City Research Online

Abstract

Underground infrastructure constructions, which consist primarily of excavations, have been focus of governmental development schemes in the recent years due to the lack of space above ground level. When excavations are performed in urban areas, there is a need for geotechnical engineers to accurately assess the likely ground movements in the surrounding area, during and after construction, so as to prevent any damage to existing assets below or above ground and to ensure sustainable designs. The highly non-linear behaviour of soils calls for advanced constitutive laws that must be implemented in numerical analyses to model realistic soil stress-strain relationships.

This research project initially carries out a literature review of the use of constitutive models to predict monitored field measurements from complex tunnelling projects in clay. The underlying equations of the Modified Cam Clay model are reviewed to create a single element triaxial test code on MATLAB to be able to do independent verifications of the outputs from ABAQUS using the default Modified Cam Clay model available in the finite elemnt software. The mathematical formulations of the Basic and Advanced Hypoplastic models are also reviewed to gain a better understanding of the models on how they predict soil failure for different initial soil stress states and loading conditions, the small strain stiffness of clays and the effects of recent stress history on clays.

Single element analyses are carried out to assess the predictions of the Advanced Hypoplastic model when back analysing undrained triaxial tests, and drained triaxial tests which involved stress reversals. Single element parametric studies are undertaken to calibrate a unique set of parameters for kaolin clay to use with the Advanced Hypoplastic model. Further verifications are performed to validate the Advanced Hypoplastic model UMAT subroutine for ABAQUS before simulating more complex geotechnical problems.

The research then uses the Modified Cam Clay and the Advanced Hypoplastic models in ABAQUS to back analyse a series of centrifuge tests that was done at City, University of London, which consists of propped retaining wall excavations in lightly overconsolidated kaolin clay and single and side by side twin tunnel constructions in overconsolidated kaolin clay. The constitutive models are appraised by looking at their abilities to predict the surface and subsurface ground movements and the changes in pore-water pressures that were measured in the centrifuge tests. The strengths and weaknesses of the Modified Cam Clay and the Advanced Hypoplastic models are identified, explained and summarised.

The good performance of the Advanced Hypoplastic model, when predicting the real clay behaviour of clay during excavations in centrifuge tests, gave the motivation to create brand new finite element analyses to investigate twin tunnel interaction further when the tunnels are placed in a piggyback setup instead. The layout and the parameters chosen for the piggyback twin tunnel analyses are similar to the side by side twin tunnel centrifuge test to enable consistent comparisons and so that it can later be tested in the centrifuge apparatus at City, University of London. Predictions of the vertical and horizontal displacements, at the surface and further down at subsurface levels, using the Advanced Hypoplastic model are compared with the greenfield profiles calculated by well-known empirical methods. Investigative element analyses are then undertaken to give further and new insight on side by side and piggyback twin tunnelling in terms of changes in stress path, stress-strain behaviour and shear stiffness response. Essential conclusions from this research are then drawn while identifying the limitations and the potential work and research that can be done in the future.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Departments:	School of Science & Technology > Engineering School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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