Soft rock's softening in water is its important failure feature. It is also a main cause of deformation failure of slope and deep excavation in red-bed soft rock. There is one important reason accounting for the failure of red-bed soft rock

a large number of initial fissures exist in rock mass

such that water is able to permeate into the rock mass and activate physical and chemical reactions which lead to propagation of cracks as well as decrease of strength of soft rock. To investigate crack propagation of soft rock with fissures

two batches of soft rock samples with different initial fissures are dedicately prepared for the examination through lab tests into their softening and cracking in water. Hereinto

one batch of samples are intact rock mass without initial fissures

the other batch of samples contains only one initial crack. Crack propagation laws for red-bed soft rock was revealed through studying on crack propagation of two groups of samples mentioned above

including: ① Cracks of samples without initial cracks propagate from outside to inside

and samples with initial cracks has its cracks propagating along the original initial cracks

so cracks of samples with one initial crack propagate more quickly than the samples without initial crack; ② It was found that from micrographs that crack propagation of soft rock generally bypasses the quartz crystals. Based on the experimental study

the micromechanical mechanism of cracks propagation of soft rock in water was elaborated and its failure process was analyzed as well.