In this study, the failure behaviors of debris flows were studied by flume model tests with artificial rainfall and numerical simulations (PFC^3D). Model tests revealed that grain sizes distribution had profound effects on failure mode, and the failure in slope of medium sand started with cracks at crest and took the form of retrogressive toe sliding failure. With the increase of fine particles in soil, the failure mode of the slopes changed to fluidized flow. The discrete element method PFC^3D can overcome the hypothesis of the traditional continuous medium mechanic and consider the simple characteristics of particle. Thus, a numerical simulations model considering liquid‐solid coupled method has been developed to simulate the debris flow. Comparing the experimental results, the numerical simulation result indicated that the failure mode of the failure of medium sand slope was retrogressive toe sliding, and the failure of fine sand slope was fluidized sliding. The simulation result is consistent with the model test and theoretical analysis, and grain sizes distribution caused different failure behavior of granular debris flows. This research should be a guide to explore the theory of debris flow and to improve the prevention and reduction of debris flow.

Coarse-grained soils are used extensively in engineering applications. The breakage of coarse-grained soil particles may have a great effect on their mechanical characteristics. It is important to fully understand the phenomenon of particle breakage and comprehend its effect on engineering properties. The aim of this study was to investigate the process and mechanism of spherical particle breakage under ball–plane contact conditions. Particle contact tests and corresponding simulations based on the discrete element method were performed. The mechanical properties and breaking morphologies of gypsum balls, as well as the significant feature of the existence of a cone core under the contact point, were obtained by the experiments. To enable particle crushing in a numerical simulation, noncrushable elementary particles were bonded together to represent the specimen. The numerical model, which was validated by the unconfined compression test and splitting test, was well fitted with the experiment by applying flat-joint contact. More importantly, the combination of the simulation and experiment demonstrated the role that the cone core plays during particle breakage and revealed the mechanism of the formation of the cone core and its effect on particle breakage.

The stress path and strain-softening effect have a great influence on the strength and deformation of soils. Former test results show that the strength and deformation behaviors under lateral unloading conditions are quite different from those under axial loading conditions, especially for the structured soils. Based on the deformation mechanism of structured soils and incremental linear elasticity theory, a four-parameter stress–strain curve model is proposed by analyzing the stress–strain curves of axial loading and lateral unloading consolidated undrained shear tests. The model contains four important parameters: ultimate residual strength, curve shape factor, initial tangent modulus parameter, and peak strength parameter. All the parameters can be determined easily by conventional triaxial tests. Besides, comparisons of predicted and measured results of shear tests of different sands and soft soils indicate that the newly built model can describe the stress–strain curves with extensive adaptability and high accuracy, and it provides a unified nonlinear elastic model for both strain-hardening and strain-softening soils.