Influence of the microstructure and roughness of weakness planes on the strength anisotropy of a foliated clay-rich fault gouge

J.J. Martínez-Díaz,J.L. Sánchez-Roldán,M. Tsige,E. Rodríguez-Escudero,J.M. Insua-Arévalo

doi:10.1016/j.enggeo.2021.106186

J.J. Martínez-Díaz, J.L. Sánchez-Roldán + Show 3 more

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https://doi.org/10.1016/j.enggeo.2021.106186

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Journal: Engineering Geology	Publication Date: May 11, 2021
Citations: 9	License type: cc-by

Affiliation: Universidad Complutense de Madrid

Abstract

Cataclastic rocks, as clay-rich fault gouges, are commonly present in brittle rock masses when fault zones appear during geological engineering projects. Highly deformed rocks that are of poor mechanical quality can lead to technical, safety, and economic problems in rock engineering. The aim of this study is to characterise the resistant behaviour of a highly deformed clay-rich gouge >40 m wide with a marked tectonic fabric that indicates strength anisotropy. We present the results of consolidated-undrained (CU) triaxial tests that were performed at low confining pressures (50, 150, and 300 kPa) on several sets of foliated gouge specimens with four different orientations in the tectonic fabric. Specimens were collected from the encapsulated rock cores of two research boreholes drilled through the Alhama de Murcia Fault (AMF), a main regional fault located in SE Spain. The strain–stress relationships and failure modes were established, indicating that the gouge behaves as hard soil or very soft rock. The test results were adjusted at each orientation using the non-linear Hoek and Brown criteria by considering the fault gouge as an intact material or as a tectonised rockmass. Here, we use the Geological Strength Index (GSI) as an indicator of the rockmass strength that depends on the direction of the tectonic fabric. However, the results from specimens with tectonic fabric that is oriented most favourably for failure were not the weakest in terms of rock strength. Such an anomalous result could be the result of asymmetry in the roughness of the weakness planes that is related to the original gouge microstructure characterised by the strong reorientation of clays in an S-C′ like tectonic fabric.Our results will be useful for practical applications that are related to the stability of slopes and/or shallow underground excavations in brittle fault zones, and provide an inexpensive and easy way to preliminarily evaluate the anisotropic behaviour of this type of brittle fault zones for future engineering projects.

Highlights

The presence of fault zones in brittle rocks targeted by engineering geology or mining projects can be decisive when considering technical and economic feasibility (e.g. Brenner Base Tunnel, Alps, Austria, Sausgruber and Brandner, 2003; Guadarrama Base Tunnel, Central System, Spain, Lopez Sopena, 2005; Schubert et al, 2006; Coal mining support, Huainan, China, Kang et al, 2018)
This paper presents the results of triaxial tests that were performed on undisturbed core samples from a fault gouge in the Alhama de Murcia Fault (AMF), for which specimens were prepared at four different orientations of the tectonic fabric to characterise the strength anisotropy
Our results reveal that in addition to the angle of orientation between the main stress and the weak planes, the strength anisotropy of the foliated fault gouges can be controlled by the roughness geometry of the weak planes along the direction of the displacement that was induced during the triaxial tests

Summary

Introduction

The presence of fault zones in brittle rocks targeted by engineering geology or mining projects can be decisive when considering technical and economic feasibility (e.g. Brenner Base Tunnel, Alps, Austria, Sausgruber and Brandner, 2003; Guadarrama Base Tunnel, Central System, Spain, Lopez Sopena, 2005; Schubert et al, 2006; Coal mining support, Huainan, China, Kang et al, 2018). Cataclastic rocks as fault gouges are commonly present in fault zones affecting brittle rocks and always have very poor mechanical quality, leading to problems in rock engineering. Gouges could be considered stiff soils, fault gouges usually comprise tectonically induced fabrics, implying aniso tropic mechanical behaviour within bands reaching a width of up to 500 m (Juliaand Santanach, 1998). The anisotropy in the mechan ical behaviour of cataclastic rocks, such as fault gouges, has not yet been studied in depth. One of the main reasons for this is that it is difficult to characterise such rocks because of the difficulties that arise in: i) obtaining undisturbed samples during field investigations, ii) specimen

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