Abstract

When dealing with tunnels in difficult ground conditions, the knowledge of the rock mass parameters is of utmost importance for the selection of appropriate excavation methods and support measures. Sections with a high content of fault material or 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 bimrocks an extensive laboratory program was carried out. Artificial bimrocks were fabricated for both direct shear tests and large oedometer tests, covering a wide range of possible block proportions and block orientations. The laboratory tests were accompanied by in-situ tests, allowing the identification of differences between the small- and large-scale tests and the determination of upscaling factors. A straightforward evaluation method is presented, highlighting the effect of block orientation and block proportion on the shear behavior, shear strength and deformation behavior of bimrocks.

1 Introduction

Tectonic faults are usually composed of lens-shaped, relatively competent rock blocks surrounded by finely grained cataclastic material (e.g. Medley 2001 and 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.

To study the principle mechanical properties of fault material an extensive laboratory program was conducted on both artificial block-in-matrix rocks and real fault material (Pilgerstorfer 2014). For the examination of the mechanical behavior direct shear tests were performed, allowing investigation of the behavior of bimrocks exposed to large strains. Another important issue is the knowledge about the stress dependency of the deformation properties, especially for TBM-advances in weak rock masses. The amount of displacements, which are expected, and the risk for a shield-TBM getting stuck should be known a priori.

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