Abstract

The Alhama de Murcia Fault (AMF) is an active fault located in the SE of Spain. This fault developed a ~80 m thick band of highly deformed clay rich gouge with a marked tectonic fabric (foliated gouge) that constitute a notable anisotropy.

In this work we present the result of consolidated-undrained triaxial tests (CU) performed under low confining pressure (50, 150 and 300 kPa) to several sets of core specimens carved with different orientation of the tectonic fabric. We analyse strain-stress relationships of each test, as well as failure modes, obtaining a brittle behaviour at lowest confining pressure, and ductile at highest. The resistance results show that the gouge of the AMF behaves as a hard soil or very soft rock. Those results are adjusted by the non-linear Hoek&Brown criteria for every orientation by considering two working hypotheses: (i) considering the fault gouges as intact rock, and (ii) considering the fault gouge as tectonised rockmass. Both working hypotheses are consistent, reaching maximum values of resistance when axial load (σ1) is applied perpendicular to the foliated planes of the gouge, decreasing for lower angles with the anisotropy planes which favour failure. Anisotropy index Rc has been estimated in a range between 1.40 and 3.16, which classify the fault gouge as low to medium anisotropic behaviour. Nevertheless, results from specimens with the tectonic fabric oriented most favourably for failure occurrence do not fit such a pattern, since strength for this orientation is greater than intermediate orientation. Such an anomalous result could be related to an increase in the roughness of the failure planes controlled either by the presence of larger fragments of protolith or by an asymmetry on the roughness of the weakness planes related to the original microscale structures characterized by a strong reorientation of clays.

These results will be useful to applications related to slope and tunnel stability, and also to the understanding of the relationships between aseismic creep deformation and earthquakes nucleation within the AMF.

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