Mechanical Properties Of Compacted Loess Considering The Structure Change And Effect Of Drying And Wetting Cycles

There are many gullies in the Loess Plateau,and the amount of filling works is very large.Loess as filling material is the most widely used for filling engineering.After fully compaction under the optimal moisture content,the strength is significantly improved and the collapsibility has been eliminated.Due to the limitation of construction site conditions,the range of soil moisture content during compaction is large,which leads to the change of soil structure and engineering properties.In large-scale filling projects,the degree of compaction does not meet the design requirements,and the soil is affected by the drying and wetting cycles which caused by rainfall infiltration and evaporation,so there are many diseases in the filling engineering,especially in the shallow filling site.Therefore,it is necessary to study the engineering properties of soil considering various adverse factors.In this dissertation,the mechanical properties of compacted loess are studied considering the influences of structure and drying and wetting cycles.A series of soil water characteristic curve test,triaxial shear test,and dynamic triaxial test were carried out for compacted loess samples with different various sample moisture contents.The results indicated that the compacted loess with various sample moisture contents had obvious structure which made the mechanical properties of compacted loess show obvious differences.The triaxial shear structural parameter m_εrand dynamic structural parameter m_drwere defined to reflect the structural difference caused by different moisture content of loess during compaction.The SEM tests were carried to study the microstructure evolution characteristics of compacted loess under different moisture content and different drying and wetting cycles.Under the same sampling water content,the pore structures of compacted loess were evolved from small and micro pores to large and medium pores with the dry density decreasing.Under the same dry density,with the sampling water content increasing,the pore structures were evolved from large and medium pores to medium and small pores,and then to large and medium pores.Under the action of drying and wetting cycle,some large and medium pores gradually were evolved to medium and small pores.The edges and corners of soil particles gradually decreased.During the drying and wetting cycles,the structural damage and compaction strengthening of compacted loess occurred simultaneously.The critical number of drying and wetting cycles was the boundary between the damage of the whole structure of compacted loess and the compaction strengthening of the whole structure.With the drying and wetting cycles increasing,the triaxial shear strength of compacted loess underwent degradation process,slow enhancement process,and finally tends to be stable.The critical number n_cof drying and wetting cycles was the boundary between the decrease of strength index and the gradual increase of strength index.The piecewise function could better reflect the deterioration process.The worst moisture content was defined to evaluate the strength deterioration degree of compacted loess.It was suggested that the strength of filling soil in long-term operation should be evaluated according to the stable value of triaxial shear strength under drying and wetting cycles.The development of cracks was the main reason for the strength deterioration of compacted loess during the wetting and drying cycles,and the increase of compactness caused by dry shrinkage was the main reason for the strength increasing gradually.The dynamic triaxial tests were carried out to study the dynamic characteristics of compacted loess under drying and wetting cycles.The results showed that the dynamic stress-strain relationship under the effect of drying and wetting cycles accorded with hyperbolic model.The initial dynamic shear modulus G₀and the maximum dynamic shear stressτ_yof compacted loess decreased at first and then increased with the number of drying and wetting cycles increasing.The damping ratioλincreased linearly with the dynamic strainε_dincreasing in the semi logarithmic coordinate.The dynamic strength index decreased first and then increased with the number of wetting and drying cycles increasing,and which was change with the wetting and drying amplitude different.Based on the analysis of triaxial shear stress-strain relationship and dynamic stress-strain relationship of compacted loess under drying and wetting cycles,triaxial shear structural parameters and dynamic structural parameters were defined to establish structural Duncan-Chang model and dynamic structural Hardin-Drnevich model.The comparison results showed that the calculated value of the constitutive model was basically consistent with the experimental value,which verified the rationality and applicability of the structural constitutive model.Based on the constitutive relation and parameters in this dissertation,taken a loess filling project as an example,the calculation and analysis showed that the final deformation of the filling foundation under the influence of drying and wetting cycle was about twice that without considering the influence of drying and wetting cycles.