Research On Experiment And Numerical Simulation Of The Failure Process Of Specimens With 3D Internal Cracks Under Biaxial Loading And Water Pressure

Engineering rock mass is commonly a brittle medium containing lots of primary joints or fissures. Under the effect of fissure water and stress redistribution during construction, the crack initiation, propagation, and coalescence may cause the strength and stiffness degradation of such medium. And these have a remarkable impact on the stability of rock mass. However, due to the complexity of tests on rocks containing 3D embedded cracks and the difficulty in direct observation of opaque rock-like materials, the test results on propagation and coalescence of 3D crack set have seldom been reported. In the previous studies, penetrated cracks or surface cracks were usually created on a plane rock sample. The propagation process of single or multiple cracks under uniaxial or biaxial loading and the accompanying physical phenomena were observed. In recent years the mechanism of 3D crack propagation has become a concernful topic in the field of rock mechanics, and the mechanism of 3D crack propagation under complex stress condition is badly in need of further exploration. In addition, the instability of rock engineering is often closely related to groundwater, therefore it’s meanwhile of great importance to study the failure laws of fractured rock mass under fissure water. In this paper, by adopting approaches such as theoretical analysis, laboratory tests and numerical simulation, we have investigated the crack initiation, propagation, and failure process of rock-like specimens with internal cracks under uniaxial compression, biaxial compression and water pressure. Finally, the elastic-brittle numerical simulation model is applied to studying the crack propagation process and stability of a slope project. The work this paper completed involves the following aspects:(1) A new transparent rock-like resin is developed, whose brittleness value (ratio of tensile to compressive strength) can reach 1/6.6 at-10～-15℃, while that of previous scholars is just 1/3. Besides, it’s mechanical parameters are relatively close to those of real rock materials, such as marble and sandstone. Therefore, the resin specimen can simulate these rocks to a great extent. Moreover, its transparency has been greatly enhanced and resulting photos are much clearer than before. Afterwards, using this new resin material, we also have developed the specimen which contains an internal hollow crack together with corresponding experimental equipment and test procedure. Then experiments (uniaxial compression and biaxial compression) are carried out using the specimen under water pressure condition, and the crack growth process and failure laws of the specimen under different water pressure and lateral stress are investigated.(2) The crack initiation, propagation, and failure process of rock-like specimens with internal cracks under uniaxial compression and biaxial compression have been investigated. It is shown by the testing results that the failure process of specimens can be divided into four stages. And the four stages are basically similar to the macroscopic phenomena observed during the four stages in conventional compression tests on rock specimens. In physical tests, besides common forms of 3D cracks, such as wrapping wing crack, piebaldness-shaped crack and petal-shaped crack, a number of cracking modes that have never been reported in the previous studies are revealed in this study, including the wrapping anti-wing crack, the quasi-wrapping cracks, the large quasi-wrapping shape fracture and petal-shaped cracks due to the enhanced material brittleness.The resin specimen is homogeneous, isotropic and there is no flaw inside the specimen except the pre-embedded crack. As a result, it can be regarded that the specimen’s failure process is just caused by internal cracks with no other defect interference. Comparing the experiment of heterogeneous material with homogeneous material, their crack initiation and growth in the preliminary stage are approximately the same, yet their failure forms are often different. The heterogeneous defects inside heterogeneous material will show a strong effect in the middle and later stage.(3) The uniaxial and biaxial compression tests under water pressure condition are carried out using the specimen that contains an internal hollow crack. And the crack growth process and failure laws of the specimen under different water pressure and lateral stress are investigated. It is shown that the failure process of specimens under water pressure is different from that with no water pressure. First of all, there are both wrapping wing cracks appearing at the tips of major axis of the pre-embedded crack. Moreover, petal-shaped crack will not be formed and the secondary crack initiation stress and the peak strength of resin specimen both decline greatly under water pressure.(4) The elastic-plastic model in FLAC3D was modified as an elastic-brittle model and superfine meshes were adopted for numerical simulation. The brittle failure process of a specimen containing 3D cracks has been successfully simulated. The numerical results are basically consistent with the laboratory tests. It indicates that the new numerical simulation method in this paper may replace or make more supplements to the physical experiments for further investigation. It also provides a new method for future studies on crack propagation. The secondary crack initiation stress, the dilatational point stress and the peak strength of rock specimen containing 3D pre-cracks increase as the lateral pressure increases.(5) By adding the fluid-solid coupling model in FLAC3D to the above-mentioned elastic-brittle model adopted in no water condition, the failure laws of the specimen under water pressure are investigated. The crack growth processes under different water pressure and stress conditions are simulated. The higher the water pressure is, the lower the secondary crack initiation stress and the peak strength will be. It’s also shown by numerical results that the final cracking surface is always perpendicular with the minimum principal stress. When the vertical principal stress is lower than the horizontal principal stress, the final cracking surface will be horizontal; meanwhile, if the vertical principal stress is much higher, the final cracking surface will be vertical.(6) Finally, the improved elastic-brittle model and fluid-solid coupling model are comparatively studied in analyzing the crack propagation process and stability of a slope project. The results can basically illustrate the failure laws of the slope under excavation. It’s also worth mentioning that byapplying mesoscopic calculation method, the results can show the crack growth process and the influence of water pressure on slope well.