Demonstrated by a series of triaxial compression tests

the mechanical properties of claystone are related to its stress state. The relationship between them shows that the larger the confining pressure is

the higher the claystone samples’ peak stress is and the later it develops. This is subject to the tendency of microcrack closure within specimens under high confining pressure

suggesting that high confining pressure has certain inhibitory effects on the development of microcracks. Therefore

the peak stress appears in higher magnitude and at a later stage. However

the degree of the stress drop during the softening process is not heavily affected by the variation of confining pressure. The reason is the microcracks within the samples under different confining pressures gradually connect to each other to generate shear planes of similar type

so that the mechanical properties of the specimens are weakened to the same extent. Based on the experimental study

the paper introduces a damage evolution equation considering varying stress states by taking microcrack as the damage element (because the development of microcracks is affected by stress states). Moreover

a new constitutive model is established based on the test results and numerically implemented in Abaqus and its subroutines. Comparisons between the simulation and experimental results demonstrate the model adequacy for effectively describing the mechanical properties of claystone. The study provides theoretical guidance for the engineering construction of underground infrastructure in claystone.