Abstract: The cylindrical cavity expansion theory is an effective method to analyze various geotechnical problems, such as pile installations and pressure meter tests. However, most of existing studies which assume that the soil is an infinite medium are not suitable for the case of finite medium. The critical state model is used to describe the stress-strain relationship of unsaturated soils and the constitutive model for water phase which represents the relationship between the specific volume and suction is introduced. With the boundary effects considered, the semi-analytical solutions for cylindrical cavity expansion in unsaturated soil under different drainage conditions are deduced. Intensive parametric studies have been conducted to analyze the effects of boundary, suction and drainage conditions on the results of cavity expansion. The results show that the cavity pressure increases continuously with the increase of cavity radius under the effects of boundary, which is different from the cavity pressure that tends to be a stable value in infinite medium. The greater cavity pressure is required to expand the same cavity radius for a smaller boundary size. In addition, the cavity pressure increases and variation of specific volume caused by the same stress increment decreases with the increase in initial suction, which can be attributed as the suction hardening effect. The saturation of soil at the same position around the cavity is higher, and the suction and effective stress components are lower in the case of undrained condition than drained condition. It is more obvious when the distance from the cavity wall decreases.

Key words: cylindrical cavity expansion, unsaturated soil, boundary effect, suction hardening effect, semi-analytical solution