End Bearing Capacity of Piles in Calcareous Sands

Abstract

ABSTRACT In this study, laboratory tests simulating the end-bearing of piles founded in silica sand and two calcareous sands at different densities were conducted. Bearing capacities are compared with field test values and theoretical cavity expansion pressures. Limit values for end-bearing based upon compressibility are suggested. INTRODUCTION The end-bearing capacity of deep foundations in sand is a complex and poorly understood subject. No recommendations exist which have been proposed by the certification authorities for more compressible sands of different mineralogy notably the biogenic calcium carbonate (calcareous) sands found mostly, although not exclusively, on continental shelves in tropical and sub-tropical regions. Pub1ications within the 1ast fifteen years have shown that pile capacities in such materials are much lower than in silica sands at similar relative densities. This paper presents the results of model pile axial load tests carried out in a calibration chamber upon a silica sand and two incremented calcareous sands in dense and loose states. The laboratory results are then compared with published field test data and suggested design values are proposed. Current Methods of Determining End-Bearing Current guidelines for offshore pile design in silica sands (API, 1989) are based upon an extensive back-analysis of pile load tests at penetrations of up to 30m (Dennis and Olson, 1983). Unit end-bearing is expressed as : (MATHEMATICAL EQUATION AVAILABLE IN FULL PAPER) Where ?y' is the effective overburden pressure and is a bearing capacity factor. It has long been recognized that does not increase linearly with depth, but that the rate of increase reduces with depth. Limiting values used in design are equivalent to values of from equation (1) at an effective overburden pressure of approximately 240 kPa. Another approach includes an empirical method based upon high quality pile load tests, (Coyle and Castello, 1981) avoiding the use of limit values, which is [I1ore realistic, but taking no account of effective stress levels, being in terms of relative penetration. Another method (Randolph, 1985) links qp to Nq using recent correlations which properly model reductions in friction angle with mean effective stress. No recommendations exist in the API guidelines for more compressible sands, although research carried out on behalf of API (Dennis and Olson, 1983) equates medium dense ca1careous sand with loose silica sand and loose calcareous sand with very loose silica sand. A semi empirica1 method (Nauroy et a1., 1986) 1inks unit end-bearing to compressibility and proposes limiting values of qp related to the value of a compressibility index