Experimental study on shear creep mechanical properties of plate surfaces under loading and unloading conditions

The slaty cleavage, a structural weak plane in slate, is of particular concern in the context of engineering projects due to its significant influence on the long-term stability of these projects. In order to reveal the influence of loading and unloading paths on the shear creep behavior of slate cleavage planes, sequential tests were conducted, including direct shear tests, normal unloading direct shear tests, direct shear creep tests, and normal unloading creep tests. The findings suggest that there are substantial discrepancies between the shear strength and long-term strength exhibited under unloading paths and those observed under loading paths. In conditions of low normal stress ( <10.3 MPa), the shear strength exhibited by the unloading paths is lower than that observed under loading paths. This disparity increases gradually as the normal stress decreases, reaching a maximum of 30% at the lowest levels of stress. With regard to long-term strength, the unloading path demonstrates lower values in comparison to the loading path, with discrepancies of up to 20% being observed. For both loading and unloading paths, the creep specimens did not demonstrate significant deformation acceleration after entering the third stage. Instead, fracturing occurred abruptly at a specific point, thus exhibiting typical brittle characteristics. In consideration of the aforementioned points, a viscoelastic brittle model was proposed, with a particular emphasis on the material's progression to the third stage of creep and the associated time of failure. The findings of the present study demonstrate that this model is capable of effectively describing the creep characteristics of the cleavage plane.