Abstract:
The Top-Loaded Bi-Directional Test (“ TLBT” ) and the conventional Bi-Directional Load Test (“ BDLT” ) are full-scale static load test methods. In both methods, the test loads are applied bi-directionally to the deep foundation. The advantages of the conventional BDLT have been well established for testing large diameter, high capacity drilled foundations. A conventional BDLT applies the bi-directional test loads using an embedded jack and bearing plates. The TLBT method also applies the bi-directional test load to embedded bearing plates but does so using reusable high-strength threaded bars attached to an above-grade hydraulic jack and load transfer assembly. The new TLBT method is well suited for load testing shafts of a moderate diameter, length, and nominal resistance or ultimate load. TLBT advantages include economic considerations due to the method’ s reusable load application components and constructability considerations as it readily accommodates a center concrete tremie pipe or pump line location.
This paper presents comparison results from full-scale load tests performed by both bi-directional load testing methods. The BDLT and TLBT shafts were both 3 feet in outside diameter and were drilled 30 feet apart center-to-center. Details regarding subsurface conditions, test shaft construction, and installation inspection tests are included for the comparison of the TLBT and BDLT results. Corresponding test data and analyses for both test shafts are presented and discussed in detail. The BDLT shaft had a maximum uni-directional load of 240 kips at upper and lower bearing plate displacements of 0.5 and -3.8 inches, respectively. The TLBT had a maximum uni-directional load of 300 kips at upper and lower bearing plate displacements of 1.0 and -5.9 inches, respectively. These results as well as the load-transfer behavior and equivalent top loading curve results from the adjacent BDLT and TLBT drilled shafts are discussed. The paper also discusses the strain gage instrumentation on the TLBT load application bars and on the embedded bearing plates.