Study of carrying capacity by rock socketed region of bored cast in-situ piles

Abstract

Bored cast in-situ pile socketed in to hard rock is the widely used method to transfer the heavy super structure loads to the rock through soft over burden soil. Finding carrying capacity of the socketed region is the most significant part in the design of in situ piles. There are many empirical formulae and factors shall be used to find the ultimate loads. Finite element analysis offers an excellent opportunity to study pile-soil interaction, pile response and soil movement under vertical loading in different sub surfaces. The common geotechnical software PLAXIS 2D which is capable for analyses different subsurface and bed rock with different loading conditions used for the research studies. This research was attempted to find out the vertical carrying capacity with settlement of the bored cast in-situ piles socketed into bed rock and embedded through weathered rocks. The PLAXlS 2D software was examined for its validation to use for the research studies. The Mohr-Coulomb model, axi-symmetric problem is simulated for single pile. The applied load is simulated as uniform pressure at the top. The following publications were examined and has been confirmed its validation; such as ''Numerical simulation of vertical loaded piles" (proceedings of Indian Geotechnical Conference December 15-17, 2011, Kochi (paper No. N-118)), "Load distribution curves in Rock Socketed Piles", (Based on Kulhawy & Goodman, 1987) and "Distribution of side wall shear stress curve in relation to socket length and modulus ratio" (after Osterberg and Gill). For these research studies bed rock parameters were extracted from the table 3.5: Rock Mass Rating System (After Bieniawski, 1989) for RMR; 20, 40, 60 and 81 rocks. The parameters are given in Table 3.6 to Table 3.9. Weathered rock, completely weathered rock and soft soil parameters are assumed based on Tomlinson and given in Table; 3.13. Axi-symmetry finite element models are developed with the use of PLAXIS 2D software. The FEM models are developed and run for all type of rocks considered for this studies with varying embedded lengths. The ultimate carrying capacities and the settlement of the pile top are found. The theoretical end bearing capacities are estimated for the bored piles socketed into bed rock using the method proposed in BS 8004.The equation proposed by William et al.(1981) (after Tomlinson, 1995) F= a.fJquc and chart proposed by William et al.(1981) and Rosenberg & Journeaux (1976) are used to estimate the ultimate shaft resistance for the same. The theoretical settlement was estimated for bed rocks and weathered rocks using equation proposed by Poulos and Davis (1996) and the pile head settlement proposed by Tomlinson. Results from the FEM analysis and the values determined from the empirical formula e were compared. From these studies very reasonable agreements were found for carrying capacities and the settlements for both bed rock and weathered rocks. The vertical carrying capacity of the pile embedded in to completely weathered rock is significant value.