Modeling Of Structural Effect Of Non-persistent Jointed Rock Masses Under Different Stress Conditions

The discontinuities in rock masses cause structural effect influencing their mechanical behavior. The structural effect significantly increases the difficulty of evaluating the rock material properties as well as of rock engineering design. The non-persistent joint commonly occurs in rock masses, from which new fractures initiate, develop and coalesce into macro failure plane driven by loading. Therefore, it is of importance to investigate the structural effect of non-persistent jointed rock masses for rock engineering design and stabilization. This thesis incorporating with the National Basic Research 973 Program of China (Grant No.2013CB036003) and the National Natural Science Foundation of China (Grant No.51374198) focuses on solving the scientific problem of estimating mechanical properties of engineering rock masses. Accordingly, the physical and particle flow modeling was undertaken to systematically investigate the structural effect of non-persistent jointed rock masses under varied stress conditions. The main results are present as follows:(1) 184 samples of non-persistent jointed rock mass were carefully prepared by using artificial similar materials. These samples were subject to uniaxial compression test to investigate the effect of joint spacing and joint overlap on mechanical behavior of jointed rock masses and the mechanism of parallel joint interaction. The test results demonstrate that the interaction of parallel joints has significant to negligible effect on failure mode, strength behavior and deformation behavior of jointed rock mass models, depending on the joint dip angle, joint spacing and joint overlap. With the reduction of joint spacing, the normalized strength value and the maximum displacement value at peak stress decrease. Moreover, when the joint dip angle is no more than 45°, the normalized strength value and the maximum displacement value at peak stress decrease with the reduction of joint overlap.(2) On the basis of aforesaid physical experiment, the particle flow modeling method was then utilized to further investigate the structural effect of jointed rock masses induced by joint geometrical parameters. First the micro-mechanical parameters for synthetic intact material were calibrated through a trial and error process. Then a back analysis procedure was used to determine the joint gap and joint micro-mechanical parameters applied in the particle flow model. In accordance these micro-mechanical parameters, a parametric study was carried out to look at the effect of joint dip angle, joint persistence and joint gap on mechanical behavior of non-persistent jointed rock masses. The modeling results indicate that the joint dip angle and joint persistence dominate the mechanical behavior of jointed rock masses, while the effect of joint gap displays dependence of joint dip angle. Also, the interaction of joint surfaces has significant influence on rock mass behavior.(3) Another 84 artificial samples were prepared in laboratory to investigate the structural effect of non-persistent jointed rock masses with lateral confinement. The anchored bolts were utilized to provide constant normal stiffness from the lateral sides of the samples. Meanwhile, the particle flow modeling was conducted to investigate the structural effect of non-persistent jointed rock masses with lateral constant confinement stress. The study shows that even though the lateral confinement stress has negligible influence on rock mass failure mode, it has pronounced effect on rock mass strength behavior. As the joint dip angle equal 30°,45° and 60°, the anchored bolts increase the resistance capability of jointed rock masses. When the joint dip angle are 0°,15°,75° and 90°, the increasing support stress from the lateral side enhances the resistance capability of jointed rock masses. Furthermore, application of preload on the anchored bolts is an efficient measure to enforce the jointed rock masses.(4) By using particle flow method, the structural effect of non-persistent jointed rock masses induced by joint orientation was further investigated subject to the stress path similar to underground tunnel excavation. According to the modeling results, without support stress on the free surface the non-persistent jointed rock masses display three different failure modes, i.e. intact material failure, step path failure and planar failure. The intact material failure corresponds the high strength values, while the step path failure and planar failure result in low strength values. What is more, the convergence of free surface was found to be in three phases after the excavation, and each of them significantly depends on the joint dip angle and support stress. The increment of support stress on the free surface enhances the compression strength of the nonpersistent jointed rock mass. To some extent, the increment of rock mass strength reaches the 10 times the corresponding applied support stress value.(5) Under triaxial stress condition, the structural effect of non-persistent jointed rock mass induced by joint mechanical parameters was investigated through particle flow modeling. With a bonded smooth joint model, the effect of joint normal stiffness, joint shear stiffness, joint friction coefficient, joint friction angle, joint cohesion and joint tensile strength were systematically studied on the mechanical behavior of jointed rock mass. Under triaxial stress condition, the shear and slide response of pre-existed joints dominates the failure of the rock mass. The joint mechanical parameters which are relevant to joint shear strength has significant effect on mechanical behavior of rock masses having non-persistent joints.