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Ground movements induced by deep excavation: Physical and analytical models

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Very deep excavation pits have become increasingly common in modern underground construction projects. They often serve as an effective means to accommodate massive underground complex and efficient mass transit systems. However, the excavation process could lead to undesirable ground movements, which might damage buildings and lifeline systems nearby. In this regard, designers often use multi-strutted structural systems as an economical way to limit ground movements. Although the general soil response to excavation is understood in principle, designers still lack simple and practical methods for calculating undrained ground movements. Judgments are generally empirical, and serious accidents are common.

This seminar presents a new apparatus for the centrifuge model testing of deep excavations in soft clay, which emulates the field construction sequence of a multi-propped retaining wall during centrifuge flight. A comparison is given between the new technique and the previously used method of draining heavy fluid to highlight the importance of correct modeling of passive soil stress in soil excavation process. Moreover, digital images taken of a cross-section during the test are analyzed using Particle Image Velocimetry (PIV) to illustrate ground deformation and soil-structure interaction mechanisms.

A decision-making tool based on an extended Mobilizable Strength Design (MSD) method which permits the designer an extremely simple method of predicting ground displacements during construction is validated by a highly non-linear finite Element Analysis (FEA) of deep excavations. This newly extended MSD approach accommodates a number of issues which are important in underground construction between in-situ walls, including: alternative base heave mechanisms suitable for different excavation geometry; the influence of support system stiffness in relation to the sequence of propping of the wall; and the capability of dealing with stratified ground. In addition, a simplified MSD framework is proposed for analyzing a database of 155 deep excavation case histories worldwide. The approach examines the governing factors controlling deformation in deep excavations and offers simple guidelines for designing support structures for deep excavations. These developments should make it possible for a design engineer to take informed decisions on the relationship between ground movements and the influence of wall stiffness, or on the need for and influence of a jet-grouted base slab prior to conducting project-specific FEA .

This talk is part of the Engineering Department Geotechnical Research Seminars series.

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