The influence of a thin weak soil layer on the pile bearing capacity.

Most pile design methods for estimating the pile bearing capacity, are based on data from the cone penetration tests (CPT), such as the Dutch (Koppejan) method, the Belgium (De Beer) method or the French (Bustamante & Gianeselli) method. In all these methods, the unit cone resistance of the CPT has to be averaged within an influence zone around the pile tip, in order to be able to calculate the pile tip bearing capacity. The influence zone is defined as the zone in which the parameters of the soil have an influence on the pile tip bearing capacity. For the French method this zone is between 1.5 times the pile diameter, D, above and below the pile tip; while for the Dutch method this zone is between 8 D above to a maximum of 4 D below the pile tip. Numerical simulations on piles installed in homogeneous sandy soil, show that these definitions of the influence zone are not accurate. The simulations show an influence zone from 1 or 2 D above, to 5 or 6 D below the pile tip, depending on the soil parameters. In this research, also the influence of a weak thin soil layer on the pile bearing capacity has been studied. All simulations are made for two different thicknesses of the thin soil layer. Also the soil strength parameters and soil stiffness parameters have been varied, in order to show the influence of this thin soil layer. This influence zone has been confirmed by laboratory tests.