Study Of Engineering Properties And Improvement Of Loessial Sulfate Saline Soil In The Northwest Area

China’s saline soil area is very wide, reaching thirty-six million nine hundred and thirty thousand hectares, the northwest region, such as Xinjiang, Ningxia, Gansu, Qinghai, Inner Mongolia, Shaanxi and other provinces, the saline soil distribution is more concentrated, as many as about 60% of the total area of saline soil. In recent years, with the rapid development of the western region, project constructions such as high grade highways, airports, power transmission towers, oil pipelines, and wind farms in Northwest saline soil area increased sharply, the geotechnical hazards of saline foundation soil, such as the salt expansion of the saline soil, dissolved depression, corrosion are becoming more and more prominent, these hazards impacted on the safety operation of engineering in Northwest China seriously, and caused huge economic losses. Saline soil in Northwest China is mostly distributed in semi arid and semi desert regions in the Yellow River basin, and the distribution of loessial sulfate saline soil is very extensive. Therefore, it is very important to study the engineering characteristic and improvement method of loess-sulfate saline foundation soil in Northwest China.In this paper, the author carried out research on the compaction performance, evaluation of compaction degree, salt-frost heaving accumulation and improved method of northwest loessial sulfate saline soil on the basis of the research projects: "The research on engineering characteristics and disease prevention of coarse grained saline foundation soil of Hexi wind farm"( 2011620111000485) and " Experimental study on the leaching deformation characteristics of Hexi coarse sulphate saline foundation soil"(2013620111000751) which funded by the Gansu Provincial Electric Power Design Institute,and obtained the following main research results.(1) Through laboratory tests, the compaction property and mechanism of loessial sulfate saline soil were studied, and the influence law of different conditions of sodium sulfate on the compaction properties of the loessial sulfate saline soil were obtained, and according to analysing of the microstructure of compacted soil, the mechanism of the influence law were revealed.(2) The influence factors and the law of the compaction performance of loessial sulfate saline soil were studied by artificial imitation of natural loessial sulphate saline soil, on the basis of this, the suggestions of indoor compaction test method and reasonable evaluation of compaction degree of the loessial sulfate saline soil were given.(3) Through indoor simulation tests, the influence law of the dry density, the load, salt content, water content of loessial sulfate saline soil on it’s freezing temperature and salt-frost heaving accumulation was studied under the condition of freezing and thawing cycle. By analyzing the changes of soil structure and dry density after freezing and thawing cycles, the mechanism of salt-frost heaving accumulation was revealed. Based on the test results, the calculation expression of salt-frost deformation which is suitable for the northwest in seasonal frozen loess sulphate saline soil under the condition of repeated freezing and thawing cycle was obtained.(4) Based on laboratory tests, the mechanical behavior and influence factors of the lime improved and lime-cement improved loessial sulfate saline soil were studied. On the basis of this, the strength change rule of lime improved and lime-cement improved loessial sulfate saline soil in the environment of sulphate saline soil was further studied, the stability of lime improved and lime-cement improved loessial sulfate saline soil in the dry-wet cycle was analyzed, and applicable conditions, corresponding technical parameters and the problems to be noted of using lime improved and lime-cement improved loessial sulfate saline soil in Saline soil foundation treatment were given.(5) Based on typical sulphate saline soil foundation of Dunhuang airport, the causes and engineering geological condition of saline soil in Hexi region were analyzed through the field test. By replaced saline soil foundation with different salt content of lime improved and lime-cement improved loessial sulfate saline soil, the bearing capacity, thaw collapsibility and water-resisting property of treated foundation soil were studied, analysis results show that it’s feasible and remarkable for the saline foundation soil treatment method using lime improved and lime-cement improved loessial sulfate saline soil.