Clay- and marlstone are low permeable rocks suitable to host radioactive waste. However, their strong layering might cause stability problems, at least when oriented at an unfavourable angle to σ1. We carried out compressive strength tests on these rocks to constrain this "weak bedding plane" effect. We tested plugs whose long axes (parallel to σ1) were oriented with varying angles to bedding, to find out at which angle between lamination and σ1 these lithologies are weakest and strongest, respectively. We observed weak bedding planes in claystone, but not in marlstone. The reason for this is the respective lack or presence of calcite. The presence of calcite in marlstone cements this rock much stronger than this is the case in claystone. Consequently, weak bedding planes occur favorably in fine grained purely clastic rocks and not in chemical-clastic rocks.

1 Introduction

Clay-, silt- and marlstone are considered to be suitable host rocks for a radioactive waste disposal site (RWDS) due to their low permeability. However, they are also highly anisotropic rocks because of their strong lamination. Bedding planes may act as planes of weakness, in particular if oriented at an unfavourable angle to the principle compressive stress σ1. We carried out numerous tests on clay-, silt- and marlstone of differing origin to quantify this "weak bedding plane effect" and to find out if there is a general pattern that can be applied to all these fine grained, low permeable and relatively weak rocks. Namely we investigated the Jurassic Opalinus Clay and Effingen Formation from Switzerland, as well as a Dogger siltstone (Dogger ß and γ) from the North German Basin and – for comparison reasons – also a Dogger ß sandstone from the same area. We did compressive strength tests on cylindrical plugs whose long axes (simultaneously the direction of axial loading σ1) were oriented at various angles to bedding (see also Figure 1), to find out at which angle between lamination and σ1 an RWDS host rock is weakest and strongest, respectively.

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