Response of Ground and Buildings to Deep Excavations and Tunnelling

Kok Hun Goh, Cambridge University
Geotechnical Engineering Group

Abstract

Activities associated with deep excavation and tunnelling will inevitably cause ground movements that may affect buildings within their zone of influence. The current approach for building damage assessment is based on tensile strains estimated using deflection ratios and horizontal strains from the response of the ground at the foundation level. This approach ignores the inherent building stiffness and is conservative. Recent researchers have included the influence of building stiffness in modifying the deflection ratio and horizontal strains induced by tunnelling construction. However, the method is restricted to tunnelling and there is no systematic way to include the stiffness of buildings adjacent to deep excavations.

The PhD dissertation describes the work to develop design guidance to incorporate the influence of building stiffness in modifying the deflection ratios and horizontal strains induced by deep excavations. Using the finite element method, a multi-propped excavation was simulated adjacent to a building, which was modelled first as an elastic beam and then as a frame structure. Parametric studies were conducted where the structural stiffness and geometry of the building were varied and the characteristics of the excavation changed. As a result of the rigorous study, new definitions of relative building stiffness have been proposed and design guidance to estimate the building modification factors are developed from which the building deflection ratios and horizontal strains may be estimated from the greenfield condition. Furthermore, using structural analysis, methods to include the influence of frame action were also developed for framed buildings on continuous footings and on individual footings.

Two new case studies were formulated and analysed in the dissertation the response of the Singapore Art Museum under the influence of movements induced by adjacent deep excavations, and the response of the Pasir Panjang shop houses under the influence of movements induced by bored tunnelling for the new Singapore Circle Line. Other than identifying some important behaviour of building response to tunnelling- and excavation-induced movements, these case studies together with previously completed case histories were also used to validate the findings from the numerical study.