Rock mechanics effects of cement grouting in hard rock masses

Large effort has been put into understanding how cement grout penetrates and hydraulically seals a fractured hard rock. Whether the grout sealing will endure the stress re-distributions due to rock excavation is, however, sparsely discussed. This paper presents a research program that investigated the principal rock mechanical differences between grouted and ungrouted joints to obtain input data for modeling. From direct shear tests of grouted and ungrouted rock joint replicas, it was found that cement grout acts basically as a 'lubricant' in a joint subjected to shear stress. The 'hydraulic failure' of a grouted rock joint was found to be a function of dilation and coincides with the joint peak strength. Numerical modeling was performed to study the consequences in tunneling. Under adverse geological conditions, failure of grouted joints may propagate into the surrounding rock mass up to twice the tunnel diameter. The investigation showed that the shear resistance of a grouted hard rock joint was lower than an identical ungrouted joint. At low normal stress levels, an initial strengthening cohesive effect was noted.