Study On Strength Degradation And Mechanism Of Loessial Subsandy In Qian'an Mud Forest Under Wetting-drying And Freeze-thaw Cycles

Jilin Qian'an mud-Forest National Geopark is located in Qian'an County of Jilin Province,Songyuan City.Its unique mud forest landscape attracts plenty of tourists.Simultaneously,mud forest landscape has high scientific research value due to its rare geological characteristics.However,in recent years,the phenomenon of slope damage and soil erosion in this area has resulted in a large reduction in the landscape of the mud-forest,which needs to be treated urgently.Qian'an area is located in the seasonally frozen region,and the mud forest is affected by repeated freeze-thaw cycles.In the period of summer rainfall concentration,the mud forest is affected by the wetting-drying cycles,resulting in the deterioration of the strength of mud forest soil and frequent destruction of mud forest landscape.Therefore,the study on the strength degradation and mechanism of loessial subsandy under freeze-thaw wetting-drying cycles can provide theoretical basis for the protection project of mud forest landscape.Qian'an mud forest soil belongs to loessisal subsandy,it belongs to saline soil due to its total amount of soluble salt is about 0.52%,and it is in the state of unsaturated soil for a long time.After freeze-thaw and wetting-drying cycles,obvious strength degradation occurs.Based on the above properties of mud forest soil,through the analysis of the test results of direct shear test,matrix suction test and scanning electron microscope test of mud forest soil,this study quantitatively studies the structure change and strength degradation mechanism of mud forest soil under the action of freeze-thaw and wetting-drying cycles.The main conclusions are as follows:(1)The content of silt and fine sand is high,the salt content is heavy,the natural water content is low,and the porosity is large.According to this,the relevant parameters of remolded sample preparation are determined.(2)In first five freeze-thaw cycles,the shear stress of the mud forest soil did not change significantly,but decreased gradually after five cycles;after the first wetting-drying & freeze-thaw cycle,the shear stress of the mud forest soil decreasedsignificantly,and the shear stress of the mud forest soil decreased more significantly under wetting-drying & freeze-thaw cycles and tended to be stable earlier.After freeze-thaw and wetting-drying cycles,the cohesive force of the mud forest soil decreased gradually,and the internal friction angle increased slightly.With the increase of salt content,the shear stress increases first and then decreases.When the salt content is 0.5%,the shear stress is the largest,the cohesive force of mud forest soil decreases,and the internal friction angle increases;with the increase of water content,the shear stress decreases,the cohesive force decreases,and the internal friction angle is almost unchanged.(3)With the increase of freeze-thaw and wetting-drying cycles,the matric suction of mud forest soil decreases gradually,which is similar to the change rule of shear strength.The matric suction and shear strength parameters are put into the semi logarithmic rectangular coordinate system,and the fit is good,which shows that the change rule of shear strength of mud forest soil is consistent with the change rule of matric suction.It is speculated that the change of soil particle pore leads to the strength of mud forest soil strength deterioration.(4)Through SEM test and quantitative analysis by IPP6.0 software,it is confirmed that after freeze-thaw and wetting-drying cycles,the pores of mud forest soil increase,the connection between soil particles decreases,and the pore water pressure decreases,which leads to the degradation of shear strength and matrix suction of mud forest soil.