Kin Man Lee

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Experience has shown that ground movement invariably occurs above tunnels constructed in soft soils. At present, this movement cannot be adequately predicted during the design stage. In this thesis, a sophisticated theoretical technique for predicting settlements above a tunnel in soft clay is presented. This theory consists of an elasto-plastic three-dimensional finite element approach which is able to take into account the construction procedure, soil-liner interaction and material non-linearity of soil. This technique is used in an extensive study of the general ground reaction and deformation behaviour of shallow tunnels. The technique and experience gained from this study are then utilized in the development of a simple method for calculating the maximum "gap" parameter. This parameter takes into account the ground loss as a function of strength and deformation behaviour in the elastic and plastic state, the physical clearance between the lining and the excavated surface and workmanship factor. This "gap" parameter can be used in conjunction with plane strain finite element method or other simplified techniques to predict the resulting surface settlement.;The validity of the proposed three-dimensional approach is then investigated by comparing the results of analysis with the detailed results of field measurements at the Thunder Bay Tunnel. It is found that the proposed procedure yields reasonable agreement with the observed overall displacement pattern including the magnitude and distribution of surface settlement.;The applicability and practicality of the proposed simplified technique in estimating the "gap" parameter are examined by four additional tunnelling case histories in various soil and construction conditions. Comparison of the calculated displacement with available field observations showed reasonable agreement. It is suggested that the "simple method" may be used effectively in preliminary design considerations.;Furthermore, the effects of elastic and strength anisotropy on surface subsidences induced by the construction of shallow tunnels are examined. Finally, the implications of stress path with respect to the selection of elastic and plastic parameter are discussed. An approximate scheme for determining soil parameters suitable for use in the preliminary analysis of deformations caused by tunnelling in soft clay is suggested.



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