Settlement predictions in granular soils: a probabilistic approach

Abstract

It is believed that, for typical shallow foundations in granular soils, settlement considerations are more critical than bearing capacity considerations, especially when the foundation width is greater than 1.5 m (Jeyapalan & Boehm, 1986; Tan & Duncan, 1991; Berardi & Lancellotta, 1994). The settlement prediction exercise in Texas, USA, in 1994 clearly demonstrated the inadequacy of the current state of the art of settlement prediction (Briaud & Gibbens, 1994). In spite of having abundant soil data through extensive laboratory and in-situ tests, the predictions were quite poor. In a typical design situation, where the engineer has only access to very limited soil data, the problems can only be compounded. Probabilistic/reliability studies and risk assessments have become increasingly popular in geotechnical engineering over the past few decades. Geotechnical engineers will continue to use their preferred settlement prediction methods, but some guidance on the risk associated with the predictions will be very valuable. The objective of this note is to propose a simple probabilistic model with design charts, for four different settlement prediction methods, which enable the designer to quantify the probability that the settlement will exceed a specific limiting value. For the same input data, the settlement predictions can be quite different depending on the method employed. At present there is no rational procedure for comparing these different predictions. The probabilistic design charts proposed herein will provide a baseline from which all settlement prediction methods can be compared against each other.