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Summary

Geotechnology

As with other programs, the FHWA program in geotechnical research is supported through staff research, and contract research. The FHWA maintains onsite a Geotechnical Laboratory Complex, which consists of soil mechanics, soil behavior, and foundations testing facilities. The FHWA geotechnology team includes Carl Ealy and Michael Adams.

The following is a brief summary of ten of the most recently completed or nearly completed studies in the program.

1. Durability of Geosynthetics - Earth Engineering Sciences, Inc. The objective of this study is to develop procedures for predicting long term strength losses of geosynthetics used in highway applications. The study is divided into a laboratory durability evaluation (hydrolytic and oxidative degradation) and a field study. Laboratory tests will be performed on both new material and old material exhumed from the field. New material tests will include damaged (simulated construction damage) and undamaged samples. The effects of soil pH, ion conditions, moisture availability, and temperature will also be investigated. Field tests will be performed to verify lab results. As research information becomes available it will be incorporated into FHWA Demonstration Project 82.

2. National Geotechnical Test Sites - United States Geo-Council. The objective of this on-going study is to develop a national system of designated experimental sites that will consolidate research efforts at a few highly efficient locations. Geotechnical tests sites that have been extensively tested will be used for R&D projects that require an accurate knowledge of insitu soil conditions. The use of these sites will increase the accuracy and consistency of future research while substantially reducing their cost. This cost reduction, and the information available in the data repository, will allow researchers the opportunity to perform accurate and effective work. A data repository has been developed. Under this contract, 81 candidate tests sites have been evaluated and rated. Originally, nine test sites were approved: two level one sites, Texas A&M, and Treasure Island (San Francisco); three level two sites, University of Houston, Northwestern, and the University of Massachusetts (Amherst); and four level three sites, I-95 Embankment (Saugus, Massachusetts), Norfolk (Virginia), Imperial Valley Wildlife Refuge (California), and Hamilton Air Force Base (California). The top five sites were selected for special funding to improve soil profile documentation and site conditions. Most of this work is completed and the sites are now operational. In 1998 a fourth level two site was added to the system at Auburn University.

3. Permanent Ground Anchors - Schnabel Construction Company. The objective of this research study was to evaluate and improve existing design and construction guidelines for permanent ground anchors. This study is now complete. Model and full scale tests were conducted and the results used to determine rational allowable loads for permanent ground anchored walls. Measurements were made on the anchors, soldier piles, and the back side of the wall. The results of these measurements were also used to determine soil-anchor interaction, earth pressure on the wall, grouting requirements, bond zone load transfer, angle of wall friction, and soldier pile load transfer. A computer program for the design of anchor walls was also developed under this contract. The following reports are available from the FHWA Report Center.

4. Pile Load Test Data Base - This staff study will provide a high quality pile load test data base for driven, drilled, and vibration installed piles. Test data and soils information from thousands of pile load tests have been collected and evaluated for inclusion into the data base. New data was also generated by installing instrumentation and conducting load tests on active bridge construction projects. The data base program includes: static load test results, complete with load settlement plots; dynamic test results; pile instrumentation results; pile driving records; boring logs; SPT testing; ECPT testing; Dilatometer testing; laboratory testing; and several methods for static analysis. At this time approximately 1600 good data sets have been input into the data base. The data base was distributed for beta testing in early 1998.

5. Shallow Foundations Research - Geotest Engineering, Inc., and Texas A&M University. The scope of this study includes:
   • development of a spread footing performance data base similar to the deep foundations data base;
   • spread footing load tests;
   • a settlement prediction symposium (ASCE & FHWA);
   • dynamic testing of spread footings;
   • evaluation of existing prediction methods and possible development of a new settlement prediction method.
   
   This study is complete, and the final report listed below is available from the FHWA Report Center.

6. Synthesis of Micropile Research - Nicholson Construction Co. This study is a critical review and analysis of current design and construction techniques for using micropile technology for new bridges and rehabilitation and/or repair of existing bridges including seismic retrofit. The study is completed. The final report was issued in 1997 as a four volume summary report, and are available from the FHWA Report Center.

7. Seismic Behavior of Micropile Systems - Polytechnic University of New York. The objective of this study is to develop improved recommendations for the design of micropile systems to withstand seismic forces applied to bridge foundations. An evaluation of current seismic design practice is being performed concurrently with the development of an analytical and laboratory investigative program on isolated piles and groups of piles. Centrifuge and shake table testing has also been included in the laboratory evaluation. Later research under this study will evaluate physical and numerical modeling of the response of reticulated micropile or "network" systems to seismic loading in selected soil types.

8. Large Model Spread Footing Load Tests on Reinforced Soil Foundations - Under a staff study the potential benefits of geosynthetic reinforced soil foundations are being investigated using large scale model footing load tests. A total of 34 load tests were performed in the TFHRC test pits to evaluate the effects of single and multiple layer of reinforcement placed below shallow spread footings. Two different geosynthetics are being evaluated; a stiff biaxial geogrid and a geocell. Parameters of the testing program include: number of reinforcement layers; spacing between reinforcement layers; depth to the first reinforcement layer; plan area of the reinforcement; type of reinforcement; and soil density. Test results indicate that the use of geosynthetic reinforced soil foundations may increase the ultimate bearing capacity of shallow spread footings by a factor of 2.5. The final report is being processed for publication.

9. Performance of a Prestrained Geosynthetic Reinforced Soil Bridge Pier - As part of a staff study a full-scale instrumented bridge pier was constructed and load tested at the Turner Fairbank Highway Research Center (TFHRC) in McLean, Virginia. The pier was 5.4m high and, at it's base, was 3.6m x 4.8m. The pier successfully sustained a maximum load of 980kN (2200kips) which is equivalent to a pressure of 900kPa (9.4tsf). The purpose of the experiment was to demonstrate the performance and ability to construct a reinforced soil bridge pier with segmental blocks. This method utilizes closely spaced high-strength geosynthetic reinforcement and quality compacted road base. The pier was instrumented to monitor load, lateral deformation and vertical settlement. Several layers of the reinforcement were instrumented with strain gauges to monitor creep and strain in the fabric. A method of prestraining the reinforced soil mass was also tested. Changes in properties of the reinforced soil due to prestraining are being evaluated. The pier was hydraulically loaded to squeeze the reinforced soil vertically. The final report is being processed for publication. As an extension of this project, two abutments and approach fills with modular block walls are being constructed in the same manner as the geosynthetic reinforced soil pier. A full-scale bridge superstructure will be supported by these "fortress" type abutments at a site not too far from the pier. The vertical spacing and strength of the geosynthetic reinforcing elements are varied in the abutments and walls, plus instrumentation was installed to measure deformation and monitor response.

10. Design and Construction of Backfill Envelopes, Foundations and End Treatments for Underground Structures a.k.a. Buried Pipe Installation - University of Massachusetts. Through an interagency agreement between the FHWA and the National Science Foundation this analytical and experimental (lab and field) study has investigated the fundamental interactions between pipe, backfill and in-situ soil that occur in the process of excavating a trench, preparing the sub-grade, installing the pipe, and placing and compacting backfill. RCP, CMP and HDPE pipes were tested in the lab and in wide and narrow trenches in cohesive and granular soils. Measurements were taken of soil density and moisture content, pipe-soil interface pressure, horizontal soil pressure between backfill and in-situ soil, vertical soil pressure on pipes, soil and pipe wall strains, and the pipe profile. The first three reports for this study, which essentially document the experimental work, have been published by UMass. The fourth report will be the Final Report. It is being revised for editorial comments. The last part of the study, recommended revisions to the installation provisions of the AASHTO LRFD bridge code, is in preparation. It is expected that the study will be completed in 1999.