A study of the behaviour of vertical rock anchors using the finite element method
A literature survey on rock anchor design revealed that current rock anchor design methods are empirical and conservative in nature. The stability of a rock anchor depends on the assumed yield surface which has been found to be difficult to predict. A simple conical yield surface is currently adopted in practice. In this approach important rock parameters such as shear stress on the surface of the cone are usually ignored. Although both theoretical and experimental evidence are available to indicate that bond distribution along the fixed anchor length is highly non-linear at the tendon/grout/rock interfaces, current design still assumed the bond to be uniformly distributed along the whole fixed anchor length. Anchor interactions group effects, debonding at the proximal end of the anchor, the effect of tendon spacing and spacers, the effect of lateral pressure on the fixed anchor are not considered in current rock anchor design. To provide a more fundamental understanding of rock anchor behaviour a simple isoparametric finite element program has been written and thoroughly tested. The initial stress method with associated flow rules was used to simulate rock anchor behaviour. The plastic potentials used for simulating the tendon and, the grout and the rock, were respectively the Von-Mises and the octahedral shear stress yield criteria. Double nodes at interfaces provided a method of simulating a perfectly smooth interface without having to resort to special interface elements. The stress strain distribution pattern in the fixed anchor zone has been obtained for a modular ratio of 2.1, The results produced have indicated, that, in addition to physical material parameters, anchor behaviour is also dependent on the applied load, the slenderness ratio and the fixed anchor diameter. The most severe position of shearing has been found to be at the grout/rock interface. Analyses show that partial debonding is not a serious problem. Surcharge tends to reduce the shear stress at the grout/rock interface however the effect is negligible. A design curve is proposed for consideration.