Static Versus Dynamic Resistance Of Pilesinclay
Authors: Sulaiman Ibrahim H. , Coyle Harry M. , Hirsch Teddy J. 
Published: October 2008

Static and dynamic tests were conducted at the time of driving on each of two instrumented piles driven into cohesive soils to embedded depths of 64 and 74 feet. The tests were repeated 11 days later in order to investigate the response of the reconsolidated soil.

The purpose of this study was to investigate the time dependent force displacement response of cohesive soils induced by dynamic pile loads and to compare dynamic response with static response. All past attempts to achieve such comparisons were based either upon an assumed dynamic soil model and/or a simulated applied dynamic force estimated from the weight and speed of the driving hammer. In order to eliminate such assumptions in this study, the induced dynamic force was measured at five strategic locations along each instrumented pile. An accelerometer was used to record the motion at the head of the pile. This motion, together with the induced forces, were utilized in a multi degree spring-mass, system to directly compute the displacements of the masses representing various pile segments. The result of this analysis showed that when the applied loads were maximum, each pile experienced What may be described as rigid body motion. Consequently, a rigid pile model was utilized in a single-degree of freedom system to compute the dynamic soil resistance.

Since both the dynamic soil resistance and the pile motions were known, a rigorous, or closed form solution of the equation of motion leads to a direct computation of the static component of soil resistance. This static component was found to approximate the static ultimate bearing capacity when the pile had experienced significant relative motion with respect to the soil.

Introduction

The bearing capacity of piles supporting Offshore structures is often determined by the so called static method. It is noteworthy that this method relies on correlations between soil shear strength, as determined from laboratory or in situ tests, to skin friction along the pile shaft and to end bearing below the pile point. Such relations have been reported by Terzaghi and Peck (15). Skempton (13), Tomlinson (16), Peck (9). Woodard et al. (18). and by Coyle and Reese (2). All of these studies undoubtedly advanced the state of the art. but uncertainties still remain. The overriding cause for uncertainty in the static method is that pile driving changes the shear strength of the clay, and it is difficult to estimate the strength of the clay after driving. In contrast to land piles, it is difficult to verify the design of offshore piles by static load tests, due to prohibitive costs and to high design loads that are difficult to attain.

An alternate approach to the static field load test would consist of utilizing the resistance of the pile during driving as a dynamic field test to compute the static ultimate bearing capacity. The dynamic test may be conducted at any time after driving, and may therefore include some of the effects of soil reconsolidation and setup. However, dynamic test methods are not problem free for several reasons. First, the forcing function induced between hammer and pile is difficult to evaluate theoretically. Second, the relation between static and dynamic resistance is usually empirically defined by use of a damping coefficient which is held constant throughout the duration of impact. Even though this is an accepted procedure in the analysis of 1ineari1y elastic solids, it may not be sufficient for ultimate bearing capacity determinations in clay, because the material properties are vastly different.