Pre-excavation grouting in rock tunneling: Dealing with high groundwater pressures

In rock tunneling, pre-excavation grouting (or, pre-injection) refers to systematic treatment of the rock mass ahead of an advancing tunnel face via pressure grouting. The main purpose of pre-excavation grouting is to reduce water inflows, or to improve geomechanical properties of the rock mass. This paper summarizes approaches for treating competent rock masses with systematic pre-injection of cementitious grouts through boreholes drilled from the working tunnel face. The focus is on traditional cement based grouting technology and drill-and-blast tunneling under high static groundwater pressures (> 20 bar). Post-injection, where drilling and grouting is performed somewhere behind the tunnel face, provides an additional means for achieving a high degree of watertightness, but is beyond the scope of this paper.