Macro-meso shear properties of alluvial-diluvial soil-rock mixture (ADSRM) subgrade fillers based on field investigation and N-method

Alluvial-diluvial deposit (ADSRM) is a special soil-rock mixture, which is composed of gravels, boulders, pebbles, and fine-grained soils carried by rivers or temporary floods. Its mechanical properties are quite different from regular soils and rocks. Based on the investigation results of alluvial-diluvial deposits in Luonan, Shaanxi Province, China, and the theory of the N-method, the shear mechanical properties of ADSRM samples with different N-values were analyzed using large-scale direct shear tests and image processing technology (IPP). Investigation results show that alluvial-diluvial deposits in the Luonan area are widely distributed, and most of complex origins and structures. The fractal dimension values of ADSRM samples only have one scale range of particle size and decrease with the increasing n value. The equivalent cohesion, internal friction angle, shear strength, and dilation value of the ADSRM sample with n = 0.55 are higher than those of other samples. The proportion of pore area, pore abundance, total crushing area, the fractal dimension of rock block contour, and equivalent diameter and abundance of rock blocks were used to characterize the pores and rock block morphology on the shear plane. The shape of most pores on the shear plane is oblate, secondly near-equiaxed, and the least long-strip. The crushing area of rock blocks on the shear plane of n = 0.55 sample is the highest, and the shape of most rock blocks is oblate and tends to develop to near-equiaxed. In summary, the macro-meso shear properties and engineering characteristics of the n = 0.55 sample are better than those of other samples designed by the N-method.