A method for determining grouting pressure and stop criteria to control grout spread distance and fracture dilation

Abstract

Determining optimum pressure in a grouting procedure affects a project's outcome as well as associated costs. Application of relatively low pressure may lead to insufficient grout spread around the borehole and prolongation of grouting time. On the other hand, a pressure higher than in-situ stresses can lead to jacking of the fractures and consequently increase of usage of grout, and in much higher pressures, ground heaving and uplifts. In a review of current practices, the ‘apparent lugeon’ method uses low pressure regardless of geology and function of the grouted zone. The theoretical approach of RTGC estimates the grout spread from grouting borehole in a fracture with constant size aperture, which obligates the usage of a pressure that avoids any deformations. On the other hand, the GIN method appreciates the usage of higher grouting pressures while the induced energy is controlled. However, the controlling Grout Intensity Number is determined based on experience and provides no information regarding the distance that grout mix is spreading and the state of fracture during grouting.The present study discusses an analytical approach that defines stop criteria that allow the spread of grout to a certain distance while controlling deformations to the extent that ensures fracture jacking remains beneficial. To elaborate this approach, first the concept of fracture jacking and approaches for detecting onset and duration of it are reviewed. This is followed by a discussion of the positive and negative effects of fracture jacking and the determination of thresholds for limiting these deformations. The paper goes on to explain the analytical solution for establishing the Deformation Limiting Curves as a stop criterion and estimation of the grout spread distance in a continuously dilating fracture. The significance and benefits of this solution are argued through case studies involving different functions and geological conditions. It is concluded that understanding the function of the grouted zone and geological conditions to define the grouting requirements, recognizing the risks associated with under or over spreading of grout and jacking of rock fractures, and using proper stop criteria to control these risks, would allow the use of an optimum grouting pressure.