The characterization of the bedrock by means of a geotechnical model as a basis for the design and the execution planning of rock structures is essential for safe and economical construction in rock. Simplified methods for the classification of rock, developed in the last years in the international domain, cannot replace a design that is based on a geotechnical model and are only suitable under certain conditions for simple problems in rock engineering.
An efficient design and an economical and on-time execution are a prerequisite for successful construction. In rock engineering, a correct description of the bedrock by means of a geotechnical model forms an important basis for this. The importance of such a model is shown in the present paper. In the last years, a number of methods for the classification of rock have been developed internationally and applied to the design of rock structures (Barton et al. 1974, Bieniawski 1976, Deere et al. 1965–1967, Ramamurthy 2002, Rawlings et al. 1995). These methods, however, contain simplifications, and some factors are neglected. They are therefore no replacement for a solid design and are only suitable under certain conditions for simple problems in rock engineering.
The procedure for designing and building rock structures on the basis of geotechnical models is shown in Figure 1. The exploration first results in a fabric model (Fig. 2) containing the relevant information regarding the fabric and orientation of grains and discontinuities as well as information on the spacing and properties of the discontinuities. It forms the basis for a quantitative representation of the mechanical and hydraulic properties of jointed rock mass (Wittke 1990). In this context, the often anisotropic behavior of jointed rock mass has to be taken into account. For example, the deformability of a rock mass perpendicular to a fabric plane may be significantly higher than parallel to it (Fig. 3). The strength of jointed rock mass is generally anisotropic as well, since it is determined to a considerable degree by the shear and tensile strength on the discontinuities. This anisotropy is shown in Figure 4 for the example of an unconfined compression test performed on a rock block with a discontinuity set dipping at varying angles. For loading perpendicular and parallel to the discontinuity set the strength of the rock block is determined by the uniaxial compressive strength of the intact rock. Deviations from these loading directions result in shear stresses on the discontinuities. With increasing deviation, the shear strength on the discontinuities determines the rock mass strength and causes in this example a reduction of the uniaxial compressive strength by appro 60 % (see Fig. 4). If several discontinuity sets exist, they have to be superimposed correspondingly (Wittke 1990). The water permeability of jointed rock mass is generally anisotropic as well. The water permeability of the intact rock can usually be neglected as compared to the one of the discontinuities.