Shear behaviour of a rock bridge sandwiched between incipient joints under the influence of hydraulic pressures

The rock bridges sandwiched in incipiently jointed rock mass were considered as barriers that block the fluid seepage,and provide certain shear strength reservation.For better revealing the influence of hydraulic pressure on the failure behaviour of rock bridges,direct shear tests were carried out through a newly proposed method on rock samples that contain two parallel incipient joints.By developing the gypsum-silicone pad coupling samples,a conventional triaxial test system was qualified to implement direct shear tests with satisfied sealing capability.The results showed that the rock bridges could be failed through the tensile failure,shear failure and mixed failure mechanism.The hydraulic pressure would facilitate the tensile failure mechanism and induce rougher fracture surfaces;while the normal stress would facilitate the shear failure mechanism and induce less rough fracture.The hydraulic pressure reduced the global shear strength of the rock block through reducing the efficient normal stress applied on the rock bridge area,which was highly dependent on the joint persistence,k.Moreover,because of the iterating occurrence of the hydraulic pressure lag with the fracture propagation,the rock bridge failure stage in the shear stress-shear displacement curves displayed a fluctuation trend.

Numerical analysis on the crack propagation and failure characteristics of rocks with double fissures under the uniaxial compression

The fissures and rock bridges with different dips had different contributions to crack's initiation,propagation,convergence and penetration.In this paper,based on the rock fracture theory,the crack's propagation and evolution process on rock specimen with double fissures under uniaxial compression was simulated.As a result,the crack propagation and evolution law of rocks with different fissure dips(α=0°,15°,30°,45°,60°,75°,90°;β=45°)and different rock bridge dips(β=0°,30°,45°,60°,90°;α=45°)was obtained by numerical tests.Meanwhile,the fissure and rock bridge dips influence on the macro mechanical properties of rock was analyzed.Besides,the paper investigated the influences of different fissure dips and different rock bridge dips on the bridge transfixion.The study is of great significance to reveal the impact of different dips on the mechanical mechanism of multiple-fissures rock under specific conditions,and it also has important theoretical significance for the research on multiple-fissure rock.

Study on Toughening on Crack Prevention of Jointed Rock Masses with Different Pre-Stress Anchor Cables

The major reason of the failure of jointed rock-mass is the formation of the plastic zone near the crack tip of I-II mixed crack which leads to the growth, propagation of the branched crack under load condition, rn the paper, the failure judgment of mini-plastic zone＇s displacement is derived by the Mises yielding rule. The anchor cable is simulated by the different link elements and inflicting pre-strains according to the difference of mechanism of the consolidated segment and free segment. The stress and strain fields near crack tip of twain collinear cracks of different angles and consolidated conditions are simulated by iso-parametric element with eight nodes. The iso-parametric element with eight nodes is degenerated to singular element at crick tip to simulate crack. It is shown that the mini plastic zone＇s displacement near the crack tip begins to increase, then decreases with the increase of the angle of the crack. The better consolidated condition is, the smaller the angle of crack tip is when the mini plastic zone＇s displacement near the crack tip arrives at the biggest value. The mini plastic zone＇s displacement near the crack Tip 2 is bigger than that near the crack Tip 3. The crack is easier to failure with the increase of load.