Research On Shear Strength And Microstructure Of Subgrade Soil Under The Effect Of Freeze-thaw Cycles

Seasonal frozen area in China is very widely distributed, accounting for more than50%of the land area. Roads in seasonal frozen area has been deeply influenced by frost damage,such as subgrade frost heave and frothing after thaw. After repeated freezing and thawing,most of the mechanical characteristics of subgrade to an unfavorable direction. The shearstrength decides subgrade bearing capacity, then the study of freeze-thaw impact on it mustbe taken seriously enough. For this reason, this paper studied the shear strength of subgradesoil under the action of freeze-thaw cycles. The change of the microscopic structure is thenature of the macroscopic behavior change, so this article also studied the microstructurechanges of subgrade soil under the action of freeze-thaw cycles, and analyzed its relationshipwith the shear strength.First this paper studied the shear strength of subgrade soil under the action offreeze-thaw cycles, test soil samples were taken from three different regions of Changchunwith plasticity index for10.7,15.97, and21.94, respectively named for the1st soil,2st soil,and3st soil. After the basic physical indicators of the three types of soil samples weremeasured, specimens with the greatest degree of compaction were made in the optimummoisture content. Under the Simulation of closed freeze environment, we applied0to7freeze-thaw cycles to specimens. Triaxial compression test was applied to specimens afterfreezing and thawing, and the variation of shear strength, angle of internal friction andcohesion with the freeze-thaw cycles were acquired. The results showed that the shearstrength of subgrade soil after freeze-thaw cycles will appear first decreased and thenstabilized variation. With different plasticity index, specific variation is not the same, thehigher plasticity is, the later decreased period and stabilization period of shear strengthcome. The overall law of cohesion variation after freeze-thaw cycles is basically consistentwith shear strength, but when the shear strength is basically stable, cohesive force, there isstill some fluctuation. The frictional resistance of high plastic remodeling clay compared to low plasticity clay relatively contribution more to shear strength.The microstructure state is divided into four structural elements, that is particlemorphology, particle arrangement, pore characteristics and particles contact characteristics,then the four structural elements are divided into nine categories (12) microstructureparameters. The same samples of triaxial compression test with0to7times freeze-thawcycles were choosen, and scanning electron microscope (SEM) was used to get themicroscopic structure of the soil sample image. Then we analyzed geometric characteristicsof the microstructure image by digital image processing technology and fractal dimensiontheories to acquire the basic geometric information of micro-structure.By quantitative analysis of the variation of12microstructure parameters afterfreeze-thaw cycles, we found changes in the microstructure parameters have their ownunique laws. Soil of No.2and No.3with larger plastic have a certain similarity in the totalor partial on variation curves of many micro-structural parameters and have a quitedifference with the soil of No.2with lower plastic. The author speculate there is a plasticboundary, soil samples which have larger plastic than it will render a similar micro-structuretendency in the freeze-thaw cycles and have a quite difference with soil samples with lowerplastic. Here the plastic boundary is called "freeze-thaw plastic limit".In order to study the relationship between the macro and micro, the gray relationalmethod was used to evaluate the relevance between cohesion or internal friction angle and12microstructure parameters under the action of multiple freeze-thaw cycles, and then inferthe correlation between shear strength and microstructural parameters. After a study we findthat9microstructure parameters are associated with the shear strength under the action ofmultiple freeze-thaw cycles, that is average particle size, particle size fractal dimension,average abundance, average circularity, directional probability entropy, particle sizedistribution fractal dimension, average pore size, pore size fractal dimension and poredistribution fractal dimension. Considering normal stress conditions, the RBF neuralnetwork was used to establish the relationship model between shear strength andmicrostructure parameters, and simulation and validation,proving the9preferredmicrostructure parameters is stable impact to shear strength, is the underlying reason to determine the shear strength.