Study On The Mechanism Of Rapid Weathering And Disintegration Of Salt-bearing Sandstone In Heijing,Yunnan

The sandstone is susceptible to weathering and engineering geological hazards under the influence of water.The salt-bearing sandstone of Heijing,which is rich in salt minerals such as mannite,is susceptible to rapid weathering under the action of rainfall and groundwater,resulting in different degrees of cracking and deformation or even disintegration of the rock body.Therefore,it is important to study the weathering mechanism of sandstone for engineering construction and development of sandstone in sandstone areas.In this thesis,a total of 50 specimens of corresponding sizes were prepared in the sandstone field according to the specification requirements for different test specimen sizes,and firstly,the basic physical and mechanical properties of the Heijing saline sandstone were tested to analyze its basic physical and mechanical parameters,mineral composition and microstructure,and secondly,the R group,M group and S group were set according to the three dehumidification temperatures of room temperature,34℃and 105℃.The physical and mechanical properties of the Heijing salt-bearing sandstone during weathering were analyzed by conducting 10 dry-wet cycle tests in three groups,namely,R group,M group and S group,according to the three desiccation temperatures of room temperature,34℃and105℃.(1)The basic physical and mechanical properties of the Heijing saline sandstone were measured as 8.43%natural water absorption,1.4%water content,2.35g/cm~3 natural bulk density,2.23g/cm~3 dry bulk density,33.31MPa uniaxial compressive strength in the natural state,31.59MPa uniaxial compressive strength in the dried state,30.34MPa uniaxial compressive strength in the saturated state.The deformation process of the salt-bearing sandstone in Heijing was classified into four stages:compaction,elastic deformation,cracking and disintegration according to the stress-strain curve of uniaxial compressive strength;the mineral composition of the original rock was mainly quartz,dolomite,sodium feldspar,aragonite,gypsum and calcite by XRD(X-ray powder crystal diffraction)and SEM energy spectrum analysis;the original rock was mainly calcareous calcite by SEM and polarized light microscopy.The rock is mainly composed of calcareous calcite and other cemented materials,which are cemented by contact with quartz and other debris,and the structure of the rock is tight without obvious gaps,with a small number of pores less than 5μm in diameter.(2)The images of surface changes,mass loss rate and conductivity of humidified solution were obtained by dry-wet cycle tests.The test results show that the damage behavior of the weathering of the salt-bearing sandstone in Heijing is mainly surface pulverization and crack expansion,and the weathering effect is more obvious the higher the dehumidification temperature is;the sandstone weathering process is summarized into stable and developing periods according to the change law of crack parameters,and the fractal dimension of the crack increases with the increase of the number of dry-wet cycles,and the change is more obvious the higher the dehumidification temperature is;the mass loss rate of the specimens in the M and R groups is related to the number of dry-wet cycles.The mass loss rate of specimens in M and R groups varied as a polynomial function of 5 times with the number of dry-wet cycles,the mass loss rate of specimens in S group varied as a power function with the number of dry-wet cycles,and the solution conductivity of specimens in all three groups varied as a polynomial function of 5 times with the number of dry-wet cycles;S group disintegrated and broke up during the humidification process in the 7th cycle,and the natural density of disintegrated material was 1.18g/cm3,which was 0.5 times of the initial density of specimens.(3)The changes of uniaxial compressive strength and cutline modulus of the weathering process of salt-bearing sandstone in Heijing were obtained by uniaxial compressive test,and it was found that the uniaxial compressive strength was negatively correlated with the number of dry-wet cycles as a whole,and the cutline modulus was positively correlated with the number of dry-wet cycles,and three significant cases of the effects of dry-wet cycles and temperature on the strength deterioration of sandstone were summarized according to the change process of strength change curve:strength deterioration due to dry-wet cycles,strength deterioration due to 34°C thermal cycles,and strength deterioration due to 105°C thermal cycles,respectively.The strength deterioration caused by dry-wet cycling,the strength deterioration caused by 34℃thermal cycling,and the strength deterioration caused by 105℃thermal cycling were summarized.Based on the uniaxial compressive strength change law,three proportional relationships were proposed for the strength change rate,strength deterioration ratio,and weathering rate to characterize the strength change magnitude,the effect of temperature and dry-wet cycling on the rock during weathering,and the weathering rate of the salt-bearing sandstone in Heijing,respectively;the results showed that the strength change rate of uniaxial compressive strength of the salt-bearing sandstone in Heijing was positively correlated with the number of dry-wet cycles,and the weathering rate was negatively correlated with the number of dry-wet cycles overall.The results show that the rate of strength change of uniaxial compressive strength of salt-bearing sandstone in Heijing is positively correlated with the number of dry-wet cycles,and the weathering rate is negatively correlated with the number of dry-wet cycles.