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Technical paper: Settlement and heave of overconsolidated clays – a simplified non-linear method of calculation

AS O’Brien and P Sharp, Mott MacDonald. This paper was first published in GE’s October 2001 issue.

Introduction

Settlement at working load is usually the critical design consideration for vertically loaded foundations on stiff overconsolidated clays. Foundation settlement is usually calculated by assuming that the ground behaves as a linear elastic material. The reliability of this calculation is mainly dependent on an appropriate choice of elastic modulus.

However, modern laboratory testing techniques have identified the highly non-linear stress strain characteristics of overconsolidated clays, for example, refer to Jardine et al 1984 and Figure 1. This explains the wide range of values for “linear elastic” moduli, which have been reported in the technical literature; for example E„/Cu ratios of between 150 and 1500 have been quoted in CIRIA SP 27. As a result the selection of an appropriate value for a linear elastic modulus becomes a matter of considerable engineering judgement. The recent Ground Engineering article exemplifies some of the difficulties that engineers face in selecting appropriate deformation moduli.

An additional problem is that linear elasticity incorrectly predicts the pattern of settlement adjacent to and beneath a loaded area. For example, if total settlement of a structure is correctly predicted, settlement at depth or remote from the structure will be incorrectly predicted. In view of these problems, the applicability of linear elastic calculations for overconsolidated clays is limited. However, the alternatives to conventional linear elastic calculations, such as non-linear finite element techniques, can be complex, expensive and time consuming, requiring high level expertise and considerable engineering interpretation.

Hence, there is a need for a simplified method which enables the engineer to gain an understanding of foundation deformation behaviour under loads of varying intensity. This paper describes a method which enables the non-linear stress strain behaviour of overconsolidated clays to be modelled in a manner which is relatively simple and is appropriate for routine design calculations. The calculations are most conveniently undertaken by computer. however, the method is readily amenable to hand calculation. Undrained and total settlement (or heave) can be calculated under foundation loading of any shape and of varying intensity using a varying ground stiffness – depth profile. 

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