Characteristics Of Salinization In Northwest Qaidam Basin And Its Risk Evaluation Of Railway Engineering

Qaidam basin is an important central point connecting Tibet,southern Xinjiang and Hexi corridor,due to the demands of China's western development and comprehensive modernization strategy,the number of regional railway projects has increased greatly.As a typical arid region in northwest China,the northwest qaidam basin is in the forefront of the soil salinization threat due to its special natural environment.As one of the special rock and soil,the saline soil in this area has great harm to the construction and operation of railway engineering.Limited by technical means and cost of treatment,"Be targeted to treat the areas with serious salinization threat?BTAST?"is bound to become the development trend of treatment of saline soil in the future.Therefore,it is urgent and necessary to build an effective salinization risk evaluation system.This master's thesis takes an region in the northwest of the abdomen of qaidam basin as the research area.On the basis of field investigation data,the physicochemical properties of saline soil and the salt driving factors of saline soil in the study area were systematically analyzed.On the basis of fully understanding the source,nature and forming mechanism of the saline soil in the study area,following the evaluation idea of combining qualitative and quantitative,and exploring a set of applications in the northwest qaidam basin the evaluation method of the salinization risk in the area.And combined with the evaluation system,the prevention and control measures of railway engineering under the threat of salinization are studied.The research results are as follows:?1?Based on the previous literature,the main classification thoughts of saline soil were put forward,the corresponding classification principles were summarized,and the conversion relationship between modern saline soil,residual saline soil and latent saline soil is proposed.Based on the preliminary analysis of the hydrological conditions and salt content of saline soil in the study area,the saline soil in the study area is divided into three categories:salinization related to surface water,salinization related to groundwater and primary saline soil.Based on the research requirements of this paper,on the basis of relevant specifications,the classification standard of saline soil in the research area was established by analyzing the data of sample experiments:Non-saline soils?<0.3%?,weakly saline soils?0.3%?1.5%?,moderately saline soils?1.5%?6%?,strongly saline soils?6%?10%?,hypersaline soils?>10%?.?2?Based on different data processing software,starting from three angles of saline soil acidity and alkalinity distribution,saline soil development degree distribution and saline soil engineering type distribution in the study area,three indexes were selected such as PH,salinity and Cl^-/SO₄^2-and spatial distribution characteristics were analyzed based on Fisher's classical mathematical statistical analysis method from the longitudinal and planar scales of the study area.Combined with the natural geological situation of the study area,starting from the perspective of"source-pathway-accumulate"of salt in the saline soil,the driving force factors in the study area are divided into geological factors,hydrological factors and natural factors,and the influence of each factor on soil salinization in the research area is analyzed in detail.?3?Put forward the definition of salinization risk prediction evaluation,put forward the characteristics of salinization risk prediction evaluation system and the selection principle of evaluation system index.Based on the construction characteristics of the evaluation system and the principle of index selection,combined with previous research data,the index system for the prediction and evaluation of salinization risk in the arid region of Northwest China is proposed,it contains the following 11 indicators:dynamic condition,groundwater table,groundwater salinity,vegetation coverage,landforms,steamed drop ratio,land use types,land irrigation type,organic content,salty minerals and soil texture.According to the expert's suggestion and the sample's experimental data,the indexes were classified into the range of salinization risk by qualitative and quantitative methods,and there are four levels,and the higher the risk level,the greater the harm to railway construction:??no or low risk?,??medium risk?,??higher risk?and??highest risk?.?4?Analytic hierarchy process and entropy weight method are selected as the evaluation index subject and objective weight calculation methods,game theory is selected as the subject and objective fusion weight calculation method,and the contact cloud model is selected as the evaluation index membership degree calculation method.These calculation methods construct a"game theory-contact cloud"soil salinization risk prediction evaluation model and apply it to the study area for regional case evaluation,the evaluation results show that:Sloping alluvial floodplain in front of the mountain the risk of soil salinization in the future is between low risk and medium risk,smaller overall.Yadan landform the risk of soil salinization in the future is varies widely,it is currently between medium risk and highest risk,and there is also the possibility of a shift to low risk in the future.Balunma lacustrine basin the risk of soil salinization in the future is highest risk,belongs to"BTAST"primary goal.?5?Based on the results of salinization risk evaluation,this paper puts forward the idea of arrangement of prevention and control measures for railway engineering under the threat of salinization and defines it.Propose the flow of prevention and control measures,and on this basis,put forward the selection of prevention and control measures.Finally,based on the preliminary discussion on the prevention and control measures of the three participating regions in the study area under different risk levels.The results show that:Sloping alluvial floodplain in front of the mountain is not need the arrangement of prevention measures.But if the region is rated high or very high risk,according to the calculation results of the index membership degree in this region,the corresponding preventive measures are proposed mainly for the steamed drop ratio and organic content.Yadan landform is dominated by combining preventive measures and governance measures.The preventive measures are aimed at the steamed drop ratio and surface salty minerals grains.According to the former,the surface is paved with a thin medium-thick gravel or cover with film or shading ball.In view of the latter,which are proposed to be removed by physical measures and covered with film on the planned road sections of the project.For the areas with moderately and strongly saline soils,the treatment measures are mainly to isolate the soil from the contact with water,and at the same time to enhance the soil strength.Balunma lacustrine basin is dominated by combining preventive measures and governance measures.And four preventive measures are put forward in terms of dynamic condition,groundwater salinity,vegetation coverage and landforms:change local landforms,planting salt-tolerant vegetation,digging ditches for drainage,fresh water leaching and grass planting in the later stage of road engineering.In terms of the treatment measures,for the hypersaline soils,it is suggested that the soil can be replaced at the surface for 30 cm first,and then the new soil can be consolidated to increase the strength and water insulation of the soil.