Guorui Feng,Pengfei Wang

Faculty of Mining Engineering,Taiyuan University of Technology,Taiyuan 030024,China

Keywords:Stress environment Angle of break Gob-side Numerical simulation Double-yield

ABSTRACT Angle of break(AOB)is the acute angle created by the coal seam bedding plane and caving line formed by roof strata movement after extraction of a longwall panel.It has a significant influence on stress redistribution both in the gob and abutment.Throughout numerical simulation investigations up to now,little attention has been paid to it or an AOB of 90° was used,which however,is not realistic.This paper presents a detailed numerical modelling incorporating the AOB against Zhenchengdi Coal Mine.The AOB was obtained through cross-measure boreholes.Hoek-Brown constitutive model was used to simulate the rock masses.Double-yield constitutive model,which was best fitted by Salamon’s model,was used to simulate the gob.The results show that a ‘‘/ shape”shear failure zone develops around the gob.The shear failure in the floor along the panel edge is due to opposite shear of rock mass on two sides of the caving line,and the number of yielded zones within the gob floor close to the gob edge is smaller.According to the research,the entry was determined to be driven under the gob edge employing splitlevel longwall panel layout (SLPL).The other numerical simulation for SLPL shows that stress around the god-side entry is much smaller than pre-mining stress,and the area of intact rock mass at the elevating section is larger than conventional layout.Numerical modelling was then validated by field observation.

1.Introduction

To solve low recovery and ground control problems in longwall mining with top coal caving(LTCC)due to large gate pillar or chain pillar,gob-side entry approach is becoming increasingly popular[1-3].Gob-side entry is actually a combination of the gateroad,pillar,and gob,which is a typical panel layout with slender pillar or artificial pillar or wall.The abutment load has a profound impact on stability of the gate pillar,as well as the gob-side entry.Better estimates of abutment loading profiles will result in improved evaluations of design of pillar and gob-side entry [4].

Angle of break(AOB)is the acute angle created by the coal seam bedding plane and caving line formed by roof strata movement after extraction of a longwall panel,as shown in Fig.1.It has a significant influence on stress distribution,both in the gob and abutment [5,6].AOB can provide an estimate of the amount of overpanel weight that is transferred to the abutment.Gob loading is also closely related to the AOB.However,throughout numerical simulation investigations up to now,little attention has been paid to the AOB.Researchers who have addressed it have used a 90°AOB,which is not realistic.In addition,in many numerical modellings,only the void space of the coal seam or coal seam and immediate roof are backfilled with gob material[8,9].However,the caved zone is normally 2 m to 8 m high(here m=mining height),depending on the properties of the immediate roof.For LTCC,as the mining height is large,the caved zone would be larger[10].

This paper presents a detailed numerical modelling incorporating the AOB against Zhenchengdi Coal Mine to study the stress environment of entry driven along gob-side.

2.Background

Zhenchengdi Coal Mine is located at Gujiao,which is the northwest of Taiyuan,the capital of Shanxi province,China (Fig.2).The annual output of coal is 1.9 Mt.The case study longwall panels are 22,202 and 22,204 panels in #2-4 coal seam.The generalized stratigraphy is shown in Fig.3.The major tectonic structure in the area is a simple mono-synclinal structure.The panels were 680 m long along the strike and 130 m wide along the dip.The average coal seam thickness was 5 m thick.The average cover depth and seam dip angle were 230 m and 4°,respectively.The gas content was low.ZFS3000/16/25 shields were used for roof support.The shearer cut 0.6 m thickness of coal for each web.The mining height was 2.3 m,and top coal caving height was 2.7 m.The top coal was induced to cave onto the rear armored face conveyor (AFC)after each web cut.

Fig.1.Illustration of AOB.

Fig.2.Location of Zhenchengdi Coal Mine.

Fig.3.Generalized stratigraphic column.

In the past,a large gate pillar (normally over 20 m)was left unmined between panels to maintain stability of gateroad of future new panels.However,ground control problems,such as floor heave,rib sloughing,etc.,were still encountered.A 5 m yield pillar was tried in order to solve these problems,as well as to improve recovery,as shown in Fig.4.Nevertheless,the gateroad performance was still not satisfactory,and support costs increased significantly.The deformation situation of the gob-side entry is shown in Fig.5.

3.Problem analysis

Fig.4.Gob-side entry layout with 5 m gate pillar.

Fig.5.Deformation of gateroads using large gate pillar.

Fig.6.Structure for conventional gob-side entry.

Fig.7.Roof structure in physical modelling.

These problems exist because the coal pillar directly contacts the immediate roof,and the immediate roof directly contacts the structures formed by the main roof,as shown in Fig.6.When the main roof stratum breaks,rotates and caves in,the immediate roof is crushed,which further crushes the coal pillar.The contacts are compacted and squashed.This causes severe deformation of the coal pillar rib and,consequently,gives rise to the overall deformation of the entry.Difficulties in supporting the entry,plastic flow,pillar punching,or even pillar failure may occur.Due to the AOB,the overburden strata overload the abutment,and the solid coal rib is squeezed out into the entry space.The convergence sometime leads to impossible access to the working face,especially at the T-junction.In addition,the shearing force along the caving line may extend right through the gob-side entry,causing severe ground control problems.A number of physical modellings also demonstrate such characteristics of strata structures at the gob edge,as one given in Fig.7 [11].Furthermore,if too much elastic energy is stored within the surrounding coal and rock mass,the likelihood of a rock burst is high [12-14].

4.Numerical modelling of conventional gob-side entry layout

FLAC3DTMcode was employed for numerical modelling study[15].The model for 22,202 panel and 22,204 tailgate using conventional longwall layout with a 5 m gate pillar was constructed.Fig.8 shows the FLAC3D model without excessive complexity and computational burden but with the capacity to capture the important rock mechanics features [7].The dimensions of the model are 300 m (length)×200 m (width)×90 m(height).A significant distance to the lateral boundaries and the bottom boundary was required to minimize model boundary effects.Model zone sizes graded from small around material boundaries to large at the center of a material domain.Suitably fine zone resolution in the vicinity of the coal seam unit,gob,and gateroads was generated for accurate prediction of progressive yielding and deformation.A uniform stress of 4.84 MPa was applied to the top of the model corresponding to 179.1 m of overburden strata by assuming the overlying unit weight was 0.027 MN/m3,and gravity force was applied.The side boundaries were roller constrained,and the bottom boundary was fixed both horizontally and vertically.Based on pre-mining stress measurements in the mine,a ratio of horizontal to vertical stresses (K)of 1.1 and 0.8 was input in the model inplane and out-of-plane directions,respectively.In addition,discontinuous model interfaces,representing bedding planes,capable of yielding and separating,were built into the model at the contact of each stratum using FLAC interface logic.The methods and parameters proposed by Wang et al.,for interfaces were employed to achieve separation between different units and between gob and surrounding rock mass [16].According to the geological data provided by the coal mine,the estimated GSI value and Hoek-Brown parameters for roof and floor strata used for numerical modelling are given in Table 1.

To date,gob modelling methods are relatively new and are developing.Some researchers filled the gob area with a very soft elastic material to approximately simulate the support capability of the fallen rock from the roof [17].Song used artificially pre-set force against the roof according to theoretical gob stress distribution to simulate the support capacity of the gob material employing Phase 2D software [18].Using the finite element package,ABAQUS and based on the Terzaghi’s model,Morsy and Peng developed a numerical gob model [19].Esterhuizen and Peng used equivalent gob elements that follow the hyperbolic stress-strain curves to model the gob compaction and response and carried out model calibration for simulation coal pillars,gob,and overburden response [20].Yavuz et al.used Salamon’s model to simulate the gob [8,21].

However,none of these studies took the AOB into account,and gobs are commonly filled by replacing the coal seam or coal seam and immediate roof.According to many mining engineers’ and authors’ field experience and observation,the AOB is developed after extraction of a coal seam,and in many cases,several roof strata contribute to fragmented rocks in the gob.Therefore,the generalized gob area is determined by field observation rather than only taking the coal seam or coal seam and immediate roof as the gob area.To be more realistic,the angles of break that were obtained by cross-measure boreholes were incorporated into the numerical model.Built-in double-yield constitutive model in FLAC3D was used to simulate the gob.The cap pressure for the double-yield model is estimated by Salamon’s equation,which is expressed by Eq.(1)[22].

Table 1 H-B criterion rock mass parameters.

Table 2 Cap pressure for the double-yield model.

According to cross-measure borehole drilling,the height of the caved zone was about 16.9 m above the coal seam reaching 5.8-m-thick siltstone.Hence,according to Eq.(1),the bulking factor,maximum strain,and the initial modulus of the gob materials are calculated as 1.3,0.23 m/m and 48.9 MPa,respectively.Cap pressure for the double-yield model is given in Table 2 and is expressed by Eq.(2).

In order to obtain the parameters for the gob and make sure that the stress-strain relationship agrees with Eq.(2),a simple model with dimensions 1 m(length)×1 m(width)×2 m(height)was built (in order to display stress and strain contours).Loading was simulated by applying a velocity on the top surface with the bottom surface fixed vertically and four side surfaces fixed horizontally.The input parameters were fitted by iterative changes in the bulk and shear modulus,angle of dilation,angle of friction,and density of gob material.By trial and error,the final properties are given in Table 3.The volumetric strain,vertical stress contours,and stress-strain matching results are given in Fig.9.It shows that numerically obtained data agree very well with Salamon’s equation.It is notable that the final set of inputs is not unique,a different combination of input values may equally satisfy the Salamon’s equation.The modelling results are given in Fig.10.

Fig.10a shows that the stress distributed within the gob is highly stratified,and the gob pressure increases from near 0 MPa around the fringe of the caved gob to the maximum 3.1 MPa at the center of the mined panel where the gob is more compacted.The stress concentration zones are developed within surrounding rock masses on two sides of the gob (blue parts).The gob-side entry is located right at the bottom of the over-stressed zone on the right.Therefore,the gob-side entry is actually driven in a stress concentration zone.Within these zones,the upper strata are subjected to higher stress concentration than the lower strata.Fig.10b shows that the failure extends in the same direction as the caving line,and the failure modes are tensile and shear failure.Within the middle of the overlying strata is the tensile failure mode.Fig.10a and b demonstrate that rock mass at the middle of the overlying strata above the caved zone and rock mass close to the caving line undergo tensile failure,resulting in corresponding destressed zones around them.The area of the yield zone within the coal seam increases from the seam top to the seam bottom.The failure of the floor strata at the gob edge is also along the direction of the caving line.However,the number of yielded zones within the gob floor close to the gob edge is smaller.It is concluded that a ‘‘/ shape”shear failure zone develops around the gob.The shear failure in the floor along the panel edge is due to opposite shear of rock mass on two sides of the caving line (denoted by the opposite yellow arrows).

Fig.10c and d show that,after excavation of the gob-side entry,the extent of the yield zone increases.The extension of the yield zone is also along the caving line.The gate pillar shown in Fig.10d is yielded but is still subjected to high ground pressure,shown in Fig.10c,that may result in bursts.A high stress concentration zone is generated close to the gob-side entry and at the right lower part of the gate pillar.A high stress concentration zone at the solid coal side of the entry also develops,which is higher.This may be due to the superposed influence of the opposite shearing of rock masses on two sides of the caving line and opposite shearing of rock masses on two sides of the pillar rib,as shown in Fig.10c.The concentrated stress and plastic flow of coal pillar and floor strata may contribute to the floor heave.

5.Split-level longwall panel layout

In view of the problems encountered in the field and numerical analysis above,many coal mines have decided to adopt split-levellongwall layout(SLPL)in which the gob-side entry is driven under the gob edge,as shown in Fig.11 [23-25].SLPL locates gateroads on either end of the panel at different elevations in a coal seam,one along the floor and the other one along the roof.Therefore,there is a curved section on one end of the panel.After extraction of the two panels,an approximate triangular coal loss is developed.Since the two panels were overlapped at one end,the width of the panel increases (Fig.11)and 22,202 panel was about 140 m.

Table 3 Parameters for gob material.

Fig.9.Iteratively fitted results.

Fig.10.Modelling results of conventional longwall layout with a 5 m gate pillar.

6.Numerical modelling of SLPL

The FLAC3D model is shown in Fig.12.The similar steps described in numerical modelling for conventional longwall layout are taken to simulate SLPL system.The results are listed in Fig.13.

Fig.13a shows that the stress distributed within gob is also highly stratified,and the gob pressure increases from near 0 MPa around the fringe of the caved gob to the maximum 3.3 MPa at the center of the mined panel.The reason for the larger maximum gob pressure at the center of the mined panel may be the width of the panel increased from 130 to 140 m,although 140 m still does not reach the critical width of extraction since the pre-mining stress is 6.21 MPa.The stress concentration zones are also developed within surrounding rock masses on two sides of the gob(blue parts).However,this is different from the conventional layout because the stress concentration zone is located 10 m to the right of the gob-side entry and mostly above the coal seam.The entry itself is within the destressed zone.Fig.13a shows that the stress around the entry is around 1 MPa.Therefore,the gob-side entry employing SLPL is actually driven in a destressed zone where stress is far less than pre-mining stress.Fig.13b shows similar failure mode and directions with Fig.10b,which was also concluded to be related to the AOB.‘‘/shape”shear failure within the floor also develops along the panel edge due to the opposite shear of rock mass on two sides of the caving line.However,the most surprising finding is that an intact zone in the curved section is observed,and part of the gob-side entry is excavated within this intact zone where the stress is far lower than pre-mining stress.

Fig.13c shows that,after excavation of the gob-side entry,the surrounding rock mass is still in destressed state,and the stress field hardly changes.Fig.13d also shows that the yield zone hardly changes due to excavation of the gob-side entry.This indicates that,within the destressed rock mass,the excavation of the gobside entry employing the SLPL does not cause much perturbation of environment of surrounding rock masses.Compared with Fig.10 and Fig.13,we can see that the gob-side entry in Fig.10 is an all-yield-zone environment,while that in Fig.13 is still a large area of intact zone.It is revealed through numerical modelling that the environment for the SLPL gob-side entry is much more favorable than the conventional layout.This also demonstrates that the AOB provides a protective environment for SLPL gob-side entry.

7.Field study and validation of the numerical modelling

Fig.12.Numerical model for 22,202 panel and 22,204 tailgate employing SLPL.

The numerical study suggests that the gob edge is the area where pressure is the smallest in the entire longwall system.Therefore,ground control problems are minimum.Due to the unique panel configuration,the mining operations known as‘‘triple section mining operation,”shown in Fig.14,is unique[12].The operation for section A of panel 22,202 is the same as that in LTCC,and only the curved section needs to be elevated incrementally by adjusting inclination of each section of the AFC,shields,and other production machines.The configuration of the shields at the curved section is shown in Fig.15.The face shields within section B have wire mesh laying device on the top for meshing operation to form an artificial roof for the overlapped section of 22,204 panel.#10 steel wire mesh was used.22,202 and 22,204 panels were extracted sequentially.The 22,204 gob-side entry was developed under the artificial roof with steel wire mesh on the top.The 22,204 gob-side entry used #11 I-beam steel sets for support spaced at 1.2 m.The support design is shown in Fig.16.The surrounding rock mass of 22,204 gob-side entry consists of the artificial roof with steel wire mesh on the top and of two sides of solid coal(one side is the rib of the triangular coal pillar,and the other side rib of the solid coal).The entry was supported by steel sets with wire mesh on the top,preventing the caved rock fragments from falling.The support system thus only has to be designed to carry only the load of the caved rocks above the entry and within the AOB.

The stress of the entry was monitored,including the stresses within pillar rib,solid coal rib,and roof.The strain gauges were inserted between the surrounding rock mass and I-beam immediately after the steel sets were installed with the advance of the gateroad heading to measure stress changes.Roof pressure of the gob-side entry indicates the gob pressure above the entry since the entry is located under the gob edge.Measurement points were approximately at the center of the roof and ribs.The obtained data and numerical data are plotted in Fig.17.The comparison demonstrates that numerical modelling is consistent with field measurement.Overall,simulated results were in close agreement with the field measurement data,and the numerical modelling is,therefore,calibrated.

Fig.17 shows that the support load in the gob-side entry is much lower than pre-mining stress.This corroborates that the gob-side entry employing the SLPL is located within the destressed zone.It is believed that this is attributed to the protective function of the AOB.This is similar to multi-slice longwall mining where the panel gateroads in the lower seam are located directly underneath the gob of the immediate super-adjacent seam.Thus,the gateroads in the lower seam are located within the destressed zone of the super-adjacent panel [10].In addition,the data also show that the roof pressure is maintained at low level without many fluctuations.This proves that the gob-side entry is independent of the side and front abutment pressures.Nearly no convergence was observed during the service of the gob-side entry.The support situation is shown in Fig.18.

This demonstrates that the overall stress environment in the gob-side entry employing SLPL is improved.Ground control problems,such as roof falls,bumps,and outbursts,are minimized.

8.Applicability of the approach

SLPL is preferential for coal seams with low gas content since the gob-side entry is located directly under the gob.However,controlled gas in multi-slice longwall mining technology can be used with proper measures,such as pre-drainage of gas through gob wells or cross-measure drill holes,grouting,employing outer offset configuration with a slender pillar in between,or leaving a coal sheet between the two adjacent panels for thicker coal seams,shown in Fig.19.Therefore,the approach is preferably adopted for coal seams whose thicknesses is more than 5 m.SLPL has been used for coal seams whose dip angles are more than 52° [26].The two adjacent panels must be extracted sequentially.The development of the entry of the future panel should be done at least six months (preferably one year)after the previous panel is mined.The author proposes using geosynthetic materials (such as membranes)to isolate the entry from the gob,which needs further study.Furthermore,coal mines currently using SLPL are all single-entry or two-entry systems.Therefore,additional research should be done to assess the performance of three-,four-,or more-entry panel systems.

9.Conclusions

The stress environment of the entry driven along the gob-side was studied through numerical simulation incorporating the AOB against Zhenchengdi Coal Mine.The AOB,obtained through cross-measure boreholes,was incorporated into numerical modelling to be more realistic.

The numerical simulation results for conventional 5 m gate pillar gob-side entry show that the stress concentration zones develop within surrounding rock masses on two sides of the gob.The gob-side entry is located right at the bottom of the overstressed zone on the right.The gob-side entry is driven in a stress concentration zone.The number of yielded zones within the gob floor close to the gob edge is smaller.A ‘‘/ shape”shear failure zones is developed around the gob.The shear failure in the floor along the panel edge is due to opposite shear of rock mass on two sides of the caving line.The 5 m gate pillar is yielded but is still subjected to high ground pressure that may result in bursts.There is a high stress concentration zone close to the gob-side entry and at the right lower part of the gate pillar.The high stress concentration zone at the solid coal side of the entry is higher.This is associated with the superposed influence of the opposite shearing of rock masses on two sides of the caving line and opposite shearing of rock masses on two sides of the pillar rib.The concentrated stress and plastic flow of the coal pillar and floor strata may contribute to the floor heave.

Fig.13.Modelling results of SLPL.

Fig.14.Triple section mining operation.

Fig.15.Configuration of shields at the curved section.

Fig.16.Support design for the gob-side entry.

Fig.17.Obtained data and numerical data of stress within roof and ribs.

Fig.18.In-situ support situation.

Fig.19.Other commonly used SLPL forms.

According to the problems encountered in the field and numerical analysis for 5 m gate pillar gob-side entry,SLPL was employed.SLPL locates gateroads on either end of the panel at different elevations in a coal seam,one along the floor and the other one along the roof.Therefore,the gob-side entry is driven under the gob edge.

The numerical simulation for SLPL shows that the stress around the entry is around 1 MPa,which is much smaller than pre-mining stress,and the area of intact rock mass at the curved section is larger than the conventional layout.The stress field and area of the yield zone hardly changes before and after the excavation of the gob-side entry.The AOB provides a protective environment for a gob-side entry employing SLPL.

Acknowledgments

This work was supported by the National Natural Science Foundation of China,Young Scientists Fund (No.51804209);NSFC-Shanxi Joint Fund for Coal-Based Low-Carbon Technology(No.U1710258),and Shanxi Applied Basic Research Programs,Science and Technology Foundation for Youths (No.201801D221363).THX.