FLAC3D Model Research Articles

Numerical simulation of abnormal roof water-inrush mechanism in mining under unconsolidated aquifer based on overburden dynamic damage

Separation layer poses a potential threat to safety mining under an unconsolidated aquifer. A dynamic load will be generated when the upper stratum in the separation layer is broken, leading to an abnormal roof water-inrush. To explore this phenomenon, the 7130 coalface in Qidong coalmine is selected as an example, and the dynamic damage analysis is performed for overburden rock during mining by numerical simulation. A model is presented based on the continuum damage mechanics for simulating the failure of overburden rock under the dynamic load. A new dynamic damage variable is introduced into the model. Subsequently, a user-defined (U-D) model is implemented under the C++ language environment in FLAC-3D. The U-D model is applied to study the abnormal roof water-inrush of the 7130 coalface, compared with the Mohr-Coulomb (M−C) model in FLAC-3D. The research results show that a larger dynamic failed zone than the M−C model under the dynamic load exists. Additionally, the height of water-conducting fractured zone (WCFZ) by the U-D model is 68.6 m and conforms to the in-situ data of 56.9–80.2 m, while the height of WCFZ by the M−C model is only 35.6 m. It can be inferred that the effect of overburden dynamic damage can lead to the upward development of WCFZ, resulting in disasters of roof water-inrush. The study will be helpful to the prevention and control of roof water-inrush.

A methodology to investigate fluid flow in sheared rock fractures exposed to dynamic normal load

Understanding the flow characteristics in sheared rock fractures is with significance for many engineering projects (e.g., geothermal reservoirs, dams, oil or gas explorations, etc.). This study reports a FLAC3D FEM-based fluent CFD - methodology to investigate the fluid flow patterns in sheared rock joints exposed to complex stress states. First, a direct shear experiment of a rough basalt fracture, subjected to dynamic normal force is performed. A FLAC3D numerical model is setup to simulate the experimental process. Using a self-designed algorithm, the fracture contact area, aperture size, and fracture volume are analyzed at different shear displacements in the numerical model. The two-dimensional apertures, obtained from the FLAC3D model, are applied to FEM-based fluent CFD to analyze variation patterns of the relevant fluid flow during the shear process. Results show that the flow characteristics including the generation and development of eddy currents, and the estimated value of the Reynolds number, flow rate, and apparent permeability, are associated with both the shearing and the dynamic normal force. These findings provide a new horizon on features of aperture flow in rock mass under complex geological stresses.

Numerical investigation of dual-row batter pile wall in deep excavation

To evaluate the performance of dual-row batter pile wall (DRBPW) in deep excavations, a three-dimensional numerical model was established. The plastic hardening (PH) constitutive model in FLAC3D was adopted, considering the hardening property of the soil. A series of parametric study was conducted to investigate the effect of different factors. By analyzing the maximum lateral displacement, the pile shaft deflection and bending moment of both the leading and trailing row, the effect of inclination angle, pile diameter, penetration depth, and row spacing on the behavior of DRBPW was determined. The following conclusions could be drawn: (1) The bearing capacity of DRBPW is quite larger than that of the dual-row vertical pile wall (DRVPW). With the increase of inclination angle, a substantial reduction of lateral displacement could be observed. However, the beneficial role of batter pile is almost negligible when the inclination angle is not larger than 5°. The recommended inclination degree of the batter pile is 10° to 15°. (2) The maximum displacement of the leading and trailing rows lies at a distance below the pile cap, while the maximum bending moment is located at a small distance below the dredging line. The maximum bending moment of the leading pile is approximately twice that of the trailing one, thereby indicating that the leading pile bear more lateral earth pressure than the trailing pile. (3) The pile diameter, penetration depth and row spacing should be maintained in a suitable range; otherwise, the DRBPW may fail earlier than expected. The slender ratios is recommend to be set not larger than 30. The penetration ratio should not be less than 0.22. The optimal row spacing for the DRBPW is four times the diameter.

Modelling of fault reactivation mechanisms and associated induced seismicity in rocks with different elastic materials

The mechanical failure of fault plane during fluid injection can be conveniently approached by numerical methods, and the results can be applied in fault slip analysis to determine the corresponding magnitude of induced seismicity. During hydrofracturing, when faults are present and the fluid is injected into the fault, micro-seismic events are possible, although the magnitude is often somewhat larger than those associated with micro-seismic events produced from regular hydraulic fracturing because of the larger surface area available for fault rupture. This study considers the rate at which the changing elastic properties of materials influences the magnitude of seismic event during fault injection. The simulation is carried out under varying injection flow rates from 0.18 kg/s to 0.3 kg/s, and the thermo-hydro-mechanical (THM) model in FLAC3D is adopted. As the material elastic moduli increase significantly under isothermal injection, the resulted non-uniformity in the fault slip timing affects the magnitude of injection-induced seismicity. Rocks with lower moduli produced higher slip distance and seismicity during shear failure. However, in the coupled thermal case, the magnitudes of seismicity during injection are largely enhanced at lower elastic properties, which suggests that the energy of accumulated fluid pressure produces a larger rupture and longer slip displacement in cold injection than in the isothermal case. The resulting volumetric strain, both in the fault zone and in the matrix, is higher in lower moduli, meanwhile, it is much developed in non-isothermal injection as a result of the rock's response to the sum effect of thermal strain and the stress-induced strain.

The 3D numerical model of the stone column in soft clay soils

An end bearing stone column is one of the most common types of stone columns, which completely penetrates through a layer of soft clay soils, resting on much stiffer soils. This article presents a numerical model (analyses) of the stone column used to strengthen the soft clay soil. The analyses were performed using a three-dimensional model in FLAC3D. To investigate the effect of the stone column on strengthening soft clay soils, a dimensionless parameter called the settlement improvement ratio (Ks) is used. The results show that, the settlement improvement ratio (Ks) increases with an increasing diameter of the stone column. The lateral deformation of the stone column decreases with the increasing stone column diameter, and the maximum lateral deformation was occurred at approximately (1.0 to 1.5) diameter from the top of the surface.

Investigation of pillar damage mechanisms and rock-support interaction using Bonded Block Models

In this study, Bonded Block Models (BBMs) are used to investigate the pillar damage mechanisms and rock-support interaction in massive-to-sparsely-fractured rockmasses. Hypothetical granite pillar models of width-to-height (W/H) ratio of 1, 2 and 3 are developed, and the input parameters are constrained by matching the stress-strain response of the BBMs to the stress-strain curves from FLAC3D models that were previously calibrated to an empirical pillar strength database. Two different block representations are also considered – elastic and inelastic. It was found that inelastic blocks are necessary to capture the behavioral transition from strain-softening to pseudo-ductile with increase in pillar W/H.Post-calibration, different rockbolt combinations are tested in the BBM and their influence on the pillar strength and lateral deformations are analyzed. It was found that as the support density is increased, the peak pillar strengths also increase but the effect is dependent on the W/H. Deformation of the outer stress-fractured region and bulking systematically decreased with increasing support density, but the exact trend evolved as the pillars were loaded to various points on their stress-strain curves. Lastly, a BBM pillar was developed with explicit intra-block fracturing capability (i.e., individual blocks could break) and the support analysis was repeated. The goal was to understand if the continuum representation of damage within the inelastic blocks led to some underestimation of the rock-support interaction mechanism. It was ultimately concluded that the continuum inelastic representation of smaller-scale damage within individual blocks allows for a more appropriate representation of the rock-support interaction than the explicit intra-block representation.

The Stability Study and Simulation of Soil Landslide Based on Bishop Method

Landslide is one of the most common disasters in nature and is a kind of slope failure. Many countries in the world suffered, because of its wide distribution, frequent occurrence and great harm. In China, the national economy and people’s lives and property suffer heavy losses due to the landslide disaster. In this paper, first of all, we used the Bishop method to theoretically analyze the stability of a landslide in Sichuan Province, and then we established a simulation model in Flac3D, the rock and soil mechanics calculation software developed by ITASCA company in the United States, to analyze the stability of the landslide under complex conditions. Finally, the results of theoretical analysis and simulation are consistent, which can provide ideas and methods for geological disaster prevention in the future.

Mechanism and Application of Static Fracturing Technology on Deep Working Face

Static fracturing technology uses chemical expansion agents to fracture roofs. With the aim of fracturing corner roofs on deep working faces, in this study, the static fracturing technology was investigated through theoretical analysis, laboratory experiments, numerical calculations, and field practice. The theoretical analysis and experiments demonstrated that the swelling force increased with a decrease in the fracturing hole spacing, and the optimal water‐cement ratio was 0.33. Twelve groups of FLAC3D models were designed using SPSSAU. The results revealed that the optimal fracturing effect was achieved when the hole diameter was 60 mm, hole spacing was 40 cm, and hole depth was 6 m. The fracturing effect of hard corner roofs was monitored by peering into the borehole and evaluating the support resistance. Thus, it can be concluded that within the fracturing range, internal fissures in the rock stratum are developed and linked to each other. The support pressure was the highest, 7 h after grouting, with a value of approximately 26.1 MPa, and then decreased gradually to 17.58 MPa, indicating that the static fracturing technology attained the expected results.

Analysis of aWater-Inrush Disaster Caused by Coal Seam Subsidence Karst Collapse Column under the Action of Multi-Field Coupling in Taoyuan Coal Mine

Minin-induced water inrush from a confined aquifer due to subsided floor karst collapse column (SKCC) is a type of serious disaster in the underground coal extraction. Karst collapse column (KCC) developed in a confined aquifer occurs widely throughout northern China. A water inrush disaster from SKCC occurred in Taoyuan coal mine on February 3, 2013. In order to analyze the effect of the KCC influence zone’s (KCCIZ) width and the entry driving distance of the water inrush through the fractured channels of the SKCC, the stress, seepage, and impact dynamics coupling equations were used to model the seepage rule, and a numerical FLAC3D model was created to determine the plastic zones, the vertical displacement development of the rock mass surrounding the entry driving working face (EDWF), and the seepage vector and water inflow development of the seepage field. The hysteretic mechanism of water inrush due to SKCC in Taoyuan coal mine was investigated. The results indicate that a water inrush disaster will occur when the width of the KCCIZ exceeds 16 m under a driving, which leads to the aquifer connecting with the fractured zones of the entry floor. Hysteretic water inrush disasters are related to the stress release rate of the surrounding rocks under the entry driving. When the entry driving exceeds about 10 m from the water inrush point, the stress release rate reaches about 100%, and a water inrush disaster occurs.

Investigating different methods used for approximating pillar loads in longwall coal mines

Accurately estimating load distributions and ground responses around underground openings play a significant role in the safety of the operations in underground mines. Adequately designing pillars and other support measures relies highly on the accurate assessment of the loads that will be carried by them, as well as the load-bearing capacities of the supports. There are various methods that can be used to approximate mining-induced loads in stratified rock masses to be used in pillar design. The empirical methods are based on equations derived from large databases of various case studies. They are implemented in government approved design tools and are widely used. There are also analytical and numerical techniques used for more detailed analysis of the induced loads. In this study, two different longwall mines with different panel width-to-depth ratios are analyzed using different methods. The empirical method used in the analysis is the square-decay stress function that uses the abutment angle concept, implemented in pillar design software developed by the National Institute for Occupational Safety and Health (NIOSH). The first numerical method used in the analysis is a displacement-discontinuity (DD) variation of the boundary element method, LaModel, which utilizes the laminated overburden model. The second numerical method used in the analysis is Fast Lagrangian Analysis of Continua (FLAC) with the numerical modeling approach recently developed at West Virginia University which is based on the approach developed by NIOSH. The model includes the 2D slice of a cross-section along the width of the panel with the chain pillar system that also includes the different stratigraphic layers of the overburden. All three methods gave similar results for the shallow mine, both in terms of load percentages and distribution where the variation was more obvious for the deep cover mine. The FLAC3D model was observed to better capture the stress changes observed during the field measurements for both the shallow and deep cover cases. This study allowed us to see the shortcomings of each of these different methods. It was concluded that a numerical model which incorporates the site-specific geology would provide the most precise estimate for complex loading conditions.

Integration of three-dimensional continuum model and two-dimensional bonded block model for studying the damage process in a granite pillar at the Creighton Mine, Sudbury, Canada

Bonded block model (BBM) has shown potential in replicating rock mass behavior as well as the rock–support interaction mechanism, but their practical application is limited to two dimensions due to the high associated computational demand. To allow for the use of BBM in simulating three-dimensional (3D) problems, this study proposes an integrated 3D continuum–two-dimensional (2D) discontinuum approach, in context of rock pillars. A cross-section of a granite pillar was simulated using a BBM with a load path from a calibrated mine-scale FLAC3D model. Pillar support as employed in the mine was also incorporated in different stages during the simulation. The model was calibrated by varying the input parameters until the displacements at six locations within the pillar matched those measured by a multi-point borehole extensometer (MPBX) in the field. The calibrated model was subsequently used to understand how the support and load path influenced the damage evolution in the pillar. The shear component of the load path was found to have a major effect on the severity and extent of the damaged regions. When the support density was increased in the model, the lateral displacements along the pillar walls were significantly suppressed in a somewhat unpredictable manner. This was explained by the interaction between the supports and the damaged regions at the corners, which ultimately modified the stresses along the pillar periphery. The amount of displacement reduction obtained by increasing the support density illustrates the potential of BBM to be used as a support design tool.

Analysis of the effect of dam impoundment at the Baihetan site using coupled fluid-mechanical elasto-plastic simulations

Contractional valley deformation is a phenomenon quite specific to high dams in the southwest of China. Numerical simulations with coupling of stress and seepage were performed to study the mechanisms of valley deformation after impoundment and to determine whether and when valley contraction could develop at the Baihetan site. The initial generic study highlighted two competing mechanisms, a “Mattress Effect” and a “Swelling Effect”, associated with dam impoundment. The comprehensive FLAC3D model incorporates detailed rock mass property values, a grout curtain, an impounding plan, and the drainage volumes. The large-scale coupled seepage-mechanical simulations show that the convergence mechanism is activated by fluid diffusion in the valley. At sites upstream and up to about 400 m downstream of the dam, the valley deformation occurs shortly after the impoundment water level is raised; this behaviour reflects the impact of the mattress effect caused by the weight change of the standing impoundment water. However, deformation also occurs during periods when the impoundment level remains the same; this behaviour is caused by the large-scale water seepage associated with pore-pressure changes under and around the dam in the valley banks. The maximum convergence along the monitoring lines does not exceed 5 cm in the simulations, where affected by the presence of faults and bedding planes.

Effect of Principal Stress Field on the Development of Plastic Zone ahead of the Gateroad

The distribution of a plastic zone ahead of a gateroad plays a significant role in maintaining the long-term stability of mining spaces. For a long time, the principal stress field such as the values, the direction, etc. have been observed to have impacts on plastic zone development, but has not been looked into deeply and systematically. To this end, the influence of principal stress field including the maximum principal stress (P1), the angle between the P1 direction and the Z-axis (α), the minimum principal stress (P3), and the ratio of maximum principal stress to minimum principal stress (P1/P3) on the expansion of the plastic zone ahead of the gateroad is investigated by the (Fast Lagrangian Analysis of Continua) FLAC3D models. The results show that: (1) The plastic zone volume increases first and then decreases with the increase of α, and the direction of butterfly-shaped plastic zone ahead of gateroad is rotating with the evolution of α. (2) The plastic zone volume ahead of excavation face increases gradually with the increase of P1/P3. Mutagenicity of butterfly-shaped plastic zone occurs ahead of the gateroad under a certain value of P1/P3. (3) With the increase of P1 and decrease of P3, the plastic zone volume is of exponential growth. The plastic zone volume approaches infinity when the critical value of maximum principal stress ([P1]) and the minimum principal stress ([P1]) is obtained. (4) The study of the effect of principal stress field on the expansion of plastic zone ahead of the gateroad is helpful for revealing the mechanisms of coal and gas outbursts. The critical stress state of butterfly-shaped plastic zone mutagenicity ahead of the gateroad can be used as an important indicator for assessing the risk of coal and gas outburst. The research can also guide the prevention of coal and gas outburst ahead of the gateroad.

Evaluation for p-y Method for Offshore Large Diameter Monopile in Sand

Large monopile is widely used in offshore wind farm due to their relatively simple installation. Monopiles are customarily designed by applying the p-y method. That being said, there is no detailed and uniform interpretation on the reasons for limitations in the current p-y method. An elaborate study is performed to determine the main reasons of such limitations. In this study, a FLAC3D model is presented to evaluate the current p-y method for piles in sand of three different relative densities under a static load, in which the non-linear modulus is taken into account. The results show that the initial stiffness of the p-y method is overestimated and even more magnified in the large depth. The diameter effect should be taken into account under diameters exceeding 3m in loose and medium sand, as well as diameters more than 5m in dense sand.

Numerical Investigation on Roadside Backfill Area Roof Failure Mechanism Subjected to Roadside Backfill Body: A Case Study

In retained gob-side entry (RGE) of coal mining, the immediate roof above roadside backfill area (RBA) plays a key part in transferring the main roof load downward and transferring the roadside backfill body (RBB) resistance upon the main roof upward. Numerical investigation was performed to evaluate the stress distribution, deformation characteristics, and plastic zone distribution of the immediate roof above RBA based on a field case of RGE in a Chinese colliery. The double-yield model for gob, strain-softening Mohr–Coulomb model for RBB and strain-softening Mohr–Coulomb model for the roof above RBA were proposed to improve the reliability of the 3D global numerical model in FLAC3D. The numerical results of the calibrated global numerical model indicate that the vertical stress of the immediate roof above RBA shows a V-shaped distribution when the RBB width is less than 2.4 m. The horizontal stress of the immediate roof above RBA reaches the maximum when the RBB width is 2.0 m. The vertical displacement and horizontal displacement of the immediate roof above RBA positively are associated with the RBB width. In addition, the tensile failure area of the immediate roof above RBA, and the ratio of the tensile failure depth of the immediate roof above RBA to the tensile failure depth of the road-in immediate roof increases with the increase of the RBB width while the tensile failure depth of the immediate roof above RBA decreases. The proposed modeling procedure can be repeated in RGE to evaluate surrounding rock stability with different geological and engineering conditions.

An Approach for Numerical Modeling of Gob Compaction Process in Longwall Mining

A field representative modeling of the gob behavior is critical for the understanding of the associated mechanics of strata behavior in longwall workings. The findings of theoretical models and laboratory tests on the simulated gob material have shown the strain hardening behavior of the gob material. The double-yield material model, which is a built-in constitutive model in FLAC3D, has significant potential to simulate the gob compaction process as it represents materials which undergo irreversible compaction in addition to shear yielding. Apart from this, several researchers have attempted using modified elastic model or applied nodal force in reaction to the strain in the gob area as per the well-known theoretical relations to meet this requirement. Although these models can simulate the mechanical response of the gob for subcritical and critical extraction widths, they do not apply to super-critical extraction width as the gob stress may exceed the in situ stress level in such cases. For overcoming these limitations, a standard approach has been developed and validated in this paper, which can be used for simulating the gob behavior in super-critical longwall workings as well. The complete study is divided into two stages wherein the laboratory scale compression test on the simulated gob material has been numerically replicated to develop preliminary confidence followed by a parametric study of the double-yield constitutive parameters for a rational estimation of their values in different conditions. The calibrated model has been used for verifying its results by plane strain modeling of two different longwall workings, using well-established empirical and analytical findings and field observations. The approach suggested in this study can be used for the efficient modeling of the gob behavior of longwall panels under varying geo-mining conditions without compromising the accuracy of the numerical model. The meticulously validated model can be helpful in rational estimation of stress redistributions around the gob area apart from load transfer on the barrier pillar.

A case study of the stability of a non-typical bleeder entry system at a U.S. longwall mine

Longwall abutment loads are influenced by several factors, including depth of cover, pillar sizes, panel dimensions, geological setting, mining height, proximity to gob, intersection type, and size of the gob. How does proximity to the gob affect pillar loading and entry condition? Does the gob influence depend on whether the abutment load is a forward, side, or rear loading? Do non-typical bleeder entry systems follow the traditional front and side abutment loading and extent concepts? If not, will an improved understanding of the combined abutment extent warrant a change in pillar design or standing support in bleeder entries? This paper details observations made in the non-typical bleeder entries of a moderate depth longwall panel—specifically, data collected from borehole pressure cells and roof extensometers, observations of the conditions of the entries, and numerical modeling of the bleeder entries during longwall extraction. The primary focus was on the extent and magnitude of the abutment loading experienced due to the extraction of the longwall panels. Due to the layout of the longwall panels and bleeder entries, the borehole pressure cells (BPCs) and roof extensometers did not show much change due to the advancing of the first longwall. However, they did show a noticeable increase due to the second longwall advancement, with a maximum of about 4 MPa of pressure increase and 5 mm of roof deformation. The observations of the conditions showed little to no change from before the first longwall panel extraction began to when the second longwall panel had been advanced more than 915 m. Localized pillar spalling was observed on the corners of the pillars closest to the longwall gob as well as an increase in water in the entries. In addition to the observations and instrumentation, numerical modeling was performed to validate modeling procedures against the monitoring results and evaluate the bleeder design. ITASCA Consulting Group’s FLAC3D numerical modeling software was used to evaluate the bleeder entries. The results of the models indicated only a minor increase in load during the extraction of the longwall panels. These models showed a much greater increase in stress due to the development of the gateroad and bleeder entries--about 80% development and 20% longwall extraction. The FLAC3D model showed very good correlation between modeled and expected gateroad loading during panel extraction. The front and side abutment extent modeled was very similar to observations from this and previous panels.

基于FLAC3D多节理模型的顶底板采动变形破坏特征研究

基于FLAC3D多节理模型的顶底板采动变形破坏特征研究

Cooperative load‐bearing characteristics of a pillar group and a gob pile in partially caved areas at shallow depth

AbstractStress concentration in partially caved goaf is the main cause of dynamic pressure accident in the lower seam mining. Aimed at the characteristics of caving in shallow partially caved goaf (PCG) and the specificity of interior load‐bearing structures, a classification criterion of goaf caving was proposed. The characteristics of cooperative load‐bearing in pillar group and gob pile were revealed by numerical calculation, physical simulation, and theoretical analysis. Taking intermittent mining as an example, combined with the deformation behaviors of the main roof and the loading characteristics of waster rock mass, the characteristics of the load‐bearing of gob piles were described in the form of a piecewise function. A FLAC3D model of intermittent mining is modeled to evaluate the stability of pillars, the long‐term stability and distribution laws of overlying loads were revealed through a refined model. The results show that the width of the caved zone bearing the load in the intermittent goaf is about 20 m, and the maximum load‐bearing capacity is 1.055 MPa. The maximum depth of the plastic region in one side of the pillar is 1.48 m, and the load on elastic zone is about 11.33 MPa. This study provides a method for the study of the caving characteristics and load uncertainty in PCG, and the results provide important theoretical values for the safe mining of lower coal seams.

Back-Analysis of the Bingham Canyon South Wall: A Quasi-static Complex Slope Movement Mechanism

A 610-m-high portion of the South Wall of the Bingham Canyon open pit has experienced slow-moving slope deformations several times during spring melt until the movement was stabilized approximately 7 years ago. To develop and optimize life of mine slope designs, an understanding of the mechanism(s) and associated material strength parameters, was required. A back-analysis and calibration of the strength parameters for the salient rock mass and structural features was undertaken. The back-analysis consisted of understanding the conditions and trigger for the slope movement and adjusting strength parameters to match available monitoring data (time-domain reflectometer cables, inclinometers, interferometric radar data, etc.). The trigger for the movement was attributed to the spring high-perched water levels in the upper part of the wall. A FLAC3D model was built to back-analyze and capture the complex behavior of the slope during the high deformation period (August 2011). The back-analysis was consistent with the conceptual model and indicated that the slide was composed of mixed mechanisms, namely, a structurally controlled mechanism for the upper wall and a rock mass controlled mechanism for the lower wall (toe). The analysis supported by a converging FLAC3D model, were used to guide mining engineers in developing robust pit design in the South Wall. Today, the O-Slide instability is being fully managed by rigorous monitoring, implementation of a toe buttress, successful dewatering efforts, and unloading of the movement mass as another slice of mining advances down the South Wall.