| With the further development of the "Belt and Road" strategy,the northwest inland region will get a new development opportunity,and a lot of major projects,such as highway,railway,and high-voltage transmission lines will cross typical coarse-grained salted soil site.With the improvement of engineering safety level,a series of new geotechnical engineering problems have triggered in salted soil sites.The most prominent core problem is the subsidence deformation and salt freeze heave deformation of coarse-grained sulfate saline soil foundation.Based on this,we depended on the research project “Research on diseases and prevention & cure of saline soil foundation”(Project No.2013620111000751)and selected a typical coarse-grained salted soil site near Dun Huang Airport.It took an in-situ test and laboratory simulation test of saline soil b y unsaturated soil mechanics and porous media theory.Various research methods and microscopic observation techniques are used to reveal the physical-mechanical properties of this kind of foundation soil.And the deformation characteristics changed by boun dary conditions under annual cyclic variation are systematically analyzed.The cause of formation and bearing capacity characteristics of "salt-rich soil layer" which widely distributed near the surface were established.The calculation formula of soil sub sidence deformation and the evaluation method of soil subsidence are improved,and the process of loss water in the soil is revealed from the microscopic point of view.The most important thing was researched the variation law of soluble salt phase changed in the process of cooling,and it simulated the variation law of soil porosity structure by PFC3 D.Meanwhile,the calculating method of the max salt frost force and the limit salt frost depth of coarse-grained sulfate saline soil are established.The resu lts not only provide a scientific basis for determining the design parameters in such land foundation treatment process but also has a certain theoretical value and s ocial and economic benefits.The main work、innovation and conclusion of the thesis.The main conclusions are as follows:The water content and temperature boundary condition of the shallow layer foundation soil was in periodic fluctuation state under annua l cyclic,and it induced the soluble salt to locate in the shallow layer.It was the reason that the "salt enriched soil layer" which be situated near the surface in 1.2m~2.0m depth range.A majority of "salt enriched soil layer" developed and bounded by t wo soil layers in the first place,and the permeability of the lower layer was almost less than the upper layer,it caused salt solution in the soil was blocked migrating to the deeper under the action of latent erosion.Because of the "salt enriched soil layer" existed,a large number of soluble salt located in the Shallow surface,it increased the chemical force between particles of soil,and accelerated the change of physical-mechanical properties,this change would threat to the stability of engineering foundation in varying degrees.(2)For the breccia and gravel layer,the soil which is enriched with salt is easily formed due to its good gradation and low permeability coefficient compared with coarse sand.According to the field subsidence test of this soil layer,the dissolution filtration deformation coefficient increased from 3% to 8% while the load was raised from 200 kPa to 300 kPa,the results showed that the "salt enrichment layer" in the stratum is an unstable layer,which presented the characteristics of brittle failure.In the process of settlement deformation,the influence of load factors is significant.(3)The main influencing factors of karst deformation of sulfuric acid saline soil are as follows: water content > salt content > load > porosity ratio.The results showed that the permeability coefficient was diminished along with the salt content increased during the sensitive salinity range were 1% ~ 3%.Meanwhile,the permeability coefficient was climbed up and then decline in the wake of the increase in porosity,while it was significantly enhanced in the condition of e>0.48.With the increase of pore ratio,the deformation coefficient of filtration first increased and then declined,and the maximum value corresponds to a range of pore ratio between 0.43 and 0.52.We furnished a simple method for evaluation of dissolubility which was based on the neural network way,and the calculation model of the filtration deformation of sulfuric acid saline soil was also established.(4)Through the simulation experiment of salt frost heave deformation under the condition of single decrease of water content in crude sal ine soil,SPSS software is used to fit the empirical formula in the variation of salt expansion rate with pore ratio,salt content,temperature,and initial water content.The results would provide the theoretical reference for the salt expansion evaluatio n of saline land in the future.Simultaneously,the TEM technology was applied to analyze the changes in microstructure phase state for the samples,which revealed micromechanism of the soil deformation and strength change.(5)The discipline was summarized based on the physical parameters of in-situ soil samples.We measured the salt and frost heaving force of remolded samples in the laboratory under the condition of limiting deformation during the single cooling process.PFC3 D software was applied to simulate the change of soil pore structure in this process.The calculation method was established or the maximum frost heaving force of sulfuric acid saline soil and the boundary depth of salt frost heaving,which would afford the quantitative basis for foundation treatment in the area of coarse-grained sulfuric acid saline soil. |
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