Research On Optimization Of Seepage Reduction Scheme For Sandy Riverbed Of Yongding River

The total freshwater resources of China are 2.8 trillion cubic meters,which accounting for 6%of the global water resources and ranking the sixth in the world.According to the statistical yearbook of China(2018),the total water used of China in2017 was 604.34 billion cubic meters,with per capita water resources only one quarter of the world’s average.China has become one of the poorest countries of per capita water resources in the world.Water resource is the most basic factor of China’s economic development and plays an extremely important role in production and life.The northern part of China belongs to arid and semi-arid region,which leads to more impoverishment of water resources.With the rapid increase of population,continuous increase of water used for industry and agriculture,water waste,pollution and other reasons,more water shortages have emerged in recent years.The areas with serious overdrawing of groundwater and happened the serious problems mentioned above have even seen river breaks which seriously affects the health of river ecosystem.In order to meet the basic needs of river ecological runoff,maintain the basic forms and functions of the river,and create a habitat environment suitable for the growth of plants and animals,the treatment of river seepage reduction is particularly important under the condition of limited water resources.Taking the ecological restoration of Yongding river in Beijing as the background,this paper selects Lugou bridge and Qujiadian as the typical areas to study the seepage reduction scheme suitable for sandy riverbed.Based on field investigation and data collection,this paper proposes six different soil treatment methods to reduce the seepage.The permeability reduction effect of the material was studied through indoor infiltration test of one-dimensional soil column,and the Image was scanned with the help of ESEM.The microstructure of the scanned Image was analyzed with Image-pro Plus analysis software,Investigate the micro-mechanism of the permeability coefficient change.And finally,through analytic hierarchy process(ahp)to find the optimal seepage reduction scheme which suitable for sandy soil.The main research contents and results are as follows:(1)Through field investigation and laboratory test,the basic physical properties and permeability coefficient of soil samples were studied,so as to understand the current situation of bed matrix in yongding river,and then the river reach where seepage reduction measures should be taken was identified:Lugou Bridge to Lifa Village section.(2)Based on the infiltration test of one-dimensional soil column,six permeability reduction schemes of sand soil with cement,bentonite,clay,lime,notoginseng ash,and28-ash were studied,and the mix ratio with the best economic efficiency and good permeability was selected respectively for each scheme:Cement content 8%,bulk density 1.8g/cm~3;bentonite content 5%,bulk density 1.6g/cm~3;clay content 30%,bulk density 1.8g/cm~3;lime content 20%,bulk density 1.8g/cm~3;3:7 lime-soil content of lime soil is 30%and the bulk density is 1.8g/cm~3;2:8 lime-soil content of erba lime soil is 30%and the bulk density is 1.8g/cm~3.(3)Environmental scanning electron microscope(ESEM)and image-pro Plus Image analysis software were used to study the microscopic mechanism of undisturbed soil and impermeable materials in riverbed,and the above impermeability reduction scheme was verified.It is found that,compared with undisturbed soil,the pore area and average pore diameter decrease gradually with the increase of the content of seepage control material and the increase of bulk density,which is consistent with the change trend of permeability coefficient.The average pore shape coefficient decreases gradually.The fractal dimension of pores also shows a trend of decreasing,and the distribution area of pores gradually decreases.The weaker the seepage transport capacity of porous media is,the smaller the permeability coefficient will be.The characteristics of pore arrangement have no obvious regularity.(4)The analytic hierarchy process is adopted to evaluate the seepage reduction scheme by selecting six influencing factors,namely,the degree of environmental impact,the effect of seepage reduction,the project cost,the construction period,the degree of construction difficulty and the risk after operation,so as to select the optimal seepage reduction scheme suitable for sandy riverbed:the amount of bentonite is 5%and the bulk density is 1.6g/cm~3;the amount of clay is 30%and the bulk density is1.8g/cm~3.