Mechanical Responses Of Tunnels In Expansive Soils Under Rainfall Infiltration

Expansive soil reveals special mechanical behaviors such as swell-shrink, fracture and over consolidation. It poses difficult problems on both the tunnel construction and operation. At present in China, the research on the tunnels excavated in expansive soil mainly focus on the support design during the construction period, whereas the research during the operation period is inadequate. In fact, during the operation period, the safety and service life of tunnels are significantly weakened by the long-term rainfall infiltration and swell-shrink process of the expansive soil. Therefore, in this thesis, based on the project of Pingshan tunnel in Nanjing, the influence of the rainfall infiltration on the safety and the mechanical response of the tunnel during the operation period is investigated. The research mainly includes two parts:(1) A review of the mechanical behaviors of expansive soil is presented. On the basis of the laboratory test, the free swell ratio and swell ratio without loading are measured. By use of the existing classification of the swell levels, the swell level for the expansive soil of Pingshan tunnel is determined. The soil strengths with various moisture contents are evaluated. Meanwhile, from the test data, a relation of the cohesive force, friction and moisture content of the soil is best fitted.(2) In accordance with the similarity of differential equations for temperature field and humidity field, the finite difference software FLAC3 D is adopted, so as to simulate the effect of rainfall infiltration on the shallow tunnel. Firstly, the influence of the softening and moistening behaviors of non-expansive soil on the support stress and the additional crown settlement is discussed. Then, the influence of the swell ratio, the deformation moduli, and the depth on support stress and safety factor in the moistening process is analysed. At last, the effect of wetting-drying cycle on the safety storage factor of soil, the distribution of support stress, the support safety factor, and the additional crown settlement are studied. The result shows that: the support stress and the additional crown settlement of shallow tunnel excavated in expansive soil are greatly influenced by the the softening and moistening behavior, swell ratio and wetting-drying cycle. At worst, the safety factor reduces for 34.5% with the increases in swell ratio. Therefore, these three influence factors should be fully considered in the support design of tunnels excavated in expansive soil, and the safety factor should be estimated in a conservative manner.