Analysis Of Existing Cyclic Vertical Load Tests For Piles In Clay

ABSTRACT A data base is collected to study the behavior of piles in clay under cyclic axial loads generated by ocean waves. The data base includes 16 studies on cyclic full scale pile load tests in clay of which 4 studies are proprietary. In a first part, general conclusions are drawn from inspection of these studies. Then a power law model is used to quantify the soil stiffness degradation as the number of cycles increases. The parameter for the model is backfigured for each case of the data base and general trends are observed. INTRODUCTION This article is the partial result of a study dealing with the behavior of piles in clay subjected to cyclic axial loads generated by ocean waves (2). As part of this project an extensive search of existing data on the cyclic behavior of piles in clay was performed. The data was collected and analyzed, and the results of the analysis which are presented in this article served as guidelines for the subsequent simple shear, rod shear and theoretical modeling programs for the study. DEFINITION OF CYCLIC PARAMETERS Figure 1 shows a general cyclic response, curve. The parameter y (vertical axis) can represent the skin friction or the load in a pile load test. The parameter × (horizontal axis) is the vertical displacement for a pile load test. The cyclic stress ratio R1 and R2, used to describe the cyclic stress amplitude and the mean cyclic stress respectively, are defined as follows: (Mathematical equation available in full paper) where all parameters are defined on Figure 1. The ratio RM1 + R2 represents the peak cyclic stress. The secant stiffness GS(N) and the cyclic stiffness GC(N) are defined as follows: (Mathematical equation available in full paper) where all parameters are defined on Figure 1. In order to describe the decrease in stiffness of the soil-pile interface or the soil-pile assembly due to cyclic loading, the secant stiffness degradation parameter ? and the cyclic stiffness degradation parameter ? are used: (Mathematical equation available in full paper) The rate of variation of ? and ? with the number of cycles is a measure of the rate at which "'damage" occurs. In order to quantify the rate of variation of ? and ? with N the following models were used (7): (Mathematical equation available in full paper) where a and b are the secant and cyclic degradation exponents which will be evaluated in this article. A critical value of the peak cyclic stress ratio (Rl + Rt) t called the "threshold", is defined. Below the threshold, some degradation occurs but increasing the number of cycles does not produce considerable damage' ("failure") to the pile-soil interface. Above the threshold, damage to the soilpile system is severe and causes "failure" (2).