The roots of the Federal Highway Administration’s (FHWA) geotechnology research program can be traced back to the 1970’s when FHWA field personnel and State Highway Agency (SHA) engineers requested assistance in solving numerous soil behavior and foundation engineering problems. The FHWA Office of Research responded by establishing three geotechnical related research projects, plus a major geotechnical task in the Tunneling Research Project. Together, these research projects and a few other "stand-alone studies" were grouped to form the FHWA Geotechnology Research Program.

The first geotechnical research project dealt with soil and rock behavior problems such as soil stabilization, compaction, frost action, expansive clays, and deteriorating shales used in embankment fills. The second project dealt with bridge foundations, including piles, drilled shafts, and spread footings. The third project covered specialized ground improvement techniques for compacting, draining, and reinforcing ground materials to withstand heavy loads under typical highway applications. The geotechnical research under the tunneling project covered site investigation, soil parameters for design, instrumentation monitoring techniques, plus ground movement prediction and control. Chapter Five covers the separate research studies that did not fit well under the four major projects previously mentioned.

This report gives an overview and summarizes the results of the research conducted under the four geotechnical projects established during the 1970’s. It describes the efforts and results of 25 years of research spanning three decades from 1973-1998. The main purpose of the report is to provide a summary of the FHWA geotechnical research activities over the last quarter of the 20th century. It is intended for the general engineers and administrative managers of FHWA and the SHA’s.

Also presented are descriptions of the various problems that were addressed; and the report discusses the objectives and scope of each project in detail, except for the Tunneling Project, which is reported elsewhere (see appendix B). A review of each project’s organization and approach is presented before the results are noted and evaluated. Technology transfer and future research needs are also covered separately to highlight the important nature of each topic.

This report demonstrates that the state of the art was significantly advanced by the many contributions from this research program. It also provides a concise reference for other researchers and practicing engineers concerned with designing and constructing geotechnical structures for highway applications.

1.1 Background

During the 1970’s, a series of FHWA studies determined that various segments of the field of highway geotechnology needed significant improvement in design and construction applications. This was especially important considering that bridge foundations, retaining wall systems, embankments, and cut slope operations account for well over 50 percent of the total cost of most highway projects. It is therefore imperative that accurate and rational guidelines be developed for geotechnical related design and construction applications to ensure safe and efficient highway structures.

The assessment studies found that pile design was more guesswork than it was scientific, especially for group behavior of piles. Other foundation systems such as drilled shafts and spread footings were starting to replace piles in a few cases, but piles were usually selected in the vast majority of projects, although in some cases they may not have been the best choice. Reasons most often cited were the lack of adequate performance records for the alternative choices and/or the need for better design and construction guidelines.

Also, at this time, there was a significant influx of innovative geotechnical methods to retain earth masses and/or improve ground materials to withstand heavy loads or resist environmental effects in typical highway projects. Some of these ground improvement technologies were imported from foreign countries where it was less important to understand how the improvement mechanism worked, only that it had a strong history of being successful and it carried a written guarantee from the specialty contractor. The lack of specific guidelines and specifications was slowing their adoption here in the United States where our society is more content to specify and control construction than to accept guarantees from the contractor.

Early in the life of this program it was discovered that geotechnical engineering for foundations and earth structures lagged behind most other highway engineering disciplines in evolving from an art to a science. Many of the commonly used design techniques of the early period suffered from a lack of precise definitions and a very imperfect understanding of fundamental behavioral mechanisms that govern geotechnical structures. Also, the difficulty and expense associated with properly defining soil and rock behavior under foundation loads significantly impeded the development of rational theoretical solutions, thus fostering the growth of empirical methods of design and analysis.

Most of the difficulty and expense of defining soil and rock behavior involves the inconsistencies and uncertainties associated with applying engineering principles to non-homogeneous ground materials. Predicting the response of soils to bridge loads that are transferred by various piles in a pile group or tensile elements in a reinforced soil mass are two special cases where more precise definitions of soil behavior and the failure mechanisms would lead to more economical designs.

1.2 Objectives

The objectives of this program were to develop improved predictive techniques for foundation design and soil behavior, and improved design and construction guidelines for ground improvement techniques, such as reinforced soil, stone columns, dynamic compaction, soil nailing, tieback anchors, and prefabricated vertical drains.

1.3 Scope

This program dealt with research and development of improved design and analytical prediction procedures for foundation systems such as piles, spread footings, and drilled shafts, as well as the development of improved design procedures for using ground improvement techniques on retaining walls, embankments, and highway cut slopes. Research efforts were also directed toward developing construction guides to complement the improved design procedures.

The development of improved analytical techniques requires accurate measurement of the stresses and strains in expensive full-scale models that are load tested to failure to serve as a benchmark for less expensive mathematical and reduced-scale physical modeling studies. Knowledge of the appropriate scaling effects is also required in some cases, because structural interaction with soil is strongly dependent on the properties of the soil that are very difficult to reduce in scale. For example, research has shown that the response of different size model piles is not generally defined in any simple, direct relationship, such as those derived by ordinary dimensional analysis.

1.4 Approach

The basic research approach was to begin the study of each topic or technique with a state-of-the-art investigation to collect all available information on use, design methods, construction practices, case histories and performance evaluations. Laboratory studies to evaluate physical and engineering properties and some small-scale model testing in the laboratory were used to supplement the physical testing and subsequent field tests. Full-scale field tests and performance evaluation studies were conducted for some of the most promising techniques, especially those that were found lacking in well-documented field data. Design and construction guidelines were developed for each technique on the basis of lab and field test results.

Because many of the newer techniques were proprietary and/or only performed by highly skilled specialty companies, the researchers coordinated their efforts with skilled specialists in the various areas. Personal interview programs were conducted during the early stages of development and draft manuals were later submitted for their review and evaluation.

State highway departments were encouraged to work with us and assist in this research and development effort. In many cases it proved to be economically attractive to "piggyback" the research efforts onto an ongoing State highway department construction project to reduce the costs of mobilization and capitalization of materials, equipment, and labor.

During the conduct of the early studies of this program, it became increasingly apparent that advancements were coming too slowly and with great difficulty because of two major obstacles or shortcomings: 1) the lack of comprehensive data bases containing research-quality information on the behavior of geotechnical structures subjected to both working stress and failure conditions, and 2) the lack of research-quality test sites available for testing developing technologies and new products at locations where soil and site conditions are well-known, and therefore can provide a standardized base with which to compare the results. Of course, more money and personnel would also have helped to speed progress as well.

When these lessons were learned, the program was modified in mid-stream to divert some funds and resources to correct these deficiencies. The development of these major resources has also been coordinated with the development of an Automated Geotechnical Information and Design System (AGIDS), which will incorporate all of the design improvements resulting from the research efforts, and also will make use of the FHWA databases in retrieving information for various correlations, analyses, or predictions that can be done with AGIDS. A detailed description of AGIDS is presented in section 5.2. The establishment of a system of research-quality test sites is described in section 5.3

FIGURE 1.  Significant shimming beneath bearing device.