadway
right of way to avoid expensive haul distances. Consequently, highway engineers
need methods for evaluating and upgrading available materials to properly
support the pavement or bridge structure being designed.
During the
height of the Interstate construction period, many difficult soil and site
conditions caused highway engineers to search for better ways to eliminate or
ameliorate these situations so that roads and bridges could be built in a safe
and efficient manner. Many areas of the United States had common problems of
unstable soils and poor compaction techniques; but some had regional concerns,
such as frost action in the northern tier of States, and expansive soils in the
south. Some areas even had problems with rock materials that seemed hard during
excavation, but later would deteriorate or exhibit expansive soil behavior after
the passage of time while inside an embankment fill structure.
The
solution to these problems was universally recognized by FHWA and SHA's as a
priority research need. Thousands of miles of highways were being constructed
annually in areas where swelling soils, frost susceptible soils, deteriorating
shales, hard to compact soils, and otherwise unstable soils were causing
expensive damage to correct. Even a small reduction in the unit costs to repair
these damages would result in annual savings of several million dollars.
4.1 Background
In the 1970's it was estimated that the annual cost of damage to highways and
streets caused by expansive soils exceeded $1 billion. In addition, similar
estimates were made concerning frost action and thaw weakening damage in
northern states. At the same time, many States were spending millions to repair,
rebuild, or replace highway embankments that were initially constructed as rock
fills and later failed as though they had been built with soft clays. Still
other SHA's were seeking solutions to the high cost of compacting weak soils or
using chemical soil stabilizers to improve strength and/or reduce volume change
tendencies.
The energy crisis was also causing prices of asphalt to
skyrocket out of control. Many places were also concerned about the high
utilization factors for quality granular materials, and efforts were needed to
conserve limited sources of these materials. Research to increase the use of
chemical compaction aids that would reduce the cost of compaction, and research
to increase the effectiveness of existing soil stabilization systems featuring
lime, cement, fly ash, and bitumens were given very high priority.
4.2 Objectives
The objectives were to develop methods for (1) predicting the performance of and (2) eliminating the engineering deficiencies of soil and rock materials used in earth structures, subgrades, base courses, and as support for highway structures.
4.3 Scope
Research efforts in this project were directed toward developing methods for predicting the volume stability of expansive and frost-susceptible materials and to provide methods for identification, characterization, and evaluation of materials that exhibit unaccountable instability or excessive compressibility when tested and handled by existing techniques. Research was conducted to provide physical and chemical treatment of soil materials to enhance their value and availability for engineering use. Special emphasis was placed on the development of methods for eliminating or relieving pavement distress due to excessive volume changes of expansive subgrades, inadequate strength or durability of subgrade or base components, and frost heave and subsequent weakness in foundation layers.
4.4 Project Description
The major research efforts were in five tasks:
4.5 Expansive Clays and Shales
The first important decision in the design and construction sequence for a
highway is the route selection. Route selection is often influenced by social,
economic, environmental, and/or political considerations prevalent at the time
of design. Often the geologic materials (and the associated problems) traversed
by the selected route are not considered until the collection of parameters for
the pavement design. For expansive soils, it is important to recognize the
existence of theproblem and have a qualitative indication of the extent of the
potential swell problem as early in the design and construction sequence as
possible.
Volume change of expansive soil subgrades resulting from
moisture variations frequently cause severe pavement damage (figure 37).
Highways constructed in the Southwest, Western Mountain, Central Plains, and
Southeast geographical areas are particularly susceptible to these types of
damage. A survey of U.S. highway departments indicated that 36 States have
expansive soils withintheir geographical jurisdiction. Expansive soils are so
areally extensive within parts of the United States that alteration of the
highway routes to avoid the material is virtually impossible.
FIGURE 37. Pavement damage due to expansive subgrade soils.
Because of the billions of dollars of damage done in the United States to
pavements and buildings each year, many requests for technical guidelines for
expansive soils were generated by SHA's and building owners. Several workshops
and technical conferences were held to discuss the problem and to develop a plan
of action and list of research needs. It was determined at first that the
procedures for the design and construction of pavements on expansive soils did
not systematically consider the variety of factors and conditions that influence
volume change, as evidenced by the continued occurrence of warped and cracked
pavements in areas where expansive soils exist. Thus more accurate methods were
needed for identifying, testing, and treating expansive clays to improve highway
design, construction, and maintenance techniques.
It was also decided
that a comprehensive study was needed to achieve the following goals: (1) the
establishment of physiographic areas of similar natural sources and
manifestations of swelling behavior, (2) the development of expedient procedures
for identifying expansive clays, (3) the development of testing procedures for
quantitatively (amount and rate of volume change) describing the behavior of
expansive clays, (4) the development and evaluation of innovative technologies
for prevention of detrimental swell under new and existing pavements, and (5)
the development of recommended design criteria, construction procedures, and
specifications for the economical construction of new pavements, and maintenance
or reconstruction of existing pavements on expansive soils.
On the basis
of these requests and a series of research recommendations developed by FHWA
personnel, a contract research study was initiated. In the study, the
distribution of potentially expansive soils was defined and their relative
expansivity established to provide a summary of potential problem areas. Various
methods for qualitative and quantitative evaluation of expansivity and pre- and
post-construction methods to minimize the detrimental effects of subgrade volume
change were also reviewed under the study. In addition, field sites were
selected, samples taken, and field monitoring plans developed to evaluate
selected methods for predicting expansivity.
The final reports summarize
the major research results and present the details of the research efforts.
Volume I presents the text and summary figures relevant to the discussion of the
results of the research tasks. Volume II presents the laboratory data collected
on samples from 22 field sampling sites and the monitoring data from 8 field
test sections located in 5 different States (48).
Conclusions drawn from
the research results provide better criteria for identifying and classifying
potentially expansive soils; more accurate and reliable procedures for
characterizing and predicting the behavior of expansive soils; guidelines for
application of pre- and post-construction treatment alternatives for minimizing
volume change of expansive soils; and practical design, construction, and
maintenance recommendations for minimizing moisture infiltration into an
expansive soil subgrade.
The results of the study that should be
implemented are presented in a manual titled Technical Guidelines for
Expansive Soils in Highway Subgrades. Technical guidelines are presented on
the location of potentially expansive soil areas using occurrence and
distribution maps, as well as alternative sources of information; field
exploration and sampling of expansive soils; identification and classification
of potentially expansive soils using index and soil suction properties; testing
of expansive soils and prediction of anticipated volume change; selection of
appropriate treatment alternatives; and presentation of design, construction,
and maintenance recommendations for new and existing highways. Appendixes to the
technical guidance report describe the soil suction test procedure, and include
a standard procedure for odometer swell tests, a bibliography on treatment
alternatives, and standards for field monitoring data.
When the research
was completed, FHWA's Technology Transfer program funded an effort to implement
the results. An executive summary report of the research reports was developed
along with various training materials to be used at two training workshops. The
executive summary report (49) was distributed at the workshops along with a
participant's workbook and the technical guidelines manual (50).
4.6 Soil Compaction
Compacted soil is an essential element of highway construction. Soil density
and moisture content is used almost exclusively by the highway industry to
specify, estimate, measure, and control soil compaction. This practice was
adopted many years ago because soil density and moisture content can be
determined very easily via weight and volume measurements.
This doesn't
mean that soil density and moisture content are the most desired engineering
properties, because they are not as important to know as the soil modulus or
stiffness characteristics. The latter properties were much more difficult to
measure, so it became standard practice to measure the former indicators
(density and moisture) in order to provide an indirect measurement of stiffness
at a much reduced cost.
In addition to property measurement techniques,
engineers and builders both were seeking some magical chemical to alter the soil
properties to make it more readily compactible. Chemical compaction aids began
to surface in large quantities purporting to be the ideal product to accomplish
this highly desired objective, thus requiring an evaluation protocol to
determine which, if any, of these products were legitimate sources of help with
this problem.
4.6.1 FHWA Compaction Aids Research – A laboratory staff
study on the evaluation of two proprietary materials as compaction aids was
completed and a final report published. The testing program used to evaluate the
materials was developed by FHWA and endorsed by the manufacturers prior to its
initiation. It was concluded that neither product produced sufficient alteration
of soil properties to be of any practical utility for acidic soils. Other
researchers have concluded from similar, but broader, studies that these
products may only be effective with neutral or alkaline soils.
A more
extensive FHWA contract research study on "Chemical Compaction Aids for
Fine-Grained Soils" was completed wherein the feasibility of improving the
compaction characteristics of fine-grained soils by chemical treatment was
determined for a number of chemicals and several proprietary products. The study
was divided into three phases: (1) an office evaluation, (2) laboratory
investigations, and (3) field evaluations of the more promising
chemicals.
The results of this study indicate that the effectiveness of
chemicals for improving compactibility cannot be generalized according to
classes of chemicals and soil types because it is a function of many
interrelated variables. The effectiveness of a given chemical must be evaluated
with the soil to which it is to be applied. Laboratory techniques and evaluative
procedures that appear suitable for predicting field performance of chemical
compaction aids were developed to facilitate this "one-on-one" evaluative
approach, which was partially validated by field studies in Iowa and New Mexico.
Although several of the 20 chemicals evaluated in the study improved some of the
engineering properties of a few soils, none of the benefits derived were
generally or practically significant.
The research results are presented
in a two-volume report Chemical Compaction Aids for Fine-Grained Soils.
Volume I of this report includes an extensive review of appropriate subject
literature and the laboratory moisture-density-strength study of 20 chemicals
with 8 soils of varying origin and mineralogy. Also included is a theoretical
discussion of possible mechanisms of chemical compaction aids, properties of the
26 soils used in the laboratory investigations, and data from supplemental tests
designed to improve understanding of the influences of chemicals on fine-grained
soils. Six chemicals were selected for the more extensive laboratory evaluations
with 18 additional soils (51).
Volume II includes
moisture-density-strength screening tests performed on several additional
chemicals and an evaluation of the standard AASHTO T-99 moisture-density test
results performed on soil specimens prepared under varying conditions of drying,
pulverization, and re-use. Also presented are the results of a laboratory
moisture-density-strength study of chemicals selected and evaluated through both
qualitative and statistically related procedures, laboratory compaction growth,
and 7-day moist cure results (51). A discussion of the mechanisms of chemical
compaction aids as evaluated through the assistance of infrared spectrography,
vapor pressure osmometer, and zeta potential tests is also included in Volume
II. Based on the total study, an "ideal" compaction aid is described. Volume II
also presents results of field trials conducted on a roadway embankment near
Knoxville, Marion County, Iowa, and a soil-aggregate base near Villanueva, New
Mexico (51).
4.6.2 Soil Stiffness Gauge (SSG) – As previously mentioned,
soil stiffness information is more valuable to designers for evaluating subgrade
support capacity than are density measurements; however, the difficulty and
expense of obtaining quality stiffness data have traditionally caused engineers
to rely on density tests to check quality assurance and control soil compaction.
In the early 1990's, construction engineers began to search for something that
was safer and more economical to use because accidents and production delays
were increasing at an alarming rate. Current methods for testing soil compaction
in the field are slow, labor intensive, unsafe, and of uncertain
accuracy.
Because of the labor and time involved, construction sites are
often under-sampled, causing some problems to go undetected, or providing data
too late for cost-effective correction of problems. Sometimes the opposite is
true, because some contractors frequently over-compact in order to ensure
passage of acceptance tests and thereby avoid rework at a later date. Also,
engineers tend to over-specify compaction requirements in order to allow for the
significant variability in a noncontinuously monitored compaction process.
Excessive over-compaction can have significant impact on site preparation
costs.
The SSG (figure 38) measures the in-place
stiffness of compacted soil at the rate of about one test per minute. The SSG
weighs about 11.4 kg (25 lb), is 28 cm ( 11 in) in diameter, 25.4 cm ( 10 in)
tall, and rests on the soil surface via a ring-shaped foot. The stiffness is the
ratio of the force to displacement: K=P/d. The SSG produces soil stress and
strain levels common for pavement, bedding, and foundation
applications.
In addition to time and cost advantages, a portable
compaction device that is quick and easy to use will save lives and reduce
exposure to injuries by allowing the technician to make measurements at the rate
of one in-place stiffness test per minute. Numerous deaths have been reported
where technicians were preoccupied with performing a nuclear density test or
other quality assurance method, and did not see or hear a heavy construction
vehicle before it ran over them. In one incident, the U.S. Nuclear Regulatory
Commission inspectors were called in because the gauge containing Cesium and
Americium sources became exposed when the unit was crushed. The technician was
killed. The paperwork and safety precautions are tedious enough under normal
operations, but they are an order of magnitude higher when accidents occur. A
non-nuclear method is in great demand.
In response to this need for a
faster, cheaper, safer, and more accurate compaction device, the FHWA
researchers joined with scientists from the U.S. Department of Defense's
Advanced Research Programs Administration (ARPA) to cosponsor a study to
investigate the possible use of military technology to solve this problem. As
part of the defense reinvestment initiatives, and using funds from the
Technology Reinvestment Project, ARPA authorized FHWA researchers to supervise
the redesign of equipment that was built to locate buried land mines for armed
forces personnel. The military device used acoustic/seismic detectors to locate
the buried land mines, and included U.S. Navy sonar acoustics and
electromagnetic shaker technology developed under another contract to DOD.
The prototype
model was modified to make a soil stiffness gauge that is portable, lightweight,
and safe to use. It rests on the soil surface via a ring-shaped foot and
produces a vibrating force that is measured by sensors that record the force and
displacement time history of the foot. It is a practical, dynamic equivalent to
a plate load test. Figure 39 is a schematic of the SSG showing the major
internal components, except for the D-cell batteries that power it.
The
device has been "Beta" tested by FHWA and several SHA's. Thousands of soil
stiffness measurements have been successfully made at highway embankment and
pipe backfill sites on sand, clay, and sandy loam soils. When converted to
density values using correlation charts, these measurements are within plus or
minus 5 percent of companion measurements made with a nuclear density gauge.
Production devices are being made for further evaluation at sites representing a
cross-section of U.S. applications and soils. Future models will include onboard
moisture measurement instruments and a global positioning system (GPS).
4.7 Soil Stabilization
The concept of soil improvement or modification through stabilization with
additives has been around for several thousand years. At least as early as 5000
years ago, soil was stabilized with lime or pozzolans. Although this process of
improving the engineering properties of soils has been practiced for centuries,
soil stabilization did not gain significant acceptance for highway construction
in the United States until after World War II. Today, stabilization with lime,
lime-fly ash, portland cement, and bituminous materials is very popular in some
areas of the country.
One of the major concerns has been the shortage of
conventional aggregates. The highway construction industry consumes about half
of the annual production of aggregates. However, this traditional use of
aggregates in pavement construction has resulted in acute shortages in those
areas that normally have adequate supplies. Other areas of the country have
never had good quality aggregates available locally. Metropolitan areas have
also experienced shortages. The reasons include lack of the raw materials,
environmental and zoning regulations that prohibit mining and production of
aggregates, and land-use patterns that make aggregate deposits inaccessible.
These factors, and others, combine to produce an escalation of aggregate cost,
with a resultant increase in highway construction and maintenance costs.
Consequently, there was a great need during the early stages of this research
program to find more economical replacements for conventional aggregates. A
natural result is that attention must be focused on substitute materials such as
stabilized soils.
Another area of concern had been the energy crisis
brought on by the temporary shortage of petroleum. It rapidly became a practice
to consider the energy demands of a project as well as cost. In terms of highway
construction materials, the trend was toward the use of materials that required
less energy input in their production, handling, and placement. A study revealed
that the energy requirements for producing the materials for various asphalts,
crushed stone, and portland cement concrete pavements ranged from 30 to 96
percent of the total energy required for production, handling, and placing of
various pavements. Since relatively small quantities of binders such as lime,
cement, fly ash, and asphalt, could be used to improve pavement layers using
stabilization technology, total energy demands may be reduced as well as
costs.
The major objective of the soil stabilization research project was
to develop information on materials that, when applied to soils of inadequate
natural stability, would be capable of achieving stabilized soil subgrades and
surfaces of sufficient strength to satisfy certain highway construction needs.
It was also considered important to establish specific soil stabilization
requirements to guide the development and evaluation phases of the soil
stabilization research program. These requirements are expressed in terms of
both the strength and thickness parameters of the stabilized-soil layer that
will satisfy anticipated construction needs, and include consideration of
certain limiting initial soil conditions that might be encountered in highway
operations. Desirable maximum limits of curing time and quantity of stabilizer
necessary to achieve the stabilization objectives were also needed.
4.7.1 Soil Stabilization Manual – A two-volume user's
manual was developed for FHWA to provide guidance for pavement design,
construction, and materials engineers responsible for soil stabilization
operations associated with transportation systems. Volume I of the manual
Pavement Design and Construction Considerations describes a method for
selection of the type of stabilizers as well as pavement thickness design
methods and construction information. Quality control, guide specifications,
cost, and energy considerations are contained in the appendices
(52).
Volume 2 of the manual Mixture Design Considerations was
prepared for materials engineers. This volume describes methods for selection of
the type and amount of stabilizers. Methods of estimating stabilizer contents
are presented as well as detailed test methods, mixture design criteria, and
typical mixture criteria (52).
The manual is directed to the engineer who
is reasonably familiar with pavement technology, but who has limited experience
with stabilized soil construction. Current technology of soil stabilization is
presented in a complete but concise format such that the engineer can grasp the
key elements and apply the information to specific needs. Suggested additional
references are provided so that the reader may follow up on details of interest
that are beyond the scope of this manual.
Basically, the manual was
developed to provide guidance to design and construction engineers of highway
agencies when using soil stabilization in lieu of high-quality aggregates for
base and subbase layers in pavement structures. The manual illustrates
techniques and advantages of using soil stabilization as a means of meeting
shortages of local aggregate supplies and demonstrates the cost effectiveness of
such utilization through examples.
Specifically, soil stabilization as
addressed in the manual is limited to the following stabilizers: lime, lime-fly
ash, portland cement, asphalt, and some combinations of these. The advantages of
each stabilizer, mixture design procedures, characterization for structural
design, and construction methods are covered for each stabilization method. In
addition, a chapter is devoted to structural design utilizing stabilized
materials in several pavement design methods, including AASHTO and elastic
layered systems. Examples of specific situations are provided to illustrate the
use of the manual as well as demonstrate how stabilized layers may be
substituted for conventional granular materials.
4.7.2 Lime Stabilization Research – The results of an FHWA
contract research study on the "Role of Magnesium in the Stabilization of Soils
with Lime" indicated that, for all practical purposes, dolomitic or calcitic
hydrated lime are equally effective for producing strength gain. However,
calcitic lime was recommended for use with all soil types if reducing plasticity
was the purpose of lime treatment.
The final report, The Role of
Magnesium Oxide in the Lime Stabilization, is presented in three volumes
(53). This report presents the results of a laboratory study to evaluate the
relative effectiveness of calcitic and dolomitic lime for the stabilization of
fine-grained U.S. soils. Evaluation of the relative effectiveness of lime,
either obtained from commercial sources or manufactured in the laboratory, was
based on the ability of the lime to reduce soil plasticity and increase its
confined compressive strength.
Thirty-five clay soil samples,
representative of major U.S. soil series, were treated with the various limes
and were examined by x-ray diffraction, scanning electron microscopy, chemical
analysis, petrography, and differential thermal tests to identify the specific
soil or lime properties that govern the response of soil to lime
treatment.
4.8 Frost Heave and Thaw Weakening Damage
The ravages of frost action on roads and streets is well documented by the
news media each spring and by county and district maintenance engineers
throughout the Northern States. For example, The Road Information Program (TRIP)
estimates that more than $2 billion is required to rebuild the thousands of
miles of pavement that are destroyed each winter in the United States. This
expenditure is in addition to the cost of filling potholes and surfacing
pavements with minor damage. According to TRIP, automobile repairs resulting
from rough pavements and increased cost to transport goods because of detouring
to avoid damaged roads should also be added to this cost to fully assess the
impact of severe winter weather.
The problem of maintaining roadways and
airfield pavements in areas of seasonal frost has long been a major concern to
pavement design engineers. Although pavement designers have attempted to provide
protection against the detrimental effects of frost action, severe winters have
demonstrated that a basic and rational methodology for analyzing and designing
pavement systems in cold regions did not exist during the early stages of this
research project.
The seasonal variation in the serviceability of a
pavement is very pronounced in areas subject to alternating freezing and
thawing. The combination of freezing and thawing of pavement subgrades is
commonly called frost action. Differential pavement surface heaving (poor
rideability) frequently is the effect of freezing, and subsequent thawing may
lead to a greatly reduced load-carrying capacity due to thaw weakening. Many
potholes and other pavement breakups (distress) result from thaw
weakening.
Some soils are more susceptible to frost action than others,
and the amount of heave that occurs is not a good indication of how much
strength loss will occur during the thaw period. It is also probable that a soil
of lower frost susceptibility will experience greater heave than a soil of
higher frost susceptibility if placed under more adverse temperature and water
conditions. In fact, a highly frost-susceptible soil will not heave at all if
either one of these two conditions (temperature and water) is missing.
Conversely, clean granular materials not normally classified as frost
susceptible will heave if the temperature and water conditions are sufficiently
adverse.
4.8.1 Differential Heaving – Because the amount of heave is dependent on three conditions that can be quite variable—frost susceptibility of the soil, freezing temperatures, and access to groundwater—uniform heaving cannot be expected (figure 40). The differential heaving that results causes surface irregularities and general surface roughness in the form of bumps, waves, and distinctive cracking. Severe cases of differential heave will usually reduce traffic speeds significantly and may cause damage to vehicles or loss of control of the vehicle. The potential for abrupt differential heave at cut-to-fill transitions or culverts requires special design considerations.
FIGURE 40. Pavement damage due to frost heave.
The amount of heave that occurs is not entirely a result of the expansion of free water in the soil voids when freezing temperatures penetrate the subgrade soil mass. This can often be a small percentage of the total heave. In severe cases of heaving, the extent and rate of growth of ice lenses are determined by the soil's ability to draw water from below by capillarity and also by the rate and depth of penetration of the freezing temperatures. The formation of ice lenses responsible for heaving is governed by the interaction of heat and mass transfer (moisture movement) in porous soil media—a very complicated phenomenon. The growth of ice lenses also results in decreased soil density. After several cycles of freezing and thawing, the soil fabric can be adversely changed depending on the type of soil and the amount of ice lens buildup.
4.8.2 Thaw Weakening – Thaw weakening is considered by many
to be the more critical manifestation of frost action. It is just as complicated
as the heave problem and it is probably more difficult to evaluate or predict.
This problem occurs when the ice lenses formed in the subgrade during freezing
begin to thaw from the surface downward. This thawing results in melt water
being trapped between the pavement and the still frozen portion of the subgrade,
which, accompanied by loading of the soil in its loosened states, generates
excess pore water pressure and a corresponding decrease in load-carrying
capacity. The duration and frequency of load application (static versus dynamic)
may also impact load-carrying capacity; that is, dynamic loading may be more
destructive to soil structure, thereby reducing strength.
The increase in
water content resulting from freezing and subsequent thawing is more detrimental
for some soils than for others. The stability of fine-grained soils is more
sensitive to changes in moisture content than is the stability of granular
materials; that is, very slight increases in moisture content for silts and
clays significantly decrease their stability while similar changes in water
content do not affect the performance of granular materials. However, soil
moisture content cannot be used as an indirect measure of thaw weakening because
certain clay soils lose significant support capacity during thawing without
significant increase in bulk moisture content. A different set of physical and
environmental factors influence heave and cause subgrade weakening, thus
requiring separate evaluations.
The frost action problem consists of a
series of interdependent factors or parameters that vary over a wide range of
values. Understanding the mechanism of frost action in soils requires a
knowledge of soil behavior including soil physics. The frost susceptibility of
soils is still a relatively unknown quantity; however, it is generally
recognized that a soil is susceptible to frost action only if it contains fine
particles. Most studies have shown that soils free of fines (particles smaller
than the 200-mesh sieve) do not develop significant ice lens buildup or ice
segregation. As a result, the engineering community has used various indirect
measures or indicators based on particle-size distribution, pore-size
distribution, grain shape, and plasticity characteristics. All of these
contribute to frost susceptibility or ice lens buildup in varying degrees.
4.8.3 Coordinated Research Efforts – Due to the expense and
large extent of evaluating all of the prior FHWA and SHA research studies on
frost action, it was decided to develop a comprehensive research project that
would utilize the combined resources of FHWA, the Federal Aviation
Administration (FAA), and the Office of the Corps of Engineers (OCE) in the
Department of Defense. A contract was signed with the OCE's Cold Regions
Research and Engineering Laboratory (CRREL) in Hanover, New Hampshire, to
evaluate, validate, and refine certain selected frost action
techniques.
The CRREL study included an evaluation of the various devices
for measuring frost susceptibility that were developed earlier by FHWA and the
SHA's, and the development of full-scale field test sites to provide correlative
data. The field data were also used to validate and refine a mathematical model
and laboratory predictive techniques for assessing strength loss due to thaw
weakening. Three test methods for predicting frost susceptibility were checked
against the field test data for eight soils studied at the field test sites
(54).
A computer model of frost leave and thaw weakening was developed
from existing data obtained from earlier studies. It is a finite element model
that couples heat and moisture transport in freezing soil water systems and
provides quantitative predictions of the frost heave and thaw weakening a soil
will experience under different moisture and temperature conditions (55).
A laboratory soil column device was designed and built by CRREL to
nondestructively measure moisture content, density, and soil suction changes
that occur during unidirectional freezing of the soil sample. A dual gamma
energy source was used to simultaneously monitor the changes that occur as the
soil sample was gradually frozen and thawed, without having to periodically stop
a particular test to take a moisture and/or density sample. The soil column
results were used to refine the computer model while researchers waited for the
measured results of the field test sites.
Another laboratory segment of
the project involved repeated load triaxial compression tests on undisturbed
samples of the test site soils. The samples were obtained by coring the frozen
subgrade soils during the first winter cycle of the field test program. The
samples remained frozen until tested and, upon completion of the repeated load
tests, the samples were allowed to thaw and then retested in the same manner.
The test loads were stopped before the samples failed so that they remained
suitable for retesting. The next tests were run on the same samples at different
stages of recovery from thaw weakening.
Thaw recovery was artificially
produced by desaturating the thawed specimen to various partial saturation
points. The CRREL researchers monitored moisture content, density, and stress
state variables including moisture tension. The deformation moduli were
expressed in terms of these variables. Various resilient moduli for the soils at
several depths in each test section were analyzed to determine which values gave
surface vertical displacements that matched the measured field
values.
The CRREL researchers and the sponsors (FHWA, FAA, and OCE)
selected two field test sites to verify the analytical and laboratory predictive
techniques developed under this project: an off-road test site on highway
department land at a district maintenance depot in Winchendon, Massachusetts,
and an FAA site at the Albany County Airport.
To minimize site-associated
difficulties, it was decided that a number of different soils would be studied
under the same environmental conditions by importing selected problem soils to
the Winchendon site and placing them in prepared trenches (cells). The
Winchendon site was chosen because it has a high natural groundwater table,
granular subsurface soil with a relatively high permeability, and a relatively
deep frost penetration. Twelve different soils were obtained from various parts
of Massachusetts and several neighboring States. Each soil was placed in a cell
at two different water table elevations and capped with a thin bituminous
surface course.
The data that was collected in the field tests are as
follows:
In addition to the above data, Repeated Plate Bearing (RPB) tests were run by
CRREL and FHWA during each seasonal variation to obtain appropriate pavement
response data (deflections) under certain selected loads.
The Albany
County Airport site involved the construction of an extension to an existing
taxiway and an existing, but little-used, taxiway pavement that had experienced
detrimental frost effects. The sampling and testing programs at the Albany site
were very similar to those for the Winchendon site.
These carefully
controlled and well-documented laboratory and field test studies provided a
valuable data base that was used to develop new methods to solve the frost heave
and thaw weakening problems. A comprehensive set of design and construction
guidelines for pavements in seasonal frost areas was developed by the CRREL
engineers with financial assistance provided by FHWA's Technology Transfer
program (56).
4.9 Performance of Problem Ground Materials
During the most prolific construction years of the U.S. Interstate Highway
System, many large highway embankment sections were built with shale materials
taken from cut sections or borrow sources and deposited in the embankment prism
as if they were rock materials that did not require thin lifts and compaction
processing. In many situations, the shales were of a type that appeared to be as
hard and durable as rock while contained in their natural environment, but were
subject to large-scale deterioration pressures when exposed to oxygen and other
factors while functioning as a fill material in a large highway construction
project.
Some of the clay shales deteriorated so fast that they reverted
back to a soft soil that caused the embankments to fail (figure 41), especially
if they were placed on steep slope angles as most rock fills are constructed.
During one period of the 1970's, many of these landslide-type failures occurred
in several States, prompting an outcry for comprehensive research studies to
determine causes and remedies for this problem scenario.
In response to
these requests, FHWA researchers studied the work of previous investigators and
developed a comprehensive research plan to study the shale deterioration
problem. Initial findings determined that compacted shale embankment problems
can generally be divided into two categories: (1) those involving settlement and
possibly lateral movement and (2) those involving slope instability. Both
problems arise in part from the fact that embankments are commonly constructed
as rock fills with material placed in 0.3-m (3-ft) to 1.2-m (4-ft) lifts and
compacted only by hauling and spreading equipment. This practice can result in
large voids within the fill, which tend to collect material when the
deterioration process begins. The introduction of water into the fill
accelerates the deterioration process and causes a reduction in the void spaces,
resulting in settlement within the fill. A reduction in shear strength also
results from the deterioration process, causing slope failures to occur in some
extreme cases.
FIGURE 41. Shale embankment failure on I-64 in Indiana.
The obvious remedy for problems associated with shale materials is to
mechanically process the materials during construction so that additional
deterioration occurring during the service life of the embankment will not cause
any significant embankment distress. The ease or difficulty of breaking down the
shale material will determine the cost of the required processing. For example,
a shale material that is mechanically hard when it comes out of the source area
and nondurable (considerable slaking with time) is the most expensive shale
material to use in an embankment because it is difficult to process to the point
where subsequent deterioration will not cause excessive, nonuniform settlement
and/or slope failure.
It is the consensus of engineers and researchers
that the highway designer finds degradation and slaking to be the most important
shale properties. Degradation is the reduction in particle size that results
from construction processing, and slaking is the decomposition of the shale
materials due to weathering within the new environment (embankment). In both
cases, the amount of deterioration of fresh material (parent shale from the
borrow or cut source) must be predictable to allow a proper design of the
embankment. The degree of slaking that will occur after placement in the
embankment will affect the geometry of the embankment and/or the amount of
processing the shale will require during construction. The ease or difficulty of
breaking down the shale material determines the cost of the required
processing.
4.9.1 Shale Embankment Research – Based on the initial
results, FHWA contracted to perform a comprehensive 4-year study to develop
design and construction guidelines for using deteriorating shale in embankment
fills. A separate contract was also issued to conduct several workshops in
regions of high shale availability to teach SHA's and FHWA field personnel how
to recognize shale deterioration problems and offer practical solutions. An
executive summary report of the various research reports would be developed and
distributed at the workshops to help field engineers implement the prominent
findings of the research project. A participant's workbook was also to be
developed and distributed along with the executive summary and a copy of the
technical guidelines manual to be developed by the researchers.
The
results of this study definitely indicate that compacted shale embankments can
be designed and constructed economically to preclude subsidence and shear
failures. The underlying cause of excessive settlement and slope failures in
highway shale embankments appears to be deterioration or softening of certain
shales with time after construction. The main difficulty is determining which
shales can be placed as rock fill in thick lifts (0.6 m to 1 m (2 ft to 3 ft)),
and which shales must be placed as soil and compacted in thin lifts (0.2 m to
0.3 m (8 in to 12 in).
The researchers developed procedures for
anticipating the performance of shales in embankments from simple slaking
indexes and delineated procedures for characterizing the shale materials,
determining durability indexes, compaction tests including criteria for oversize
shale gradations, and a simple test on compacted samples to assess expected
compressibility of saturated shales for estimating long-term settlement
potential.
The final report provides guidance on geological
investigations, durability classification of shales, design features, and
construction procedures unique to compacted shale embankments for highways.
Guidance is also given on techniques for evaluating existing shale embankments
and remedial treatment methods for distressed shale embankments. Index tests and
classification criteria for determining shale durability, techniques for
evaluating excavation characteristics, and alternative procedures for
excavation, placement, and compaction of shales to achieve adequate stability
and minimum settlement are described. The use of drainage measures, selective
excavation, and placement of nondurable shales in thin lifts with procedural
compaction provisions based on field test pads is emphasized (57,58).
4.9.2 Rockfall Hazard Mitigation – Public highway agencies are expected to provide a safe and efficient ride for its users. The traveling public not only expects it, they demand it. When serious consequences result from an unsafe situation, litigation is sure to follow. Rockfall is a common highway safety hazard in mountainous terrain (figure 42). The loss of life and property, serious injury, disruption of traffic, and expensive maintenance are major problems in many States. Costly litigation actions against States due to rockfall-caused accidents defer large sums from highway budgets that are needed for roads and bridges.
FIGURE 42. Bus badly damaged by large boulder at rockfall site.
Keeping roadways clear of rockfall debris is made more difficult in
constricted transportation corridors where rock cuts are required. In some
States rock slopes are rare but in more mountainous States, many miles of
roadway pass through steep terrain where rock slopes adjacent to the highway are
common. Some of these manmade slopes are very high. Many are situated near the
base of rugged natural slopes that extend hundreds of meters further
upslope.
There is an inherent rockfall potential at these sites. This
potential is compounded by the way our highway systems have evolved over many
years. In the past it was normal construction practice to use overly aggressive
blasting and ripping techniques to construct rock slopes. Although this
facilitated excavation, it frequently resulted in slopes more prone to rockfall
problems. In some cases, uninformed designers were creating these unsafe rock
slopes in order to satisfy architectural desires for a natural looking rock
face. In addition, cut slopes are subjected to a broad range of climatic
conditions that also affect the overall slope stability. Where these conditions
exist, agencies are faced with the monumental task of reducing rockfall
hazard.
It is estimated that a combined outlay of several hundred million
dollars is spent each year to mitigate rockfall problems in this country. One
recent incident occurred on July 1, 1997, on Interstate 40 in western North
Carolina after a heavy rainfall caused a large rockfall to close all four lanes
of the Interstate highway for over 4 months. Two cars were trapped and five
people were injured. In addition, seven people were killed on the detour route
in truck-related accidents that were attributed to the poor level of service
provided by the lower standard highway route used as the detour facility. The
rockfall slide area was approximately 100 m wide and contained more than 300,000
cubic meters of rock materials that had to be removed at a cost of more than $3
million.
An additional $1 million was spent to place a protective rock
curtain over the cleared slope and construct a barrier wall at the toe of the
slope to block further rockfall debris. The tourist industry claimed that it
lost $40 to $50 million in business revenue during the shutdown period. A review
of the remaining 32 km (20 miles) of this particularly dangerous highway section
has resulted in an estimate of $40 million to correct the remaining potential
rockfalll problems on this highway that was constructed in the early 1960's
using construction blasting methods that blew the rock formations to
unnecessarily high proportions.
The extreme danger to the traveling
public and the significant economic impact of falling rocks on the roadway
require an accurate assessment of rockfall probability and extent. In
recognition of this problem, FHWA and several SHA's pooled their resources to
initiate a program to develop a "proactive" system to improve an agency's
ability to identify and respond to adverse rockfall situations. Prior to this
cooperative approach, SHA's relied on a reactive system of identification and
treatment to prioritize rockfall projects and allocate available repair funds.
Experience has shown that a proactive system is more legally defensible than one
that reacts to accident history and annual maintenance costs at a site to fix a
problem situation.
In addition, it was discovered that a reactive system
had several engineering deficien-cies, and may not reflect the true potential
for future rockfall events. The annual maintenance costs in a rockfall section
generally represents the cost to clean out the catch ditch and to patrol the
highway for rock debris on the roadway. However, if an adequately designed catch
ditch performs well (no rock on the roadway) but needs regular cleaning, the
maintenance cost may be high while the hazard to the motoring public is low.
This would indicate that these two items are not sufficient by themselves to
develop a rockfall priority list. In addition, this technique relies on
information reported by highway maintenance crews, law enforcement personnel,
the general public, emergency response personnel, etc. Such a diverse group is
not adequately trained to systematically document or evaluate rockfall
events.
The first step in developing a proactive system to address the
rockfall problem was to develop a Rockfall Hazard Rating System that could be
used by all agencies confronted with serious rockfall problems. Using resources
from its own research budget and several SHA's including Arizona, California,
Idaho, Massachusetts, New Hampshire, New Mexico, Ohio, Oregon, Washington and
Wyoming, FHWA arranged with the Oregon DOT (ODOT) to develop a generic Rockfall
Hazard Rating System (RHRS).
The RHRS is a tool for managing rockfall
sites along highway routes. The system contains six main features:
These features are discussed in detail in the research report and engineering
manuals developed by ODOT for use in a series of workshops funded by FHWA's
Office of Technology Applications (59, 60,61).
This new process developed
by ODOT appropriately places the responsibility for slope evaluations and design
concepts with properly trained and experienced staff. In Oregon's case, that
responsibility rests with its staff of engineering geologists. Utilizing their
expertise and judgment, they have demonstrated that reasonable and repeatable
slope ratings can be achieved. In addition, appropriate state-of-the-art design
concepts were advanced for project development consideration.
Experience
with the RHRS has been very favorable. Courts have ruled that it is unreasonable
to expect an agency to have at its disposal all the funds necessary to deal with
all the rockfall safety related issues at once. It is necessary though, to have
some kind of a proactive system in place that provides quality information to
designers for project development and helps management make rational decisions
on how to prioritize projects to best allocate scarce repair funds. The response
by agency management has been one of relief and acceptance. Managers believe
that public safety is now best being served and that greater legal protection is
afforded the agency by having the RHRS in place in their State.
4.10 Publications and Implementation Items
Research and development activities for soil and rock behavior areas were very successful. Numerous quality reports were generated that give valuable guidance on how to treat various problem soil and rock materials, and new ideas and methodologies are documented in each report. Many technology transfer items were developed and disseminated to practicing engineers. Workshops and training courses were also conducted to enhance the implementation process.