The anisotropic behaviour of schistose rocks is rarely considered in the conception of retaining structures (diaphragm wall, soldier piles wall …) in this type of rocks. Brioverian shales are often encountered underground the city of Rennes (Brittany France) and its surrounding area. They are fractured, altered and folded along a vertical plane which lead to lateral variations of facies and alteration changes on short distances. Consequently of the numerous construction projects, including the excavation of the second subway line, the physical and mechanical knowledge of this material has to be improved to help structure design.

This paper presents the results of physical and mechanical characterization on schistose rocks extracted from several locations in the Rennes area. Some of the methods employed were classical geotechnical tests (X-ray diffraction, thin sections observations, uniaxial compression tests and shear tests along open joints) whereas others were modified tests usually used in the road construction fields (grindability test based on LCPC abrasiveness test and Micro Deval test used on prismatic samples). This characterization showed an important heterogeneity of the mechanical properties of the Brioverian shales despite homogeneity in their mineralogical contain. The important sensitivity to water of the material was revealed.

1 Introduction

Brioverian shales can be encountered undeground a large part of Brittany (West of France, Jegouzo 1973, Dabard 1990). The unpredictable anisotropic behaviour of these rocks during construction projects can be a real problem. Indeed, those shistose rocks present a high level of fracturation and alteration that leads to important lateral variations on short distances. The most common problems encountered are: deviation during drilling (piles used as foundations or retaining structures, diaphragm wall), collapsing between vertical elements of a retaining structure during excavation, higher ground pressures than expected in calculations leading to unacceptable displacements or moments of the structure…

Numerous underground construction projects are under view in the Rennes Metropole area and the construction of a second subway line will begin in 2014 through these schistose formations.

Considering those expected projects in this type of ground, the physical and mechanical knowledge of this material have to be improved to help structure design. Rock samples, from different sites in the city of Rennes, were extracted during excavation steps of different construction sites. The physical characterization consisted in X-ray diffraction analyses on shale powders, thin section observations, grindability tests on aggregates and a modified Micro Deval test on prismatic samples. The mechanical characterization used uniaxial compression tests and direct shear tests along open joints. An overview of the results obtained so far is presented in the next pages.

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