Study On The Shear Behavior Of Alluvial-diluvial Filling Subgrade Based On N-value Method

As one of the main types of loose deposits,alluvial and diluvial deposits have the characteristics of loose structure,good roundness of stones and large change of particle size,and their engineering properties are different from those of ordinary loose deposits.Gradation is an important factor affecting to the mechanical properties of loose deposits,the paper takes the alluvial-diluvial deposits in Shaanxi Luonan as the research object,and applies the grading theory N method which evolved from the theory of maximum compactness curve to design the gardation of the alluvial-diluvial roadbed fillers.Combined with laboratory experiments and image processing techniques to analyed the effects of gradetion to the the shear behavior of alluvial-diluvial roadbed fillers.1)The main distribution characteristics and types of loose deposits along luonan highway in shaanxi province were investigated(the starting point is located in the Bao an town of Luonan County,the end point is located in Mu Cha Gou).The results show that the alluvial-diluvial deposits is the main type of loose deposit in the area,and the impact of the alluvial-diluvial deposits as the subgrade filler on the engineering characteristics is analyzed.2)This paper introduces the theory,calculation method,application and advantages and disadvantages of N method,analyzes the feasibility of applying N method to the alluvial and diluvial gradation design,and finally selects 5 gradations to study the influence of gradation on mechanical properties of alluvial and diluvial subgrade,and studies the fractal characteristics of particle size under different n values.3)Based on the basic physical propertie test of the alluvial-diluvial fillers under different gradation,the compaction characteristics of the subgrade fillers under different gradation were studied.The results show that the optimum water content of the alluvial-diluvial subgrade filler decreases with the increase of n value,and the maximum dry density increases first and then decreases with the increase of n value.When n=0.55,The dry density is the largest,and the dry density is larger in the range of 0.5 to 0.55.4)Based on the direct shear test,the stress-strain characteristics,volumetric strain characteristics of the subgrade fillers under different grades were studied and the shear strength,cohesion and internal friction angle were analyzed;The degree of influence of gradation and stone content on shear strength was studied by comparing the stone content control sample with the grading control sample.The results show that the shear strength,cohesion and internal friction angle of the alluvial-diluvial subgrade filler are higher than other gradations when the grading is n=0.55,and the influence of gradation on shear behavior is higher than that of stone content.5)Based on the IPP image processing technology,the mesomorphic morphology of the shear surface is studied for different grades of the alluvial roadbed filler from different shear stages and the aspects of the porosity and the status of the blocks in different shear stages,and the influence of the change of the microstructure of the shear plane on the shear strength was analyzed.On this basis,according to the macroscopic and microscopic results,the optimal gradation interval and key sieve holes are found out,which can provide technical support for the design and construction of alluvial-diluvial subgrade in this area.