Parametric Study of Open Core-Hole on the Behavior of Drilled Shafts Socketed in Soft Rock

Abstract

Rock socketed drilled shafts are frequently used in very soft rocks, with compressive strength less than 2.5 MPa (26 tons/ft 2 ), to support structures. Due to the lack of understanding of the behavior of rock socketed drilled shafts in very soft rocks, the design capacity has to be verified by performing full-scale load test and/or sampling and testing of the rock at the bottom of the borehole using a core-hole sampler, before installing the production drilled shafts. Hence there is a need to better quantify the performance of drilled shafts with and without a core-hole in very soft rocks. In this investigation, field test results are used to first calibrate the end bearing capacity of rock socketed drilled shafts in soft rocks using the finite element method. Effects of rock cohesion and friction angle on the end bearing capacity were investigated and unit end bearing-displacement (q-w) relationships have been developed. Linear and non-linear relationships between ultimate unit end bearing and rock strength have been developed. Effect of the open (not filled with material) corehole diameter on the ultimate bearing capacity, unit end bearing-displacement relationships and the bearing capacity factor has been quantified. For a 0.1D and 0.4D open core-hole diameter the bearing capacity factor was reduced by 4% and 40% respectively.