Study On Permeability Of Modified Loess Under Dry And Wet Cycle Conditions

Loess is widely distributed in China,and it has strong water sensitivity and wet sinkability,and the soil structure will be wet sink after water infiltration,which will cause harm to engineering buildings.At present,the engineering construction in loess area is increasing,and it is very necessary to solve the engineering disaster caused by wet sink of loess.Seepage is one of the important causes of wet subsidence in loess,which can cause settlement and landslide disasters in building foundations and slopes in loess areas.Therefore,this paper studies the permeability of modified loess to provide a reference basis for engineering construction in loess areas.This paper takes Lanzhou loess as the research object,and adopts commercial sodium bentonite to modify Lanzhou loess,and then conducts dry and wet cycle test,GDS permeability test and NMR test in the room to analyze the effect of dry and wet cycle and bentonite dosing ratio on the permeability coefficient and internal pore structure of modified loess.Finally,a numerical model with different depths of treatment on the slope surface is established to study the water infiltration on the slope surface under different types of rainfall.The main research works are as follows:（1）The cracks on the surface of modified loess after dry and wet cycles were processed and analyzed by using the image processing software PCAS.The variation of surface fracture pattern with the number of wet and dry cycles and bentonite dosing ratio was investigated,and the fracture rate after wet and dry cycles was fitted by quadratic function.（2）Using GDS permeameter to conduct permeation tests on the specimens under each working condition,the effects of the number of dry and wet cycles,bentonite doping ratio,percolation time,surrounding pressure and head difference on the permeation coefficient were investigated.Among them,with the increase of bentonite dosing ratio and surrounding pressure,the permeability coefficient gradually decreases,and the influence is larger when the dosing ratio and surrounding pressure are smaller;with the continuation of percolation time,the permeability coefficient shows the trend of"fast decreasing-slowly decreasing-reaching stability";with the increase of the number of wet and dry cycles,the permeability coefficient gradually increases;the head difference has a greater influence on the permeability coefficient of remodeled The influence of head difference on the permeability coefficient of remodeled loess is large.（3）The pore structure of modified loess was studied by NMR technique with the number of wet and dry cycles and the variation of bentonite dosing ratio.The T₂spectra showed a bimodal structure under different conditions,with the increase of bentonite doping,the number of small pores was increasing and the total area of T₂spectra was decreasing,with the increase of the number of dry and wet cycles,the results were opposite;the mathematical parameters contained in the T₂spectra were analyzed by mathematical statistics,and the permeability was calculated based on each characteristic parameter to reveal the influence of dry and wet cycle conditions and the bentonite doping ratio on the permeability from a microscopic perspective.The mechanism of the influence of bentonite dosing ratio on the permeability of modified loess was revealed from a microscopic perspective.（4）Geo-Studio was used to analyze the infiltration of loess slopes under different working conditions,and to investigate the changes of unit flow rate and volume water content in the soil under different rainfall types and different slope treatment depths.In the process of rainfall infiltration,the intensity of rainfall has a great influence on infiltration,among which the forward and uniform rainfall types have a greater rainfall intensity at the beginning of rainfall and the unit flow rate of infiltration is larger,followed by the single-peak type and the smallest post-peak type;with the increase of rainfall duration,the gap between the unit flow rate of infiltration of the four rainfall types shows a trend of first increasing and then decreasing;with the increase of slope surface treatment depth,the depth of rainfall infiltration gradually decreases When the surface treatment depth of the slope reaches 100cm,the depth of rainfall infiltration can be reduced to half of that of the remodeled loess,indicating that the surface treatment of the loess slope with bentonite can achieve better anti-seepage effect.