Predicting ground displacement in clay during construction

Ashraf Sir El Khatim Osman, Cambridge University
Geotechnical Engineering Group


The work reported in this dissertation is a contribution to the development of a new design method, based directly on the data of accurately chosen soil tests, conceived within the framework of plasticity theory, which will enable geotechnical engineers to design more reliable foundations and basements, limiting their displacements. The objective behind the introduction of the Mobilisable Strength Design (MSD) method is to achieve a simple design methodology, which could satisfy both safety and serviceability, in a single simple step of calculation. In conventional term, this offers a rational procedure for selecting safety factors according to the stress-strain behaviour of soil.

A new plastic deformation mechanism for the indentation of a circular punch in undrained clay has been developed. In this mechanism, a continuous displacement field has been imposed to avoid discontinuities and to satisfy incompressibility. This solution is used to create a relation between the penetration force and displacement of materials which are strain-hardening. The validity of this solution is examined against complex non-linear numerical analyses. Comparisons are also made with field studies including pad loading tests at Bothkennar soft clay site and a pad loading test at the Kennigar site. Comparison is also made with five cases of shallow foundations on stiff overconsolidated London clay.

The applications of the MSD method in the design of cantilever retaining walls supporting an excavation in clay are illustrated for a spectrum of soil conditions and wall flexibilities. New plastic deformations mechanisms with continuous displacement fields have been developed for stability calculations of braced excavations in soft clay. From these mechanisms a method for estimating displacements around braced excavations in clay has been developed. The validity of this method is examined against comprehensive finite element analyses in which MIT-E3 model was used and against five previously published case histories of propped excavations in soft clay.

These examples from the professional field of Civil Engineering demonstrate the applicability of modified plasticity theory to predict and control deformations in a boundary value problem for non-linear strain-hardening materials.

Key words: Plasticity theory, displacement, design, shallow foundations, retaining walls.