A New Way to Predict Capacity of Piles in Clay
Authors: Focht John A. , Vijayvergiya V.N. 
Published: October 2008

According to a new concept proposed for correlating results of pile load tests with soil parameters, the total frictional capacity of pipe piles embedded entirely in clay is expressed as Qs = ?(_?m + 2 cm)As, where _?m is the mean effective vertical stress between the ground surface and the pile tip, cm is the mean undrained shear strength along the pile As is the pile surface area, and ? is a dimensionless coefficient. Using this concept, values of ? were determined for 43 previously reported load tests on steel pipe piles in clay and 4 additional tests not previously reported. For these tests including piles 8 to 333 ft long with capacities of 6 to 1760 kips, the frictional capacity coefficient ? was found to have a close correlation with embedded pile length. Application of the new criteria to typical clay profiles indicates lesser capacities for very stiff to hard clays the new criteria will predict capacities greater than the API procedure. Prediction of pile capacity by this new procedure based on effective vertical stress as well as undrained shear strength is shown to have greater reliability than by precious methods based solely on shear strength.

INTRODUCTION

The static method of predetermining the bearing capacity of piles is based on empirical data gathered from model studies and field load tests and interpreted in accordance with accepted soil mechanics theories. The ultimate pile capacity, Q, at a given penetration is the sum of Qs, the skin frictional capacity, and Qp, the end bearing or point capacity, so that(mathematical equation)(available in full paper)

Where dAs and Ap represent, the embedded elemental surface area at any depth and the cross sectional area at the pile tip, respectively; f and q represent the unit skin friction and the unit end bearing, respectively. Various methods have been suggested previously for the prediction of f based on the shear strength of the clay. Mansur (10), Seed(18) and Tomlinson(19) have suggested that the unit skin friction, f, along the pile shaft in soft to firm clays is approximately equal to the undrained shear strength of the clay. From a comprehensive study of comparative behavior of friction piles, Peck (16) concluded that friction piles in stiff clays do not develop the full strength of the clay. Based on large number of pile load tests, Tomlinson(19) suggested that skin friction may be a function of pile material and therefore should be limited by adhesion between pile and soil; the adhesion values suggested were considerably lower than the shear strength of the clay. Woodward (23) reported that skin friction observed during a series of pile load tests was greater than the adhesion values suggested by Tomlinson, though it was less than the shear strength of the clay. In a later paper(20), Tomlinson concluded that the mobilized adhesion is also influenced by the sequence of soil strata into which the pile is driven. McCleland, Focht and Emrich(13) suggested a general predictive criteria giving depth some consideration in addition to shear strength. For stiff over-consolidated clay, the unit skin friction by this procedure is computed as 100 psf or 33 percent of the effective overburden pressure.