Mechanical properties and cracks evolution law of sandstone containing two filled fissures

In this paper, the mechanical experiments were conducted on red sandstone containing two filled fissures under different rock bridge parameters by triaxial compression to analyze the influence of the fissures filling types, rock bridge lengths, and rock bridge inclinations on the characteristic stresses of sandstone, as well as the cracks propagation and coalescence modes of red sandstone, revealing the influence mechanism of the filling types on the internal fracture propagation and failure modes of the two filled fissure red sandstone rocks. The results showed that the characteristic stress of the filling double fracture specimens under different filling material types gradually decreased when the length of the rock bridge was smaller, and the inclination angle of the rock bridge was larger. Additionally, under different conditions of rock bridge parameters, the weakening degree of unfilled double-fissure specimens was greater than that of mud-filled ones and much greater than that of sand-filled ones. The number of main fissures through the upper and lower ends of the filled specimen was fewer than that of the unfilled one, but with a greater number of branch fractures. The external load caused the delamination of the mud and sandstone fillings, with the maximum width of interlayer fissures reaching 50 μm. Sand crystal particles are the main reason why the mechanical properties of sandstone sand-filling specimens are stronger than those of mud-filling ones.