The anisotropic behaviour of most geological masses has well-recognized implications upon the design of underground excavations. Most of our experiences derive from near-surface structures and conventional stress levels. Accumulating evidence, however, indicates that the traditional inference of quasi-isotropic under high stresses can be misleading. This paper presents the results from phsyical model tests on circular openings in simulated highly stressed formations with anisotropic mechanical characteristics. It is shown that, even under extreme stress conditions, the secondary stresses and deformations are controlled by the mass structure. Relevant numerical modelling concepts are briefly discussed and some comparisons with experimental results are made.
The mechanical response of discontinuous geological masses to perturbations in their initial stress environment induced by construction on or in the mass, is influenced largely by the structure of the matrix, i,e. jointing, schistosity, bedding, etc. These structural features constitute compliant elements within the mass and introduce marked mechanical anisotropy. The latter causes directional transformation of the stresses. The literature reveals limited information directly relevant to underground excavations in anisotropic materials under very high stresses. The stability of openings in weak, plastic materials is usually studied within the context of shallow tunnel construction in problematic soils. Experimental research deals almost exclusively with homogeneous materials (e.g. Attewell and Boden 1971, Atkinson and Orr 1976, Casarin and Muir 1981). More recently, the theoretical effects of anisotropic behaviour in the stability of deep wellbores has attracted some interest (Aadnoy,1987). Fundamental experimental investigations on the stability of tunnels in discontinous competent rocks generally refer to conventional levels of stress (e.g. Barton and Hansteen, 1979, Langof, 1983). However, structure-amrolloo instabi Ii ty rmy be mooonteroo in various types of deep engineering structures, sud1as mining tunnels, wellbores and shafts (Zischinsky 1984, Barton and Bakhtar 1983, Kaiser ard Malony 1987). In this paper, some further evidence will be presented, demonstrating that anisotropic behaviour may prevail at levels of stress approaching the strength of weak and competent materials, containing adversely oriented single interfaces of variable frictional characteristics. Physical models of circular openings in simulated highly stressed formations have been revelq:ed and tested to investigate:
the secondary stress regime
the modes of deformation
the failure mechanisms
the material / lining interaction Two types of geological formations will be simulated herein, namely, transversely fissured weak shales and regularly jointed competent rock.
Experimental results from high confining pressure polyaxial tests on 5×5×l0cm prismatic specimens of a horrogeneous marble containing a single interface, have demonstrated that anisotropic behaviour is effective, even when the normal stress (σn) component across the interface, approaches the confined strength of the intact rock. In this case, σn reached a level of ~ 150MPa. The uniaxial compressive strength of the rock was 60–80 MPa. Depending upon the value of β, failure occured either through the intact rock or along the interfaces, or in combined mode. Such behaviour was observed with interfaces represented by saw-cut surfaces or extension induced fractures.
