Using the Pressuremeter Curve To Design Laterally Loaded Piles

Abstract

ASTRACT The P-y curve is a soil model used in the design of laterally loaded piles. A method is presented to obtain the P-y curve from the pressure meter curve. This method distinguishes between the friction model and the front resistance model. Using this approach, comparisons are made between the predicted and measured behavior of two laterally loaded piles; one in a soft clay and one in a medium to dense sand. The comparisons show very good agreement and point out that a distinction may have to be made between displacement piles and non displacement piles. INTRODUCTION In the analysis of a laterally loaded pile, the soil model is one of the key elements. The most commonly used method of analysis is the subgrade reaction method which is based on the solution of the governing differential equation by the finite difference method. The elementary soil model is the P-y curve which describes, at any depth z, how the soil resistance per unit length of pile P (lb/in.) varies with the lateral displacement of the pile y (in.) In this paper a rational method of obtaining the P-y curve from the pressure meter curve is presented. This method is then evaluated by comparing the predicted behavior with the actual behavior of two piles which were loaded laterally: one in a soft clay and one in a medium to dense sand. THE F-y/Q-y MECHANISM A vertically loaded pile derives its capacity from the point bearing capacity and from the friction along the pile shaft. The same two components, point bearing and friction, exist when a pile is loaded laterally. The point bearing will be called the front resistance Q and the friction resistance will be called F. Figure 1 gives an example which shows the distinct existence of the two components. A 3 foot diameter drilled shaft was loaded laterally in a stiff clay with an undrained shear strength from unconfined compression tests averaging 2000 psf. Pressure cellswere installed along the shaft as shown on Figure 1 in order to record the mobilization and distribution of the front pressure. The shaft was loaded and the resulting top load top movement curve is shown onFigure 1. At a horizontal load of 43 tons, the soil resistance due to front reaction was calculated from the pressure cell readings (1). Considering front resistance only, horizontal and moment equilibrium cannot be obtained. After including the friction forces (Figure 1) corresponding to the full shear strength of the stiff clay(l), both horizontal and moment equilibrium are approximately satisfied. This example tends to indicate two points:the friction resistance is an important part of the total resistance,the friction resistance is fully mobilized before the front resistance. These two points lead one to think that a fundamental soil model must distinguish between friction and front resistance and that at working loads the friction is all important. THE PROPOSED METHOD One obvious application of the pressure meter test (PMT) is the design of laterally loaded piles because of the analogy of loading between the PMT and the pile.