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<p><strong>Abstract:</strong> Uniaxial and biaxial tests are performed to investigate the evolution mechanism of crack propagation and coalescence through developing newly cement mortar materials with horizontal and inclined pre-existing flaws. Additionally, a new numerical method-CDEM (Continuous discrete element method) is employed to analyze the evolution laws of stress field of crack tips under hydraulic coupling. The results reveal that the maximum principal stress of the wing crack tip gradually decreases with increase of internal water pressure, and the initiation stress, initiation angle and peak strength show decreasing trend. The results of crack propagation and coalescence obtained by numerical simulation is consistent with laboratory results. With the water pressure increases, prior to the occurrence of wing cracks, the coplanar cracks firstly initiate around inclined flaws. Under the coupling action of uniaxial compression and internal water pressure, the lateral pressure would limit the initiation of the wing cracks, while the increasing water pressure weakens the inhibition of lateral pressure on wing cracks.</p>

<p><strong>Abstract:</strong> Based on laboratory direct shear tests and discrete element theory, the crack propagation and coalescence mechanism and numerical simulation of cement mortar specimens considering the combined actions of internal hydraulic pressure and shear force were carried out. We completed the filling of the internal hydraulic pressure in the cement mortar specimens with preexisting flaws, and performed the direct shear tests on the specimens. In the numerical analysis, the pipe domain model in the two dimensional particle flow code (PFC2D) was modified owing to the high brittleness and low permeability of the cement mortar particles in the numerical model. We also modified the calculation rules of the interaction between the fluid and cement mortar particles, and proposed an improved fluid-solid coupling model which is more suitable for the high brittle cement mortar. Under the action of internal hydraulic pressure, a tensile region existed at the tip of the preexisting flaws of the cement mortar specimen, which can also explain the crack initiation and propagation along the horizontal shear direction during the stage of crack initiation. However, the fissure water pressure was not completely dissipated because of the high brittleness of the cement mortar and the existence of a large number of micro-cracks in the failure area, which finally resulted in a relatively concentrated horizontal compressive stress, and roughly formed a compressive region with a smaller stress along the horizontal shear direction.</p>