MICRO-STRUCTURAL CHARACTERISTICS AND SOFTENING MECHANISM OF RED-BED SOFT ROCK UNDER WATER-ROCK INTERACTION CONDITION

Under the long-term of water-rock interaction, the red-bed soft rock shows the characteristics of poor diagenesis, easy weathering and softening, which brings challenges to the soft rock engineering design and construction. Firstly, in order to analyze the mechanical properties of soft rock, uniaxial and triaxial compression tests are carried out on the samples of mudstone taken from the red-bed soft rock area of central Yunnan in the natural state and saturated state. Then the microstructure of soft rock in different water-bearing states is studied by X-ray diffraction. Finally, based on the experimental results, the softening mechanism is discussed. The results show that the mechanical properties of soft rock are obviously deteriorated under the action of hydro-rock, and the compressive strength of rock peak value is significantly reduced, the failure degree of rock mass increases. When the soft rock clay minerals absorb water, the microstructure changes from compact granular structure to loose and porous irregular flake structure. Clay mineral particles such as illite react with water, resulting in uneven stress and a large number of micropores, which causes the destruction of the internal structure of soft rock. The water-softening mechanism of red-bed soft rock includes the dissolution and secondary action of soft rock minerals, the water-absorbing expansion and disintegration of clay minerals, and the failure of intergranular bonding caused by the interaction between soft rock and water.