Study On The Weathering Erosion Monitoring And Characteristics Of Pisha Sandstone

The Pisha sandstone is a special loose interbedded layer,which is composed of sandstone,sand shale and argillaceous sandstone,in the middle and upper reaches of the Yellow River in China.Due to its poor degree of cementation between the particles and weak structural strength,the Pisha sandstone collapses quickly when encountering water.In the case of freezing and thawing,gravity,hydraulic and other external forces,the phenomenon of collapse,diarrhea and so on will happen,so the vegetation in the Pisha sandstone area is seriously degraded and the soil erosion is severe.It is the most serious area of soil erosion in China and the main sand-producing area in the middle reaches of the Yellow River.At present,the research on Pisha sandstone erosion mainly focuses on freezing and thawing,hydraulic power,gravity,etc.,but there are few studies on the weathering erosion caused by environmental factors?temperature,humidity,etc?.On the basis of referencing the results of previous studies,this study takes the unique Pisha sandstone from the Ordos Plateau in Inner Mongolia as the research object.Through a series of experiments and combining theoretical analysis,the microstructural damage evolution process was monitored,and the macroscopic mechanical behavior of the Pisha sandstone under the dry-wet cycle loading were studied.Based on the basic theory of damage mechanics,the constitutive relationships of the Pisha sandstone under the coupling of dry-wet and load were established.It provides theoretical guiding significance for studying the stability of the sandstone slope,predicting the life of the rock mass,preventing the occurrence of disasters and reducing environmental damage.The main contents of this dissertation are as follows:?1?The X-ray diffractometer and ultra-depth three-dimensional observation system were used to determine the mineral composition and microstructure of the Pisah sandstone at different depths.The weathering characteristics of the Pisah sandstone at different depths were studied,which can provide a theoretical basis for the sampling of dry and wet cycles experiments in the later investigation.The experimental results show that the Pisha sandstone at different depths have different degrees of weathering,and the mineral composition contents and the degree of crack development are different.The shallow Pisha sandstone is affected by the external environment,and the mineral components undergo hydrolysis reaction,which enlarges the pores between the particles and deepens the weathering.?2?The field emission environment scanning electron microscope?ESEM?and X-ray diffractometer were used to monitor the weathering process and study the weathering properties of the Pisha sandstone after different wet-dry cycles loading,and the relationships between the mineral composition,microstructure and dry-wet cycle times of the Pisha sandstone were established.Mesoscopic damage evolution mechanism of the Pisha sandstone under dry-wet cycle loading was analyzed.The monitoring results show that with the increase of the times of dry and wet cycles N,the water absorption rate of the Pisha sandstone is gradually reduced,the secondary mineral composition is increased,the pores between the particles are increased,and that the connection between the particles is weakened.The changes of the parameters are more significant in the early stage of the cycle,and the later changes are gradually slow.?3?The uniaxial compression experiments were carried out on the Pisha sandstones after different wet-dry cycles loading by universal testing machine.The failure modes,stress-strain curves,compressive strength and elastic modulus of the Pisha sandstone were analyzed after different wet-dry cycles loading.The effects of the mechanical properties of the Pisha sandstone after dry-wet cycles were studied.It is found that when the dry-wet cycles times N increases,the angle between the main macroscopic failure surface and cross section of the Pisha sandstone is gradually increased.In addition,the damage degree of the fracture surface of the rock sample is gradually severe,accompanied by the generation of parallel secondary cracks.Besides,the compressive strength and elastic modulus are gradually reduced when the dry-wet cycles times N increases.The decay rate is faster in the early stage of the cycle and slows down in the later stage.The function Y=A+Bln?(N+D)can be used to fit the relationships between the dry-wet cycles times N and peak stress or the elastic modulus.It is shown that the dry-wet cycle loading has caused irreversible damage to the Pisha sandstone,and the early damage is more obvious.?4?According to the results of the uniaxial compression experiments of the Pisha sandstone under the dry-wet cycle loading,the improved Duncan-Chang model for rock was used,damage variable based on the Weibull distribution was established,according to the basic theory of damage mechanics and statistics.Finally,two-stage nonlinear constitutive model of the Pisha sandstone were established,and the cumulative damage constitutive model for the Pisha sandstone under the dry-wet cycle is proposed.