Numerical Modelling of Subsidence due to Tunnel Excavation

Examensarbete för masterexamen
Nagy, Frida
The West Link Project, Västlänken in Swedish, is an infrastructure project in Gothenburg. The main goal of the project is to increase the capacity of the railway network, by constructing eight kilometres of railway tunnel and three new underground stations. The tunnel will go through both rock and soft soils, such as clay. The parts driven through clay will be constructed with the cut-and cover method and will therefore entail deep excavations well below the groundwater table. As the excavations will be both easier and safer to construct in dry conditions, may groundwater lowering actions during the construction time occur. When lowering the groundwater table, the surrounding soil can be affected. A change in the pore water pressure in the soil could result in consolidation settlements, called subsidence. Subsidence can cause differential settlements and other severe damage, affecting surrounding buildings and infrastructure. When reviewing the groundwater conditions, complex conditions were found at the Korsvägen site, where one of the underground stations is to be constructed. It was then decided that the aim of the thesis would be to analyse and model the subsidence due to groundwater lowering actions in an excavation at Korsvägen constructed as part of the West Link project. To carry out the analyses a representative cross section was created, using geometry and soil data from the Korsvägen site. A model was then created in Plaxis 2D to study the subsidence using representative constitutive models and consolidation analysis. The model was also used to investigate how the uncertainties in determining soil parameters affect the results. Furthermore, the influence radius of the subsidence was examined, to see if adjacent buildings and infrastructure would be affected. When reviewing the stratigraphy, a rather horizontal soil layering was found, with a clay layer approximately 20 meters thick. In the middle of the clay layer a permeable layer of friction material was found. The bottom layer is a stiff friction layer, which mainly consists of sand and overlies the bedrock. The uncertainties of the model were mainly related to the two friction layers, specifically permeability and stratigraphy. Furthermore, the strength and stiffness of the bottom friction layer was varied. The chosen solution for the retaining wall was a back-anchored secant pile wall. Groundwater management will be complicated since the groundwater conditions are very sensitive to change. The secant pile wall is deemed impermeable and to avoid seepage a grout curtain was needed to be injected underneath the wall. The influence of the grout curtain was further studied, by running a fully coupled flow-deformation analysis to investigate the groundwater flows. When reviewing the result could it be seen that the magnitude of settlements and wall displacements were most affected when the stratigraphy and the strength and stiffness parameters were varied. Varying the permeability mainly influenced the pore pressure distributions. When reviewing the grout curtain was it seen that a prolonged curtain efficiently cut off the groundwater flow beneath the wall. In an urban environment a small variance in settlements or differential settlements can cause severe damages. Subsidence could therefore be an issue at Korsvägen, mainly since the area round the future excavation is so densely built-up, which could be problematic since the potential subsidence is largest close to the excavation.
Subsidence, West Link Project, Plaxis 2D, Uncertainties in Parametric Determination
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