Geological materials which can be classified as weak rocks are frequently encountered in geotechnical engineering situations. The mechanical behaviour of such weak rocks is characterized by their ability to exhibit creep effects under the action of externally applied or gravitationally induced loads. The creep behaviour of such geological materials has been characterized by various theories of material behaviour which range from elementary non-Newtonian creep to more com- Plicated theories such as viscoplasticity. An alternative characterization utilizes the theory of linear viscoelasticity in the treatment of such creep phenomena. The results of laboratory investigations and large scale field observations indicate that the creep behaviour of soft rocks such as shale, mudstone, limestone, and other geological materials such as coal, rock- Salt, potash and oilshales can be described by the theory of linear viscoelasticity. These studies are summarized in a recent article by Selvadurai(1978). On account of the inherent weakness of these geological materials, rarely can constructions such as excavations, tunnels, etc" be made Without the provision of temporary or permanent soil support. For the stabilization of such geotechnical constructions it is customary to use ground or rock anchor systems. These anchors can be embedded Plates or regions of the rock mass which are grouted to form the anchoring region. In the present paper the anchor region is idealized as a rigid disc shaped region.
Such anchor regions can be created by the hydraulic fracture of the weak rock under the pressure of the cement grout.
The importance of time dependent effects associated with the performance of anchors located in creep susceptible geological media is discussed by, among others, Lang (1962),Stefanko and Cruz (1964),Gasschalk and Taylor (1970),Hobst and Zajic(1977) and Littlejohn (1980) (see also Ground Engineering, 1980). These authors emphasize the loss of load in prestressed anchors due to creep deformations of the anchoring region. Recently, Selvadurai (1978, 1979) has provided a theoretical basis for the estimation of prestress loss in deep and near surface anchors located in geological media susceptible to creep. These latter investigations clearly indicate the importance of accounting for creep effects when estimating long term efficiency of anchorages located in creep susceptible media.
This paper is concerned with the theoretical analysis of the creep response of a flat rigid disc-shaped anchor region embedded in bonded contact with a geological medium, the mechanical behaviour of which can be characterized by linear viscoelasticity. As a first approximation the anchor region is considered to be a flat circular region. The analytical techniques outlined here can be extended to include other disc anchor shapes (e.g. elliptical, rectangular etc.). These extensions are beyond the scope of this article (see e.g. Selvadurai, 1981). The flat disc shaped anchor region is subjected to an inclined anchor load which acts at the edge (Fig. 1). The results of interest to geotechnical engineering concern either the evaluation of the displacement of the anchor region due to a step function of load or the relaxation of the load in an anchor rod due to the application of a step function type displacement.
