Physical modeling of a rock mass under a true triaxial stress state

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Tiwari, R P
Rao, K S
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Civil engineering activities at the ground surface can be mainly governed by a confining pressures i.e. minor (s3) and intermediate (s2) principal stresses are negligible or zero. However, at depth the stress state will be significantly true triaxial, i.e. polyaxial. In the literature, strength enhancement in intact rock (Haimson and Chang, 2000) and in the rock mass at site (Singh et al., 1998) has been reported due to s2: Therefore, a study was planned on 77 15 cm size block mass models similar to Singh et al. (2002), having three orthogonal continuous smooth saw cut joint sets.Joint set-I has varying inclination ‘y’ equal to 0, 20, 40, 60, 80 and 90 with the x-axis; joint set-II has interlocking, s ¼ 0:5 times width (b) of the 2.5 cm size small cubical blocks forming the specimen; and joint set-III was always vertical (Fig. 1a). The specimens were prepared with sand-lime brick model material of uniaxial compressive strength 13.5MPa and representing the ‘EM’ category on Deere–Miller’s classification chart. Its suitability to use as brittle model material was established by preliminary tests and XRD, SEM analyses (Tiwari and Rao, 2003). The specimens were tested in true-triaxial system (TTS) developed by Rao and Tiwari, 2002 (Fig. 1b) under a s1 > s2 > s3 stress state. The test results show that with an increase in s2 in the dip direction of joint set-I, the strength of the rock mass (s1) and deformation modulus (Ej ) increase significantly, which is confirmed by fracture shear planes developed on the s2 face of the specimen dipping in the s3 direction and steepening with an increase in the s2 values. The strength enhancement was maximum (309.2%) for inclination of joint set-I, y ¼ 60 and minimum (24.2%) for y ¼ 90: The axial stress–volumetric strain curves show onset of dilatancy in the rock mass and dilatancy stress (in % s1peak) at which dilatancy in the specimen initiates is an increasing function of s2: Most of the specimens failed in shearing corresponding to y ¼ 0; 20, 80 and 90 with sliding, joint dilation mixed with shearing for other cases (y ¼ 40 and 60). The effect of interlocking and rotation of principal stresses s2 and s3 on the strength and deformation response was also investigated for a few specimens with y ¼ 60: Finally suitable failure criteria were also evolved in both the triaxial and true-triaxial stress states taking into account s2; s3; joint configuration, joint and intact material properties.
True triaxial system, Rock mass, Physical modeling, Intermediate principal stress, Dilatancy, Scale effect