Soil structure and in-site shear test of moraine soil near the Xingkang Bridge over the Daduhe River in Luding

A thick layer of moraine sediments exists on the Kangding bank near the Xingkang Bridge over the Daduhe River in Luding along the Ya′an-Kangding Expressway. In order to study the shear strength characteristics of the moraine soils, six groups of field shear tests were carried out at different locations and depths. Based on the geological survey and test results, the structural characteristics, shear strength and deformation characteristics of the marine soils and their relationship with soil structure were analyzed. The results show that the grain size of the moraine soil is mainly composed of coarse grain and huge granular group. Soil skeleton particles are suspended, mixed, disorderly accumulated and filled with matrix between the skeleton, forming a skeleton suspended compact structure. According to the composition of particles and weathering degree of skeleton, four types of mesoscopic structure can be divided, such as the suspended skeleton dense structure, weathered suspended skeleton dense structure, lenticular sand-gravel enrichment structure and boulder enveloped structure. There are four main shear failure modes of moraine soil: shear fracture zone, enclosing boulder boundary, serrated shear and cutting through weathering framework. Under the shear load, the linear elastic deformation stage with large shear stiffness is obvious, and the yield stage is short. The suspension compaction structure formed by grain skeleton and matrix is the internal controlling factor of strength and deformation characteristics of the marine soil. Under the shear loading, the skeleton particles interact with the cemented matrix, when shear failure occurs, the matrix is crushed and the skeleton is dislocated. The strength and deformation characteristics of the marine soils are affected by the strength of skeleton particles, the distribution of large skeleton particles and the degree of matrix cementation. The shear strength, shear stiffness and dilatancy of the marine soils increase with the increasing skeleton strength and matrix cementation, while the ductility decreases.