Enzhi Wang

Fluid-driven fractures in granular materials

The initiation and propagation process of a fluid-driven fracture in granular materials is inherently a hydro-mechanical coupling problem. The bonded-particle method (BPM) was utilised to simulate the hydraulic fracturing process in granular materials, and different failure mechanisms were evaluated by analysing the formation of microcracks. Hydraulic conductivity is determined by pore size and connectivity in the direction of flow. A strain-dependent formulation was presented to highlight the inherent link between hydraulic conductivity and pore size. The results show that the BPM is capable of realistically predicting fluid-driven fractures in granular material. Using the BPM, the numbers of fluid-driven fractures induced by different failure modes can be determined. It is concluded that for consolidated formations, the initiation and propagation of fluid-driven fractures are dominated by tensile failure, which has been recognised in the field of geology and geomechanics. However, for unconsolidated formations, shear failure seems to be more important during the hydraulic fracturing process. As described in this article, the number of shear failure cracks is twice that of tension failure cracks, which has not been widely recognised. Overall, the simulation results of the fluid-driven fracture are in accordance with the experimental data observed by other researchers.

Numerical Simulation Analysis of Reservoir Bank Fractured Rock-Slope Deformation and Failure Processes

Abstract: The fluctuations of reservoir water levels significantly change the hydrogeological conditions of reservoir banks and thus affect the steady state of the bank slope. Reservoir landslide stability is governed by the complex interactions between the rock and the water (both reservoir and pore water). This paper establishes the constitutive coupled model of seepage and stress field for the E20 bank slope in the Haizhou Open Pit Coal Mine hydropower station’s lower reservoir. Using the geotechnical engineering three-dimensional fast Lagrangian analysis of continua (FLAC-3D 4.0) program as a computational analysis tool, the E20 bank slope deformation mechanism was analyzed. The simulation model, calculated by using the Boit consolidation and pore-water continuity principles, was built using FEMs. The numerical simulation results showed that cyclical fluctuations and speeds of the reservoir’s water level affect its bank slope stability. When the water level drops at a speed of 12 m/day, the simulation...

Modified governing equation and numerical simulation of seepage flow in a single fracture with three-dimensional roughness

Roughness and tortuosity influence groundwater flow through a fracture. Steady flow through a single fracture can be described primitively by the well-known Cubic Law and Reynolds equation with the assumption that the fracture is made of smooth parallel plates. However, ignoring the roughness and tortuosity of the fracture will lead to inaccurate estimations of the flow rate. To obtain a more accurate flow rate through a rough fracture, this paper has derived a modified governing equation, taking into account the three-dimensional effect of the roughness. The equation modifies the Reynolds equation by adding correction coefficients to the terms of the flow rates, which are relative to the roughness angles in both the longitudinal and transverse directions. Experiments of steady seepage flow through sawtooth fractures were conducted. The accuracy of the modified equation has been verified by comparing the experimental data and the theoretical computational data. Furthermore, three-dimensional numerical models were established to simulate the steady flow in rough fractures with the triangular, sinusoidal surfaces and the typical joint roughness coefficient (JRC) profiles. The simulation results were compared with the calculation results of the modified equation and the current equations. The comparison indicates that the flow rate calculated by the modified equation is the closest to the numerical result.

Dam Safety Evaluation Based on Multiple Linear Regression and Numerical Simulation

The Shimenzi roller-compacted concrete arch dam was built in a soft-rock region with faults and intercalations. The abutments are divided into several potential sliding wedges by major faults and intercalations. In this study, the safety of the dam was analyzed on the basis of measurements of the displacements, strains, and stresses in the concrete. A hydrostatic, temperature, time-displacement model was used to quantify the contributions of water level variations, temperature in the concrete, and time effects, to the dam’s upstream–downstream displacements. A multiple linear regression was used to train and test the relationships among the hydrostatic conditions (temperature, time, and displacements) and to evaluate the dam deformation characteristics and stability. Numerical models of dam stresses were then established and calibrated, which allow quantitative evaluation of dam safety by using statistical methods. The results indicated that (1) the dam is currently in an elastic deformation state; (2) the contribution of temperature to dam displacement differs with elevation; and (3) the state of stress is below the limits of material strength.

The Quantified Characterization Method of the Micro-Macro Contacts of Three-Dimensional Granular Materials on the Basis of Graph Theory

We have attempted a multiscale and quantified characterization method of the contact in three-dimensional granular material made of spherical particles, particularly in cemented granular material. Particle contact is defined as a type of surface contact with voids in its surroundings, rather than a point contact. Macro contact is a particle contact set satisfying the restrictive condition of a two-dimensional manifold with a boundary. On the basis of graph theory, two dual geometrical systems are abstracted from the granular pack. The face and the face set, which satisfies the two-dimensional manifold with a boundary in the solid cell system, are extracted to characterize the particle contact and the macro contact, respectively. This characterization method is utilized to improve the post-processing in DEM (Discrete Element Method) from a micro perspective to describe the macro effect of the cemented granular material made of spherical particles. Since the crack has the same shape as its corresponding contact, this method is adopted to characterize the crack and realize its visualization. The integral failure route of the sample can be determined by a graph theory algorithm. The contact force is assigned to the weight value of the face characterizing the particle contact. Since the force vectors can be added, the macro contact force can be solved by adding the weight of its corresponding faces.

The Stability Analysis Method of the Cohesive Granular Slope on the Basis of Graph Theory

This paper attempted to provide a method to calculate progressive failure of the cohesive-frictional granular geomaterial and the spatial distribution of the stability of the cohesive granular slope. The methodology can be divided into two parts: the characterization method of macro-contact and the analysis of the slope stability. Based on the graph theory, the vertexes, the edges and the edge sequences are abstracted out to characterize the voids, the particle contact and the macro-contact, respectively, bridging the gap between the mesoscopic and macro scales of granular materials. This paper adopts this characterization method to extract a graph from a granular slope and characterize the macro sliding surface, then the weighted graph is analyzed to calculate the slope safety factor. Each edge has three weights representing the sliding moment, the anti-sliding moment and the braking index of contact-bond, respectively, E1E2E3E1E2E3. The safety factor of the slope is calculated by presupposing a certain number of sliding routes and reducing Weight E3 repeatedly and counting the mesoscopic failure of the edge. It is a kind of slope analysis method from mesoscopic perspective so it can present more detail of the mesoscopic property of the granular slope. In the respect of macro scale, the spatial distribution of the stability of the granular slope is in agreement with the theoretical solution.

Contributions of Flexible-Arch Configurations in Shimenzi Arch Dam: New Evidence from Field Measurements

This paper presents a retrospective investigation into the performance of a new type of flexible-arch configurations in Shimenzi arch dam based on the past ten-year-long field measurements. The flexible-arch configurations are mainly comprised of artificial short joints at the middle downstream surface and a middle contraction joint with hinged well and enlarged arch ends with bending joints. Fundamental design considerations of these components are provided, and their contributions to the performance of Shimenzi arch dam are discussed in detail using the monitoring data from joint meters, strain gauges, and thermometers. Some elementary numerical studies have been conducted on a typical arch structure with different arrangements of artificial joints. Both the field data and numerical results prove well the effectiveness of the purposely built short joints and the middle contraction joint on the relaxation of tensile stress mobilization. Field survey data also clearly demonstrate the significance of the hinged well at the upstream side of the middle joint for a continuous arch force transfer.

Simulation on Reservoir-Induced Seismicity Considering Thermo-Hydro-Mechanical Couplings

Reservoir-induced seismicity (RIS) might happen when impounding over a critical level, changing the water load and seepage field. Statics show that the depth of seismic source increases gradually after thousands of seismicity inside or near the reservoir, implying that water might be a factor to break the initial balance and propagate fractures. The process is a coupling of multiple fields, such as stress field, permeability field and thermodynamic field. This paper presents a 2D Finite Element Model (FEM) to simulate the effect of Thermo-Hydro-Mechanical (THM) coupling on a 2-m pre-existing crack placed at a different inclination. Elastic and damage model are introduced to simulate generation and propagation of crack under the conditions of different temperature at the corresponding depth. A discover is summarised that inclination of fracture could determine length of crack propagation and thermal field definitely has influence in reservoir-induced seismicity, while the deformation of crack depends on thermal expansion and softening of rock. When the inclination of fault is above 45°, either wall will have obvious movement. Therefore, fault will be triggered to active if the angle is above 45°, relative displacement will rise with the increasing temperature.

Study on the Fractal Characteristics of Fracture Network Evolution Induced by Mining

The evolution and distribution of fracture network induced by mining is essential to determine the mechanical properties and permeability of disturbed rock mass. In this paper, the similar material model tests are employed to simulate the stress variation, cyclic breaking, and fracture formation and distribution status of the overlying strata with different loading conditions, rock properties, and mining process. The fractal dimension of mining-induced fracture network varied with mining advancing, and the evolvement laws of fracture network with mining advancing and different mining advancing footage are concerned and obtained. By establishing the relationship between the fractal dimension and the mining length in different horizontal and vertical zones, it demonstrates that the fractal dimensions in horizontal and vertical zones have a self-similar characteristic, and the distribution of the fractal dimension of the mining-induced fractures shows generally the “W”-type trend.

The Dynamic Response of Brittle Materials under Impact Loading

Abstract In order to make sense of the dynamic response of brittle materials under the certain range of impact strength, the numerical simulation for two kinds of representative ones glass and ceramic are conducted, in which the elastic micro-crack damage model is used. The plane impact experiments of ceramic and glass are summarized, which are used to compare with the simulation results. The simulation results show that the dynamic responses of brittle materials, failure wave and the plastic-like response appeared in glass and ceramic respectively are depended on their micro-cracks distribution in meso-scale. And moreover, both of failure wave and the plastic-like response are controlled by the same mechanism, and the different phenomena are just influenced by the size and distribution of the micro-cracks.