Improving Foundation Design in Rock: Load Test at Burma Road Overpass: FHWA-WY-09/10F
This report describes the results of a bi-directional load test on a drilled shaft foundation in weak sandstone. The test was conducted in conjunction with construction of a new bridge at Burma Road Overpass on I-90 in Gillette, Wyoming.
The purpose was to provide much needed information on side resistance and base resistance in weak sandstone of the Tertiary Wasatch Formation. Load test results are compared to design equations for both soil and rock. Design equations based on treating the weak sandstone as cohesionless soil provide close agreement with side resistance values measured by the load test. Design equations based on treating the sandstone as rock also provide reasonable agreement with the load test results, but comparisons were limited by the inability to obtain representative intact core samples suitable for measuring the uniaxial compressive strength of the sandstone.
Unit base resistance mobilized in the load test exceeds by a significant amount the value of unit base resistance predicted using AASHTO and FHWA design equations. The load-displacement response of the test shaft is analyzed by fitting to an analytical model, providing a practical tool for evaluation of trial designs to satisfy service limit states. Finally, results of the load test are used to illustrate the application of AASHTO LRFD methodology to design of drilled shafts for the bridge at Burma Road Overpass.