On the resistance provided by grouted rock reinforcement to shear along bedding planes and joints

Design of wide span caverns in the horizontally bedded Hawkesbury Sandstone supported by rock anchors and shotcrete is governed by serviceability considerations; these normally being maximum crown sag and maximum sidewall movements. The crown sag may be set as a target of, say, 50mm with an upper bound of 100mm; the actual values being defined from consideration of shotcrete cracking or delamination, or separation from connection to rock anchor heads.

In order to constrain crown sag to the serviceability targets it is necessary to limit shear on the near horizontal bedding planes. This is achieved by the shear resistance of the anchors or bolts at small displacements of about 10mm. Pretensioning the rock anchors provides substantial shear resistance, but currently such tensioning is limited to small amounts by safety considerations. Therefore dowel action of rock reinforcement is an important consideration.

This paper gives experimental data from full scale testing of one type of Megabolt anchor and suggests how the results can be used in design analyses.