Lateral loads exerted on offshore piles by subbottom movements

Abstract

Very weak marine clay sediments may be subjected during storms to considerable movement. A theoretical procedure is developed to determine the loads that the moving sediments exert on pile foundations. The lateral loading per unit pile length F derived from the consideration of a plastic slip -line model is : F= 10.10 D C(subscript u) where D is the diameter of the pile and C(subscript u) is the undrained shear strength of the soil. Results obtained with model test equipment showed that the above equation is valid for all cases studied if C(subscript u) includes the effect of the velocity through an equivalent strain rate and the influence of confining pressure. A decrease in load and shear strength was observed with an increasing number of cycles. Evidence is presented suggesting that this degradation is associated with a build-up in pore pressures caused by the cyclic straining of the mud around the pile. A method is proposed for estimating the pile loading based on the in-situ measurement of shear strength at one strain rate and the value of the liquidity index. Suggestions for further work are included.