Parameters Affecting Axial Capacity of Piles In Clays
Authors: Bea Robert G. 
Published: October 2008

Pile-soil interface strength or the frictional capacity of a pile is primarily a function of the effective lateral pressure between the pile wall and the surrounding soils. These pressures will be affected by the state of vertical and lateral effective stresses in the soil prior to the pile installation and by the pile's installation and loadings.

Laboratory tests indicate that the interface strength 'is strongly related to the clay's composition and the pile's roughness and relatively insensitive to the rate of loading and load cycling.

INTRODUCTION

There are four classes of parameters that may influence the axial capacity of piles which support offshore platforms:Soil Conditions - the type of soils Or foundation materials encountered, their physical properties and stress state, and vertical and horizontal variability.Loading Environment - the vertical and lateral forces developed by longterm dead loads and transient live loads, their cyclic and directional characteristics. In some cases, the loading environment may cause important changes in the soil IS supporting characteristics, e.g., scour or cyclic loadings causing decreases or increases in the soil modulus and strength.Structure Characteristics - the diameters and lengths of the piles, the pile material, shape, and roughness, spacing (interaction effects), and over-all group properties (redundancy influencing the ultimate load capacity of the entire pile system).Installation Methods - how the piles are driven, vibrated, drilled, grouted, or jetted into place, and the behavior of the piles during installation (displacement, non-displacement, partial displacement, doglegging). Particularly important are the stresses induced in the soils during and after installation, and the influence of the installation method on the soil properties.

This paper will focus on results from laboratory tests intended to investigate parameters which control the skin friction componen1 of the axial capacity of driven steel pipes in clays. The tests were direct shear tests (soil on-soil and soil-on-steel) and triaxial rod shear tests. These latter tests consisted of inserting a small diameter steel pipe pile into a large cylindrical soil sample, confining the sample triaxially, and loading the pile axially to failure.

Results from the laboratory tests suggest that skin friction or interface strength is primarily a function of the effective lateral pressure between the pile and soil at the time of loading. It is secondarily related to the soil's composition and roughness of the pile. It is relatively insensitive to the rate of loading and to load cycling. If sufficient time, pressure, and drainage are allowed, undisturbed and remolded soils have virtually the same interface strengths.

The interface strength relationship developed from the tests is as follows:(Available In Full Paper)

For typical Gulf of Mexico clays (Plasticity Index, PI, 40 to 70), the soil-steel friction angle is approximately 14 and 6 degrees for the direct shear and rod shear tests, respectively. For silty clays (PI, 20 to 30), *ss is approximately 20 and 10 degrees. For dense sands, *ss is approximately 30 and 20 degrees. The Differences in results produced by the two types of tests is attributed primarily to the different stress states induced t the soil-steel interface by the test apparatus.