Experimental Study On Deformation And Damage Of Loess Slope Under Excavation And Unloading Effect

The geological environment of Loess Plateau is fragile,the engineering geological conditions are complex,and geological disasters are frequent,especially the loess landslide triggered by engineering excavation has become a common geological disaster in the region,and its evolution process has certain lag and concealment,which increases the difficulty of early identification and forecast and early warning of disasters,and brings serious threat to the safety of local people and properties.This paper focuses on the hot issue of excavation footing induced loess landslides,aiming to explore the intrinsic causal mechanism of excavation unloading type loess landslides in the region.Taking the typical excavation and unloading landslides in Yan’an and Xi’an,Shaanxi Province as the prototype objects,we investigate the relationship between the macro-mechanical properties and microstructure of landslides under the unloading effect by using indoor triaxial tests,SEM tests,NMR tests and numerical simulations,and investigate the multi-scale damage deformation and mechanical response mechanism of loess slopes under the unloading effect by combining site investigation and sampling.The main research results are as follows(1)Through the triaxial consolidation and drainage test of loess,the effects of water content,unloading rate ratio and surrounding pressure on the macroscopic mechanical properties of loess were analyzed,the damage forms of loess under different stress paths were studied,and the stress-strain changes of in-situ loess and remodeled loess under different unloading rate ratios were compared and revealed.The stress-strain curves of the remodeled soil samples with low water content were found to be strain-softening type,and the degree of strain-softening was positively correlated with the unloading rate ratio,while the remodeled soil samples with high water content were of continuous hardening type,and there was no peak value.Combining the stress-strain curves and the morphology of the soil samples after the damage,the damage of loess under the test conditions is mainly divided into three types: overall damage,central bulge and lateral bulge.(2)The relationship between the microstructure indexes of loess and water content and unloading rate ratio was analyzed qualitatively and quantitatively by using scanning electron microscope test,nuclear magnetic resonance test and image analysis software,and the results showed that at low water content,with the increase of unloading rate ratio,the percentage of microporosity of soil gradually increased compared with that before damage,while the percentage of medium and large porosity decreased;at high water content,with the increase of unloading rate ratio,the percentage of both microporosity and medium and large porosity increased.(3)Based on the numerical simulation test,the influence law of different unloading rate ratios on the macroscopic shear damage of loess is derived,and the damage deformation mechanism of excavation unloading type loess landslide is discussed by combining the triaxial test and numerical simulation results,and the damage deformation characteristics of loess slope under unloading effect are revealed,and it is found that the strain hardening trend of soil sample is more obvious with the increase of unloading rate ratio.(4)In general,the damage and deformation of excavation and unloading loess landslide is mainly due to the excavation of the foot of the slope,which leads to the local destruction of the slope and extrusion in the direction of the hollow surface,and with the continuous excavation of the foot of the slope,the local stress concentration appears and develops to the middle and rear of the slope.