The objective of this study is to determine shear strengths of factures under constant normal loads (CNL) and constant normal stiffness (CNS) conditions as affected by displacement rates. The rock specimens are prepared from Saraburi marble with the nominal dimensions of 100×100×180 mm3. The fracture area is about 100×90 mm2. The fractures are artificially made in the laboratory by tension induce method. The direct shear test is performed with constant normal stresses at 0.5, 1.0, 1.5 and 2.0 MPa using the triaxial loading frame. The applied shear displacement rates are 10−4 to 10−1 mm/s. The results indicate that the higher shear displacement rates have affected to the peak shear stresses are increase under CNL and CNS condition. The higher initial normal stresses have affected the shear strength and the normal displacement than the lower initial normal stresses for CNL and CNS condition. The CNS condition shows higher peak shear strength than the CNL condition. The higher initial normal stress and higher shear displacement rate give the higher peak shear strength. The CNS condition shows the effect of rock joint roughness more than the CNL condition.
The presence of joints in the rock can affect its mechanical behavior, depending on the underground situation. When dilation of the rock joint during shearing is constrained or partially constrained, an increase in the normal stress over the shear plane occurs, which substantially increases the shear resistance. An underground excavation is potentially unstable rock blocks are constrained between two parallel dilatant rocks (Indraratna et al., 1999). The sliding of such blocks inevitably increases the normal stress, and also, dilation becomes significant if the joint surfaces are rough.