FLAC3D Software Research Articles

Mining-Induced Stress and Ground Pressure Behavior Characteristics in Mining a Thick Coal Seam With Hard Roofs

The hard roof of coal mines has the characteristics of high hardness, good integrity, and large layer thickness, which leads to many ground control problems. To reveal the influence of a hard roof structure on the stress performance and coal pillar stability during mining operations, the 8,104 and 8,105 working faces in the Tongxin coal mine were considered as the research object to analyze the stress behavior during the working face advance. Numerical simulation software FLAC3D was used to establish the numerical model of the longwall face under hard roof conditions. The stress distribution laws and coal pillar stability under different roof strengths were analyzed so as to explain the impacts of the hard roof on the stress distribution at the working face. The results show that during the second face proceeding, the influence zone of the front abutment pressure under hard roof conditions is 6 m wider than that under soft roof conditions, and the bearing stress at the working face is 10.4 MPa higher. At the mining position, the plastic zone of the pillar under hard roof conditions is 11 m wider than that under soft roof conditions, and the peak vertical stress is 5.13 MPa higher than that under soft roof conditions. At 25 m ahead of the working face, the plastic zone of the pillar under hard roof conditions is 6 m wider than that under soft roof conditions, and the peak vertical stress is 24.84 MPa higher than that under soft roof conditions. Additional overburden pressure produced by the uncaved hard roof increased pillar stress and plastic zones. Therefore, the hard roof is the main cause of strong ground pressure behavior in the Tongxin coal mine. Aiming at the strong mine pressure behavior, it is suggested to adopt the pre-splitting technology to reduce the influence of the hard roof on mine pressure.

Numerical Stability Analysis of the Dam Foundation Under Complex Geological Conditions at Great Depth: A Case Study of Kala Hydropower Station, China

The stability of dam foundations at great depth has always been a challenging problem concerned by designers. This study built a three-dimensional numerical model of the dam and foundation under a complex geological condition in the project area of the Chinese Kala hydropower station. The numerical model was imported into the finite difference software FLAC3D to conduct a numerical simulation. Overload and strength reduction methods were adopted to evaluate the stability of the dam foundation at great depth. The results indicated that the structural faults had an essential impact on the stability of the dam foundation. The stability analysis showed that the safety factor of the dam foundation is greater than 2.5, which met the standard’s requirements. The simulation results can provide a reference for the design and engineering reinforcement of the Kala dam foundation.

Influence of Sublevel Unloading Excavation with Deep Consideration of the Superposition Effect on Deformation of an Existing Tunnel under an Intelligent Geotechnical Concept

The construction of a deep and large foundation will inevitably impose stress and deformation on the existing shield tunnel structure. Based on the two-stage analysis and the Pasternak foundation beam model, the analytical solution for the existing tunnel deformation is derived by taking the grouting pressure and water buoyancy into account in the total tunnel deformation. Using FLAC 3D software, based on the soil-structure interaction model, the variation law of the tunnel uplift value under partition excavation of the foundation pit is studied and the variation law of the stress field and displacement field of the tunnel under the MJS method is analyzed and compared. The results of this paper are worthy of reference for similar projects.

A Numerical Model for the Compressive Behavior of Granular Backfill Based on Experimental Data and Application in Surface Subsidence

Granular backfill is generally confined in stopes to bear underground pressure in metal mines. Its mechanical behavior under lateral confinement is vital for controlling stope wall behavior and estimating surface subsidence in backfill mining operations. In this paper, an experimental apparatus has been developed to explore the bearing process of granular material. Pebbles were selected to model granular backfill. A series of compression experiments of pebble aggregation were performed under lateral confinement condition using the experimental apparatus. The bearing characteristics of the pebble aggregation with seven gradations were analyzed. Based on the experimental data, a constitutive model that takes the real physical characteristics of granular material into account was proposed with variable deformation modulus. The constitutive model was implemented into the FLAC3D software and verified basically by comparison with experimental results. The surface subsidence in backfilling mines was studied using the proposed model. The effects of the particle size of the granular backfill and the height and buried depth of mined-out stopes on surface subsidence have been clarified. The research results are of great significance for guiding backfill mining and evaluating surface subsidence and movement.

Nonlinear creep model of salt rock used for displacement prediction of salt cavern gas storage

To explore the creep response and unloading rebound effect of salt rock, uniaxial compression creep and unloading rebound tests are performed at different axial stresses on salt rock specimens. The test results indicate that the instantaneous strain generated by salt rock mainly contributes by unrecoverable instantaneous plastic strain under a higher axial stress, and the creep strain mainly consists of unrecoverable visco-plastic creep strain. The ratio of visco-plastic creep strain to the total creep strain gradually increases with increases in time and axial stress. A hardening viscous body and a damage viscous body are proposed to describe the hardening effect and damage effect of rock during the creep process. Subsequently, a new five-component nonlinear creep model is established by connecting the two viscous bodies with the conventional elastic body and Kelvin model in series. The creep and unloading rebound test data of salt rock are utilized to testify the reasonability of the nonlinear creep model. The results indicate that the theoretical curves of the nonlinear creep model exhibit good coherence to the test data in the creep and unloading stages. Based on the secondary development platform of the FLAC3D software, the nonlinear creep model is redeveloped to obtain the computer application program. Thus, the creep and unloading rebound test of salt rock is simulated using the secondary development program of the nonlinear creep model. In general, the results from the numerical simulation and creep and unloading rebound test of salt rock are in good agreement, and this proves the accuracy of the secondary development program of the nonlinear creep model. Finally, the computer application program of the model is utilized to predict the surrounding rock deformation of a salt cavern gas storage during operation.

Strength Tests and Numerical Simulations of Loess Modified by Desulfurization Ash and Fly Ash.

Desulfurization ash and fly ash are solid wastes discharged from boilers of power plants. Their utilization rate is low, especially desulfurization ash, most of which is stored. In order to realize their resource utilization, they are used to modify loess in this paper. Nine group compaction tests and 32 group direct shear tests are done in order to explore the influence law of desulfurization ash and fly ash on the strength of the loess. Meanwhile, FLAC3D software is used to numerically simulate the direct shear test, and the simulation results and the test results are compared and analyzed. The results show that, with the increase of desulfurization ash’s amount, the shear strength of the modified loess increases first and then decreases. The loess modified by the fly ash has the same law with that of the desulfurization ash. The best mass ratio of modified loess is 80:20. When the mass ratio is 80:20, the shear strength of loess modified by the desulfurization ash is 12.74% higher than that of the pure loess on average and the shear strength of loess modified by fly ash is 3.59% higher than that of the pure loess on average. The effect of the desulfurization ash on modifying the loess is better than that of the fly ash. When the mass ratio is 80:20, the shear strength of loess modified by the desulfurization ash is 9.15% higher than that of the fly ash on average. Comparing the results of the simulation calculation with the actual test results, the increase rate of the shear stress of the FLAC3D simulation is larger than that of the actual test, and the simulated shear strength is about 8.21% higher than the test shear strength.

Research on Goaf Retaining Technology of Double‐Layer “Combined” Filling Body in High Gas Mine

In high gas mines, the safety accidents of coal seam spontaneous combustion and gas leakage in goaf caused by fresh air flow entering goaf due to loose roof connection of filling body beside goaf are easy to occur. In order to solve such problems, this paper proposes the gob‐retaining technology of the “combined” filling body at the side of the roadway. The upper layer of the “combined” filling body is constructed with high‐water material as the “connecting top layer” so that the deformation form of the filling body is consistent with the broken characteristics of the roof, and the lower layer is constructed with concrete as the “supporting layer” to achieve sufficient support strength. The “combined” filling body can not only ensure the stability of roadway surrounding rock but also a timely closed goaf. The gob‐retaining technology of the “combined” filling body was applied in the field of coal mine 2603 working face. Based on the engineering background, the mechanical model was established to calculate cutting resistance of the filling body, and Flac3D software was utilized to optimum parameters of the “combined” filling body. According to the field monitoring results, the “combined” filling wall at the side of the roadway effectively controls the stability of surrounding rock and the gas concentration in the roadway.

Prediction of the geomechanical state of the rock mass when mining salt deposits with stowing

The technogenic impact of mining on the environment is analyzed and the transition to geotechnology with stowing to reduce the impact of mining operations is proposed. The results of the research work devoted to the justification of parameters of the development of salt deposits with stowing and the definition of the influence of stowing on the dynamics of deformation of the underworked rock massif are presented. The relevance of research aimed at creating a safe and efficient technology for the transition from systems with natural maintenance of stoping space to systems with stowing has been substantiated. The results of studies on qualitative and quantitative assessment of the state of the rock massif (by the finite element method using FLAC3D software), worked out by combines, are given and the dynamics of the impact of mining operations on the rock mass and the change in the maximum stresses during the hardening of the stowing in the chambers are revealed. The numerical modeling method is used to analyze the conditions of change in the state of the underworked rock mass, to establish the mechanisms of its deformation at various stages of development. It is recommended to use this approach for geotechnical assessment of the rock mass state in conditions of using development systems of different classes.

Research on the Effective Control of Ground Settlement during Double‐Layered Foundation Pit Dewatering Based on Seepage Control‐Recharge Coupling Model

This study focuses on the settlement of buildings around the foundation pit caused by foundation pit dewatering. Within the study context, a test is conducted on the control of ground settlement during double‐layered foundation pit dewatering based on the seepage control‐recharge coupling effect. Moreover, a large indoor seepage test device is developed to accurately simulate in‐situ three‐dimensional seepage conditions of double‐layer foundations. The study results showed that compared to the antiseepage conditions, upon the additional recharge, the head of the upper row of the pressure measuring tubes outside the foundation pit is raised by nearly 10.7%, and the water level of the lower row of pressure measuring tubes is raised by about 3.1%. Additionally, the head of the outer side of each pressure measuring tube is higher than that of the inner side by about 2.75% on average, indicating that the recharge outside the pit can effectively increase the head height in the area above the soil layer at the bottom of the recharging well outside the foundation pit and reduce the settlement of the building outside the foundation pit. The numerical calculation of the settlement outside the pit using FLAC3D software further confirms the effect of the seepage control‐recharge coupling model on the ground settlement during foundation pit dewatering. The research results are important for the infiltration deformation and foundation settlement control in composite foundations precipitation.

Influence of Natural Gas Hydrate’s Property Weakening from Dissociation on Subsidence of Seabed Sediments: A Simulation Study

The dissociation of natural gas hydrate in the seabed during hydrate production can lead to substantial weakening in soil mechanic properties, which in turn results in the deformation and even destruction of the seafloor sediments, thereby posing a potential threat to the safety of the undersea engineering infrastructures. A better understanding of the effect of soil mechanical weakening on seabed deformation will benefit the prediction and/or determination of geological conditions for safer hydrate production. In this work, a three‐dimensional model of the submarine geological body was established and engineering properties of hydrate reservoir under different degrees of hydrate dissociation were determined. Such model and engineering parameters were applied for simulation calculation on examining the effect of hydrate dissociation degree on soil subsidence using FLAC3D software. The simulation results show that the subsidence of sediments is centrally distributed within the dissociation area and the subsidence of hydrate‐bearing sediment’s top surface is much more severe than that of the overlying layer. Subsidence on the horizontal section above the hydrate reservoir presents an essentially symmetric dish‐like subsidence trough, with the greatest subsidence in the centre and approximately constant subsidence beyond the trough range. The radius of the subsidence trough increases with the increase of the degree of dissociation, and the increasing extent in the early dissociation stage (lower dissociation degree) is greater than in the middle and the later stage (higher dissociation degree). As the degree of hydrate dissociation increases, subsidence of both the overlying layer and the HBS increases. Especially when the decomposition exceeds a certain threshold, the seafloor subsidence increases rapidly. The simulation results in this work are of guiding significance for deformation monitoring and infrastructure deformation prevention in practical hydrate production projects.

Analysis of the Influence of Whipstock Point Position of Directional Drilling through Embankment on the Stability of Embankment Slope against Sliding

With the increasing number of river‐crossing pipeline projects, many studies have focused on the safety and stability of embankment slopes in horizontal directional drilling crossing embankment construction. In addition, it is challenging to study the influence of key processes in directional drilling construction on the antisliding stability of embankment slopes. A model of horizontal directional drilling crossing the Shaying River embankment was developed by using the numerical simulation software FLAC3D, and the strength reduction method is used to analyze the antisliding stability of the embankment after crossing the embankment. In this study, the effect of the position of the whipstock point on the antisliding stability of the embankment slope was considered. The analysis shows that when the inclined point is located below the center of the embankment, the anti‐sliding stability of the embankment is the most unfavorable, and the construction should try to avoid the whipstock point below the center of the embankment. It was also determined that when the vertical distance between the oblique point and the crest of the embankment is exceeding 13 m, the impact on the antisliding stability of the embankment can be ignored.

Slope Stability Considering the Top Building Load

Slope stability is one of the most important subjects of geotechnics. The slope top-loading plays a key role in the stability of slopes in hill slope areas. When the building load is too large or the point of action from the shoulder is too close, the shear stress of the slope will be significantly greater than its shear strength, resulting in reduced slope stability. Therefore, it is of great importance to study the relationship between the building load and the stability of the slope. This study aims to analyze the influence of different building loads applied at different distances on the top of the slope and deduces their effects on the slope stability. For this purpose, a three-dimensional slope model under different building loads with different distances to the slope shoulder was established using the finite-difference analysis software Flac3D. The results show that the loads applied at different distances on the top of the slope have different effects on the slope stability. The slope factor of safety (FOS) increases with the increase of the distance between the top-loading and the slope shoulder; it varies from 1.37 to 1.53 for the load P = 120 KPa, 1.27 to 1.53 for the load P = 200 KPa, and from 1.18 to 1.44 for P = 300 KPa, resulting in the decrease of the coincidence area between the load-deformation and the potential sliding surface. The slope is no longer affected by the potential risk of sliding at approximately 20 m away from the slope shoulder.

Rotation of the Stress Tensor in a Westerly Granite Sample During the Triaxial Compression Test

We simulated the spatiotemporal modelling of 3D stress and strain distributions during the triaxial compression laboratory test on a westerly granite sample using finite-difference numerical modelling implemented with FLAC3D software. The modelling was performed using a ubiquitous joint constitutive law with strain softening. The applied procedure is capable of reproducing the macroscopic stress and strain evolution in the sample during triaxial deformation until a failure process occurs. In addition, we calculated focal mechanisms of acoustic emission (AE) events and resolved local stress field orientations. This detailed stress information was compared with that from numerical modelling. The comparison was made based on the 3D rotation angle between the cardinal axes of the two stress tensors. To infer the differences in rotation, we applied ANOVA. We identified the two time levels as the plastic deformation phase and the after-failure phase. Additionally, we introduced the bin factor, which describes the location of the rotation scores in the rock sample. The p values of the test statistics F for the bin and phase effects are statistically significant. However, the interaction between them is insignificant. We can, therefore, conclude that there was a significant difference in the time between the rotation means in the particular bins, and we ran post hoc tests to obtain more information where the differences between the groups lie. The largest rotation of the stress field provided by the focal mechanisms of AE events from the numerically calculated stress field is observed in the edge bins, which do not frame the damage zone of the sample.

Investigation of Deep Shaft-Surrounding Rock Support Technology Based on a Post-Peak Strain-Softening Model of Rock Mass

To control the large deformation that occurs in deep shaft-surrounding rock, the post-peak strain-softening characteristics of deep jointed rock mass are discussed in detail. An equivalent post-peak strain-softening model of jointed rock mass is established based on continuum theory and the geological strength index surrounding rock grading system, and numerical simulations are performed using FLAC3D software. The convergence-constraint method is used to analyze the rock support structure interaction mechanism. A composiste support technique is proposed in combination with actual field breakage conditions. During the initial support stage, high-strength anchors are used to release the rock stress, and high-stiffness secondary support is provided by well rings and poured concrete. This support technology is applied in the accessory well of a coal mine in Niaoshan, Heilongjiang, China. The stability of the surrounding rock support structure is calculated and analyzed by comparing the ideal elastic-plastic model and equivalent jointed rock mass strain-softening model. The results show that a support structure designed based on the ideal elastic-plastic model cannot meet the stability requirements of the surrounding rock and that radial deformation of the surrounding rock reaches 300 mm. The support structure designed based on the equivalent joint strain-softening model has a convergence rate of surrounding rock deformation of less than 1 mm/d after 35 days of application. The surrounding rock deformation is finally controlled at 140 mm, indicating successful application of the support technology.

Numerical Simulation of Ultra-Shallow Buried Large-Span Double-Arch Tunnel Excavated under an Expressway

The temporal and spatial effects of a complicated excavation process are vital for an ultra-shallow buried large-span double-arch tunnel excavated under an expressway in service. Numerical simulations are urgent and necessary to understand the effect of the total construction process. Taking Xiamen Haicang tunnel as a research object, the total excavation process of three pilot tunnels and the three-bench reserved core soil method of an ultra-shallow buried large-span double-arch tunnel with a fault fracture zone under an expressway was simulated using software FLAC3D. The deformation of the surface, surrounding rock, underground pipelines, tunnel support structure and partition wall of the three pilot tunnels and the main tunnel was analyzed, and the dangerous areas and time nodes were obtained. When the tunnel was excavated to the fault fracture zone, the deformation of the surface and surrounding rock increased significantly. The rock and soil within 20 m behind the excavation surface of the pilot tunnel were greatly disturbed by the excavation. During the excavation of the main tunnel, the horizontal displacement of the middle partition wall moved slightly towards the main tunnel excavated first. The research results can provide a reference for the construction design of double-arch tunnels.

A semi-analytical method for elastic-plastic analysis of a deep-buried elliptical tunnel

The elastic–plastic problem of an elliptical tunnel is of great significance in engineering. Previous studies were mainly focused on the circular tunnel. An elastic–plastic analysis is performed for determining the plastic zone around a deep-buried elliptical tunnel subjected to far-field uniform in-situ stresses. The yielding of the rock mass obeys the Mohr-coulomb criterion. The elastic stress field is given by the complex variable method and the plastic stress field is numerically determined by the slip line theory. The solution is obtained according to the stress continuity condition along the elastic–plastic interface. Correspondingly, a set of nonlinear equations involves the mapping function coefficients is established and solved with the help of differential-evolution algorithm. The validity of the method is verified by comparing the results with the numerical simulation of FLAC3D software. Estimates of the size and shape of the plastic zone are then presented for varying parameters including in-situ stress, cohesion, internal friction angle and axis ratio.

Influence of horizontally variable soil properties on nonlinear seismic site response and ground motion coherency

AbstractThe objective of this study is to quantify the effect of the spatial variability of soil properties on site response and ground motion coherency. A 2D random field theory is used to simulate soil parameters, including shear wave velocity (Vs) and nonlinear stress–strain curves. Different levels of spatial variability are controlled by coefficient of variation (COV) and correlation length (CL). The wave prorogation is simulated through the 2D finite difference software FLAC2D. Different types of analysis conditions, such as 1D or 2D model, linear or nonlinear analysis, and input motion, are investigated. The increase of Vsvariability leads to a decrease in the mean amplification ratio (surface response spectrum to bedrock response spectrum) but an increase in its variation. The effect of the variability of soil nonlinearity on the mean amplification ratio is marginal compared with that induced by Vsvariability. However, as the ground motion intensities increase (or more nonlinearity induced), the standard deviation of the simulation normalized by the baseline response increases. Compared with the 2D result, the 1D site response analysis that is commonly used in practice may potentially underestimate the mean amplification ratio but overestimate its variability due to the ignorance of the horizontal soil property variability. In the evaluation of the spatially correlated site response, the ground motion coherency decays significantly, as the COV of Vsand the ground motion intensity increase. The effect of CL on ground motion coherency is minor and limited to a separation distance within two‐times of CL. Therefore, a coherency model, which eliminates the CL term but includes the ground motion intensity (i.e., nonlinear effect) parameter, is proposed in this study. It is revealed to yield a better agreement with the simulation results than other coherency models.

Investigating the Effect of Geological Heterogeneity of Strata on the Bearing Capacity of Shallow Foundations Using Markov Random Field

Geological heterogeneity is a result of the natural deposit process of strata with a highly irregular and uncertain distribution of soil mass in the real world. However, compared with the effect of inherent spatial variability of soil properties on geostructures, the effect of geological heterogeneity has received less attention so far. In view of this limitation, this paper adopts a Markov random field model to stochastically simulate the geological heterogeneity for assessing the bearing capacity of a shallow foundation with quantified uncertainty. By mapping the generated stratigraphic realizations in the commercial software FLAC3D version 7.0, an ensemble of numerical models is generated. Computational results show that geological heterogeneity has a significant influence on the estimated capacities. Information entropy is used to characterize the geological uncertainty. It is not surprising that the increase of boreholes could reduce the average information entropy, and thus reduce the standard deviation of bearing capacities. But the effect of increasing the borehole numbers on the uncertainty reduction exhibits a clear nonlinear form with a potential to select an optimal borehole number. Results from the proposed stochastic analysis provide a range of calculated capacities, which can be used to mitigate the risk of insufficient bearing capacity.

Numerical analysis of a double layer rubber dam

A new rubber dam with two layers of dam bodies anchored closely into a rigid base is proposed to improve the water-retaining ability of the conventional rubber dam. Numerical studies using FLAC2D software are conducted to analyze the behavior of the proposed double layer rubber dam. Laboratory model tests are also carried out to verify the accuracy of the numerical model. It is found that the optimal internal water head in the upstream dam normalized by its cross-sectional perimeter ranges from 0.35 to 0.45, and that in the downstream dam ranges from 0.32 to 0.40. The double layer rubber dam has optimal perimeter ratio of 0.8 and optimal anchoring distance normalized by cross-sectional perimeter of upstream dam of 0.08. For same ultimate external water head, the cross-sectional perimeter of the double layer rubber dam increased by 25.7%–49.6% than that of conventional rubber dam in different design parameters. However, the requirements of the maximum circumferential tensile strength of the upstream and downstream dams decreased by 40.8%–62.9% and 51.9%–69.6%, respectively. Assuming the material cost is determined by length and maximum circumferential tensile strength of membrane materials, the total material costs of the proposed double layer rubber dam reduce at most 82.2% than those of the conventional rubber dam in different design parameters.

Research on the Controlling Effect of NPR Cables for Anti-Dip Slope Based on the Numerical Simulation

As the scale and depth of mines increase, large deformations of high-steep slopes progressively become prominent. Compared with the ordinary cables, negative Poisson’s ratio (NPR) cables can provide a constant resistance force and high deformation inhibition during slope deformation, avoiding the occurrence of slope instability hazards. Consequently, the control effects on the toppling failures of slopes were necessary to be researched. Changshanhao open-pit gold mine was taken as an example; based on the field geological investigation and rock mechanics testing, a three-dimensional engineering geological model of open-pit mine was constructed. Subsequently, the stability of open pit in current situation and final boundary situation was estimated with FLAC3D software, for the potential slope vulnerable areas to be comprehensively identified. Finally, the control effects of ordinary cables and NPR cables on the instable W13 slope section were compared and studied through FLAC3D simulations, and the reinforcement effects of NPR cable on the anti-dip slope were proved as significant; meanwhile, the corresponding reinforcement methods in the failure mine areas were proposed, laying a reference for the instability failure control and reinforcement of similar anti-dip slopes.