Drilled Shaft Load Testing during Design

Rock-socketed caisson design is governed by the side resistance and end bearing of the rock. In Philadelphia and many other regions, upper limits are set on the allowable design based on presumptive values together with unconfined compressive strength test results on intact rock specimens. When locally assigned upper limits are applied to the Wissahickon mica schist in Philadelphia, the length of the rock socket would be as long as 10 m (33 ft) under the maximum design load of 88,520 kN (19,900 kips) for a 49-story building. In order to optimize the socket design, an Osterberg cell load test of a rock-socketed caisson was planned and successfully completed during the design phase. As a result, a significant reduction was achieved in the rock socket lengths by increasing the allowable side resistance from 0.9 to 1.4 MPa (10 to 15 tsf) and the end bearing from 4.8 to 12.5 MPa (50 to 130 tsf). This case study illustrates the importance of understanding the unique aspects of the site geology such as degree of weathering, top of rock, and metamorphic texture variations, in terms of local presumptive strengths, and how that justified performing a very expensive test even though the outcome could not be guaranteed. It also highlights the importance of foliation or schistosity and fracture dip angle in characterizing the rock mass and their influence on the unconfined compressive strength of the rock cores. Without such an understanding, making the case for the load test would have proved very difficult during design.