Variation of Apparent Cohesion and Friction Angle Under Polyaxial Stress Conditions in Concrete

Abstract

Amongst many mechanical properties, cohesion (c) and angle of internal friction (φp) are probably the most widely used rock and rock-like material strength design parameters. However, unlike Mohr-Coulomb (MC) failure criterion that assumes cohesion and friction angle are intrinsic material properties and are not affected by the applied stress level, the Hoek-Brown (HB) criterion predicts a continuous change of apparent cohesion and friction angle if the induced normal stress on the fracture plane changes. That is, at low values of normal stresses, the instantaneous angle of friction will be relatively large, whereas cohesion will be a small value. As the applied normal stress value on the fracture plane increases (moving ‘up’ the non-linear Hoek-Brown strength envelope), the angle of friction reduces, and the cohesion increases. This is an important result from the HB failure model and allows a more realistic estimate of shear strength to be made at low values of normal stress, preventing potential over-design problems. Nevertheless, the HB model neglects the effect of the intermediate principal stress on material properties. Limited studies on the variation of apparent cohesion and friction angle under polyaxial stresses in concrete are available in the literature. Therefore, this paper aims to investigate the effect of true triaxial stresses on concrete cohesion and friction angle using a polyaxial strength criterion developed by Mogi. The results of concrete show that the intermediate principal stress has a pronounced effect on cohesion degradation and the mobilization of internal friction angle as the ratio of the intermediate to the minor principal stress changes. The results are expected to provide a framework for a more realistic design of underground concrete structures at depth.