Shear Creep Failure Characteristics of Cement-Grouted Sandstone Structural Planes

The rheological behavior of rock masses governs long-term stability, yet the time-dependent properties of grouted structural planes remain insufficiently quantified. Graded shear creep tests were conducted on artificially split sandstone structural planes with controlled grout thicknesses, complemented by scanning electron microscopy (SEM), to clarify creep evolution and long-term shear strength. The results show that the total shear creep displacement of grouted specimens exhibits limited sensitivity to grout thickness, while the ratio of long-term to theoretical shear strength increases by approximately 10% at a grout thickness of 2 mm; this strengthening effect, however, diminishes at greater thicknesses. Moreover, the creep rate evolution of grouted specimens differs fundamentally from that of ungrouted specimens, with about 60% of grouted samples exhibiting an accelerated creep stage characterized by a U-shaped rate curve. The failure mode shifts from asperity-controlled slip in ungrouted structural planes to damage concentrated at the grout–rock interface in grouted specimens. SEM observations further reveal that micro-defects at this interface initiate and propagate cracks, ultimately governing the macroscopic creep failure process. Overall, this study establishes an isochronous curve-based method for determining long-term strength and demonstrates that interface micromechanics critically control the long-term performance of grouted rock masses. These findings provide practical guidance for grouting reinforcement in underground engineering.