Study of Pumping Pressure and Stop Criteria in Grouting of Rock Fractures

Today practice of grouting is based on empirical approaches in that, pumping pressure and stop criteria are determined by benchmarking similar projects. Considering a maximum limit for grouting pressure would allow applying a relatively high pressure that may lead to jacking of the fracture or even uplift of the rock mass. On the other hand, keeping the pressure lower than the overburden, in order to avoid any deformation, will prolong grouting process. Determination of pumping pressure is more complicated considering the induced energy to the rock fracture due to combination of the injected volume and pumping pressure. In other word, pressurizing large volume of the injected grout with a low pumping pressure establish the same force inside the fracture as the high applied grouting pressure on small injected volume do. Therefore, an stop criterion to limit grouting volume along with grouting pressure, which is a hyperbola trimming maximum pressure-maximum volume limits and named as grout intensity number (GIN), has been defined. However, in using this stop criterion and at completion point, the state of the fracture and the distance that grout spread inside the fracture are unknown. As a theoretical approach, examining the flow of the Bingham fluid in network of fractures led to development of a numerical model and later an analytical solution, which enabled estimation of distance that grout spread in the fractures in real time. Finally, theoretical curves to limit elastic and ultimate jacking were established to limit grout pressure in correlation with depth of grout penetration by considering the state of the fracture. Despite empirical and theoretical developments, determination of optimum grouting pressure is still challenging. In this study, In addition to examining performance of the analytical solution in estimation of grout spread and distinguishing onset of fracture jacking, the goal is coming up with recommendations for selection of optimum grouting pressure, by examining mechanism of elastic jacking. For this purpose, negative aspects of fracture deformation, which are increase of grouting time and remaining transmissivity, were quantified and discussed against its positive effect on increase of penetrability. By that, application of a relatively high pressure was recommended in order to opening of the fracture to a permitted level, with purpose of increasing penetrability while considering negative effects of elastic jacking. The stop criter