FLAC3D Software Research Articles

Impact of spatial variability of shear wave velocity on the lagged coherency of synthetic surface ground motions

The spatial incoherence of ground motion during an earthquake can have a significant effect on the dynamic response of engineering structures such as bridges, dams, nuclear power plants and lifeline facilities. The main objective of this paper is to study the effect of anisotropic heterogeneities in a soil layer overlying homogeneous bedrock on the lagged coherency of surface ground motion. A set of numerical experiments is performed based on 2D spatial variability of shear-wave velocities modeled as a homogeneous stationary random field and discretized by the EOLE method (Expansion Optimal Linear Estimation). Seismic ground motions were simulated using FLAC2D software in the 1–25 Hz band for a plane wave excitation with SV polarization. The soil is characterized by horizontal and vertical autocorrelation distances ranging between 5 and 20 m and 1 and 2 m, respectively, and a coefficient of variation of the shear-wave velocity varying between 5% and 40%. The synthetic seismograms calculated for 9 parameter sets (100 realizations each) clearly show seismic waves scattering and surface waves diffracted locally by the ground heterogeneities, generating large spatial variations in coherence mainly controlled by the coefficient of variation of shear-wave velocity. Consistently with existing models and experimental data, the numerical coherency curves decrease with frequency and receiver distance, however at a rate which is lower than that observed in the experimental data. This difference is probably due to intrinsic attenuation that is not accounted for in the simulations and/or to our 2D simulations that do not reproduce the complete wavefield. The numerical average coherency curves for each parameter set exhibit maxima within narrow frequency bands caused by the vertically trapped body waves and surface wave propagation properties within the average ground model. This interpretation is supported by experimental data recorded in the Koutavos-Argostoli valley (Greece).

The segmental subsidence structure with immediate roof of gob side entry retaining in backfill mining

When gob side entry retaining is carried out in backfill mining, the roof will show different subsidence morphology due to the difference of compactness and supporting force of the backfill body at different positions. This paper analyzed the immediate roof subsidence structure under two extreme conditions, constructed the roof segmented subsidence structure and the mechanical model of roadside backfill body, and used FLAC3D software to investigate the roof migration and the force law of the roadside backfill body under the conditions of different goaf backfilled rates, different width and strength of roadside backfill body. Finally, the backfill practice of a mine in Shandong Province of China is taken as an example for analysis. The results show that the segmented subsidence structure of the immediate roof is related to the mechanical properties of the roadside backfill body and the goaf backfill body. When the backfilled rate of goaf decreases from 95% to 70%, the width of roadside backfill body decreases from 5 m to 1 m, and the elastic modulus decreases from 10 GPa to 0.5 GPa, the greater difference in the subsidence and inclination of the immediate roof on both sides of the roadside backfill body is, the more obvious the segmented subsidence structure characteristics of the immediate roof are, and the greater force on the roadside backfill body will be, the more unfavorable it is to maintain the stability of the roadway surrounding rock and the roadway backfill body. Therefore, when gob side entry retaining is carried out in backfill mining, the surrounding rock structure and the force on roadside backfill body should be considered comprehensively.

The effects of dynamic pressure on the stability of prepared drifts near the working surface areas

Due to the effects of dynamic pressure, the stress distribution of rock mass is very complex. The reason for this could be a risk of stability loss for an auxiliary tunnel system constructed within the study area. In this article by using Flac3D software the author simulated two adjacent working faces with the thickness of 5 m natural coal pillar. Three factors: the upper working face excavation process, auxiliary tunnel mining process, and the location of lower working face, affected by deformation, stress distribution, safety of lower floor area and surrounding rock mass of tunnel. The research results show that during the excavation, the mechanical behavior of the rock mass surrounding the auxiliary tunnel showed displacements, volatility, and phase characteristic. The displacement on the auxiliary tunnel boundary in both excavation and working face cases showed that the roof and the left side wall displacement was greater than the right side wall and the bottom. Therefore, the distance between the auxiliary tunnel and the empty mining space needs to be computed to meet technical and economic requirements.

Influence of Internal Friction Angle and Interface Roughness on Shear Behavior of Mortar-Rock Binary Medium Joint

The damage of the interface between mortar and rock often occurs in engineering projects. The stability of the binary medium joint is the key factor in judging the stability of a hydraulic structure foundation. The shear characteristics of the flat mortar-rock joint were numerically simulated under the direct shear test by finite element using FLAC3D software. The results showed that the internal friction angle of mortar has little influence on the shear strength, its corresponding displacement, strain softening degree and residual shear strength for flat joint. Therefore, considering the roughness of the structural surface, the joint with regular serrated mortar-rock binary medium was established in this paper, and the direct shear test under the condition of constant normal stress was carried out for comparative study. The results showed that the shear strength, residual strength, peak strength displacement and strain softening degree of the serrated structure were strongly correlated with the internal friction angle of mortar. For the regular serrated binary joint, improving the mortar performance can improve the shear performance and stability of the whole structure more effectively than the flat joint. The relationship between the shear strength, residual strength and the friction angle were established respectively.

Study on Instability Mechanism of Extraction Structure under Undercut Space Based on Thin Plate Theory in Block Caving Method

The instability of extraction structure under the undercut space in the block caving stope presents specific characteristics: rib spalling and floor heave in ore-loading roadway and collapse of major apexes. In order to study the stress and displacement evolution law of extraction structure under undercut space and reveal the instability mechanism of extraction structure, the numerical simulation model of block caving stope was established using the finite difference software FLAC3D. According to the postundercutting strategy in Tongkuangyu Mine in China, extraction structure was formed first in the simulation process, and then the undercut level was divided into eight units for excavation step by step. The stress and displacement of extraction structure after each step of undercutting were monitored and analyzed. Based on the thin plate theory, the mechanism of stress change and deflection deformation of extraction structure was revealed. The research results show that, under the action of high horizontal tectonic stress and vertical stress, the extraction structure under undercut space produces vertical upward bending deformation after undercutting during the block caving. The tension stress concentration gradually appears in the side wall of the ore-loading roadway and the tip of the major apexes; with the increase of the undercutting area, the degree of tensile stress concentration gradually becomes strong; when the tensile strength of the rock mass in extraction structure is exceeded, extraction structure presents instability. It is necessary to make the overlying ore collapse on extraction structure as soon as possible after undercutting, which is beneficial to release the tension stress in the extraction structure under undercutting space.

Construction Technique of Vertical Shafts Excavation Above Subway Tunnel

Taking the foundation pit construction of the section K0 + 700 ~ + 900 of Shuangjiehe Road, Tinghai Road and other underground roads in Qianhai District of Shenzhen as an example, this paper introduces a new key construction technology of deep and large foundation pit excavation in the coastal area of reclamation to control the uplift and deformation of underground subway, which effectively utilizes the principle of space–time effect. Based on the number of finite difference software FLAC 3D numerical analysis, the skip excavation method and key construction parameters are simulated and optimized. The monitoring data show that the excavation method, sequence and construction protection measures have significant effect on the deformation control of subway tunnel, which can provide reference for similar projects.

Numerical Analysis of Twin Tunnels Lining under Different Seismic Conditions

The last seismic events showed that tunnel lining may suffer extensive damage. Employing numerical modeling has a great importance in predicting the seismic performance of tunnels. This paper tests the tunnel lining of the Zaouit Ait Mellal (ZAM) twin tunnels located between the cities of Marrakesh and Agadir in Morocco. Dynamic analysis was adopted by FLAC 2D software using the finite-difference elements. Four soil cross-sections were chosen, with different support devices installed along the twin tunnels, such as rock bolts and steel ribs. The seismic signals introduced as input were obtained from three different earthquakes: Al Hoceima 2004 in Morocco, EL Centro 1940 in the USA, and Kobe 1995 in Japan. The numerical results show that the deformation of the tunnel lining is more noteworthy in the sections using steel ribs compared to those using rock bolts, which is observed by the large values of relative displacement, reaching 1020 (mm) and 2.29 × 105 (N.m/m) of maximum bending moment. The analysis indicates that these sections present higher vulnerability during an earthquake, which should be considered when looking at the overall safety of the tunnel.

Stability analysis of slope in soil with strength mobilization

Considering the strength parameters cohesion c and internal friction angle < of the soil constitutive model are mobilized non-simultaneously. By gradually applying uniform vertical loads on the top of the slope, the progressive failure process of the slope is simulated by the FLAC3D software. The failure state of the slope is judged by monitoring the maximum unbalance force in the slope system, and the slope sliding surface is determined based on the maximum shear strain increment, and the safety factor is calculated by the slope critical height ratio method. The numerical simulation shows that when the strength parameters are not synchronicity come into play, the safety factor is less than that of two cases, which consider the synchronous mobilization of strength parameters and the traditional Strength Reduction Method (SRM). Finally, the influence of residual cohesion and plastic strain threshold on the position of sliding surface and safety factor are discussed by numerical analysis.

Numerical simulation of a single pile under the combined effects of axial and lateral loading in liquefiable soil

In this research, a numerical investigation was carried out using FLAC 2D software to develop an understanding of the effects of combined (axially and laterally) loading on a single pile subjected to different earthquake motions. Several parameters were studied, including vertical displacement at pile head and soil surfacelateral displacement with pile depth, and pore water pressure ratio (ru) at the near and far-fields within the soil model; these were then compared with the soil shaking table test, and the numerical model was solved with the Finn Model and Mohr-Coulomb to capture the non-linear effects of saturated sand under earthquake motions in Ali Algharbi and Kobe. The main test results from this numerical analysis showed that the lateral load on the pile headsignificantly reduced pile lateral displacement, yet had substantial effect on both pile and soil vertical displacement even at higher earthquake magnitudes. A comparison of pore pressure ratios at near and far-field between the numerical and experimental work showed a good agreement, with only a slight difference in overall values. The importance of this study and the test results imported from the numerical model lies in the generation of a reliable base for liquefaction analysis in more complex cases.

Study on Stress Characteristics of Multi-arch Tunnel Lining under Weak Surrounding Rock Conditions Based on Field Measurement

Considering the complexity of stresses on multi-arch tunnel structures under conditions of shallow depth, unsymmetrical pressure and weak surrounding rocks and the actual geological exploration of the Xiaojian Mountain Multi-arch Tunnel, the cross-sections in strongly-weathered section were selected for embedment of monitoring and measurement equipment to monitor in real time the stress of support structures during tunnel construction. In addition, the finite difference software FLAC3D was adopted to study the surrounding rock deformation during the excavation of the multi-arch tunnel. As a result, the stress state and deformation characteristics of the support structures under unsymmetrical pressure and construction disturbance were obtained. Based on the analysis of monitoring data, it can be concluded that under conditions of shallow depth and unsymmetrical pressure, reduction of stress imbalance on the support structures is of utmost importance to the structural stability of a multi-arch tunnel and that the method of cross-validation between monitoring & measurement and numerical simulation enables more accurate analysis of stability of tunnel structure and better guidance for construction.

Study on rib spalling control of shortwall drilling mining technology with a large mining height

The drilling shearer and drilling mining technology has been developed in order to effectively control the rib spalling of a shortwall with a large mining height face in a soft thick coal seam. Working face no. 1305 of the Zhaozhuang Coal Mine was taken as the research subject. The rationality and superiority of shortwall drilling mining technology with a large mining height (SDMTLMH) were explained by combining the theoretical analysis with a numerical simulation. First, based on the occurrence characteristics of the immediate roof, the mechanical model of a simple supported beam was established to calculate the maximum unsupported roof distance. The advance of the working cycle was determined. Then, based on the spatial mobilization plane (SMP) theory, the abutment pressure distribution in the reserved coal was deduced by establishing the unit mechanical model. The appropriate range of the width of the reserved coal (WRC) was determined according to the principle of conservation of energy. Finally, six drilling mining schemes were proposed according to the range of the WRC. In addition, we redeveloped the constitutive model of supports and compacted rock mass in the gob using FLAC3D software. The distribution of the abutment pressure in the drilling mining face and the control effect of the rib spalling were obtained. It was proved that the SDMTLMH can effectively suppress rib spalling and roof fall. The distribution of the strain energy and the external potential energy in reserved coal in six schemes was plotted by Surfer software. The rationality of the theoretical analysis was verified. Considering the caving effect of the reserved coal, the control effect of rib spalling, and the appropriate advancing speed of the working face, the appropriate WRC was determined to be 0.4 m. A field investigation showed that the control effect of rib spalling for working face No. 1305 was much better than that of other large mining height faces in this coal mine.

Combined support mechanism of rock bolts and anchor cables for adjacent roadways in the external staggered split-level panel layout

Using the spatial structure of the external staggered split-level panel layout, a combined support technology for adjacent roadways was developed and analyzed for a rock bolt and anchor cable mechanism. The influence of the side rock bolt and anchor cable parameters on the mechanical properties of the anchorage body and the support stress distribution of the lateral coal body were revealed using the FLAC3D software. The optimal support parameters of the side rock bolts and anchor cables were subsequently determined, and the support effect of gob-side entry in a mining scenario was verified. The results show that the support of the side rock bolts and anchor cables improves the mechanical properties and stress state of the anchorage body, producing a good protective effect on the coal body of the air-intake entry roof and side wall. This is beneficial to the stability of the side wall and the realization of the suspension effect for roof rock bolts and anchor cables, which in turn makes the surrounding rock maintenance of the gob-side entry to a thick coal seam more favorable.

Numerical stability analysis of superhigh-fill subgrade underlying acute inclined mined-out area with underground coal fire

According to the borehole drilling detection results of a superhigh-fill highway subgrade that crossed an open-pit coal mined-out area with an underground coal fire (UCF) in Urumqi, China, a numerical model of the subgrade and substrata was established. The variations of the stability characteristics of the highway pavement and the subgrade slope with respect to the substrata properties under the effects of coal-seam combustion were analyzed via thermal-stress coupled calculations using the finite-difference software FLAC3D. When the coal body underlying the subgrade underwent burnout and the combustion cavity formed, the bearing capacity of the strata decreased significantly. Under the effect of the heavy self-weight of the high-fill subgrade body and other external loads, such as the vehicle load on the pavement, collapse was likely, followed by the formation of a large sink basin on the surface of the subgrade. The displacement of the highway suffering from UCF increased gradually. The displacement development was divided into two stages: the swelling stage due to rock-mass expansion and the collapse stage due to the burnout of the coal body. The concrete geocell had a good reinforcement effect on the subgrade in which the settlement of the basin occurred under the effects of the UCF. When part of the geocell was deformed under external loads, the loads were transferred to the neighboring concrete geocell located in the stable region. Thus, the concrete geocell in the stable region constrained the geocell in the unstable region from deforming. Furthermore, a parametric analysis with varying combustion conditions indicated that a higher heat-release intensity, wider combustion region, and shallower combustion depth lead to greater stability hazards for the overlying subgrade.

An Innovative Technology for Monitoring the Distribution of Abutment Stress in Longwall Mining

Fracturing roofs to maintain entry (FRME) is a novel longwall mining method, which has been widely used in China, leading a new mining revolution. In order to research the change law of side abutment pressure and movement law of overlying strata when using the FRME, a new abutment pressure monitoring device, namely, the flexible detection unit (FDU), is developed and is applied in the field. The monitoring results show that compared with the head entry (also called the non-splitting entry), the peak value of the lateral abutment pressure in the tail entry (also termed the splitting entry) is reduced by 17.2% on average, and the fluctuation degree becomes smaller. Then, finite difference software FLAC3D is used to simulate the stress change of the solid coal on both sides of the panel. The simulation results show that the side abutment pressure of the tail entry decreases obviously, which is consistent with the measured results. Comprehensive analysis points out that after splitting and cutting the roof, the fissures can change the motion state of the overlying strata, causing the weight of the overburden borne by the solid coal to reduce; therefore, the side abutment pressure is mitigated.

Stability Analysis on the Long-Term Operation of the Horizontal Salt Rock Underground Storage

The construction of the vertical cavern in the salt dome deposit can meet the requirements of both storage capacity and tightness. However, if the vertical cavern is still used as the design shape of the salt rock underground storage in the layered salt rock deposit, the high design capacity cannot be guaranteed while the tightness is satisfied. In this case, the use of a large-span horizontal cavern as the design shape of the salt rock storage can not only effectively increase the design capacity of the storage, but also solve the problems such as the stability and tightness of the storage during the operation period by improving the structural form and working mode. Based on this, the ellipsoid-shaped horizontal salt rock underground storage is taken as an example, and a single-cavern horizontal salt rock underground storage model with different diameter-to-height ratios is established by using FLAC3D software. The change law of vertical and horizontal displacements, volume loss rate, and plastic zone distribution of salt rock storage changing with the diameter-to-height ratio are studied, and the optimal diameter-to-height ratio is determined. And then the long-term operation process of the double-cavern horizontal salt rock underground storage under the optimal diameter-to-height ratio is simulated, and the optimal pillar width is obtained.

Numerical Modeling and Stability Analysis of Surrounding Rock of Yuanjue Cave

Yuanjue Cave, located in Big Buddha Bay, Mount Baoding, Dazu Rock Carvings Area, has extremely high historical, artistic, and religious value and is an important grotto cultural relic in China. Due to the cutting action of the fissures and weak interlayers, the South Cliff of Big Buddha Bay where Yuanjue Cave is located showed signs of instability. In order to fully evaluate the stability of the rock mass around the cliff where Yuanjue Cave is located, a three-dimensional geological model of the surrounding rock of Yuanjue Cave was established by using FLAC3D software, combined with three-dimensional scanning, fissure investigation, and indoor tests. The stability of the surrounding rock mass adjacent to Yuanjue Cave has been studied by precise numerical simulation, and the results of numerical simulation and monitoring have been compared and analysed. The results show the following: (1) The west of J10 fissure, above the mudstone interlayer, is the main deformation area. The cliff displacement increases gradually from the east to the west. The independent block above the corner has the largest free space displacement, and there is a risk of independent collapse. Special attention should be paid to the stability of this block. The displacement of the upper monitoring point of the cliff wall is significantly greater than that of the lower layer. (2) In the surrounding rock block in the adjacent area, the various concentrated stresses of the body are mainly located at the entrance of Yuanjue Cave, the height of the chest of the Zhengjue Buddha statue, and the lower mudstone erosion and reinforcement zone. Among them, the stress concentration in the erosion and reinforcement area under the Zhengjue Buddha statue is the largest. The conclusions obtained can provide a useful reference for the stability assessment of the surrounding rock of Yuanjue Cave.

Seismic vulnerability of circular tunnels in sand

The paper is focused on the assessment of seismic fragility curves for circular tunnels under moderate to severe earthquakes with the aim of improving the reliability of the risk assessment of underground infrastructural networks. Non-linear two-dimensional dynamic analyses were performed on different tunnel and soil configurations by using the finite-difference method implemented in Flac2D software. The applied input motions were selected considering their amplitude and frequency content variability. The response accelerations and predominant frequencies computed at ground level, above the tunnel, were compared with the corresponding free-field results to distinguish the effects attributable to the tunnel presence from those due to the site amplification. Tunnel safety was assessed through fragility curves, taking into account the dependence of tunnel lining bending resistance on the axial force variation during the earthquake. The more effective intensity measure was investigated correlating the tunnel performance to peak ground accelerations and peak ground velocities computed at the ground level and at the bedrock depth, respectively. The resulting fragility curves showed a satisfying matching with the empirical ones, generated on the basis of the observed seismic damage on tunnels.

Повышение безопасности при извлечении водорастворимых руд путем оптимизации параметров закладочного массива

The results of the scientific and research work devoted to modeling the stress-strain state of the mining system of an underground mine using the finite element method in the FLAC3D software are presented in the article. The possibility of using room-and-pillar mining with backfill with the abandonment of chain pillars is justified. The results are presented concerning the research results of the quantitative assessment of the mass using the von Mises equation and Norton' power law of creep to estimate the contour stress. It becomes possible to determine the sequence of mining of the deposit area and the stage-by-stage filling of the stopes at the stage of mining design due to the high accuracy of modeling the stress-strain state of the mass near mining operations using FLAC3D software. The proposed approach is possible when evaluating the geotechnical state of the rock mass when using the mining systems with various methods of support of the mined-out area: natural; artificial; with the collapse of ores and enclosing rocks, as well as when calculating the combined method of deposit development. A differentiated approach to assessing the state of the mass is possible considering difficult mining and geological conditions, conditions of increased rock pressure, abundant water influx, increased fracturing of rocks by introducing additional conditions into the model depending on its geotechnical state. Variant of the comprehensive exploitation of mineral resources was proposed considering the total value of natural and man-made geo-resources of the area being mined. The possibility of rational use of the mined-out space with the use of various positive qualities of geo-resources at their manmade transformation is also considered. An assessment of the mineral and raw materials potential of the mined area is given with a proposal for the prospects of increasing the integrated approach of excavation, increasing the life of the mining enterprise and the complexity of developing this area.

Numerical modeling of twin tunnels under seismic loading using the Finite Difference Method and Finite Element Method

Considerable advances in the development of user-friendly modeling code against the limitations of analytical methods have led to an increase in the utilization of numerical methods to predict the dynamic behavior of tunnel lining under seismic loading. This paper examines the seismic performance of the ZAM (Zaouit Ait Mellal) twin tunnels located between the cities of Marrakesh and Agadir in Morocco, using the Finite Difference Method (FDM) provided by FLAC 2D software, and Finite ElementMethod (FEM) provided by PLAXIS 2D software. The adopted simulated acceleration is obtained from an artificial earthquake event with free-field boundary conditions in the numerical models. Three points are chosen in the tunnel lining: two in the corner and one in the crown. The results show the evolution displacement during the dynamic time for each point, we note different distributions of values in FLAC compared with PLAXIS, which these differences are justified by the type of interface used in both programs.

Research and Application of Hydraulic Punching Pressure Relief Antireflection Mechanism in Deep “Three‐Soft” Outburst Coal Seam

To solve the problems of gas predrainage in deep seams with “three softs” and low‐air permeability, hydraulic punching pressure relief antireflection technology is proposed on the basis of the research background of gas predrainage technology in Lugou Mine to alleviate technical problems, such as low gas drainage efficiency, in this mine. Through the analysis of the mechanism of hydraulic punching and coal breaking, combined with FLAC3D software, a hydraulic punching pressure relief antireflection model is established. Then, the fracture radii of coal rock are simulated and calculated. The results show that, under hydraulic punching with a water pressure of 10 MPa and coal outputs of 3 m3, 6 m3, 9 m3, and 12 m3, the fracture radii of coal and rock are 3.4 m, 4.8 m, 5.5 m, and 5.9 m, respectively. Using the software to fit the relationship between coal output V and hydraulic punching fracture radius R under the same water pressure, R = 2.32479 V0.3839 is obtained. The field test is carried out in the bottom drainage roadway of 32141 in Lugou Mine. The application effect is as follows: the gas concentration of hydraulic punching with a coal output of 3 m3 is twice that of ordinary drilling, and the coal output of hydraulic punching with a coal output of 6 m3 is four times that of ordinary drilling. The extraction concentration is four times that of ordinary drilling, and the extraction concentration of hydraulic punching with a coal output of 9 m3 is 6.4 times that of ordinary drilling. Combining the results of the numerical simulation and taking into account the actual construction situation on site, the coal output of water jetting from the borehole is 9 m3, and the fracture radius is 5.5 m. This outcome means that the effective half radius is 5.5 m, and the borehole spacing is 7.7 m. These values are the construction parameters for large‐scale applications. This proposal provides effective technology and equipment for gas drainage in the deep three‐soft coal seam. Consequently, it has promotion and reference significance for gas drainage in coal seam of the same geological type.