Study On Physical And Mechanical Properties Of Red Sandstone Under Dry Wet Cycle

The red sandstone is widely exposed in the Danxia landform area of Chishui,which has abundant rainfall and obvious dry wet cycle.In recent years,the engineering geological problems caused by the interaction between human engineering activities and geological environment are increasing year by year.The research on the physical and mechanical performance of the red rock and rock mass in the dry wet cycle of Chishui can provide the engineering geological problems under the similar local environment Reference basis.Therefore,this paper takes the typical red sandstone in the Danxia landform area of Chishui as the research object,and studies the problems of physical and mechanical properties degradation and rock material damage of red sandstone under the action of dry and wet cycles through indoor dry and wet cycle tests.The main research contents and achievements are as follows:(1)Through early data collection,field investigation and thin section identification of rocks,the research object was comprehensively recognized,and the mineral composition,structure,structure and basic physical and mechanical indexes of rocks were determined,which laid the material foundation for the analysis of degradation effect and mechanism of red sandstone under the dry wet cycle.(2)The physical and mechanical indexes of sandstone samples under the condition of dry and wet cycles are tested.The data results show that the physical and mechanical properties of sandstone under the condition of dry and wet cycles basically have a logarithmic degradation effect,and the degradation effect is the most obvious at the initial stage of dry and wet cycles.With the increase of dry and wet cycles,the degradation amplitude gradually decreases.The degradation coefficient is introduced into the calculation of fitting formula to carry out physical and mechanical properties.Generally,the degradation rate of physical and mechanical indexes in dry state is higher than that in saturated state.(3)The total open porosity is subdivided into large open porosity and small open porosity for pore analysis in the process of dry wet cycle.The total open porosity and large open porosity increase logarithmically with the increase of dry wet cycle times,and the small open porosity fluctuates within a certain range.In the process of dry and wet circulation,there are not only new pores,but also the trend of transition from small pores to large pores,and the trend of further expansion of large pores.Scanning electron microscopy(SEM)was used to test the sandstone samples under the condition of dry and wet cycles.The microstructure of sandstone samples gradually deteriorated with the increase of dry and wet cycles.The interstitial materials around the skeleton particles were gradually destroyed and formed micro cracks and pores,which also verified the analysis of the law of pore degradation.(4)Based on the degradation effect of macro physical and mechanical parameters and micro structure,the degradation mechanism of sandstone in the process of dry wet cycle is analyzed from physical and chemical effects.The swelling and shrinking mechanism of clay minerals in physical action is the main control factor of degradation,and the key mineral is montmorillonite with water absorption expansion and dispersing into paste;the softening mechanism of inter particle friction in physical action also causes the mechanical properties of sandstone to decrease after the clay minerals are pasted or disintegrated,and the damage mechanism is the secondary factor;the dissolution mechanism of chemical action is the possible factor,and it is the material genus There are certain requirements for sex and chemical reaction environment.(5)After the uniaxial compression test and acoustic emission parameters collection of sandstone under the condition of dry and wet cycles,the uniaxial compression process of sandstone under the condition of dry and wet cycles is divided into fracture compaction,elastic deformation and macro failure stages through the stress-strain curve,strain energy characteristics and acoustic emission characteristics,and the critical points are determined quantitatively.(6)Firstly,the damage of sandstone caused by dry wet cycle is defined based on the P-wave velocity,and then the damage variable evolution equation coupled with dry wet cycle and uniaxial compression is established based on the statistical damage theory of rock with Weibull distribution of micro element strength.Based on the Lemaitre strain equivalence theory,a damage constitutive model of sandstone under uniaxial compression in dry wet cycle is established.The model is modified in view of the fact that Lemaitre strain equivalence theory does not consider the strain release in the stage of fracture compaction and the plastic strain in the subsequent process.The effective medium theory has a more reasonable explanation for the pore compression and the skeleton strain.Based on this,the damage constitutive model of fracture compaction stage is established.Finally,according to the deformation and failure characteristics of sandstone in the process of compression under the condition of dry wet cycle,the whole process damage constitutive model is established by the way of subsection.After verification,the model can reasonably reflect the mechanical response of sandstone in the process of uniaxial compression,except for the slight error between the peak strength and the test value.