Experimental Study On The Dynamic Mechanical Properties Of Weakly Cemented Red Sandstone Under Sub-zero Temperature

Most of China’s energy supply is still completed by coal.Due to the uneven distribution of reserves and the early development of the central and eastern regions of China,the coal resources in these areas are almost exhausted,thus forming the pattern of coal development in China now,namely,the trend toward the deep and western regions.The special diagenetic environment and sedimentary process have caused a wide range of soft rock formations with low cementation degree in western China.Due to its poorly cementation,weakly-cemented-red sandstone is easy to produce fractures after being disturbed,forming water-conducting channels,and bringing hidden dangers to the construction.At present,artificial freezing method is often used for construction in this special stratum.During excavation of the frozen ground,power machinery or drilling blasting method is often used,no matter which method,frozen rock mass is often under dynamic load.The mechanical response of frozen rock mass under dynamic load is not only related to the efficient,but also affects the safety and stability of the frozen wall.The purpose of this paper is to research the properties of the weakly cemented red sandstones commonly distributed in western China,and especially studies its dynamic mechanical properties under sub-zero temperature environment.From the three aspects of loading rate,temperature effect and lateral restraint as the factors,using the improved SHPB system,we firstly study the dynamic mechanical properties of dry and saturated weakly cemented red sandstone at room temperature,and analyzes the effect of water conditions on its dynamic mechanical properties.Secondly,according to the freezing temperature on site,the test negative temperature gradient is set to 25℃～-25℃,the dynamic compression and tensile properties of weakly cemented red sandstone under negative temperature environment was carried out,and the relationship between loading rate,temperature,constraint state and its dynamic mechanical characteristic parameters were established.Based on the SHPB energy analysis method,this study also analyze and discuss the energy dissipation law of weakly cemented red sandstone under different conditions.Finally,the macro and meso failure characteristics of the rock are analyzed and corresponding explanations are given.Focusing on the dynamic mechanical response of weakly cemented red sandstone under negative temperature environment,this paper has obtained certain conclusions,which are mainly expressed as follows:(1)The physical and mechanical properties of weakly cemented red sandstone by using various testing methods such as XRD,SEM,cryogenic press,and sonic tester were tested and analyzed.The mineral composition of the weakly cemented red sandstone,the contact mode of the particles inside the rock,and the characteristics of P-wave velocity were determined through experiments.The quasi-static test results show that the mechanical characteristics of weak cementation under dry and saturated conditions have significant temperature effects.The compressive strength and tensile strength of dry rocks increase as the temperature decreases;the compressive strength and tensile strength of saturated rocks increase first and then decrease as the temperature continuously decreases,and the inflection points all appear at-15℃.Comparing the quasi-static mechanical strength and elastic modulus of dry and saturated rocks,it is found that the presence of water has a significant effect on the mechanical behavior of weakly cemented rocks.(2)To reveal the influence of the water state on the dynamic mechanical properties of weakly cemented red sandstone,dynamic compression tests of the rock under uniaxial and lateral constraints under two water states were carried out by applying different impact velocities,and the dynamic tensile test was also conducted based on the Brazilian disc test,the test result shows that the dynamic mechanical properties of the rock are closely related to the loading rate(strain rate,loading rate),and every mechanical characteristic parameter shows the strengthening effect of the loading rate.The presence of water has a significant degradation effect on weakly cemented rocks,and the dynamic strength of dry rocks is greater than that of saturated ones under corresponding load patterns.The lateral constraint has a significant effect on the dynamic mechanical properties of rocks.Under lateral constraint,the dynamic compressive strength and deformation of rocks are both significantly increased.(3)Aiming at the problem of dynamic mechanical response of saturated and weakly cemented red sandstone under negative temperature environment,a modified SHPB experimental system was used to perform dynamic tests:including compression tests with different strain rates,and dynamic split tensile tests based on the Brazilian disc test.The effect of negative temperature gradient,strain rate and constraint state on the dynamic mechanical response of saturated and weakly cemented red sandstone were analyzed and discussed.The experiment found that the rock failure in the negative temperature range changes from brittle to plastic with decreasing temperature.Saturated weakly cemented red sandstones are sensitive to strain rate at various temperatures,and their strength increases with increasing strain rate.Considering the temperature effect and strain rate effect of weakly cemented rocks,the two main influencing factors and strength are fitted.It is found that the relationship between dynamic uniaxial compressive strength and strain rate at each temperature can be approximated as:Comparing the dynamic strength of uniaxial and lateral restraints,it is found that the influence factor values of lateral restraints are greater than 1 at each temperature,which reflects the improvement of rock bearing capacity by lateral restraints.(4)The strength and dynamic elastic modulus of saturated weakly cemented rocks show significant temperature effects under dynamic loading.In the range of room temperature to-15℃,the dynamic strength of the rock increases as the temperature decreases.In the temperature range of-15℃to-25℃,the strength decreases as the temperature decreases,and it shows a double polyline in the negative temperature range.The dynamic tensile characteristics of weakly cemented red sandstone are also affected by the loading rate and temperature environment.(5)Based on the SHPB energy algorithm,the energy dissipation of weakly cemented rocks during dynamic failure is studied.The dissipation energy of dry and saturated rocks at room temperature and saturated and weakly cemented red sandstone at negative temperature are loaded with the load.The relationship between rate and temperature was analyzed and discussed.As a result,it was found that the dissipated energy of both dry and saturated samples at room temperature had a significant strain rate effect,and had a linear positive correlation with the strain rate.At low strain rates,the internal particles are the main carrier of mechanical response.When the strain rate increases,the effect of temperature on the internal structure begins to appear.In order to predict the energy dissipation capacity of weakly cemented red sandstone,the relationship between theη_Vand temperature and strain rate can be established.(6)The macro and meso failure characteristics of rocks after being subjected to coupling hydro-thermal treatment was analyzed.First,the rock fragmentation morphology under compressive and tensile loads was studied,and the effect of temperature and loading rate on rock failure was analyzed.The coupling mechanism of hydrothermal-rock is analyzed,and a meso-scale mechanism of temperature strengthening and weakening is proposed.The strengthening is mainly reflected in the shrinkage of the internal mechanism of the rock,and the water-ice phase change solid-fills the pores in the rock.The weakening effect is mainly due to the mismatch of shrinkage coefficients between particles at negative temperature,the frost heave effect caused by water-ice phase transition.For this test,in the temperature range of-5℃to-15℃the strengthening effect plays a leading role,and in the temperature range of-15℃to-25℃,the weakening effect is more significant.(7)Analysis of the SEM image of the rock fracture found that the fracture form of the weakly cemented red sandstone is mainly cement damage,intergranular failure,with a small amount of penetrating particle damage,and the temperature and loading rate have an impact on the rock fracture morphology.