Assessment of the bolt-grout interface behaviour of fully grouted rockbolts from laboratory experiments under axial loads

The bolt-grout interface of fully grouted rockbolts has been studied with the aim of determining its constitutive law. For this purpose, axial loadings acting on the bar have been considered and theoretical and experimental research conducted. The theoretical investigations link the response of the surrounding materials (bolt, grout and ground) to that of the interface, which is not directly accessible. Pull-out tests have been carried out to obtain experimental information. Combining the theoretical developments with the test results, the interface variables can be determined. This procedure proves that, in order to study the bolt-grout interface accurately, the behaviour of the surrounding materials should be well known. From the available data, a semi-empirical formulation of the interface response is proposed. The tangential behaviour suggests that friction is not the only component to the shear stress. The normal response, which is more difficult to ascertain, depends on the bolt profile.