Stability and Serviceability Driven Design for Excavations in Clays

Abstract

Deep excavations in clayey deposits present challenges of varying nature, depending on
the characteristics and mechanical properties of the soil strata. In general, stability
and/or serviceability performance aspects are the primary considerations for projects in
normally consolidated and over-consolidated clayey deposits, respectively.
A series of commonly available constitutive models have been reviewed. The suitability
of these models for use in routine soil-structure interaction analyses, in lieu of the
popular linear elastic – perfectly plastic (Mohr-Coulomb) model, has been investigated.
The research has focussed on two case histories, namely a 20m excavation in London
Clay and a 15m to 17m deep excavation in Boston Blue Clay.
For the London Clay case history, the use of a constitutive model able to capture the
progressive stiffness degradation behaviour with increasing strains is considered
essential, in order to provide accurate predictions of ground movements in the zone
surrounding the excavation. The findings of the study presented indicate that the use of
a model in which the stiffness is controlled by the mean effective stress (p’) and the preconsolidation stress (pc) would provide reasonable results in terms of both wall
deflections and overall ground movements.
Aspects of the behaviour of the Boston Blue Clay have been reviewed, based on data
from a comprehensive ground investigation. Small strain stiffness, stiffness degradation
with strain and undrained shear strength properties have been investigated in detail.
Stability of the excavation is a key concern in these ground conditions. The findings of
the study indicate that, for the case study concerned, the DSS undrained shear strength
profile can be adopted for excavation stability assessments using limit equilibrium or
finite element methods, assuming isotropic behaviour for the material. The study has
also highlighted the significance of partial consolidation mechanisms occurring during
the basement excavation, indicating that the undrained assumption made as part of
traditional stability assessments, may not be accurate, and total stress
analyses/calculations, carried out for preliminary stability assessment, should be based
on relatively conservative undrained shear strength parameters.

Date of Award	25 Jan 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Andrea Diambra (Supervisor)