Abstract: To study the influence of unloading on complicated geotechnical structures such as cast-in-place piles, tunnels, etc, a simplified solution model of cavity reverse expansion considering elastoplastic unloading in drained soil is established. In this model, the superposition method is adopted in the derivation of stresses. In the reverse plastic zone, the non-correlated Mohr-Coulomb criterion, the assumption of large strain and ignoring the elastic strain are adopted. This study shows that the reverse expansion process can be divided into an elastic and two elastoplastic stages. When the cavity pressure is restored to the initial value, the radius of both cylindrical and spherical cavities cannot revert to the initial value, and there is still a certain range of stress weak zone around the cavity. Compared with the classical in-situ expansion solution, the yield surface development, stress variation and stress field around the cavity tend to be the same when cavity pressure increases to a certain value. The assumption of ignoring the elastic strain in the plastic zone will lead to a smaller displacement and a larger ultimate expansion pressure of the simplified solution, but it has no effect on the stress-related quantities. Compared with the Tresca solution, the proposed model takes into account the effect of internal friction angle and is closer to the actual soil mass. A plane strain model of cavity reverse expansion is established by numerical simulation, and the stress analysis results are basically consistent with that of theoretical model.

Key words: elastoplastic unloading, cavity reverse expansion, cavity radius, large strain hypothesis, stress field around cavity