| Zhenlai County located in Baicheng City is a typical soda-saline soil distribution area in western Jilin Province,China.Due to the climatic conditions,the area belongs to the seasonal frozen soil area,and the periodic freezing and thawing of the pore water in the soil makes the structure of Zhenlai saline soil in a dynamic state.Zhenlai saline soil is a typical structured soil,and the constant change of the structure in the soil will seriously affect the soil strength and deformation.In engineering construction,the excavation of canals and slopes makes the saline soils that have a stable structure in the original location exposed,leading to the stress level changes.Freezing and thawing are one of the driving forces for stress redistribution and soil structure modification,which threatens the stability of slope engineering in the area seriously.Therefore,investigating the structural characteristics of Zhenlai saline soils and the mechanism of structural deterioration under the effect of freeze-thaw cycles,and establishing the structural constitutive model that can reasonably reflect the stress-strain relationship can provide a theoretical basis for the structural design of geotechnical engineering in the distribution area of saline soils,which is conducive to enhancing the stability of the project.At present,the extraction of quantitative parameters for the structural dynamic changes is the key to establish the structural constitutive model of Zhenlai saline soil.Based on this,the compression deformation properties and shear strength properties were analyzed by confining compression test and triaxial shear test,and its structural characteristics were summarized combine with microstructure test;Then,the deterioration mechanism of the structural properties of Zhenlai saline soils under the freeze-thaw cycles was analyzed from the aspects of material composition and microstructure;Based on the structural characteristics of Zhenlai saline soil,the quantitative parameters reflecting the structural changes were proposed,and the empirical equations of the quantitative structural parameters were established;Finally,the structural quantization parameters were introduced into the super-subloading surface models to establish the structural constitutive model of Zhenlai saline soil and were initially verified.The main works and results of this study are as follows:(1)The mechanical properties of Zhenlai saline soil were studied by confining compression and triaxial shear tests,and the structural characteristics were analyzed combined with the characteristics of microstructure.According to the confining compression curve patterns and stress-strain curve patterns in-situ and remodeled samples,it is found that the Zhenlai saline soil is consistent with strong structured soil characteristics.Through field investigation and mercury intrusion porosimetry,the macro and micro structures of in-situ samples and remolded samples are analyzed.It is found that the pores in in-situ samples are more dense,the embedded structure in agglomerates is better,and there is a strong solidification and cementation connection.However,a large number of fractures destroy the solidification and cementation connection in in-situ samples,resulting in that the structure yield strength of in-situ samples is lower,the compression coefficient is higher,and the rebound index is higher than that of the remolded samples under the condition of confined compression.The effect of solidified cemented connection under triaxial shear condition is more significant than that under confining compression condition,so the difference in principal stress?(?1-?3)or between in-situ and remolded samples gradually increases to a great value,and finally shear damage occurs under the action of fracture,and?(?1-?3)or continues to decrease or even becomes negative.Therefore,although Zhenlai saline soil is a strongly structured soil,the existent of a large number of fracture in the soil leads to structural properties significantly different from other strong structural soils such as loess and soft soils,and the fracture network has an obvious negative effect on the structural properties of Zhenlai saline soil.(2)The structural deterioration mechanism under freeze-thaw cycles of Zhenlai saline soil was analyzed in terms of material composition and microstructure.It was found that the structure of Zhenlai saline soil is easily modified by the freeze-thaw cycle due to the high content of hydrophilic clay minerals,clay particles and sodium bicarbonate.And the freeze-thaw cycle gradually weakens the structure of Zhenlai saline soil,including the reduction of strength caused by the structural connection failure and the reduction of the negative effect of fractures.The freeze-thaw cycling mainly expands micro-pores,small-pores and medium-pores into macro-pores,ultra-large pores and fracture,making the pore morphology and spatial distribution of the soil more complex,and destroying the cementation connection in the soil leading to the differentiation of agglomerates and the growth of fracture.The analysis shows that the freeze-thaw cycling changes the compressive-deformation curve and stress-strain curve patterns mainly by weakening the structural connection in the soil.In addition,although the effect of freeze-thaw cycling on the microstructure modification of the in-situ samples was more significant,the strength and deformation parameters of the remodeled samples changed at a faster rate,indicating that the mechanical properties of the remodeled samples were more sensitive to the changes of pore distribution and pore morphology.Coupled with the large agglomerates in the in-situ samples were differentiated,and the fractures were filled and extruded under the freeze-thaw cycling effect,and the negative effect of the fractures is weakened,resulting in an insignificant change of the deformation difference between the in-situ and remolded samples(?Sroi)with the increase of the number of freeze-thaw cycles under the confining compression conditions.Under triaxial shear conditions,the effect of the solified cemented connection in the soil is more obvious,and it is difficult to recover after broken by the freeze-thaw cycles,which leads to the difference of principal stress between the in-situ sample and the remolded sample(?(?1-?3)or)gradually approaching the abscissa axis,and the difference of the principle stress difference between the in-situ and remolded samples(?(?1-?3)os)decreasing with the increase of the number of freeze-thaw cycles.(3)Based on the special structural properties of saline soils,a quantitative parameter-differential structural parameter D,is proposed to describe the structural properties of saline soils in Zhenlai,and its reasonableness and stability are verified.The variation law of differential structural parameter with load was analyzed based on the confining compression and triaxial shear test.The results show that the differential structural parameter can reflect the destructive effect of fracture on soil structural connection and the softening effect of water immersion on saline soil structure with high accuracy and sensitivity.In addition,both the freeze-thaw cycles and the higher consolidation pressure destroy the solidified cemented connection in the soil,resulting in the decrease of structural variability D?1,the increase of structural stability D?2,and the decrease of stress differential structural parameter D?.Both the freeze-thaw cycles and the consolidation pressure have a significant weakening effect on the structural properties of Zhenlai saline soil.In order to convenient the establishment of structural constitutive model and practical engineering application,empirical formulas of differential structural parameters with axial strain,number of freeze-thaw cycles and consolidation pressure were established.(4)An elastic-plastic constitutive model of Zhenlai saline soil considering structure was established.According to the mechanism analysis of differential structural parameters,it is found that the change of differential structural parameters actually reflects the change law of cohesive strength.Based on previous studies,the evolution formula of cohesive strength of Zhenlai saline soil is proposed.Under the framework of the super-subloading surface model,the structural constitutive model of Zhenlai saline soil is established by using cohesion strength to modify the yield function and hardening law,and using the non-associated flow law.(5)The structural constitutive model of Zhenlai saline soil was validated.Comparing the simulation results of the structural modified Cam-clay model and the structural constitutive model of Zhenlai saline soil,it is found that the super-subloading surface model considering the cohesive strength can better fit the stress-strain curve and volume variable results of saline soil because it can reflect the gradual effect of cohesive strength in the initial stage of loading and the weakening effect of fractures on cohesive strength in the later stage of loading,It can better describe the weak hardening of saline soil. |
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