Study On The Swelling Characteristics Of Mudstones In The Shaximiao Formation And Suining Formation In The Neijiang-Dazhou Section Of The Sichuan Basin

Mudstone itself is a kind of strong hydrophilic minerals,which has the property of rock volume expansion and shrinkage with the change of water content when it encounters water.It has a certain degree of expansion and is one of the important factors that cause many adverse geological disasters.The red-bed soft rock has the problem of swelling after encountering water,and it is also an important topic in the field of geology and geotechnical diseases.Many engineering projects have been plagued by the red-bed soft rock disaster problem,which has a great impact on the construction and later operation of railway subgrade and other related projects.After regional excavation,the subgrade slope will be unstable,the building or structure will bulge after water absorption,and the surface cracks will be generated after water loss and shrinkage,resulting in engineering geological hazards such as slope slip deformation.The red bed soft rock high-speed railway subgrade is prone to arching deformation,and long-term continuous arching deformation will occur regardless of whether the soft rock in the area has expansive or weak expansive or even no expansive subgrade.Therefore,the water absorption and expansion characteristics of red-bed mudstone have always been a concern of scholars and experts,and the current understanding of this problem still needs more research,which not only has certain research significance,but also is a common engineering project.Geological problems provide a certain scientific basis.This paper takes the red-bed Jurassic mudstone in the central Sichuan Basin in the Dazhou-Neijiang section as the research object.The relationship between clay mineral content and expansive physical and mechanical indexes,the relationship between microstructural characteristics and expansive physical and mechanical indexes,and the expansion characteristics of Jurassic mudstone in this area under different water conditions are studied.,to explore the relationship between different water contents and rock expansiveness,and obtain the following main understandings:(1)The clay minerals contained in the rock samples in the study area include illite,kaolinite,chlorite and illite mixed layers.The illite mixed layer has the highest content of clay minerals in the rock samples,while the contents of kaolinite and chlorite are relatively low..In general,the mudstone expansive mineral content in the study area is average,and the content is 14.5-30.6%.The rocks in the study area mainly have flaky grain aggregate units,granular grain units and granular aggregate units.The connection features of the unit body exist in the form of face-to-face contact and edge-to-face contact,as well as in the form of indirect contact and the existence of connected elements between particles.The pore parameters of rock microstructure in the study area are dominated by micropores and nanopores in terms of pore diameter,with few or no mesopores and macropores.(2)The mudstone in the study area belongs to weak expansive rock.At the beginning of the test,the expansion deformation increased sharply during this time period.After a period of time after the start of the test,the expansion deformation still increases rapidly,but the growth rate is slightly slower.Finally,under the condition of sufficient water absorption of the rock,the expansion strain of the rock specimen will reach the final strain value at a certain time point,after which the rock will no longer be expanded and deformed,and the slope of the expansion curve is almost zero.(3)There is a general correlation between the total amount of clay minerals and the overall rock expansiveness.The characteristics of the rock unit body in the study area are mainly granular grain units,the denser,the greater the expansion potential.The expansion potential of flat flaky granular aggregate mudstone will be lower than other mudstones.The connection between the particle aggregates and the single-grain rock unit body will be more swellable in the indirect contact and the contact mode in which the connection unit exists between the particles.The expansiveness of the rock sample is negatively correlated with the fractal dimension of the number of pores.The fractal dimension of the pore structure is large and the expansiveness is weak.(4)The expansiveness of mudstone in the study area decreases with the increase of water content.Under different water content conditions,the expansiveness becomes weaker and weaker with the increase of water content.The rock samples with the highest water content have the strongest expansion,and the rock samples with the water content closer to saturation have the weakest expansion and almost no expansion.