Triaxial test study on the mechanical properties of TBM muck with consideration of its soil structure

TBM muck, a typical needle- and flake-shaped soil-rock mixture, develops a distinctive soil structure during stockpiling. Its deformation and strength characteristics differ markedly from those of other soil-rock mixtures. In this study, the entire dumping and accumulation process of TBM muck was simulated through physical model tests to investigate the resulting soil structure. Using GDS large-scale triaxial tests, we systematically analyzed the effects of varying rock block proportions and orientations on the mechanical behavior and failure characteristics of TBM muck. The results indicate that: (1) During the dumping and accumulation, flake-shaped rock blocks are prone to sliding, resulting in significant particle size segregation and the formation of a soil structure characterized by oriented rock block alignment and a well-graded distribution. (2) The relative density of TBM muck initially increases and then decreases with increasing rock block proportion, indicating the existence of an optimal rock block proportion. (3) As the rock block proportion increases, the internal structure of TBM muck evolves, and its failure mode transitions from a single shear band to an "X-shaped" conjugate shear band failure. Consequently, the shear strength initially increases, reaches a distinct peak, and then decreases. (4) The needle- and flake-shaped rock blocks in TBM muck primarily exhibit face-to-face contact, with limited interlocking and nesting between particles. Under compression, sliding friction predominates, resulting in a lower overall strength and higher compressive deformation. (5) With increasing confining pressure, the oriented arrangement of rock blocks suppresses the interlocking and nesting effects between particles, thereby limiting strength development and progressively amplifying the material’s inherent structural weaknesses.