Tunnel face stability in non-homogeneous soil
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Date
2015
Authors
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Publisher
Universiti Teknologi Malaysia
Abstract
This research concerns on the stability of the excavated circular face of the tunnel in non-homogeneous soils. Previous studies mostly considered homogeneous soils however in reality the ground is non-homogeneous. Two cases of nonhomogeneities, a two-layered soil and a soil with linear variation of the shear strength with depth, were considered in the analysis. Three-dimensional mechanisms based on the conical block mechanism were proposed to simulate the failure in these ground conditions. Upper bound theorem of the limit analysis has been utilized to determine the stability of the excavated face of a tunnel in both layered soil and a soil with linear increase of the cohesion with depth. In case of a closed face excavation such as shield tunnelling, the support pressure of the excavation was estimated by the use of the aforementioned failure mechanisms. In case of an open-face excavation such as NATM (New Austrian Tunnelling Method) tunnelling, the strength reduction technique in conjunction with the upper bound theorem was used to evaluate the safety factor of the excavated face of a tunnel. Dimensionless design charts suggesting the minimum support pressure and safety factor of the excavated face of the tunnel based on the upper bound study were proposed for the soil with cohesion non-homogeneity. Comparative study revealed that adopting the cohesive parameter located at the tunnel centreline (usually acquired from site investigation) for stability calculations may lead to an unsafe design, particularly when dealing with soil where cohesion varied with depth, that is, in relatively deeper tunnel, the support pressure was underestimated by 100 %. However, using the average (mean) cohesion of the whole profile may result in conservative predictions (for shallow tunnels) and overestimation (170% in some deeper tunnel cases) of tunnel support pressure. Parametric study in layered soils showed that where the shear strength of the crossed layer is less than the shear strength of the cover layer, both safety factor and support pressure take effect from the variation of the shear strength in crossed layer. Increasing the shear strength of the crossed layer resulted in the decrease of the tunnel face pressure and consequently increase of the safety factor. In this case, the cover layer had a slight influence on the results. When the shear strength of the cover layer is less than the shear strength of the crossed layer, the stability of the tunnel face is influenced by both the shear strength of the crossed soil and cover soil. Increasing the shear strength of the cover soil resulted in decrease of the tunnel support pressure. However this decrease in shear strength did not have any significant effect on the safety factor. A full three-dimensional numerical study based on finite difference method was conducted to validate the findings of the analytical solution. Results of the numerical simulations were remarkably in a good agreement with the findings of the upper bound analyses.
Description
Thesis (PhD (Civil Engineering))
Keywords
Earthwork—Research, Tunneling, Shear strength of soils