Research On The Unloading Mechanical Properties Of Red Bed Sandstone Under Dry-wet Cycle

Through the investigation of the geological background of Chi shui City,it is found that the surface water system of Chishui City is developed,the river network density is large,the river valley is crisscrossed,and the study area is strongly eroded by the river.The frequency of geological disasters in the study area is closely related to rainfall and rainfall frequency.Therefore,in order to study the influence of slow erosion unloading and water-rock interaction on the mechanical properties of the red sandstone of the Cretaceous Jiading Group widely distributed in Chishui City,the dry-wet cycle test was designed in the interior design,and the sandstone after the dry-wet cycle was used for the triaxial confining pressure test.The unloading mechanical properties,degradation mechanism and unloading constitutive model are studied,and the following conclusions are drawn.（1）According to the stress path of red sandstone and the damage deformation characteristics under external load,the stress-strain curve is divided into four stages:crack compaction stage,elastic deformation stage,unloading yield stage and brittle drop stage.With the increase of the number of cycles,the strain and crack closure stress of red sandstone have an increasing trend.The lithology has a tendency to gradually transition from brittleness to plasticity.However,with the increase of initial unloading confining pressure,these two trends can be effectively suppressed.（2）The damage stress,peak stress,total energy,elastic strain energy,dissipation energy,internal friction angle and cohesion of red sandstone decrease with the increase of cycle number,and increase with the increase of initial unloading confining pressure.The growth of each energy parameter value in the unloading yield section is more obvious,indicating that the crack propagation of sandstone in the unloading yield section is faster,and there is intense energy conversion inside.The accumulation of irreversible damage to red sandstone caused by dry-wet cycle accelerates the process of rock failure,resulting in a decrease in the total amount of unloading required for sandstone failure.（3）The influence and sensitivity of dry-wet cycle on the mechanical parameters of sandstone are as follows:unloading peak strength>elastic modulus>cohesion>internal friction angle.（4）The failure mode of red sandstone under dry-wet cycle and unloading conditions is as follows:when 0≤N≤10,the failure of red sandstone is dominated by shear failure,and the tensile cracks caused by unloading are not obvious.When 20≤N≤30.The failure mode of red sandstone with relatively high initial unloading confining pressure（15MPa）is still dominated by shear failure,and only a few tensile cracks appear locally.However,the failure mode of red sandstone with relatively low initial unloading confining pressure（5MPa,10MPa）transits from shear failure to shear-tension failure.When 30<N≤50,the failure mode of red sandstone under three initial unloading confining pressures is shear-tension failure,and the tensile cracks caused by unloading are significantly developed.（5）Red bed sandstone is mainly composed of quartz,clay minerals,feldspar minerals,etc.Feldspar minerals are easy to hydrolyze in water,and the concentration of K⁺Na⁺and Ca²⁺in the soaking solution increases with the increase of soaking times,but the growth rate gradually becomes slow after 30 cycles.The reason is that the increase of salinity of soaking solution and the adhesion of refractory minerals will inhibit the hydrolysis of feldspar cements.The hydrolysis and hydration reactions of feldspar minerals lead to the gradual reduction of cements,which in turn leads to the development of small pores and cracks.The geometric parameters and statistical parameters of pores and fissures in SEM images are calculated by image processing technology.It is found that the number and size of pores and cracks in sandstone gradually increase with the increase of the number of cycles,and their expansion direction gradually becomes disordered.（6）Water reduces the shear strength of sandstone by weakening the strength of sandstone cement and reducing the contact force between sandstone particles.The hydration,decomposition reaction of feldspar cement and the crystallization pressure of insoluble or insoluble minerals lead to the development of pores and cracks in sandstone,which leads to the deterioration of mechanical parameters of red sandstone.（7）Based on continuous damage mechanics and Weibull distribution statistical theory,the damage constitutive model of red sandstone under dry-wet cycle and unloading conditions is established.The sum of unloading damage and dry-wet cycle damage is calculated by equivalent principle.The parameters of Weibull distribution model were determined by peak point method.The mechanical parameters of different stages are obtained in sections,and appropriate parameters are introduced to replace the expressions of Poisson’s ratio and deformation modulus.The data obtained by the test are verified with the theoretical value,and the results show that the theoretical value is basically consistent with the experimental value.