True Triaxial Strength and Deformability of the Sandstone Overlying the Chelungpu-Fault (Chi-Chi Earthquake), Taiwan

We have conducted true triaxial compression tests (in which σ1 is monotonically raised to failure while holding σ2 and σ3 constant) to determine strength, deformability, and other mechanical properties of the fine-grained (70 μm), quartz- (70%) and clay-rich (20%), Pleistocene sandstone just above the Chelungpu fault, intercepted by the TCDP (Taiwan Chelungpu-fault Drilling Project) borehole at 1111 m depth. The fault zone is considered the host of the 1999 Chi-Chi earthquake. Specimens (19×19×38 mm3 rectangular prisms) were instrumented with strain gages and a strain- gaged beam for recording strains in all three directions. Five groups of tests were conducted, each for a constant σ3(10, 25, 40, 60, 100 MPa). Within each group σ2 was varied from test to test between σ2 = σ3 and σ2 approaching σ1(at failure) . Contrary to the Mohr criterion assumption, for each σ3 a consistent pattern was observed of gradually increasing strength with the rise of σ2 until a peak was reached followed by a gradual decline, with σ1(at failure) always higher than the conventional-triaxial strength (at σ2 = σ3). Strength variation with σ2 for a given σ3 is best fitted by a second order polynomial. For example, for σ3 = 60 MPa, the true triaxial strength is σ1(at failure)(MPa) = 237 + 1.65σ2 - 0.004σ2 2 (r = 0.900). The maximum strength of 400 MPa is achieved at σ2 = 206 MPa, which is 25% higher than the 320 MPa at σ2 = σ3 = 60 MPa. Integrating all the true triaxial strength data into a Mogi-modified Nadai strength criterion yields a monotonically increasing power function τoct = 2.32[(σ2 + σ3)/2]^{0.75} (r = 0.995), where τoct is the octahedral shear stress at failure. The modulus of elasticity, and the onset of dilatancy, exhibited a similar behavior to that of σ1(at failure) when subjected to a constant σ3(gradual increase followed by a decrease with rising σ2). Upon failure the sandstone developed a shear through-going fracture steeply dipping in the σ3 direction. Moreover, the fracture dip angle steadily increased with rising σ2 for unchanged σ3, between 59° and 77°. The trend of the dependence of fracture dip on the deviatoric stress state is consistent with a prediction of rock failure based on localization of deformation (Rudnicki and Rice, 1975). SEM inspection of tested specimens failed to reveal microcrack localization adjacent to and predating the through-going fracture, as previously observed in crystalline rocks tested under similar conditions.