Constitutive model for Ya'an mudstone based on mesoscopic breakage mechanism

The slope stability of Ya'an expressway in Sichuan dominated by mudstone strata, is influenced greatly by both the mechanical properties and stressstrain relationship of mudstone. In this paper, the mechanical properties of the Ya'an mudstone samples under triaxial compression conditions were studied, based on an established constitutive model under the framework of breakage mechanics to simulate the mechanical properties of mudstone. Firstly, triaxial compression tests and SEM tests at the confining pressures of 0.5 MPa, 1.0 MPa, and 2.0 MPa were carried out on the mudstone samples, and it was found that the mudstone sample undergoes strain softening and dilatancy followed by the volumetric compaction. Then, based on analysis on the breakage mechanism of the above test results, we idealized the mudstone sample as a binary medium material consisting of the bonded elements and frictional elements, of which the bonded elements are composed of solid matrix and pores, and the frictional elements are composed of broken aggregates. During the loading process, the cementation between clay minerals and non-clay minerals in the mudstone sample is first destroyed, leading to the formation of micro-cracks within the particle aggregate, that is, the bonded elements are gradually damaged during the loading process and gradually turned into the frictional elements, and the two jointly bear the external load. The bonded elements are composed of mudstone matrix and pores, which have the cementitious characteristics of mudstone, and the frictional elements are composed of the broken aggregate with the frictional characteristics of the broken particles. Based on the homogenization theory, the constitutive model for the mudstone is established, and the determining method for model parameters is also given. Finally, the results of the triaxial compression tests of the mudstone samples are predicted by the constitutive model proposed here, which can reflect the main mechanical properties of the mudstone samples.