A deep retaining structure in till and sand. Part II: Performance and analysis

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Abstract
The problem of magnitude and distribution of lateral pressure acting on a deep retaining structure supporting an excavation in till and sand is studied by an approach integrating field measurements from a case history with a finite element analysis. Also studied are ground movements associated with the excavation. The finite element analysis is based on stress path dependent testing of the soils involved.The case history is the behaviour of a deep supported wall of the underground Churchill Square Station of the recently built line of the Light Rail Transit System in Edmonton. A tangent pile wall, 17 m deep, has been placed through glacial till to underlying sands. The site has been instrumented to record displacements of the wall and of the surrounding ground as well as the loads carried by the lateral supports.A finite element analysis employing several stress–strain models was used to simulate the excavation and its sequence, the placement of lateral support, and the differential stiffness of the structural components and of the surrounding soil. Special attention has been given to the effect of different stress–strain models of soils, with a particular focus on the influence of stress paths typical for the studied structure.Agreement between the field and analytical results for displacements is accepted as a criterion of validity of the analytical results of stresses, where direct in situ stress measurements are difficult to obtain and interpret. Of special importance is the calculated lateral pressure against the wall and its relation to the stiffness of the wall and to the magnitude of associated ground movements. The calculated lateral pressure has been found to differ significantly from the semiempirical design pressure diagrams used in practice.The soil stress–strain model found to describe the field behaviour most closely has been derived from test results obtained using a plane strain apparatus. Keywords: deep supported excavation, displacement, lateral pressure, field measurement, finite element analysis.

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