Shallow tunnels in stratified clay: stability and induced ground movements

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The area of soft ground tunnelling is of particular importance to Canada. Over the past ten years, the government of Canada has invested over $12 billion on infrastructure projects with $2 billion towards large-scale projects of major national and regional significance including transportation, water and wastewater, etc. Tunnels are essential components of this infrastructure and constitute a major portion of project expenditure. Limiting the surface settlement produced by shallow tunnels in soft ground is a major challenge for tunnel engineers. Surface subsidence can be caused by several factors such as ground loss at the tunnel face, behind the tail of the shield, and through the tunnel lining. Besides surface settlement, tunneling also produces lateral deformation and longitudinal movement of the ground ahead of the tunnel face. Several studies have been devoted to develop solutions for the deformations caused by tunneling in soft ground. Most of these methods are empirical in nature and based on field observations of mainly UK tunnels. Soil stiffness and failure models required for settlement prediction in London clay (a stiff marine deposit) does not represent the ground response to tunneling in stratified clay deposits found in different parts of Canada. This ambiguity leads tunnel designers to use a technique that has a good track record rather than one that represents the actual ground conditions. Using these methods in stratified clays may result in either uneconomic or unsafe approaches. Both centrifuge and gravity scale model tests on block clay samples will be used to simulate the tunnel construction process. Theoretical investigations will also be carried out to develop a constitutive model for the unloading behavior of stratified clays. Results of this study will facilitate the interpretation of field measurements taken during the construction of several tunnels in Canada and worldwide. This research program will directly contribute to the training of three graduate students and a number of summer students.