FLAC3D Software Research Articles

The response characteristics and stability evaluation of vegetated slope under strong wind

As one of the major geological disasters in southeastern China, typhoon-induced vegetation slope instability causes significant loss of life and property each year. Despite the criticality of this issue, the response mechanism of vegetated slopes to wind loading in terms of soil deformation and stability still remains unclear. This research conducted field investigations on 330 historical landslides in Yongjia County, Zhejiang Province, analyzing their spatiotemporal distribution and developmental characteristics to establish a conceptual model. The influence of wind and vegetation conditions on slope dynamic parameters and soil strength parameters were subsequently determined through numerical simulations using FLAC3D software, model tests, and direct shear tests. The results show that: (1) The influence of wind on the dynamic response of vegetated slopes is significantly affected by wind speed. At wind forces ≤ 12 (hurricane: 32.7–36.9 m/s), vegetation plays a positive role in enhancing slope stability. At wind forces ≥ 13 (typhoon: 37.0–41.4 m/s), vegetation exerts a negative influence under the combined action of strong wind loads. (2) Based on the experimentally determined evolution of soil strength parameters, a formula c (v, w) was fitted to express the variation of cohesion with wind speed (v) and moisture content (w). (3) The optimized evaluation model demonstrates increased sensitivity compared to the traditional model, resulting in a 17.88% increase in sliding force and a 10.62–11.64% increase in anti-sliding force. This research accounts for both the direct and indirect effects of wind loading on vegetated slopes. The findings are expected to enhance the stability assessment of vegetated slopes under strong winds and facilitate the development of more accurate machine learning and statistical models in the future.

Study on the influence of anchor plate parameters on the bearing characteristics of the new large-diameter multi-plate soil anchor and creep property of anchor

The existing mechanical and grouting anchors mostly use the expansion shell method to form a cavity on the borehole wall, and the cement slurry is poured to form multiple enlarged head plates, but the operation is more difficult and the diameter of the formed plate is smaller. In this paper, a new type of large-diameter multi-plate soil anchor and its reaming cavity forming tool are proposed, which can make the operation easier and form a large-diameter enlarged head plate. In order to study the influence of the diameter of anchor plate, the number of anchor plates and the spacing of anchor plates on the vertical uplift capacity of the large-diameter multi-plate soil anchor, 25 sets of comparative models were established for simulation analysis. The finite difference method of FLAC3D software is used to simulate the model. It is found that when the length of the anchor is 6 m and the diameter of the anchor rod body is 150 mm, the optimal diameter of the anchor plate of the large diameter multi-plate soil anchor is 590 mm, the optimal number of anchor plates is 6, and the optimal anchor plate spacing is 800 mm, which means the action range of the anchor plate on the lower soil is about 5 times the diameter of the bolt. When the number of anchor plates is too small or the spacing between anchor plates is too large, the structural advantages of large-diameter multi-plate soil anchor cannot be fully utilized, resulting in a decrease in the ultimate uplift capacity. When the number of anchor plates is too large or the spacing between anchor plates is too small, the stress superposition effect occurs in the soil, and the through shear failure occurs, which leads to the decline of the ultimate uplift capacity. Under the condition that the number of anchor plates and the spacing of anchor plates are fixed, the larger the diameter of the anchor plate is, the larger the ultimate pull-out capacity of the large-diameter multi-plate soil anchor is, the smaller the vertical failure displacement of the anchor head is, but the increase of the uplift capacity is gradually reduced. The creep rate of the new large-diameter multi-plate soil anchor bolt is 0.91 mm, and the creep rate of equal-diameter soil anchor bolt is 1.69 mm. It is verified that the new large-diameter multi-plate soil anchor can be effectively applied to various projects.

Numerical Simulation Analysis of the Influence of Interlayer Quantity on the Long-Term Stable Operation of Gas Storage Facilities

Salt rock is considered as an ideal energy storage medium, and compressed air energy storage by a salt cavern can improve the utilisation efficiency of renewable energy. Salt rock in China mostly contains different interlayers, among which mudstone interlayers are the most common. At present, there are relatively few studies on the influence of mudstone interlayers on the long-term stable operation of gas storage. FLAC3D software was used to simulate the long-term operation of salt rock gas storage with different numbers of interlayers in the Yexian area of Pingdingshan. The results show that with the passage of time, the vertical displacement of the surrounding rock of the vertical single-cavity gas storage tank increases gradually. The maximum settlement value at the top of the surrounding rock is always greater than the maximum uplift value at the bottom. The horizontal displacement shows obvious symmetry with the vertical displacement at the top and bottom of the surrounding rock. The effect of the cyclic pressure interval on horizontal displacement is the same as that of vertical displacement. With the increase in the number of interlayers, the volume of the plastic zone gradually increases with the increase in the running time, and the increasing speed shows a growing trend.

Numerical analysis of shape coefficients and bearing capacity factors of shallow foundations on heterogeneous soil

We often face heterogeneous foundation soils in geotechnics, which significantly influence shallow foundations' shape coefficients and bearing capacity factors. The evaluation of these factors introduces significant uncertainties in the thickness of the upper layer is comparable to the width of the rigid footing placed on the soil surface. This ambiguity depends on the geometry of the footing, the mechanical properties of the soil, and other geotechnical factors. Conducting thorough geotechnical studies and performing specific analyses are essential to accurately assess the effect of clay layering on the shape coefficients and bearing capacity factors of circular and square footings. A numerical analysis using the FLAC 2D and 3D software is carried out to evaluate the effect of the superposition of two undrained clay layers on the bearing capacity and shape factors of circular and square footings subjected to a static axial load. The aspects of homogeneous and heterogeneous shear strength profiles were addressed at various positions of the height of the first clay layer. It was found that the critical depth of the first clay layer necessary to optimize the bearing capacity of a foundation resting on stratified soil is 2, 1.5, and 1, respectively, for strip, square, and circular footings. The shape coefficient values range between 1.14 and 1.22 for square footings and between 0.98 and 1.64 for circular footings. The results are compared to previously published values in the literature.

Deformation-related earth pressure within grid wall-pattern foundation under adjacent surcharge

This study investigated the deformation-dependent earth pressure evolution on the periphery of the grid wall-pattern foundation fully embedded into soft ground. The numerical modelling was performed using the embedded Modified Cam Clay Constitutive model in the FLAC3D software platform and numerical results were validated using centrifuge results. The induced effective vertical and horizontal stresses acquired from the numerical modelling were used to compute the corresponding deformation-dependent earth pressure at different buffer zone distances and surcharge loading. Thereafter, the parameters that influence the determination of the lateral displacement, induced effective vertical stress and deformation-dependent earth pressure coefficient were investigated using machine learning and genetic programming was used to establish the physical equations that could be used to estimate parameters for similar problems. Moreover, for the deformation-dependent earth pressure to be within Rankine's active and passive earth pressure the buffer zone distance/depth ratio should be 10% of the grid wall-pattern foundation embedment into the ground. Furthermore, it was also discovered that the structure fully embedded into the ground under adjacent loading cannot only be designed based on Rankine's active and passive earth pressure investigation on the periphery but the evaluation should be done within the entire location of the grid wall foundation.

Research on the method of identifying vertical earth pressure of hinged prefabricated culvert box culvert on the top slab

In China, several expressways have been designed as prefabricated box culverts with hinge connections, which have different structural features from the prefabricated culverts in other countries. The difference would contribute to the culvert–soil interaction of prefabricated box culverts, which could affect the earth pressure on the culvert. Based on the field test and numerical simulation method, a hinged prefabricated box culvert (HPBC) with a span of 4 m and a rise of 4 m was investigated, which was applied to the Xi-Yu expressway in China. The objective of this research was to investigate the vertical earth pressure on the top slab of the HPBC culvert at different backfill heights through the field tests. The FLAC3D software was employed to conduct further analysis of the effects of backfill height, backfill modulus, and foundation modulus on the vertical earth pressure on the top slab of HPBC. The differences between the HPBC and monolithic box culvert (MBC) were also examined. Furthermore, a revised method for calculating the vertical earth pressure on the top slab was put proposed and compared with the AASHTO method for calculating earth pressure on the top of culverts and the values taken from the Chinese culvert design code. The proposed method is capable of improving the accuracy of the earth pressure approach, making it more representative of actual conditions. Subsequently, the sensitivity analysis of backfill height, backfill modulus and foundation modulus to the vertical earth pressure concentration coefficient of the top slab was carried out by using the principle of orthogonal array analysis and the modified calculation method proposed in this paper. The findings of this study offer valuable insights into the determination of culvert top earth pressure of HPBC.

Geometrization of a 3D numerical model of an underground facility based on the results of terrestrial laser scanning

Abstract The article proposes a method of combining CloudCompare, RHINO, and FLAC3D software, aimed at building numerical models of underground objects of natural or engineering origin, based on the results of measurements made using terrestrial laser scanning technology. This technology is one of the most advanced in mine survey as it enables accurate mapping of even the most complex geometries of underground facilities. This opens wide possibilities in the construction of more accurate numerical models of the behavior of the rock mass around such underground objects. The results of simulation of the behavior of the rock mass around the analyzed excavations, obtained by performing numerical calculations, allow predicting unfavorable phenomena that may occur as a result of the destruction of the rock mass and which may threaten the safety of users of underground facilities, for example, caves, tunnels, and mining excavations. In this work, we carried out measurements using a terrestrial laser scanner and obtained a “point cloud” that reproduced the geometry of the underground facility. An example is a fragment of the adit St. Johannes, which is part of the underground tourist route “Geopark” St. Johannes Mine in Krobica in Lower Silesia in Poland in the neighborhood of Gierczyn and Przecznica. In the next step, the measurement results were processed, so that it was possible to import the generated geometry into the FLAC3D software and use it to build a numerical model of the adit, based on “brick” zones. The aim of the article is to present in detail the methodology of geometrization of numerical models of underground objects with complex geometry. The author wanted the method to be as easy to use as possible, give full control over the surface structure, and not require many numerical modeling programs.

Simulation research on blasting of an open pit mine slope considering elevation conditions and slope shape factors

This study established a numerical model that considers elevation conditions and slope shape factors by the modified Sadovsky formula to analyze the vibration attenuation law of open-pit slopes under blasting vibration conditions. The blasting excavation of a slope in a certain open-pit mine in Yunfu, Guangdong, is selected as an example. Using a numerical model that considers elevation conditions and slope shape factors by the modified Sadovsky formula, a triangular pulse load was utilized to approximate the time-history characteristics of explosion vibration with FLAC3D software. The simulation results showed the radiation range of the blasting vibration seismic wave. By comparison with field monitoring data, the numerical model that considers the slope shape factor had a relative error of ∼10%, while the numerical model that disregards the slope shape factor had a relative error of ∼15%. The relative accuracy of the calculation results of the new numerical model is higher and closer to the actual attenuation law of blasting particle vibration speed, providing more reliable results for slope stability assessment. The peak particle velocities obtained from the numerical simulation results were generally higher than the field monitoring data. These discrepancies might be attributed to the use of simplified models that disregard the discontinuous structural planes within the rock mass. This study provides an important reference for the stability assessment of open-pit slopes under blasting vibration conditions, offering guidance for improving slope stability assessment and related engineering practices.

Study on the Vibration Effects of Cyclic Blasting on Bridge Structures under Construction

This study investigates the impact of cyclic tunnel blasting on adjacent bridge structures under construction. Monitoring vibration velocities of the bridge deck, piers, and middle column adjacent to the Zhanma Tian Tunnel, a three-dimensional numerical model was developed using FLAC3D software, utilizing blasting vibration test data from the Zhanma Tian Tunnel project in Guizhou. Results show that, as the distance between the bridge and tunnel increases, vibration velocity at the bridge deck decreases more rapidly compared to the base of the pier. Peak vibration velocities recorded were 0.235 cm/s for the bridge deck, 0.081 cm/s for the pier base, and a predicted 0.209 cm/s for the middle column. The impact order from blasting vibrations on the bridge structure is: pier base, middle column, bridge deck. Peak vibration velocity induced by blasting ranged between 0.05 and 0.25 cm/s, within safe limits of bridge material strength. Eight daily blasting cycles do not compromise the safety of bridge structures located 43 m away.

Study on the sliding mechanism of the external dump slope in the open-pit mine with wind-blown sand base

Wind-blow sand (WBS) is widely distributed in the “Desert Gobi” region. This study is aimed at exploring the mechanism of how different thicknesses of the WBS layer influence the slope movement of external dumps in open-pit mines. To achieve this aim, the slope of the external dump in the open-pit mining area of Panel 3 in Daliuta Coal Mine was taken as the research object. First, similar simulation experiments were performed for investigating the failure modes and deformation characteristics of the external dump slopes under three geo-morphological conditions: loess base, 10-m-thick WBS base, and 20-m-thick WBS base, respectively. The following results were obtained from the experiments. For the slope with a loess base, its failure is mainly caused by circular sliding from the dump to the interior of the loess layer. For the slope with a 10-m-thick WBS base, the sliding mode involves circular sliding from the dump area to the interior of the WBS layer, linear sliding along the WBS base, and shearing along the foot of the dump area. For the slope with a 20-m-thick WBS base, the sliding mode is circular sliding from the dump area to the interior of the WBS layer. Besides, the sliding area of the dump slope expands as the WBS layer thickens. Furthermore, the results of similar simulation experiments were verified by the finite difference software FLAC3D based on the strength reduction method, and an equation of relationship between the safety factor of the dump slope with a WBS base and the thickness of the WBS layer was derived.

Flexural loading test of a tunnel lining concrete model strengthened with carbon-fiber composite cables

In general, incorporating fiber-reinforced polymer reinforcements into arch-shaped tunnel lining concrete (TLC) is challenging. To overcome this, the TLC can be strengthened by combining a flexible carbon-fiber composite cable (CFCC) with the near-surface mounted (NSM) technique. Thus, this study evaluated the load-carrying capacity of arched concrete members strengthened by NSM–CFCC and investigated the effectiveness of the strengthening system for TLC. To measure the load-carrying capacity of arched concrete members, a specialized loading system was fabricated to conduct flexural loading tests on TLC models. The primary test models were the reference TLC models without reinforcement. TLC models bonded with a carbon-fiber sheet (CFS), and NSM–CFCC-strengthened TLC models, and two types of NSM–CFCC TLC models were tested. Eight TLC models were constructed using the conventional NSM technique in which the CFCC was embedded into a cementitious material-filled groove. The results revealed that the TLC member was not perforated by a flexural crack, even in non-strengthened concrete, owing to the arch effect. The load-carrying capacity of the NSM–CFCC TLC was remarkably higher than that of the reference and CFS strengthening models. The ultimate failure of all TLC members was caused by the crushing of the upper concrete. A finite difference analysis (FDA) for the loading test was also performed using commercial software FLAC3D. The numerical simulation results were found to be in good agreement with the cracking behavior of the strengthened TLC members observed in the experiment. These results suggest that the strengthening system using NSM–CFCC is effective for the TLC.

Analysis of the Impact of the Former Abandoned Dregs of Mountain Highways on the Construction and Safety of New Bridge Pile Foundations

In highway reconstruction and expansion projects, the impact of the abandoned dreg body produced by the original highway on the new bridge project cannot be ignored. If the abandoned dregs are not handled properly, they will cause great threat to the operation of the new highway as well as to the safety of the surrounding environment and personnel. In this paper, based on a South China highway reconstruction and expansion project, a model of the abandoned dreg body, bridge pile and anti-slide pile is established with the help of finite difference software FLAC3D 5.0. The calculation results show that compared with burying the anti-slide pile, slope cutting is more effective for improving the stability of the abandoned dreg body. Under the action of vertical force intensity, the displacement of the top of the pile increases continuously, and the maximum bending moment of the pile body increases firstly and then decreases. The pile displacement and bending moment of the bridge pile increase significantly in the storm condition compared with the normal condition. In addition, the abandoned dreg body hazard prevention and control technical measures are studied. The impacts of different locations of anti-slide pile burial on bridge piles are analyzed and compared, and it is determined that the optimal location of anti-slide pile burial is the middle of the secondary slope.

Stability Analysis of Yebatan Underground Cavern Surrounding Rock based on Crack Evolution Theory

The deformation properties of rock are closely related to the effect of crack closure, generation and expansion in the rock. In this paper, the stress-crack strain evolution constitutive model is used to determine the relaxation damage zone of the surrounding rock based on the change regulation of the crack closure ratio during the whole process of surrounding rock damage, and the model is embedded into FLAC3D software, and then the stability evaluation of the surrounding rock of the cavern group under unsupported and supported conditions is carried out for the underground plant of Yebatan hydropower station. The excavation simulation shows that the displacement of the cavern surrounding rock increases with the excavation step, and the influence of the fault on the displacement of the surrounding rock is very significant. The high stress concentration area is distributed in the arch top of the plant, the tailwater connection hole, the arch top and the bottom of the tail chamber, and the tensile stress area is mainly distributed in the high side wall of the underground plant and the tail chamber. The crack closure ratio index was used to quantitatively evaluate the degree of relaxation damage of the surrounding rock, indicating that the relaxation damage area is influenced by the fault, and the volume of the surrounding rock damage area under the system support is significantly reduced compared with the unsupported working condition, showing the effectiveness of the support measures.

Formulation and numerical realization of anisotropic permeability coefficient for fluid-solid coupling in underground engineering

Underground engineering soil functions as a typical porous medium wherein groundwater flow and alterations in pore water pressure can influence the medium’s porosity, thereby affecting soil permeability. Moreover, changes in porosity (volumetric strain) result in anisotropy in the permeability coefficient, complicating the accurate depiction of the entire fluid-solid coupling process using a uniform permeability coefficient. Fluid–solid coupling is achieved by establishing a connection between soil and fluid parameters in response to changes in the permeability coefficient during soil deformation. Considering the limitations of the deformation mode described by volumetric strain, this paper proposes different influencing factors of anisotropic principal strains (εx, εy , and εz ) on the anisotropic permeability coefficient k i, based on the Kozeny–Carman semi-empirical equation for the permeability of porous media. Additionally, a numerical simulation method for the fluid-solid interaction effect in underground engineering is implemented by incorporating variations in the permeability coefficient with strain, using a fish-parameterized programming language within the FLAC 3D software. The coupling effect of the seepage field with the strain-dependent permeability coefficient on the mechanical field is validated using a specific calculation example. This methodology presents a novel approach for expressing fluid-solid coupling processes.

New indices of the recovery effect of the large deformation shield tunnel based on a light-disturbance grouting model test

The sudden surface surcharge is a key disturbance that causes excess transverse deformation of the existing shield tunnel underneath. The light-disturbance grouting technique, which is applied on two sides of a tunnel spring-line, is an effective way to control or even recover the large tunnel structural deformation. Based on the 1g model test of grouting for the shield tunnel in the sand layer using the Shanghai Metro shield tunnel as a prototype, a numerical model is established based on the FLAC3D software. Meanwhile, the numerical simulation method of simulating grouting involves the application of the virtual expansion stress. The grouting transfer efficiency and recovery bias are proposed in this paper and used as the evaluation indexes of the recovery effect of light-disturbance grouting on the shield tunnel. The findings indicate that irrespective of various overloading modes and symmetrical single-row grouting, the transfer efficiency remains approximately 12%, which may potentially serve as a optimization index for determining the number of grouting rows. Moreover, the observed disparity in recovery bias within tunnels experiencing asymmetric deformation underscores the significance of joint behavior, emphasizing the necessity for closer monitoring during grouting procedures.

The Stability of Compressed Air Storage Underground Gas Storage Chamber and Steel Lining Structure Analysis

According to the address characteristics and structural characteristics of an underground artificial chamber gas storage, a structural model of an underground chamber including steel lining, flexible concrete, concrete lining, and surrounding rock is established, and two limit states of large slip and no contact between steel lining and flexible concrete are simulated respectively. The stability of the surrounding rock and lining structure are calculated and analyzed by using large finite element software FLAC3D and ANSYS respectively. The results show that the maximum deformation of the surrounding rock is 1.57cm, the maximum stress of steel lining is 262.08MPa, and the changes of plastic zone, displacement, and steel lining stress of surrounding rock can meet the design requirements of this project.

Research on a Classification Method of Goaf Stability Based on CMS Measurement and the Cloud Matter–Element Model

The evaluation and classification of goaf stability are fuzzy and random. To address this problem, a new classification method is proposed. A cavity monitoring system is used to detect the goaf, 3DMine and FLAC3D software are used to conduct the 3D visual modeling of the scanning results, and numerical simulation analysis is performed on the goaf. According to the analysis results, the stability classification standard of the goaf is constructed, and the characteristics of each classification are described. The evaluation indicator system of goaf stability is constructed in accordance with similar engineering experience, and the evaluation indicator is weighted by using the analytic hierarchy process. The cloud–element coupling evaluation model is built, the field measured values of indicators are collected, the cloud correlation degree of goafs belonging to each stability level is calculated, the stability level is evaluated according to the principle of maximum membership degree, and the results are compared with the numerical simulation to analyze the reasons for the differences in the stability evaluation results obtained by the two methods and to improve the accuracy of the evaluation of goaf stability. The pillar stress and surrounding rock deformation are monitored in Room 1# of the inclined mining area of Shirengou Iron Mine. The monitoring results are consistent with the evaluation results, which proves the accuracy of the proposed goaf stability classification method.

Seismic displacement analysis of slope reinforced by frame prestressed anchor rod structure

Instability of slope induced by earthquake seriously affects casualties and property losses and hinders earthquake rescue, so the study of slope displacement under earthquake is of great significance. For the slope reinforced by frame prestressed anchor rod structure under earthquake, the calculation method of slope displacement and axial force of anchor rod under seismic action were derived by combing Newmark method and limit analysis method. This method proposed in this paper comprehensively considers the amplification effect of earthquake acceleration on the slope surface, the inertial force of reinforce structure and the dynamic change of axial force of anchor rod. The solution program of this method was compiled in MATLAB and its rationality was proved by a numerical model in FLAC3D software. The results show the slope displacement calculated by the method proposed is consistent with the variation law of slope displacement obtained by traditional methods. Regardless of whether the anchoring force is considered or not, the slope yield acceleration obtained by this method is very close to the slope yield acceleration obtained by the Bishop method. The critical sliding surfaces obtained by the two methods are very close, and the sliding points are located at the foot of the slope. Compared with the Bishop method, this method can consider the amplification effect of seismic force on the free surface of the slope and the gravity of the supporting structure. Ignoring the change of the axial force of anchor rod will make the slope displacement larger, while not considering the amplification effect of the seismic acceleration on the slope surface and the gravity of the reinforce structure will make the slope displacement smaller. This method proposed in this paper comprehensively considers the above factors and is more accurate. In the process of landslide mass sliding, the axial force of the anchor rods increase gradually, and the growth rate of the axial force of the lower anchor rods are larger than that of the upper anchor rods. The increase of anchor rod prestress and elastic modulus will reduce the seismic displacement of the slope, and the increase of the dip angle of the anchor rod will increase the seismic displacement of the slope. The dip angle of the anchor rod should be minimized without affecting the grouting effect of the anchor rod.

Enhancing gas drainage efficiency in soft coal seams: Mechanically over-excavated cavities along borehole in floor tunnels

In addressing the challenge of gas drainage in soft coal seams, this study introduces an innovative technique involving mechanically over-excavated cavities along boreholes in floor tunnels. Through a combination of laboratory experiments, numerical simulations, and field tests, we investigate the efficacy of this method in weakening the coal structure and enhancing gas extraction. The influence of cavity parameters (inclined angle, cavity length and diameter, cavity number) on the mechanical properties and AE behavior of coal specimens were investigated by biaxial compression tests. Besides, the stress distribution and permeability evolution around the cavity were studied using FLAC3D combined with COMSOL software. Finally, the field tests were conducted in LiCun Coal Mine. The results indicate that a significant reduction in the compressive strength of coal specimens with increasing cavity dimensions. Notably, the peak stress in coal specimens decreased significantly as the cavity length, diameter, and number increased, by up to 42.09%, 39.38%, and 57.83%, respectively. In contrast, the peak stress increased with the cavity inclination angle, and it was about 1.14 times higher for specimens with a 90° angle than for those with a 15° angle. It is also found that the cavity inclination reduces the damage and acoustic emission activity of the specimen, while the cavity volume increases them and weakens the rock-bearing capacity. The failure modes of all specimens are shear failure combined with surface spalling. Numerical simulations employing FLAC3D and COMSOL software corroborated these results, demonstrating enhanced pressure relief and increased permeability around the cavities. Field tests conducted in the LiCun Coal Mine provided practical evidence of the method's efficacy, with a more than 3-fold increase in gas drainage concentration and volume observed compared to traditional drilling methods. By adopting mechanically over-excavated cavities, not only is the efficiency of gas drainage significantly improved, but also the safety and productivity of mining operations are enhanced. This study offers a promising and impactful solution for addressing gas drainage challenges in soft coal seams.

Double Unloading Gas Control Technology for Fracturing Soft Coal Seams in Overlying Key Strata

Based on the ‘three highs and one low’ geological conditions of high gas pressure, high gas content, high ground stress, and low permeability in deep coal seams, this study proposes a dual method of hydraulic fracturing of key layers of overlying rock layers combined with pre-extraction of gas via large-diameter caving boreholes. The aim is to unload and dissipate the coal seam by fracturing the overlying key strata, allowing the stress and energy from the excavation working face to be transmitted and transferred to the deep coal seam. Additionally, large-diameter drilling effectively increases the effective drainage radius of the coal seam, resulting in a shorter extraction time. To validate this approach, a fracturing model and a gas extraction model were established for the key layers of the overlying rock layer using the engineering background of the 15,111 excavation working face of a mine in Shanxi. FLAC3D software v.6.0 was utilized to simulate the stress and energy changes of the coal seam before and after fracturing of the key layers, while COMSOL software v.6.0 was used to analyze the gas migration conditions, permeability, and effective drainage radius changes before and after drilling and caving drilling. The findings, combined with the engineering test results, conclude that key strata fracturing combined with large-diameter caving can effectively increase the permeability of coal seams and improve gas extraction. This study serves as a theoretical basis for guiding the design of gas drainage technology under the effects of coal seam pressure relief and permeability enhancement.