PREDICTION OF SETTLEMENT TROUGH DUE TO TBM TUNNELLING WITH AN EMPHASIS ON GROUNDWATER DEPRESSION BASED ON CENTRIFUGE MODEL TESTS AND NUMERICAL ANALYSIS

Traffic congestion problem is a common phenomenon in Seoul metropolitan region due to overpopulation. The construction of a double deck tunnel is under planning in order to solve this problem and also for various other purposes. The double deck tunnel needs to be planned as a large diameter tunnel constructed by TBM. Some of the advantages of a double deck tunnel include decreased affected zone due to construction and increased traffic allowance per unit excavated area as compared to the twin tunnels. However, the construction of large diameter tunnel inevitably causes settlement as well as damage to the adjacent structures. In urban areas, the ground settlement due to tunnelling is an important consideration in tunnel design and the main causes of settlement can be classified into lost ground as well as groundwater depression due to construction. The methods used previously for predicting the settlement do not consider the influence of groundwater depression. The purpose of this study is to predict the transverse settlement trough due to TBM tunnelling considering the combined effect of the two causes of settlement stated above. In order to analyze the characteristics of settlement trough considering the realistic values of volume loss and increased effective stresses, centrifuge model tests at 80g condition were performed according to the parameters such as volume loss, C/D ratios, and consideration of groundwater. Two types of tunnel models were developedvolume loss tunnel model for the simulation of excavation and drainage tunnel model for the simulation of reduction in pore pressure. Resultantly, different inflection points and K values of the settlement troughs were obtained according to various values of volume loss and C/D ratio. The surface settlement trough due to groundwater depression matched well with the existing curves and the values of total volume loss, inflection point, and maximum settlement increased when the superposition method was applied to the settlement trough due to excavation. Numerical analysis was performed in order to analyze the settlement trough obtained due to the combined effect of excavation and groundwater depression. Displacement controlled model (DCM) was used to obtain the realistic settlement trough according to the volume loss and the suitable convergence point matching well with the empirical relationship and experimental results was suggested. The trough parameters within a reasonable range were obtained from the analysis results. By adding a new parameter in the modified Gaussian curve, a new curve was suggested for the settlement trough due to groundwater depression in order to consider the elastic settlement at a distance farther from the tunnel centerline. The newly suggested curve matches well with the numerical analysis results. For the use in preliminary design, the empirical relationships for trough width parameter, maximum settlement, and change in inflection point with depth considering the effects of groundwater depression were suggested.Docto