Experimental Study On Disintegration Characteristics Of Jurassic Red-Bed Soft Rock In The Central And Eastern Sichuan Basin

The disintegration of red-bed soft rock is one of it’s typical special properties.In railway projects,such as Baocheng Line,Chengkun Line,Chengdu-Chongqing Line,etc.,the strong disintegration characteristics make the weathering and spalling problem of red bed soft rock particularly prominent.Problems such as subgrade mashing and other problems affect driving safety.Therefore,the research on the disintegration characteristics of red-bed soft rock has important practical and theoretical significance for the construction of railways and highways passing through the red-bed area.This paper takes the red-bed soft rocks of the Middle Jurassic Upper Shaximiao Formation（J₂s）and the Jurassic Upper Suining Formation（J₃s）in the central and eastern Sichuan Basin as the research objects,and carried out thin section identification,XRF,XRD,SEM-EDS,indoor water immersion disintegration test and rock disintegration resistance test under dry-wet cycle conditions and other experiments and analysis work,studied the basic characteristics,disintegration characteristics,disintegration characteristics evaluation index,influencing factors of disintegration and its action mechanism of red-bed soft rock,and the disintegration mechanism of red-bed soft rock was summarized.Through the research,the following main results and understandings have been obtained:（1）The disintegration of red-bed soft rock is a periodic progressive failure process.Under the condition of dry-wet cycle,the content of disintegrated particles can be roughly divided into three stages:the rapid decrease of coarse particles,the change of intermediate particles and the continuous increase of fine particles.With the increase of the number of dry-wetting cycles,the content of large particles in the disintegration rapidly decreased,the content of small particles continued to increase,the disintegration rate continued to decrease,and the disintegration gradually stabilized.The disintegration process of red-bed soft rock was mainly concentrated in the first 3 to 5 cycles.（2）A new index of small particle index is proposed to describe the growth of small particle content during rock disintegration.After function fitting,it was found that the disintegration resistance index,disintegration ratio,standard basic entropy and cycle times showed a good exponential negative correlation,and the small particle index showed a good power function positive correlation with the cycle number.The comprehensive comparison shows that the combination of disintegration ratio,standard basic entropy,disintegration resistance index and small particle index is suitable as the evaluation index of the disintegration characteristics of red-bed soft rock,and the fractal dimension is not suitable as an evaluation index for the disintegration characteristics of the red-bed soft rock.（3）The external disturbance has an acceleration effect on the disintegration of soft rock.In engineering practice,a certain external force disturbance can be applied to the red-bed soft rock to make it disintegrate rapidly.The dry-wet cycle effect has a great influence on the disintegration characteristics of red-bed soft rock.The construction of railways and highways in the red-bed area should consider the impact of disintegration and take corresponding preventive measures.The disintegration sequence of red-bed soft rock in different regions is Neijiang area>Suining area>Dazhou area.The main reason for the difference is the difference in clay mineral content and cement content and type,which cannot be eliminated by short-term external disturbance.The influence of sampling depth on the disintegration of soft rock is small,and the existence of external disturbance will further weaken its influence.In general,the higher the content of clay minerals,the stronger the disintegration of soft rock,and the existence of cementitious carbonate minerals has a certain inhibitory effect on the disintegration of soft rock.（4）The disintegration mechanism of red-bed soft rock is summarized into three categories:swelling-dissolution disintegration,extrusion-wedge disintegration and mixed disintegration.The disintegration of strongly disintegrating soft rock is mainly the result of the combined effect of the uneven volume expansion of clay minerals after water absorption and the dissolution and softening of cement.The disintegration of weakly disintegrating soft rock is mainly the result of the combined action of air compressive stress and wedge cracking pressure in the rock mass when flooded.The disintegration of medium-strong disintegrating soft rock is the result of the combined action of the above two mechanisms.