深长岩溶隧道掘进过程中掌子面和隧道两侧壁将出现大量裂隙，裂隙在远场应力与水压力的共同作用下将发生扩张膨胀，使得通过裂隙的水流量增大并最终导致隧道发生大范围的突水突泥事件。受水压致裂技术的影响，一般认为裂隙内部水流作用的机制是由水力劈裂和水压扩径造成的。基于断裂力学和流体力学，在平面受力情况下求解含水裂隙发生劈裂破坏和压剪破坏所需的临界水压值，并将两者进行比较后发现，同一裂隙发生水力劈裂破坏所需的临界水压大于其发生压剪破坏所需的临界水压，得出压剪破坏是含水裂隙破坏的主要形式。

A large amount of fissures would occur on tunnel face and tunnel sidewalls during deep long karst tunnel excavation. Due to the interaction of farfield stress and water pressure, the fissures would be expanded and the water flow rate will be increased. As a result, largescale water and mud gushing would occur.The mechanism of internal water effect of fissures is considered to be hydraulic fracture effect and hydrostatic expansion effect popularly. The critical water pressure stresses of tensoshear failure and compressionshear failure in conditions of plane strain are calculated based on fracture mechanics and fluid mechanics. The calculated results show that the critical water pressure stress of tensoshear failure is larger than that of compressionshear failure. The compressionshear failure is the main failure mode of watersaturated fissure.