True triaxial strength and deformability of the siltstone overlying the Chelungpu fault (Chi‐Chi earthquake), Taiwan

We conducted true triaxial compression tests to examine the effect of the intermediate principal stress ( σ 2 ) on brittle failure and deformability of the siltstone overlying the Chelungpu fault, Taiwan. For constant minimum stress ( σ 3 ) a pattern of increased strength ( σ 1 ) as a function of σ 2 , unpredicted by common strength criteria, was observed. Strength data are well fitted by a Mogi‐modified Nadai criterion relating the octahedral shear stress at failure to the mean normal stress on the induced shear fracture or fault. The induced fault strikes subparallel to σ 2 and dips steeply in the direction of σ 3 . Significantly, fault dip angle steadily increases as σ 2 rises for unchanged σ 3 . SEM inspection of tested specimens reveals only sporadic localized intergranular microcracks associated with the creation of the fault. The modulus of elasticity and the onset of dilatancy exhibit a similar behavior to that of σ 1 , as they increase with σ 2 when subjected to a constant σ 3 .