Study On The Effect Of Vegetation Restoration On Soil Improvement And Stability Of Mine Dump Slope In Alpine Regions

In this study,the dump site of the No.5 mine field in Jiangchang Mining area,northeast of the Qinghai-Tibet Plateau,was used as the research area.By measuring the physical and chemical properties of the soil in the study area,the variation laws of soil density,water content,p H value and seven nutrient elements(total nitrogen,total phosphorus,total potassium,alkali-hydrolyzed nitrogen,available phosphorus,available potassium and organic matter)in the slope of the dump site planted herbaceous plants(Elymus nutans Griseb.,Poa crymophila Keng)in five different growth periods were analyzed,and the mechanism of herbaceous plants improving the slope soil of the dump site was discussed.The mechanical strength characteristics of root system of herbaceous plants at different growth stages were analyzed by field pull-out test,single root pull-out test and shear test.Through direct shear tests of root-soil complex and undisturbed plain soil samples in the study area,the effect of herbaceous plant roots on shear strength of soil was discussed.The relationship between the physical and mechanical properties of soil and the number of plant roots and the shear strength of root-soil composite was studied by means of multi-factor analysis.In addition,through the finite element numerical simulation method,the deformation characteristics of the dump slope in the study area are simulated under the conditions of five different heaped load heights and seven different soft base thickness.Based on the reduction coefficient method,the stability of the dump slope is evaluated.Through the above research,the following conclusions are drawn:(1)The soil mass of the dump slope is weak alkaline.The soil density,water content and nutrient elements in the soil are significantly affected by the position of the dump slope.The soil density decreases gradually from the top to the bottom of the slope,while the soil moisture content increases gradually.The nutrient elements in the soil flow from the top of the slope to the bottom of the slope as a whole,and the nutrient elements content is relatively concentrated at the bottom of the slope.(2)With the growth of herbaceous plants,the physical and chemical properties of the soil on the slope of the dump have been improved in varying degrees.During the growth period of 1 to 5 years,the soil p H value gradually changes from weak alkalinity to neutrality.The annual increase of nutrient content(total phosphorus,alkali-hydrolyzed nitrogen,available phosphorus,available potassium,organic matter)in soil was 11.63%?26.95%.Artificial planting of herbaceous plants can improve the soil of the dump slope significantly.(3)With the increase of growth period,the pull-out resistance of Elymus nutans Griseb.and Poa crymophila Keng showed exponential function growth(R~2>0.98).The average pull-out resistance of Elymus nutans Griseb.and Poa crymophila Keng were 39.65 N and 34.65 N,respectively.In the same growth period,the pulling resistance of Elymus nutans Griseb.was higher than that of Poa crymophila Keng,and the annual average increase rates of pulling resistance were 11.84%and 11.11%,respectively,with no significant difference.There is an exponential function relationship between the pull-out resistance and the growth index(plant height,ground diameter,root diameter and root number)of herbaceous plants.The pull-out resistance of herbaceous plants increases with the increase of the growth index.(4)In the growth period of 1?5 years,the average tensile strength and shear resistance of Elymus nutans Griseb.were higher than those of Poa crymophila Keng.The average tensile and shear strength of single root of the two herbaceous plants increased with the increase of growth period,showing an exponential or linear function(R~2>0.9).The relationship between single tensile force and single shear force and root diameter shows an exponential function(R~2>0.8).With the increase of root diameter,the single tensile force and single shear force increase gradually.Tensile strength and shear strength of single root have power function relationship with root diameter(R~2>0.8).Tensile strength and shear strength of single root decrease with the increase of root diameter.(5)With the growth of herbaceous plants increasing,the cohesion of root-soil complex showed an increasing trend,with an average annual increase of 26.00%.Root system can play a relatively significant role in enhancing soil cohesion.The shear strength of root-soil composite is significantly higher than that of plain soil.Under the same shear stress,the shear displacement of root-soil composite at the same position of slope is significantly lower than that of plain soil.Water content has a significant effect on the cohesion of plain soil and root-soil composite.At different locations of the dump slope,the cohesive force of the root-soil complex is from large to small:middle of the,slope,top of the slope,bottom of the slope.(6)The numerical simulation results of two-dimensional finite element method show that the plastic deformation of the soft base layer at the lower part of the dump slope is relatively significant.The displacement deformation is prone to occur near the free surface at the bottom of the slope,and the plastic deformation of the dump slope is characterized by a"drum"ridge.The calculation results of safety factor show that when the thickness of soft base is 10 m,the ultimate heaped load height of dump slope is 65 m.There is a significant negative correlation between the slope safety factor and the thickness of the soft basement in the dump.