| In the southwest of China,especially in Sichuan and Yunnan provinces,the Upper Triassic Xujiahe Formation is widely distributed,and this set of strata is mainly thin-bedded mudstone and siltstone interbedded with thick-bedded-mega-thick-bedded sandstone.The study area of this paper is located in Zhouba,Leshan,and the coal formation of the Xujiahe Group in this area is characterized by soft and hard sandstone interbeds,monoclinic slopes,multi-layered inclined mining areas,and the rise and fall of reservoir water level.The rise and fall of the reservoir water level has a great influence on the engineering and mechanical properties of the bedrock of the slope of the lower subgrade in this area,so it is extremely important to study the change law of the soft rocks such as siltstone and mudstone in this area under the action of water.In this paper,the mineral content,mechanical property parameters,macroscopic deterioration mechanism after water exposure,and microstructural change characteristics of typical siltstone and mudstone of the Sujiahe Formation are studied through field sampling,indoor data collection and collation,indoor tests,microscopic experiments,and image analysis,and the following main results and understandings are obtained.(1)Qualitative and quantitative analysis of the mineral composition of the sandstone,siltstone and mudstone of the Sujiahe Formation was carried out by four analytical methods:polarized light microscopy,X-ray diffraction,portable fluorescence spectroscopy and scanning electron microscopy energy spectrum analysis.The sandstone is a feldspathic sandstone,with quartz and feldspar as the main mineral components and less clay minerals;the siltstone contains quartz,feldspar and other minerals,and generally contains a certain amount of clay minerals;the clay minerals in the mudstone account for more than half of all mineral contents.The results of microstructure analysis show that the unit structure type in siltstone is lamellar flocculation structure and lamellar accumulation structure with obvious orientation,and the cementation type is mud cementation,and the pore size is concentrated in 0～10μm;the mudstone as a whole is lamellar structure,but its orientation is not obvious,and the pores are mostly tiny pores,concentrated in 0～5μm.(2)Through the evaluation index of physical and mechanical properties,Richter’s hardness,the interconversion between mechanical indexes was carried out to obtain its engineering mechanical parameters.The uniaxial saturation compressive strength of the sandstone(21-ZB-01)was calculated to be 24.50 MPa,and the uniaxial saturation compressive strength of the coal-bearing mudstone(21-ZB-03-02)was calculated to be 9.33 MPa,by testing the Richter’s hardness of the samples in the study area.(3)By conducting indoor fluorescence drip tests on siltstone and mudstone,we studied the macroscopic changes of the samples after the samples were exposed to water,including the diffusion of fluorescence solution on the surface and sides of the samples,the diffusion rate of the solution on its surface and the development pattern of fractures after exposure to water,and dynamically demonstrated the process and pattern of disintegration of rock samples.(4)In order to eliminate the disintegration of the soft rocks of the Xujiahe Formation by direct immersion in water,the siltstone and mudstone samples were buried under pressure in wet sand,and the water content of the soft rocks was slowly changed by adjusting the water content of the surrounding sand to obtain in-situ samples with different water contents,while freeze-drying was used to prevent their structural damage,and the microstructural morphology was observed by scanning electron microscopy,and the internal pore structure was statistically observed by Image Pro Plus software to analyze its change pattern and discuss with its macroscopic change pattern. |
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