BACKGROUND:

During the 1970’s, a series of FHWA studies determined that various segments within 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 was therefore imperative that accurate and rational guidelines be developed for geotechnical-related design and construction applications to ensure safe and efficient highway structures. 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.

As a result of these discoveries, FHWA expanded the Geotechnical Research Program to address many of these needs. The program was divided into three main projects: Soil Behavior, Foundations, and Ground Improvement. This program, as described in the body of this report, was completed in 1998; and a new program to study innovative foundations and excavation support systems was established. The new program places more emphasis on the development of innovative methods to support bridge foundations and earth retention systems.

In the National Geotechnical Engineering Improvement Program report prepared for The Office of Engineering, the authors developed a list of research needs that relate mostly to foundations, earth retention, and excavation problems that were identified by their customers in a recent national survey. The results of the survey and a state-of-the-practice assessment by FHWA clearly shows that research is needed to develop technical guidance for some of the new technologies that have recently emerged from foreign sources or other building disciplines.

OBJECTIVES:

The objectives of this research are the development of new and/or improved support systems for bridge foundations and deep excavations for highway construction projects.

SCOPE:

The scope of this research includes analytical studies, laboratory testing, and field monitoring of construction sites in order to develop, refine, and validate new or improved designs. It includes research into a wide range of materials properties, instrumentation techniques, monitoring methods, analytical techniques, performance assessment, and design principles in much the same way as the predecessor program.

The research program will be set up to focus on two main projects: foundations and excavation support systems. The foundations project will cover innovative load testing systems, load and resistance factor design, piles, drilled shafts and spread footings, plus some innovative uses of geosynthetic reinforcing materials that are combined with modular building blocks to form bridge support piers and abutments. The excavations project will look at new and innovative methods to build earth retention systems from the top down, plus other innovative ways to support and retain soil and rock masses.

APPROACH:

The major research efforts in the foundations project are included in four tasks:

1. Innovative Load Testing Systems - In the FHWA National Geotechnical Engineering Improvement Report, it was noted that there was a large increase in the number of highway agencies that are using innovative load test systems for bridge foundations. The reasons for the increase are economy and reduced time for load testing as was demonstrated in previous FHWA research studies. However, several methods need documentation for standardized test procedures or for the interpretation of the data produced by the test. The Office of Engineering will use this information to develop a Geotechnical Engineering Circular to provide FHWA- recommended procedures for these innovative load testing systems, such as the Statnamic rapid load test, the Osterberg load cell, and several dynamic load test systems. Comparative analysis studies will correlate results from these tests with results from conventional static load tests from the FHWA load test data base developed under previous research studies.
   
   
2. Load and Resistance Factor Design (LRFD) - According to the Office of Engineering’s national report, the FHWA geotechnical research data bases are key links in their work to implement LRFD nationally. Recent efforts by them and the National Highway Institute to train engineers and implement LRFD procedures for foundations have disclosed that adequate resistance factors are not available to make an orderly transition to LRFD methods. The authors of the report suggest that the resistance factors can be developed from one segment of the FHWA research data bases and then verified with data from other segments of the data bases. In addition to using the data base to verify the reliability of the various factors and computational procedures, theoretical correlations of existing procedures with the research quality databases will be required to convince customers of the reliability of the new LRFD procedures.
   
   
3. Micropile Technology - The Office of Engineering has requested that recently completed research efforts in this area be expanded to investigate use in seismic retrofit situations and for slope stabilization purposes. According to its survey, the popularity of micropiles is increasing, with more proprietary systems being developed for both foundations and earth retention. In addition, three recent failures of micropile systems on design- build projects have caused concern among the FHWA engineers, their partners, and customers about current design practice. Both vertical (compression and tension) and lateral resistance (structurally and geotechnically) of micropile systems must be investigated before FHWA launches Demonstration Project 116 on micropile technology.
   
   
4. Automated Geotechnical Information and Design Aid System - A comprehensive effort is required to integrate all of the FHWA research-quality data bases and recently developed design improvements into a comprehensive design aid system to allow bridge engineers to quickly and economically obtain information and evaluate design alternatives from a centrally located computer source. The approach to be taken will involve development of commonality features and the design of a user interface application for performing cross queries, correlations, and engineering analyses. Several of the data bases already contain modules for performing correlations, predictions, and analyses, but they need to be linked through a multi-user workstation that contains an interactive system for automatically generating design solutions based on interactive user input. Such a system will take most of the guesswork out of geotechnical design and replace it with an objective, quantitative system that supports sound management decisions.

The major research efforts in the Excavation Support Systems project are included in three tasks:

1. Soil Mixing - The process of deep and shallow soil mixing with cement and lime additives is increasing at a rapid rate, especially in large urban areas near large bodies of water containing very soft soil deposits. The two largest highway construction projects in the United States (Boston Central Artery and the I-15 corridor in Salt Lake City) are employing different types of soil mixing to stabilize critical ground conditions. These soil-mix designs were introduced into these projects through value engineering or design-build contracting approaches. At present, neither FHWA nor AASHTO have any published design guidance for these techniques, which originated in other countries.
   
   Research will develop soil-mix design criteria and construction quality control procedures to permit rational use by FHWA customers. Some preliminary research by FHWA has clearly shown that these methods have significant potential to reduce costs and time delays if rational guidance for strength, deformation, and durability concerns can be developed.
   
   
2. Top-Down Construction Techniques - The use of soil nailing, ground anchor tiebacks, and other top-down construction techniques, such as slurry walls, continue to be a very popular way to support deep excavations, especially since FHWA research results have been disseminated through implementation manuals, training courses, and other technology transfer functions. Further refinements to optimize their usage are needed in the form of increased knowledge of the load transfer mechanism between the reinforcing elements and various soil types or ground treatments. Corrosion and durability aspects are also in need of study. Most of the prior research involved granular materials to take advantage of the soil’s frictional strength along the reinforcing element’s surface area to resist deformations that could damage the structure. Recent FHWA research efforts have clearly shown that clay soils can also be reinforced with nails and other inclusions.
   
   
3. Geosynthetic Reinforcement Applications - Recent FHWA research results have demonstrated that geosynthetic materials can be economically combined with modular blocks and granular soil materials to provide foundation support for bridges and excavation support for roadways. Initial studies of this technology have resulted in questions related to mobilization of the resistance in the composite mass structure. Other design issues include the vertical spacing distances between the geosynthetic reinforcing sheets and the connection methods between the reinforcing elements and the facing blocks.
   

SUMMARY:

The future R&D program described in this appendix will provide new knowledge and technology to help ensure the safety and reliability of the Nation’s highway bridges and retaining wall systems that are exposed to such dangers as floods, earthquakes, and strong winds. The new knowledge will also help to reduce the amount of over-conservative design that often results from fear due to a lack of knowledge in how to properly design for certain contingencies. It would not be prudent to have large quantities of "buried treasures" beneath some bridges and earth retention systems in order to be sure that these structures are safe and reliable in times of crisis. We must also be sure that these systems are efficiently designed. Experience and previous research results have demonstrated that this new program can provide the opportunity to develop these innovative capabilities for improving the safety, reliability, and efficiency of these critical national assets.