Abstract:Softening mechanism of soft rocks is the basis of the mechanical characteristics of soft rocks and engineering design. Combined with the Dongjiang to Shenzhen Water Supply Reconstruction Project in Guangdong Province,a series of experiments on saturated soft rocks including red sandstones and mudstones,which are widely distributed in South China,are designed and conducted. By some testing methods,such as scanning electron microscope,polarizing microscope,energy spectrum analysis,X-ray diffraction,physicomechanical experiments of soft rocks,as well as chemical analysis of water sample,the research obtains a series of data,such as microstructures,mineral composition,physicomechanical properties of soft rocks,and chemical composition of water solution. Then,more attention is paid to the softening mechanism of the soft rocks. The research results show that the softening mechanisms of soft rocks include water-absorbing expansion of clay mineral,disintegrating mechanism,ionic exchanging and absorbing action,mineral dissolution and creation of fusible mineral,micro-mechanical mechanism of the interaction between soft rocks and water,and the nonlinear chemical dynamics mechanism. The main causes of softening of soft rocks are water-absorbing expansion of clay mineral,ionic exchanging,and absorbing action and micro-mechanical mechanism under the interaction between soft rocks and water. Mechanism of nonlinear dynamics can explain the dynamic chemical process,and it is one of the important directions of quantitative research on softening of soft rocks in the future.