This paper introduces a new numerical approach to model the strain-softening behaviour of the rock mass. The strain-softening process is simplified as a series of brittle-plastic ones and solving a strainsoftening problem becomes finding a series of brittle-plastic solutions in the proposed numerical procedure. Moreover, the cohesive and frictional strength components can be mobilized separately as functions of plastic strain. Thereby, the cohesion weakening and frictional strengthening process can be captured for geomaterials. Numerical examples are presented and the results compared with those published, and it is shown that good agreement is obtained. Different strain-softening processes including different softening rates and modes of strength components' mobilization are investigated, and plastic zones around the tunnel in strain-softening rock mass are presented and the appropriateness of the parameters are discussed.
Strain-softening rock types, which are characterized by a decrease of strength with further strain after the peak strength, are frequently encountered in geotechnical engineering, and they are studied from both micro-mechanical and macroscopic viewpoints. The onset and extension of the microcracks in the material are described and strain-softening behaviors of specimens in laboratory tests are reproduced if micro-mechanical analysis is performed. However, the macroscopic approaches based on the classical continuum theory is adopted and the obtained results are welcome for engineers when main concerns focus on the failure zone and the deformation in practice. Usually, different response will be produced by different strain-softening behaviour. The strain-softening process can be considered as brittle when the softening rate is high and it considered as perfectly plastic when the softening slope is very gentle. The displacements and plastic zones predicted in the surrounding rock mass differ greatly by different stress-strain relationship and it is necessary to study the influence of the softening rate on the tunneling conditions (Egger 2000).
