Correlations of rock bolt — shotcrete support and rock quality parameters in Scandinavian tunnels : CECIL, OS 170F, 30T, 65R. SWEDISH GEOTECH. INST. PROC. NO.27, 1975, 275P

Abstract

Field observations at 14 civil engineering rock tunnel projects in Norway and Sweden have enabled empirical correlations to be drawn between three different rock quality parameters and the rock bolt-shotcrete supports used in loosening ground conditions. The three rock quality parameters used in the investigation are average discontinuity spacing, rock quality designation (RQD), and seismic velocity ratio. Numerical values of each of these three parameters have been related to the three support classifications of maximum (two or more shotcrete applications, frequently with closely spaced rock bolts), intermediate (one shotcrete application, frequently with medium to widely spaced bolts), and minimum (none or medium to widely spaced botls). These correlations offer the most realistic approach to the selection of a shotcrete design that has heretofore been possible. Laboratory model studies have been used to demonstrate the significance of joint orientation and tangential stress on the stability of an unsupported jointed medium. Both the failure mechanism and the mechanism of stabilization of an unsupported span have been described. The influence of intact material failures on the failure mechanism is particularly noteworthy. Several simple rigid block analytical models have been used to demonstrate possible shotcrete-rock interactions. They point out the importance of the rock- shotcrete bond strength in determining the support capacity of a discontinuous shotcrete tunnel lining.