Study on Shear Failure and Crack Propagation Characteristics of Soil-Rock Mixture

Soil–rock mixture is a special geological material between homogeneous soil masses and fractured rock masses. In this study, the shear characteristics, movement and failure characteristics of particles and the evolution law of cracks were studied by direct shear tests and particle flow numerical simulations. The results show that the shear stress-shear displacement curves of the soil–rock mixture can be roughly classified into three stages: elastic stage, plastic stage and strain softening stage, and there was a "jump" phenomenon. The higher the rock content was, the more obvious the phenomenon. The shear strength and its indices of the soil–rock mixture did not increase with increasing rock content, but there was an "optimal rock content". According to the experimental and simulation results, particle breakage can be divided into three types: slight failure, partial failure and complete failure. The crack propagation characteristics can be divided into three stages, and the crack propagation depth increases with increasing shear displacement. It increases with increasing vertical stress and decreases with increasing block rock content.

Rate Effects on Peak and Residual Strengths of Overconsolidated Clay in Ring Shear Tests

Overconsolidated (OC) clay soil is widely distributed in landslide slopes. This soil is often fissured, jointed, contains slickensides, and is prone to sliding. Thus, the shear strength behavior of OC clayey soil is complicated and has received much attention in the literature and in practice in terms of evaluating and predicting landslide stability. However, the behavior of the shear strength of OC clayey soil at different shear rates, as seen in ring shear tests, is still only understood to a limited extent and should be examined further, especially in terms of the residual strength characteristics. In this study, a number of ring shear tests were conducted on kaolin clay at overconsolidation ratios (OCRs) ranging from 1 to 6 under different shear displacement rates in the wide range of 0.02 mm/min to 20.0 mm/min to investigate the shear behavior and rate dependency of the shear strength of OC clay. Variations in the cohesion and friction angles of OC clay under different shear rates were also examined. The results indicated that the rate effects on the peak strength of OC and normally consolidated (NC) clays are opposite at fast shear displacement rates. At the residual state, as with NC clay, the positive rate effect on the residual strength is also exhibited in OC clay, but at a lower magnitude. Regarding the shear strength parameters, the variations in the cohesion and friction angles of OC clay at different shear rates were found to be different at peak and residual states.

Investigation of the Shear Mechanical Behavior of Sandstone with Unloading Normal Stress after Freezing–Thawing Cycles

Freezing–thawing action has a great impact on the physical and mechanical deterioration processes of rock materials in cold areas where environmental changes are very complicated. The direct shear test under unloading normal stress was adopted to investigate the shear mechanical behavior of sandstone samples after a freezing–thawing cycle in this paper. The failure shear displacement (Dsf), the failure normal displacement (Dnf), the shear displacement of unloading (Dsu), and the normal displacement of unloading (Dnu) were analyzed to describe the evolution of shear and normal deformation during the test. The results indicated that the shear displacement increased as the freezing–thawing cycle duration increased in a direct shear test under unloading normal stress. The unloading rate and the number of freezing–thawing cycles affected the failure pattern of the rock sample significantly in both the direct shear test under unloading normal stress and the direct shear test. The three-dimensional inclination angle, the distortion coefficient, and the roughness correlation coefficient of the fracture surface are dependent on the number of freezing–thawing cycles and the unloading rate. The surface average gradient mode of the fracture surface decreased as the freezing–thawing cycle times and unloading rate rose.

Experimental Study on Gasketed Bell and Spigot Joint Behaviour of Lined-Corrugated HDPE Pipe Subjected to Normal Fault

Although structural response of pipelines has been studied in relation to different geohazards, few studies have focused on the behaviour of flexible pipeline joints. In this paper, the response of a bell and spigot joint in a 600 mm diameter lined-corrugated High Density Polyethylene (HDPE) pipe was investigated under the differential ground movements imposed using a facility that simulates a normal fault. Two experiments were undertaken in this facility. In the first experiment, the kinematic responses of the pipe joint (i.e. axial, shear displacements and rotational angles) were measured using Particle Image Velocimetry (PIV) and string potentiometers. Strains were also monitored using optical fibres. In the second experiment, the pipe was sealed and leakage of the joint was captured through monitoring of internal vacuum pressure of the pipe. The results show that axial shortening, rotational angle and shear displacement of the pipe joint increased with increasing fault offsets. The joint began to leak when axial shortening, rotational angle and shear displacement of the pipe joint were 0.65 mm, 0.44° and 3.40 mm, respectively, and the joint clearly lost its functionality when those values were 0.85 mm, 0.58° and 4.32 mm.

Experimental Study on Shear Stress-Displacement, Creep and Shear Slip Instability Behavior of Rock Fractures

Rock mass inherently contains discontinuities, and shear sliding of rock masses along discontinuities is the most common failure mode of rock mass in engineering practice (such as slope, dam and tunnel). In this study, the mechanical and failure properties of rock fractures were examined through direct shear tests, pre-peak tiered cyclic shear tests and multistage loading shear creep tests. The results show that the deformable memory of rocks can be observed from shear stress-shear displacement curves, namely reloading curves continued to increase along the loading curve of the last cycle under the pre-peak tiered cyclic shear loading. The envelopes of shear stress-shear displacement curves were similar to the variation trend of shear stress-shear displacement curves obtained in the direct shear test. Besides, the variation trend of residual shear displacement (RSD) and relative residual shear displacement (RRSD) before slip instability were obtained by the data analyses of rock mass under pre-peak tiered cyclic shear tests. It is found that the change in the friction and sliding state of rock fracture is the main reason for the fluctuation of shear stress-shear displacement curves. The Chen's method was used to process the multistage loading shear creep curves and the three creep stages were analyzed. Moreover, the complex creep models in some previous studies were discussed and the data of shear creep tests were successfully fitted to the classical Burgers model. This study provides a guidance for the study on the shear instability of rock fractures under different shear loading paths.

Influence of Shear Displacement on Fluid Flow and Solute Transport in a 3D Rough Fracture

Fractured rocks in the subsurface are ubiquitous, and the dynamics of mass transfer in fractured rocks plays an important role in understanding the problem in engineering geology and environmental geology. In this study, the influence of shear displacement on fluid flow and solute transport in a 3D rough fracture was investigated. A 3D self-affine rough fracture was generated using the modified successive random addition (SRA) technology, and three sheared fractures with different shear displacements were constructed based on the mechanistic model. A direct numerical model based on the Navier-Stokes equation and the advection-diffusion equation was developed to solve the fluid flow and the solute transport. The results showed that shear displacement had a significant influence not only on the fluid flow but also on the solute transport. A global measure of the spatial variability of the flow velocity showed that the heterogeneity became weaker with decreasing shear displacement. All measured BTCs deviated from the Gaussian profile and exhibited the typical anomalous behaviors, such as the long tail and the early arrival. Although the best-fitted results of the advection-dispersion equation (ADE) model and mobile-immobile model (MIM) were generally consistent with those of the BTCs, the MIM was more capable than the ADE model for characterizing the shear-induced anomalous behavior of the BTCs. It was found that the mass exchange process between the immobile and mobile domains was enhanced in the sheared fractures while the fraction of the advection-dominant mobile domain decreased as the shear displacement increased. Furthermore, the deviation of the Taylor dispersion coefficient from the fitted dispersion coefficient by the ADE model and MIM in the sheared fractures was confirmed due to the influence of shear displacement.

Anti-pulling Force and Displacement Deformation Analysis of the Anchor Pulling System of the New Debris Flow Grille Dam

To avoid waste from a large section space structure layout and deep burial to improve the structural strength and stability, anchor technology is introduced, and combined with the advantages of the supporting wall, a new debris flow grille dam is proposed. Starting from the force process and damage mechanism of the new debris flow grille dam, the computation formula for the anti-pulling force and the total displacement is given. The anti-pulling force includes the sidewall frictional resistance of the anchor pier and the positive pressure of the front end face of the anchor pier. The total displacement includes three parts: the elastic deformation of the cable, the relative shear displacement between the anchor pier and the surrounding soil, and the compression deformation of the soil at the front of the anchor pier. Finally, the influence of soil parameters and anchor pier size on the anti-pulling force and displacement deformation of the anchor-pulling system is analyzed by examples, and the results are compared with the numerical results. The results show that the displacement deformation decreases gradually with increasing elastic modulus of the soil around the anchor pier and increases with increasing Poisson's ratio. The change in elastic modulus mainly affects the relative shear displacement of the anchor pier and soil and the compressive deformation of the soil at the front end of the anchor pier. Poisson's ratio has the greatest influence on the relative shear displacement of the anchor pier and soil. A larger anchor pier is not better; thus, it is wise to choose the economic design dimension. Theoretical and numerical simulation results are consistent, showing a linear growth trend. The results of this paper can further improve the theoretical calculation method of the new debris flow grille dam, thus making it widely used in more debris flow control projects.