Laboratory study of bolting, comparison of pull-out test results – cases of soft rocks and hard rocks

摘要

Support systems with fully grouted bolts in rocks and soil are composed of three parts: bar, grout and natural material. These techniques are widely used in mining galleries and shallow tunnels for civil works because one can quickly modify bolting pattern in response to the ground stability variations. Consequently, these support systems require a regular design optimization. As depth modifies the in situ stress of the rock mass, it must be taken into account in design through confining pressure. For this purpose, a bench was developed in our laboratory to reproduce such conditions. It allows various choices for rock sample, bore hole wall, types of bar and grout. Two large test campaigns were carried out, one on hard rocks, the other on soft rocks. As the strain rate of the tests was small, only static conditions were investigated. In both cases, the bench was provided with gauges and pumps adapted to the pressure range of values. As for hard rocks, the compressibility of the bladder in Viton and the inertia of the oil reservoir are neglected. As for soft rocks, they must be taken into account as they have bigger impacts. Gauges are added to monitor or evaluate them. More than eighty tests on sandstone and thirty tests on hard soil are analyzed. The results show that some factors impact all test series, as the embedment length and the confining pressure. But the main difference deals with the surface of the failure which is the joint between the grout and the bolt in case of hard rocks and the interface between the grout and the rock in case of soft rocks.