Experimental investigation of steel cables for rock reinforcement in hard rock

Abstract: This thesis describes a detailed experimental investigation of the mechanical behaviour of fully grouted steel cables used for rock reinforcement in hard rock. An attempt is also made to explain the possible modes of deformation and cable failure, based on observations from the experiments. The behaviour is investigated using pull-out and shear tests. Two mechanically quite different cables, a steel wire rope and a steel strand were tested in the experiments. The cable, types and design are specified in detail, and results from mechanical tests are presented. A cement based grout was used as grouting agent for the cables. Through extensive tests of compressive and tensile strengths, different properties of grout with and without additives are defined. The effect of both the curing conditions and the water segregation are studied. Laboratory scale pull-out tests on cables with short and long embedded lengths were conducted. The tests define what are believed to be important aspects of the mechanical behaviour of a grouted cable. The effect of the embedded length, cable surface properties, curing conditions and grouts with and without additives on the mechanical behaviour of the grouted cables are given. It is shown that the cable surface properties, the curing conditions as well as type of grout significantly effect the pull-out behaviour of the cable. No general correlation between cement grout compressive or tensile strength and bond strength in pull-out tests was established. However, the bond strength increases with increasing compressive strength of grout without additives. The laboratory scale pull-out tests are compared with insitu pull-out tests to establish the validity of the test procedure. It is concluded that pull-out tests should, (unless they are conducted for purposes of comparison) be conducted under in-situ conditions where the results will be applied later, whenever this is possible. Shear tests on the steel wire rope were unsuccessful because the rock blocks, inside which the rope was grouted, failed. The shear tests of the steel strand indicated however high shear loads and large shear displacements before failure of the cable occurred. Failure of cable reinforcement in hard rock is discussed. Here, it is reasonable to assume that cable failure will be a more important factor than failure of the rock, grout or intermediate bonds. Results from further in-situ pull-out tests confirm this assumption. An in-situ test procedure for cables in hard rock conditions is proposed. The stiffness/compliance of a fully grouted cable is defined. However, it is concluded from the pull-out tests that the complete pull-out curve always should be considered, in the interpretation of the pull-out test results. Results of in-situ pull-out tests show that blast induced dynamic loading before pull-out can have a significant effect on the pull-out behaviour of the cable. Borehole / cable diameter ratios were found to have an insignificant effect on the pull-out behaviour of the cable. Finally, the results of this investigation are discussed and recommendations for cable reinforcement in hard rock are made.