| Frozen soil is a special soil that is so sensitive to temperature that its properties are unstable.The west of Jilin Province is located in the seasonal frozen soil area of northeast China.A large number of carbonate saline soil is distributed in this area.Since2003,the frequency and intensity of seismic activity in this area have increased due to the northeast segment of Fuju-Zhaodong fault zone in Songliao Basin.With the development of the western region and the implementation of the strategy of revitalizing the northeast old industrial base,the process of traffic integration in the northeast region is accelerated.The vibration load of vehicles and trains,seismic load,damage to soil caused by freeze-thaw cycle and the particularity of soil itself jointly affect the engineering geological properties of soil in western Jilin.Therefore,the study on the dynamic characteristics of frozen soil under the influence of freeze-thaw cycle,earthquake and other dynamic loads,as well as soil salinization,has important scientific significance for the engineering construction in the saline seasonal frozen soil area.In this paper,the saline soil in Qian'an County,which has typical characteristics of soil salinization,soil dispersion,seasonal freeze-thaw and seismic activity in western Jilin Province,is selected as the research object.Considering the environmental characteristics of seasonal freeze-thaw cycles in the study area,from the perspective of unfrozen water content,the dynamic characteristics of remolded saline soil under the combined effects of freeze-thaw cycles,test temperature,confining pressure,moisture content,initial deviatoric stress ratio and vibration frequency were studied and the mechanism was analyzed.The grey correlation analysis method was used to analyze the correlation between each factor and the backbone curve,dynamic shear modulus and damping ratio of soil.The BP neural network prediction model was established to predict the dynamic shear strain amplitude,dynamic shear modulus and damping ratio of remolded saline soil in western Jilin.The specific contents are as follows:1.The freezing temperature and unfrozen water content in the freezing and thawing process of soil samples under different moisture contents were obtained by temperature measurement and nuclear magnetic resonance.According to the unfrozen water curve in the process of freeze-thaw cycle,the freezing process of saline frozen soil in western Jilin is divided into four stages: micro phase change stage(0 ?-2 °C),severe phase change stage(-2 °C ?-5 °C),transition phase change stage(-5 °C ?-9 °C)and stable convergence stage(below-9 °C).The thawing process is divided into three stages: slow phase change stage(below-5 °C),accelerated phase change stage(-5 °C ? 0 °C)and pseudo overheating stage(0 °C ? 2 °C).Among them,the slight change of temperature at-2 °C will cause great changes in the content of unfrozen water in soil.Therefore,-2 °C is a very unstable change point of remolded saline frozen soil in western Jilin.2.The particle size and structure characteristics of the soil after 0 ? 120 freezethaw cycles were obtained by laser particle size analyzer(LPSA),mercury intrusion porosimetry(MIP)and scanning electron microscopy(SEM).The results show that:(1)20?m is the turning point of soil particle size under freeze-thaw cycles : within five freeze-thaw cycles,particles with particle size less than 20?m decrease,and particles with particle size 20-200?m increase;the change trend of particle size in remolded soil with different moisture content is different,but the turning point is still around 20?m after 5-60 freeze-thaw cycles.After 60 – 120 freeze-thaw cycles,the number of particles with particle size less than 20 ?m increased,and the number of particles with particle size of 20 – 200 ?m decreased,that is,the turning point of particle size under different freeze-thaw cycles was at 20 ?m.(2)Ten freeze-thaw cycles are the turning point of soil pore characteristics and microstructure.The freeze-thaw cycle belongs to the structural adjustment period of the soil within 10 times.The porosity of the soil continues to increase.The changes of pores with different pore sizes are uneven.The small particles or cements in the contact between particles collapse or fall off.The cracks between large particles and aggregates around the pores are in the state of collapse or suspension,and the original structure of the soil is gradually lost.After 10 – 120 freezing-thawing cycles,the porosity of soil decreased gradually,the number of macropores(4 – 40 ?m)decreased,and the number of mesopores(0.4 – 4 ?m)increased.Under the action of water migration in freezingthawing cycles,the pores were easier to penetrate and form fractures.The increase of fractures led to the increase of ultra-large pores(> 40 ?m).The rebound of fine particles and the decrease of porosity increased the contact area between particles,and the overall cementation was more solid.3.The backbone curve,dynamic shear modulus and damping ratio of remolded saline soil under the influence of freeze-thaw cycles,test temperature,confining pressure,moisture content,initial deviatoric stress ratio and vibration frequency were tested by GDS dynamic triaxial test.The results show that:(1)The effect of freeze-thaw cycle on the dynamic characteristics of remolded saline soil is affected by temperature.With the increase of the number of freeze-thaw cycles,the strength of the unfrozen soil decreases,the dynamic shear modulus decreases and the damping ratio increases.Among them,the properties of the unfrozen soil within 10 freeze-thaw cycles decrease slowly,and the properties of the soil have been greatly reduced by 60 and 120 freeze-thaw cycles.Therefore,it is suggested that when the foundation soil undergoes a certain number of freeze-thaw cycles,the soil reaches the equilibrium and stable structure under freeze-thaw conditions,and then the construction is carried out.-2 °C is a special temperature point of saline soil in western Jilin.At this point,the vibration load can adjust the distribution of particles in soil,and then affect the distribution of ice crystals in frozen soil.At-2 °C,the frozen soil without freeze-thaw cycle has the strongest deformation resistance,the largest dynamic shear modulus and the smallest damping ratio;the deformation resistance of frozen soil decreases,the dynamic shear modulus decreases and the damping ratio increases successively after 1,5 and 10 freeze-thaw cycles.Under the action of vibration load,the soil after 60 and 120 freeze-thaw cycles forms uniform ice crystal cementation,and the anti-deformation ability of frozen soil is improved,the dynamic shear modulus increases,and the damping ratio decreases.The development of cracks in 120 frozen soil makes its mechanical properties lower than that of frozen soil under 60 freeze-thaw cycles.For frozen soil at-5 °C and-9 °C,the strength of frozen soil is negatively correlated with its porosity.The greater the porosity of soil,the lower the strength of frozen soil,the smaller the dynamic shear modulus and the greater the damping ratio;the smaller the porosity of soil,the higher the strength of frozen soil,the larger the dynamic shear modulus and the smaller the damping ratio.(2)Confining pressure has a dual effect of strengthening and weakening on the strength of frozen soil in the violent phase transition section.300 k Pa ? 400 k Pa confining pressure has a strengthening effect on the strength of soil.400 k Pa ? 500 k Pa confining pressure has a weakening effect on the strength of soil.The weakening effect of confining pressure on frozen soil disappears with the decrease of temperature.The increase of initial deviatoric stress ratio may cause structural damage of unfrozen soil and reduce its strength,but it will improve the strength of frozen soil.The increase of vibration frequency contributes to the improvement of soil strength,and vibration contributes to the structural adjustment of frozen soil in different states and tends to be uniform.4.Based on the grey correlation analysis,the correlation analysis between the dynamic characteristics of remolded saline soil and its influencing factors is carried out.According to the correlation analysis,it is considered that the moisture content is the key factor to be considered in the design and construction of saline soil in western Jilin.The BP neural network prediction model is established by MATLAB to predict the dynamic shear strain amplitude,dynamic shear modulus and damping ratio of remolded saline soil.The predicted value of the model is the same and close to the real value.It is considered that the established BP neural network can well complete the backbone curve and dynamic parameter prediction of remolded saline soil in western Jilin. |
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