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Current Research Program
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 geotechnical research program has two main projects: foundations
and excavation support systems. The foundations
project covers 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
looks 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:
- 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.
Summary
The GT R&D program described in this section provides 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 also helps 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.
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