When dealing with tunnels in difficult ground conditions, e.g. fault zones, the knowledge of the rock mass parameters is of utmost importance to ensure a safe and economical tunnel design. Sections with a high content of cataclasites form the most challenging stretches during tunneling, and a proper geomechanical characterization is imperative. However, investigating the overall-properties is currently a challenging task, originating from difficulties in sample acquisition, sample preparation and laboratory testing.
In order to gain insight into the overall mechanical properties of cataclasite-like material and to study the shear behavior of BIM-rocks an extensive laboratory program was carried out. Artificial block-in-matrix samples were fabricated for direct shear tests. Models with six different block orientations, related to the shear direction were created. A straight forward evaluation method is presented and the results are discussed in detail, highlighting the effect of block orientation and block proportion on the shear behavior and shear strength of BIM-rocks.
Tectonic faults are usually composed of lens-shaped, relatively competent rock blocks surrounded by finely grained cataclastic material (Medley 2001, Riedmüller et al. 2001). Hence, their properties are highly anisotropic and depend on the degree of the regularity of the block orientation, the total volumetric amount of the competent lenses as well as the properties of the matrix. Cataclasite associated with tectonic faults is often assumed to produce soil-like material, and traditional soil mechanics testing procedures are normally adopted. Its mechanical behaviour is poorly understood, originating from difficulties in sample acquisition during field investigations, sample preparation and laboratory testing. However, sections with a high content of cataclasites form the most challenging stretches during tunneling, and proper geomechanical characterization is of crucial importance.
To study the principle mechanical properties of fault material an extensive laboratory program was conducted on artificial block-in-matrix rocks. Tunneling in weak rock mass is often accompanied by large displacements, hence an intended issue was to investigate the behavior of BIM-rocks exposed to large strains. Lindquist (1994) provided a comprehensive study of triaxial tests on artificial BIM-rocks. However, being a characteristic of triaxial tests the occurrence of large strains is mostly limitedand the investigation of failure surfaces is rarely possible. Hence it was decided to examine the mechanical behavior of artificial BIM-rocks in direct shear tests.