Rate of Loading Parameters for Vertically Loaded Piles in Clay

Abstract

ABSTRACT The analysis of 152 laboratory tests and 32 pile load tests confirms that for clays, the faster the rate of loading, the higher the undrained shear strength and the higher the pile capacity. The data shows that the gain in undrained shear strength due to increasing rate of loading increases with increasing water content, plasticity index, liquidity index, over consolidation ratio but with decreasing undrained shear strength A simple model is proposed to quantify the rate of loading effects on undrained shear strength. The viscous exponent n which is the main parameter of the model can be measured by conventional laboratory tests or possibly by cone penetrometer testing, or as a last resort by the proposed empirical correlations to index properties. This model is used to develop rate dependent t-z curves and a computer program to predict the response of a pile subjected to a certain rate of vertical loading. The model and the program are checked by comparing the predicted and measured behavior of two piles INTRODUCTION The present practice for the design of vertically loaded offshore piles is to calculate the pile length which will allow to resist two times the dead load and one and a half times the sum of the dead load and the extreme storm load. The design is based on static ultimate pile capacity calculations and no explicit consideration of dynamic loads is made. The data presented in this article is the partial result of an ongoing research project at Texas A&M University sponsored by the American Petroleum Institute. The purpose of the project is to study the explicit influence of rate of loading and cyclic loading effects due to waves on the capacity of long offshore piles in clays. This article is dealing with the rate of loading effect only. RATE OF LOADING MODEL An undrained shear strength Su is measured at an average rate of shearing Su/t where t is the time to failure. It has been shown as early as 1951 (Casagrande and Wilson) that the value of Su is dependent upon the rate at which the soil is sheared. Others including Whitman (1970), Bea-Audibert (1979) and Lacasse (1979) addressed this problem. A value SUI is obtained for a time to failure t1 while a value SU2 is obtained for a time to failure to Two models can be used to describe the dependency of upon. The first model is:(Mathematical equation available in full paper) This model leads to a straight line on a log-log plot and is the one proposed by Riggins (1981). Laboratory data collected from six different studies published in the literature show that Su versus t increases faster than a straight line on a semi log plot and is closer to being a straight line on a log-log plot (Briaud and Garland, 1984). As a result, the second model (Equation 2) was selected.