Soft rock is a familiar rock mass in rock engineering. One main character of soft rock is swelling and collapsing after clay mine absorbing water. By deeply researching the granularity fractal characters variation regularity of the slaking material in different slaking stages

a mathematical model to simulate the slaking process of soft rock is established based on fractal concept. Simulation results show that the same developing trend as the experiments results which proves the slaking process of soft rock is typical fractal. According to the results

parameters of the mathematical model can be back analyzed by few experiments data; and the critical value of fractal dimension of soft rock can be calculated. Furthermore the mathematical model can be used to forecast the fractal dimension of swell and collapse in long term.