| The significant climatic characteristics of strong evaporation and less rainfall has been observed to take place in Northwest China.Secondary salinization caused by water and salt migration is serious,rock saline soil is widely distributed,and coarse-grained saline soil is a typical representative.Because saline soil has engineering characteristics such as salt expansion and settlement,its mechanical properties are quite different from those of traditional subgrade fillings.At present,there are still some engineering technical and academic problems in the application of saline soil subgrade,such as lack of indepth research on filling engineering characteristics,lack of understanding of mechanical behavior,imperfect design control standards and so on.Therefore,to solve the key control indexes and technical problems of filling highway subgrade with coarse-grained saline soil in saline soil area,taking coarse-grained chlorine saline soil and sulfate saline soil as the research object in this paper,through the methods of literature investigation,theoretical analysis,laboratory test and numerical simulation calculation,the mechanical behavior of coarse-grained saline soil subgrade under different salt content,water content and stress state is studied,and the salt expansion and strength characteristics of coarse-grained saline soil and the mechanical response under load are analyzed,and the filling control standard of coarse-grained saline soil subgrade is put forward to ensure the stability,durability and controllable deformation of coarse-grained saline soil subgrade under the action of temperature,moisture,salinity and load,so as to realize the reasonable and effective utilization of saline soil.The main research work of this dissertation are as follows:(1)Study on salt expansion characteristics of coarse-grained saline soil.Taking temperature,water content and salt content as the main controlling factors,the variation law of salt expansion of coarse-grained saline soil under different test conditions was analyzed through indoor salt expansion test,the prediction model of salt expansion rate was put forward,and its salt swelling property was evaluated.And then control the coarse-grained saline soil as the applicable index of subgrade filling.(2)Study on strength characteristics of coarse-grained saline soil.The evolution rules of resilience modulus(MR)and CBR of coarse-grained saline soil affected by stress,water content and salt content are studied by indoor repeated loading triaxial test and load-bearing ratio(CBR)test.The theoretical model of resilience modulus obtained has higher determination coefficient and the model parameter prediction formula based on physical property index is established.Finally,its road performance is evaluated by double indexes,and a dynamic resilient modulus prediction model based on CBR value is proposed to control the filling standard of coarse-grained saline soil subgrade based on the road performance of fillers.(3)Control index of coarse-grained saline soil as subgrade filling.Based on indoor salt expansion test and mechanical test,salt expansibility and roadability are selected as the two major control standards of saline soil subgrade filling,and the factors affecting the two control standards are analyzed.The index limit value of subgrade failure is determined by the comprehensive failure value of pavement,and then the salt content limit value of coarse-grained saline soil used in each layer of subgrade filling under different humidity is obtained.(4)Design method and treatment Technology of coarse-grained Saline soil subgrade.Based on the test law that the salt swelling and road performance of coarse-grained saline soil are affected by temperature,humidity and salinity,the design principles and treatment requirements in the existing subgrade design codes are supplemented and summarized in this paper.Combined with the research on salt expansion and mechanical properties of existing saline soil,the treatment methods and prevention measures of salt expansion deformation of coarse-grained saline soil subgrade are put forward from the three aspects of"salt control,waterproofing and temperature control".The results show that the salt expansion of coarse-grained chlorine saline soil fluctuates little by temperature,salinity and water,while the salt expansion of coarse-grained sulfate saline soil increases gradually and finally tends to be stable with the decrease of temperature.At the same time,with the increase of water,the salt expansion increased at first and then decreased,and increased with the increase of salt content.The CBR value of coarse-grained saline soil is dependent on the force level,and it is significantly affected by water and salt,and the salt content is more obvious than that of water.The dynamic resilience modulus(M_R)of coarse-grained saline soil is significantly stress-dependent,which is obviously affected by water and salt,and the salinity is more significant than that of water.The obtained NCHRP1-28A model and Ni model have higher determination coefficients.At the same time,the determination coefficient of the prediction model of salt expansion rate and road performance index is basically more than 0.9,which has a good linear relationship with salt content and water content.Finally,the salt content limit index of coarse-grained saline soil for filling in different layers of subgrade is put forward.The use of widely distributed coarse-grained saline soil as subgrade filling can not only effectively solve the problem of lack of high-quality subgrade filling in saline soil areas,but also avoid the destruction of cultivated land and farmland,and provide a reasonable way for the utilization of saline soil.Therefore,the popularization and application of coarse-grained saline soil as road construction materials is of great significance to the construction level of road engineering in Northwest China.The research results of this paper have important reference value for the design and control standard of subgrade in saline soil area. |
