Disintegration of soft rock within water is considered as a fatal factor causing the engineering disasters. It is widely accepted that the disintegration results of soft rock from the variation of the microstructure of soft rock within the water. However

only a few studies were carried out to quantitatively describe soft rock disintegration

based on its meso-mechanism. We conducted experiments on the static disintegration of soft rock and immersion tests of soft rock debris in the laboratory. Besides

the compositions of soft rock at different soaking stages were analyzed by scanning electron microscopic as well. Finally

this study revealed the microscopic evolution of the interface between water and soft rock

according to the obtained results. The disintegration mechanism of the red-layer soft rock was generally caused by the hydration

diffusion and loss of clay particles in the water-rock interface in the mud-filled area of the soft rock fragments. Then the related shale cementation was reduced

resulting in the decrease of cohesion between debris. The cohesion between fragments of soft rock decreased exponentially with time under hydrostatic action. On the basis of the above analysis

the loss of hydrated clay particles in soft rock and the cracking type of model Ⅱ between soft rock fragments were quantitatively identified by using the interface and colloid chemistry and fracture mechanics theory. The interfacial model of soft rock with water disintegration was established

and the disintegration process of soft rock was further demonstrated quantitatively. By comparing the obtained results

the flaking time of soft rock fragments calculated by the above model was similar to that observed in the experiment

which verified the rationality of the model.