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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

This report is an archived publication and may contain dated technical, contact, and link information

Publication Number: FHWA-RD-98-139
Date: June 1999

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A Quarter Century of Geotechnical Research

Chapter 4: Soil and Rock Behavior

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:

  1. Improved Technology for Expansive Clays and Shales - The research was directed toward providing an improved methodology for predicting expansive clay characteristics and conditions responsible for pavement distress and synthesizing remedial treatments that can be used for maintaining volume stability. The validity of design and construction procedures to prevent detrimental volume change of the soils was also examined.

  2. Improved Compaction of Fine-Grained Soils by Chemical Treatment -  Increasing the density of the soil mass generally provides improved strength and performance of the specific soil material. Detailed knowledge of the changes in soil structure or fabric changes caused by the compaction process, however, was conspicuously lacking. Earlier FHWA research had provided some insight into the soil fabric of compacted soils and into factors influencing the subsequent behavior of compacted soils. Research regarding the chemical substances (elements, ions, group and specific types of compounds) that might be added to soils to provide greater compactibility, and to make laboratory and field evaluations of selected compaction-aid chemicals was conducted under this task to provide solutions.

  3. Increased Effectiveness of Existing Soil Stabilization Systems -  This task was concerned with rendering current Stabilization practices more effective, economical, and widely useful. Studies were conducted to identify and evaluate trace additives for increasing the strength, durability, resistance to frost and moisture changes, and to extend the range of soil types appropriate for Stabilization with a given stabilizer.

  4. Development and Evaluation of Frost Heave Predictive Techniques  -  The research was done to evaluate the suitability of parameters such as heave pressure, permeability, and heave rate for predicting the frost susceptibility of subgrade soils and base course materials. After selecting the most appropriate parameters, and perfection of the apparatus and procedures for measuring them, criteria for predicting the frost heave susceptibility were developed and verified by field observations. In addition, research was conducted to characterize subgrade materials with respect to changes in strength during seasonal variation (frozen, thawing, and thawed periods).

  5. Strength and Deformation Properties of Soil and Rock Materials -  This task was concerned with: (1) providing a methodology for identifying and evaluating the performance of problem materials such as shales, organic soils, and sensitive soils; (2) developing techniques for preventing distress and failures of embankments, cut slopes, and foundations of problem materials; and (3) developing techniques for estimating and controlling hazardous rock fall problems.

The Soil and Rock Behavior Project was the initial project for FHWA in the geotechnical research arena. The following five sections of this chapter provide a summary of the major contributions of this project.

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 the problem 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 within their 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.

Pavement damage due to expansive subgrade soils

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.

Soil Stiffness Gauge 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.

Schematic of SSG showing major internal components 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.

Pavement damage due to frost heave

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.

Shale embankment failure on I-64 in Indiana

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.

Bus badly damaged by large boulder at rockfall site

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 deficiencies, 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.


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