A Quarter Century of Geotechnical Research
Appendix A - Future Geotechnology Research Program
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
The objectives of this research are the development of new and/or improved
support systems for bridge foundations and deep excavations for highway construction
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.
The major research efforts in the foundations
project are included in four tasks:
- 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.
- 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.
- 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.
- 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:
- 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.
- 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.
- 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.
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.