Numerical Shear Tests Research Articles

DEM simulation of effects of particle size distribution on meso-mechanical properties of slip zone soil

Particle Size Distribution (PSD) exerts a substantial influence on the mechanical properties of geological materials such as rocks and soils, which can be viewed at a microscale as an assembly of discrete particles. An exploration into the effects of particle gradation on the properties of these materials provides valuable insights into their nature. In the study, the Discrete Element Method (DEM) was used to conduct numerical shear tests on eight distinct groups of slip zone soil, each characterized by a different particle gradation. The aim was to examine the meso-mechanical properties and shear evolution laws of slip zone soil numerical samples with both optimal and sub-optimal PSDs. Findings underscore the pivotal role that PSD plays in various aspects, including dilatancy, the evolution of the displacement field, the network of contact force chains, the principal stress, and the distribution of normal and tangential contact forces within the slip zone soil. It was observed that the network of contact force chains in the numerical samples with an optimal PSD was more complex than in those samples with a sub-optimal PSD. Additionally, the distribution of principal stresses before and after shear was more uniformly balanced. This particle size-based study offers significant reference value for future investigations into the impact of PSD on the macroscopic and meso-mechanical properties of slip zone soil. By augmenting this knowledge, a more comprehensive understanding of the fundamental behavior of these materials can be attained, leading to improved prediction and management of geological risks.

Predicting peak shear strength of rock fractures using tree-based models and convolutional neural network

It is of significance to estimate peak shear strength (PSS) of rock fractures in engineering practice, but the existing PSS criteria may not fully represent the 3D characteristics of fracture surfaces. In this study, data-driven PSS criteria of rock fractures are developed to comprehensively consider effects of surface roughness features. An effective method to create a large high-quality dataset is first proposed by combining particle-based discrete element method for numerical shear tests with diamond-square algorithm for generating random fracture surfaces. Five tree-based models are trained based on the dataset containing normal stress, rock mechanical properties and 16 explicit roughness features, while convolutional neural network is trained based on the mixed dataset containing normal stress, rock mechanical properties and relative fracture elevation. The prediction accuracy of the trained data-driven PSS criteria is examined using additional experimental data, and the results show that the tree-based models of categorical boosting and light gradient boosting machine and convolutional neural network can provide reliable prediction of PSS of rock fractures. The dominant features are ranked according to their contributions to the tree-based PSS criteria. The data-driven PSS criteria of rock fractures would have a great potential in engineering application with limited access to experimental data.

Numerical Study of the Shear Behavior of Ultra-High-Performance Concrete Beams


 
 
 In the last decades, experimental and numerical studies on steel fiber-reinforced concrete have shown that providing suitable fiber content and distribution in the matrix can improve the post-cracking strength of concrete. Although there is a large database of experimental and numerical shear tests of steel fiber-reinforced concrete beams, there are not profuse test data and numerical models available for steel fiber-reinforced concrete beams with traditional transverse shear reinforcement (stirrups). In this work, the shear behavior of ultra-high-performance fiber-reinforced concrete beams is analyzed using the plastic damaged model developed in Abaqus. For this purpose, a comparative study is previously carried out by means of an experimental test available in the literature to validate the finite element model with conventional rebars. Then, the shear behavior of different ultra-high-performance concretes without fiber reinforcement, with the addition of 13 mm steel straight fibers, with 30 mm steel hooked-end fibers, and a mixture in 50% of 13 and 30 mm steel fibers are analyzed.
 
 

Experimental and numerical study on the mechanical properties of the rock-concrete contact surface for consideration of ground–liner interaction in rock tunnelling

The mechanical properties of the rock-concrete contact surface are vital in the study of the ground-liner interaction for the tunnels embedded in rock mass. This paper mainly investigates the shear mechanical behaviour and failure mechanisms of the ground-tunnel interface with experimental and numerical approaches. Firstly, a Brazilian test was conducted to make a rough surface that represents the rock-concrete contact surface in the rock tunnelling. The split rough surface was then scanned with a 3 D scanning technique with which the digital terrain map (DTM) model of the split surface was acquired. In conjunction of the DTM model with a 3 D printing technique, synthetical samples that represent the rock-concrete contact surface were produced. These samples were put in shear test in which the shear behaviour and failure mechanism were investigated, and the shear strength criterion of the rock-concrete contact surface was presented. Ultimately, based on the discrete element method (DEM) code 3DEC, the numerical shear tests of the contact surface were performed. A sophisticated, nonlinear, continuously yielding (C-Y) model was adopted to describe the complex shear behaviour of the contact surface during the numerical shear test. The reasonable match of laboratory data with the numerical test results demonstrated the applicability of the DEM approach in reflecting the shear mechanical behaviour of the rock-concrete contact surface of liner.

Shear strength model of joints based on Gaussian smoothing method and macro-micro roughness

The accurate evaluation on the shear strength of rough rock joints has always been the pursuit of rock mechanics workers. In this paper, based on the MATLAB image processing technology, the joint surface contour was firstly numerically reconstructed. Then, the Gaussian smoothing method was used to separate the macroscopic undulation characteristics from the microscopic roughness characteristics in the contour shape, and the morphological characteristic parameters were calculated respectively. Subsequently, a new shear strength model of rock joints containing both macroscopic and microscopic characteristic parameters was established by introducing the average undulation angle of topography parameters and the tangent angle corresponding to the root mean square of slope. Meanwhile, the numerical shear tests were designed and implemented in particle flow software PFC, and the numerical shear strength was employed to verify the rationality of the established model by comparing it with the predicted shear strength. The comparative results indicate that the proposed characterization method of joint roughness is capable of well considering the influences of macroscopic and microscopic characteristics of joints, and the predicted shear strength shows a great agreement with the numerical shear strength. The research can provide a good reference for the quantitative analysis of joint roughness and the estimation of joint shear strength.

Scale effect of shear mechanical properties of non-penetrating horizontal rock-like joints

From the perspective of macroscopic scale, the majority of natural rock mass should be categorized as non-penetrating jointed rock mass. The existing researches in the field of scale effect of joint properties were mainly implemented on penetrating joints, which contradicts engineering practice, and is of high possibility to make the strength estimation of large natural jointed rock mass inaccurate, leading to serious loss of life and property. In response to such case, a series of numerical calculations of direct shear test on non-penetrating horizontal rock-like joints with different scales were carried out by PFC in this paper, to investigate the scale effect of shear mechanical properties of non-penetrating horizontal rock-like joints. First, the model microparameters were calibrated by three physical experiments to guarantee the precise reproduction of the mechanical performances of target rock and joint. Next, the particle parameters (average particle size dave and radius ratio μ) of model were changed, the effect of particle size on joint strength was studied by direct shear calculation, and the determining method for the values of dave and μ was suggested. Then, based on two distribution forms of non-penetrating horizontal rock-like joint (type I and type II joints), the numerical shear tests were conducted on jointed rock models with different persistence rations and model scales, and the variations of shear stress displacement curve and strength characteristics were analyzed. The results indicate: The lower the persistence ration λ of the joint, the more obvious the negative scale effect of joint shear strength. Besides, the scale effect of shear strength gradually decreases when λ > 0.5 for type I joints while λ > 0.8 for type II joints and the scale effects of joint strength parameters only emerge in the case of λ < 0.2.

Experimental and numerical modeling of the shear behavior of filled rough joints

Abstract The shear behavior and mechanisms of clean and filled rough joints under direct shear tests were comparatively studied using experimental and numerical approaches. Concrete was used for casting laboratory specimens with different roughness joint surface profiles, and clay was used as an infill material. In addition, a modified shear box genesis method was employed to reproduce the shear behavior of clean and clay-filled joints under different normal stress by two dimensional Particle Flow Code (PFC2D). The results show that as the normal stress increases, the shear stress increases, and the normal displacement decreases. Due to the presence of clay, the shear strength and the normal displacement of the clean joint are larger than those of the clay-filled joints, and the influence of the joint roughness on the shear stress is significantly reduced. The mechanism of the joint roughness on the shear stress and shear dilatancy mode during the shear process becomes more complicated. Furthermore, according to the bond fracture mode in the numerical shear tests, it can be concluded that the cracks of the clean joints are mainly caused by parallel-bonded shear cracks and parallel-bonded tensile cracks at an asperity, but for filled joints, the cracks are mainly caused by smooth-jointed shear cracks between block and filling, and parallel-bonded tensile cracks between the filling particles.

Numerical Shear Tests on the Scale Effect of Rock Joints under CNL and CND Conditions

The scale effect of rock joint shear behavior is an important subject in the field of rock mechanics. There is yet a lack of consensus regarding whether the shear strength of rock joints increases, decreases, or remains unchanged as the joint size increases. To explore this issue, a series of repeated and enlarged numerical joint models were established in this study using the particle flow code (PFC2D). The microparameters were calibrated by uniaxial compression tests and shear tests on the concrete material under the constant normal loading (CNL) condition. Three different normal stresses were adopted in numerical shear tests with joint specimen lengths ranging from 100 mm to 800 mm. In addition to the commonly used CNL, the constant normal displacement (CND) condition was established for the purposes of this study; the CND can be considered an extreme case of the constant normal stiffness (CNS) condition. The shear stress‐shear displacement curves changed from brittle failure to ductile failure alongside a gradual decrease in peak shear strength as joint length increased. That is, an overall negative scale effect was observed. Positive scale effect or no scale effect is also possible within a limited joint length range. A positive correlation was also observed between the peak shear displacement and joint length, and a negative correlation between shear stiffness and joint length. These above statements are applicable to both repeated and enlarged joints under either CNL or CND conditions. When the normal stress is sufficiently high and shear dilatancy displacement is very small, the shear behavior of rock joints under CNL and CND conditions seems to be consistent. However, for shear tests under low initial normal stress, the peak shear strength achieved under the CND condition is much higher than that under the CNL condition, as the normal stresses of enlarged joints increase to greater extent than the repeated ones during shearing.

Evaluation of the anisotropy and directionality of a jointed rock mass under numerical direct shear tests

The characteristics of the anisotropy and directionality of jointed rock masses are the key to simplifying it as a transversely anisotropic material. Experimental direct shear tests were performed on stratified rocks cored from granulite rocks. The influence of normal stresses and joint orientations was tested and the anisotropy and directionality of the shear strength were evaluated. Numerical shearing tests on stratified rock models and discrete fractures network (DFN) models were subsequently conducted. In addition, the shear anisotropy and directionality in the DFN models were tested and discussed. The results demonstrate that the peak shear stress was more sensitive to the inclination angles than the normal stress. The stratified rocks in the experimental test showed apparent shear anisotropy and directionality. The maximum value of directionality coincided with the direction of the strike of the bedding plane. The more apparent the anisotropy was, the higher the directionality became. The failure patterns of stratified rocks exhibited more complex characteristics under higher normal stress. Significant variation of the shear strength distribution occurred due to the different orientations of fractures relative to the directions of the shear load. The failure patterns of DFN models of varied scales differed, and no apparent convergence was discovered. The shear strength of the DFN model showed an apparent principal direction. The numerical results presented in this paper are valuable for evaluating the prediction of the failure behaviour of rock masses, and can be further applied to the study of the failure mechanism of surrounding rock in engineering.

Shear deformation and strength of the interphase between the soil–rock mixture and the benched bedrock slope surface

A series of benched excavations were typically carried out on the bedrock slope surface to improve the stability of the soil–rock mixture (S–RM) fill slope. It is difficult to devise an in situ, large-scale direct shear test for the interphase between the S–RM fill and the benched bedrock slope surface. This study introduced a comprehensive approach to investigate the shear deformation and strength of the interphase. First the soil–rock distribution characteristics were analyzed by test pitting, image analysis, and sieve test. Then the PFC2D random structure models with different rock block size distributions were built, and large-scale numerical shear tests for the interphase were performed after calibrating model parameters through laboratory tests. The stress evolution, damage evolution and failure, deformation localization (based on a principle proposed in this paper), rotation of rock blocks, and shear strength were systematically investigated. It was found that as the rock block proportion and rock block size (rock block proportion of 50 %) increase, the fluctuations of the post-peak shear stress–displacement curves of the interphase become more obvious, and the shear band/localized failure path network becomes wider. Generally, smaller rock blocks are of greater rotation angles in the shear band. The peak shear stress and internal friction angle of the interphase increase, while the cohesion decreases with growth of the rock block proportion. However, all these three parameters increase as the rock block size (rock block proportion of 50 %) increases.

Identification of material parameters in extended contiuum mechanicalmodels

AbstractA method for the identification of material parameters of constitutive models describing foamed materials is discussed in the present work. One can model foamed materials in two ways: using a lattice‐like micro‐model or alternatively a continuum mechanical approach. A beam model is used for the description of foams on a microscopic scale and alternatively an extended continuum model based on the Cosserat theory is employed on the macroscopic scale. By performing numerical shear tests at specimens of different sizes, we can observe the following effect on both models: The distribution of the rotations shows a strong boundary layer character at the top and bottom boundary of the specimen, whereas at the inner part it takes a constant value. These boundary layer effects do not scale with the size of the specimen, as a result specimens occur to be stiffer, see [1]. The goal is to perform a fitting of the Cosserat parameters by comparing the results of the macroscopic model to these of the microscopic one. The parameter fitting concerns an inverse problem which is treated within the framework of stochastic optimization methods. An evolutionary algorithm is used in combination with a response surface method for the identification of the material parameters. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)