Through its unique capabilities and expertise, the Geotechnical Laboratory (GTL) supports the FHWA Geotechnical Research Program, other disciplines, laboratories, and offices throughout the agency, as well as other organizations and agencies within the transportation community. The core mission of the GTL is to advance the state-of-the practice and develop innovative solutions for applied transportation issues.
The GTL currently evaluates the material properties of soils and structural backfills; studies the interaction with structural elements such as steel, concrete, geosynthetics, or timber that are used for bridge foundations and retaining wall systems; and assesses geostructural aspects related to pavements. Testing is also performed to calibrate models for advanced numerical modeling.
Recent Accomplishments and Contributions
The GTL contributes to transportation agencies through material testing and technical assistance. In addition, research conducted by the Geotechnical Research Program has recently led to the development of the following key accomplishments:
- Design and Construction Guidelines for Geosynthetic Reinforced Soil (GRS) Abutments and Integrated Bridge Systems (IBSs) (FHWA-HRT-17-080).
- Deep Foundation Load Test Database, version 2 (DFLTD v.2).
- Strength Characterization of Open-Graded Aggregates for Structural Backfills (FHWA-HRT-15-034 and FHWA-HRT-18-048).
- Risk Assessment for Mechanically Stabilized Earth (MSE) Walls.
- Protocols for the use of high-speed inertial profilers to quantify the bump at the end of a bridge.
- Methodologies to measure super-substructure thermal interaction for integral and semi-integral abutments.
- Inaugural Geo-Structural Aspects of Pavements for Highways, Railways, and Airfields Conference (GAP-2019).
- Constitutive soil model for crushed, manufactured open-graded structural backfills.
- Field monitoring programs and training for the Federal Lands Highway Division.
The GTL consists of an indoor testing facility, several unique outdoor testing facilities, and a numerical modeling station. The indoor facility can conduct basic and specialized index tests for characterizing soil, aggregates, nontraditional backfill materials, and geosynthetics for both research studies and field production projects. The outdoor facilities consist of two test pits to perform large-scale foundation experiments and a strong floor to test earth-retaining structures. In addition, the Laboratory functions extend throughout TFHRC with several full- and large-scale GRS structures primarily to evaluate their long-term performance under load conditions. For field work, the GTL has the ability to prepare and install remote automated field instrumentation to monitor and evaluate performance of bridges, pavements, and slopes.
Some of the basic and specialized laboratory services include, but are not limited to:
- Forensic analysis
- Material testing
- Technical assistance
- Research advice
- Compaction testing
- Automated data collection system development and installation
- Large- and small-scale load tests for structural foundations
- Soil constitutive model development
- Risk assessment
- Numerical analysis
- Quality assurance
Current activities and services performed by the GTL are to: 1) study the material properties of soil and structural backfills for pavement and earth-retaining structures; 2) advance the state-of-the-practice of geotechnical instrumentation and remote automated systems; 3) assess the long-term performance of geotechnical assets; and 4) evaluate and advance testing methods; and 5) perform load and resistance factor design (LRFD) calibrations.
Figure 1. Large-Scale Direct Shear Devices.
The indoor facility has equipment to characterize soil and aggregate materials for both research studies and production projects. Special equipment includes two 12-inch direct shear device, a 6-inch diameter triaxial unit, a 6-inch resilient modulus device, and a 20-kip universal testing machine. The Laboratory also has a variety of fixtures and auxiliary equipment to conduct a variety of specialized tests to include the evaluation of innovative instrumentation for geotechnical applications.
Figure 2. Strength Testing of Geosynthetics.
Figure 3. Large Diameter Triaxial Device.
Figure 4. Resilient Modulus Device.
Figure 5. Frictional Connection Testing: Side View.
Figure 6. Frictional Connection Testing: Top-Down.
Figure 7. Calibration Reaction Assembly.
Figure 8. Evaluation of Pressure-Sensor Technology.
Figure 9. Standard Direct Shear Device.
Figure 10. Constant/Falling Head Permeameter.
Figure 11. Walk-in Environmental Chamber.
Outdoor Laboratories: Test Pits
One of the outdoor laboratory facilities consists of two test pits that are 18 feet wide, 23 feet long, and 18 feet deep. The pits can be filled with various soil types for modeled shallow or deep foundation experiments and have also been used to conduct full-scale wall experiments and to test the tension capacity of ground anchors. The pits have reinforced concrete walls, sump pumps to control water-table levels, and anchorage systems to provide reaction loads for experiments.
The pits have a separate building to store the load-test equipment and a control room for the data-acquisition systems.
Figure 12. Mechanically Stabilized Earth (MSE) Shoring Wall Experiment.
Figure 13. Helical Anchor Tensile Tests.
Outdoor Laboratories: Full-Scale Test Sites
The Laboratory includes two additional outdoor test sites where full-scale bridge piers, abutments, and retaining wall structures were constructed for research and testing purposes. The following are a few examples of full-scale experiments in these locations to illustrate the capabilities of Turner-Fairbank Highway Research Center (TFHRC) to lead the advancement of the state of the art.
Figure 14. Geosynthetic Reinforced Soil (GRS) Test Pier.
Figure 15. Prototype Geosynthetic Reinforced Soil - Integrated Bridge
Figure 16. Long-Term Performance of GRS Test Piers.
Outdoor Laboratories: Strong Floor
The Geotechnical Laboratory has an outdoor strong floor that is also available for the construction and testing of full-scale geotechnical features on a rigid concrete platform. The spacing of the anchorage locations is 3 feet by 3 feet, each with a 300-kip capacity similar to the Structures Laboratory to allow a variety of load fixtures and arrangements.
Figure 17. Outdoor Strong Floor.
Figure 18. National Cooperative Highway Research Program (NCHRP) 12-59 Experiment on the Strong Floor.
Figure 19. Long-Term Performance of GRS Abutments with Various Geometries on the Outdoor Strong Floor.
The Geotechnical Laboratory also calibrates many different types of typical and advanced geotechnical instrumentation and develops data acquisition systems for installation in the field. Recent installations have included pressure cells, strain gauges, tactile pressure sensors, in-place inclinometers, piezometers, water content reflectometers, and survey targets. Various projects, including evaluation of bridge abutments and monitoring of pavement and slope conditions, are currently underway.
Figure 20. Pressure cell installation in Sheffield, MA.
Figure 21. Installation of automated MEMS-based accelerometer sensors and piezometers in Denali National Park, AK.
Figure 22. Solar-powered remote data acquisition system in St. Lawrence, NY.