Effect of Roadway Size and Layout on Stability of Surrounding Rock

Chengdong Tian Postgraduate Student, State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, School of Mechanics and Civil Engineering, Xuzhou, Jiangsu 221116 China; e-mail: [email protected]

Haibo Bai * Professor, State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116 China; *Corresponding Author, e-mail: [email protected]

Jing Qi Postgraduate Student, State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116 China; e-mail: [email protected]

Yanmeng Wang Postgraduate Student, State Key Laboratory for Geomechanics and Deep Underground Engineering,China University of Mining and Technology, Xuzhou, Jiangsu 221116 China; e-mail: [email protected]

ABSTRACT In order to study the surrounding rock stress, plastic zone and deformation characteristics under the influence of the size and layout of roadway, theoretical analysis and numerical simulation are adopted to analyze the impact of different section sizes and angles between tunnel axis and the maximum horizontal principal stress direction on stress and deformation characteristics of surrounding rock. The results show that the roadway section size and the angle between the tunnel axis and the maximum horizontal principal stress have a great effect on stress and deformation characteristics of surrounding rock. The decrease of span-depth ratio has a more influence on the sides of roadway, making the stress concentration degree and range increase, which is the main concern; The increase of angle between tunnel axis and the maximum horizontal principal stress can cause horizontal principal stress asymmetric on both sides, increasing the stress concentration degree in roof-floor, which makes the roadway deformation become obvious.

KEYWORDS: drift layout; span-depth ratio; maximum principal stress; FLAC3D

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INTRODUCTION

For there are many kinds of factors that affect the stability of roadway surrounding rock, including not only the in-situ rock stress field, geological structure and physical and mechanical properties of rock mass, but also the size and layout. In recent years, with the shallow coal seam less and less, deeper underground space develops rapidly, leading to the roadway be in a high stress state, adding the influence of fault, collapse column and the complex geological conditions, such as groundwater, increasing the risk of disaster in underground engineering, and bringing a great difficulty in maintaining the stability of roadway surrounding rock.

Currently, researchers in many countries have studied about the stability of roadway surrounding rock under the influence of the size and layout, and have achieved abundant results. Size and direction of horizontal stress are important factors which affect the stability of surrounding rock. Especially near the geological structure such as faults, with the mining depth increasing, there will be a strong connection between the deformation and failure of tunnel roof and floor and the direction of high horizontal stress[1-8]. Studies have shown that the maximum principal stress in surrounding rock will be greatly reduced, when the angle between tunnel axis direction and the maximum horizontal principal stress is less than 25°[9-10].

At the moment, the designers just have focused on support pattern and determination of parameters for maintaining the stability of roadway. For the section size and layout, research is inadequate. But the interplay of the two factors has a great influence on the stability of roadway surrounding rock in the process of digging. Only one of the factors taken into consideration, there will be a difference between the result and actual situation. Therefore, this paper mainly studies the stability of surrounding rock under the influence of the cooperation of roadway size and layout.

MECHANICAL ANALYSIS

Many researchers have shown that the span-depth ratio is an important factor which affects the stability of surrounding rock of roadway. Because the horizontal crustal stress is directional, it has a great influence on the mine safety. Based on the field measurement, it has shown that the horizontal stress is greater than the vertical stress in many cases, and the maximum horizontal principal stress is significantly greater than the minimum. What’s more, the maximum horizontal principal stress is 2 times bigger than the vertical stress[11-13]. So attention should be paid to the stability of surrounding rock of roadway caused by the angle between tunnel axis and the maximum horizontal principal stress. The maximum and minimum horizontal principal stress in the original rock stress q q q field are respectively designated as 1 and 2 , and the vertical principal stress is 3 .

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Analysis of Effect of Ground Stress Field on Roadway Roof and

Floor

The horizontal stress in original rock stress field is one of the important factors which influence the stability of the roadway in roof and floor. When horizontal stress is greater than vertical stress, q destruction tends to occur in roof and floor. If the maximum horizontal principal stress 1 is parallel to the tunnel axis, horizontal stress will have a minimal impact on roof and floor, thus its stability is q the best. As the angle between the maximum horizontal principal stress 1 and tunnel axis increases gradually, stress concentration will appear at one side of roof and floor, leading to deformation and q failure; When roadway axis is perpendicular to the maximum horizontal principal stress 1 , horizontal stress will have the most influence on roadway roof and floor, resulting in the stability of roof and floor being the worst.

Analysis of Effect of Ground Stress Field on Surrounding Rock at

Both Sides of Roadway

Surrounding rock failure is mainly caused by shear stress and tensile stress. The smaller the q difference between horizontal normal stress v and vertical stress on both sides of the roadway’s borders, the better for maintaining the stability of the roadway will be. For different in-situ rock stress field, the optimum angle between tunnel axis and the maximum horizontal principal stress is different. qqq>> qq= When 123, only to make v 2 , that the roadway axis is parallel to the direction of q maximum principal stress 1 ，it’s the most appropriate axis direction, which is beneficial to qqq>> qq= maintain the stability of roadway on both sides. When 312, only to make v 1 , that the q roadway axis is perpendicular to the direction of maximum principal stress 1 , it’s the most appropriate axis direction in favor of maintaining the stability of roadway on both sides. When qqq>> α 132, based on the two-direction tress on horizontal plane, there is an angle 1 between the qq= maximum horizontal principal stress and tunnel axis to make v 3 , and the angle is:

1 qq+ -2 q α = arccos 13 2 1 2-qq 13 （1）

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ESTABLISHMENT OF THE NUMERICAL MODEL

Software Introduction and Principle

FLAC3D is a three-dimensional Lagrange finite difference program. Combined with the mixed discrete method, taking the area of hexahedral element node for calculating object, by the virtual work principle, the calculation equation of motion will be solved in time domain from stress and external force, and the equations in the application of finite difference method are the following:

（1）The node equation of motion is expressed as:

∂vl Ftl () i = i l ∂tm （2） where F is unbalanced force component of node at t time in i direction, deduced by virtual work principle, m is the lumped mass of l node. （2）Compatibility equation is deduced by the rate for unit strain increment of a time step, and it is given as follows: 1 ∆=eij() vv i,, j + j i ∆ t 2 （3） where v is the speed of one point in the medium. For the boundary condition, when the model is in different stress fields, the most suitable angle between roadway axis and the maximum horizontal principal stress to maintain the stability of roadway is different. Therefore, while using FLAC3D to simulate, it is needed to determine the angle between roadway axis and the maximum horizontal principal stress by changing the stress boundary conditions of model.

Numeric Calculation Model

Simplifying the other unnecessary interference factors, two factors are mainly considered in the establishment of numerical model, which are span-depth ratio and the angle between tunnel axis and the maximum horizontal principal stress. The rock formations are regarded as homogeneous and isotropic, regardless of the unevenness and discontinuity caused by crack, structural plane and weak interlayer. Considering the limitation of computer memory and Saint-Venant Principle, only a limited range is obviously influenced in rock excavation, so the roadway’s characteristics of force and displacement must be fully embodied in the process of modeling. What’s more, the running speed of computer is important. According to the principles, numerical simulation model is established. The geometric size is 40m×40m×40m (length×width×height).The buried depth is about 350 m. The Mohr-Coulomb yield criterion is used in calculation. The coal and rock physical mechanical parameters are shown in

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Table 1. Based on the analyze of horizontal ground stress status, the maximum and minimum horizontal principal stress are respectively designated as 11.2MPa and 5.6MPa, and the intermediate principal stress is 8.6MPa. Horizontal displacement on the side of the model is restricted, while ertical displacement at the bottom of the model was limited. The upper surface is free. The model is divided into 175200 zones and 183890 nodes.The numerical calculation model is shown in figure 1. Table 1: Coal and rock physical mechanical parameters

Strate Bulk Shear Internal Tensile Cohesion Density Lithology thickness modulus modulus frictional strength c/MPa ρ/t·m-3 h/m K/GPa G/GPa angle φ/° σc/MPa Sandy 9 21.1 12.7 33 3.2 3.1 2.5 mudstone

Mudstone 7 21.6 13.6 32 2.8 3.2 2.6 Coalbed 5 6.6 2.5 23 1.5 1.6 1.4 Clay 10 223 13.4 31 2.6 2.7 2.5 mudstone Fine 9 22.0 12.7 35 3.6 3.3 2.7 sandstone

Figure 1: Numeric calculation model

Simulation Project

The shape of roadway section is selected as straight wall arch tunnel. The width of roadway is = 2R=4m, which is a fixed value. The height of straight wall is h, span-depth ratio is f2/ Rh. Taking the angle between tunnel axis and the maximum horizontal principal stress as 0°、30°、60°

Vol. 19 [2014], Bund. J 2330 and 90°, stress and displacement field will be simulated while span-depth ratios are respectively designated as 4、2、1 and 0.5.

NUMERICAL CALCULATION RESULTS AND ANALYSIS

Stress Characteristics of Roadway Surrounding Rock

（1）Stress distribution of surrounding rock influenced by the angle between tunnel axis and the maximum horizontal principal stress:

o 16 α = 0 α = 30o 14 α = 60o = o 12 α 90

8 MPa

/ 6

 4

-2 -2 0 2 4 6 8 10 12 14 16 18 20 d/m Figure 2: Variation of the maximum principal stress on roof under different angles

16 α =0o 14 α =30o o 12 α =60 α = o 10 90

MPa 6 / σ 4

-2 -2 0 2 4 6 8 10 12 14 16 18 20 d/m Figure 3: Variation of the maximum principal stress on floor under different angles

The maximum principal stress on roof and floor are shown in figure 2 and 3. From the figures it can be found that:

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The variation of the angle between roadway axis and the maximum horizontal principal stress has great impact on the maximum principal stress of roadway roof and floor. Stress unloading area ① appears on a small scale at upper roof and lower floor, while phenomenon of stress concentration arises in the deep strata of roof and floor. ②With the increase of α , the range of stress concentration increases, but the change is not obvious. With the distance to roof and floor further, the maximum principal stress gradually reaches to the in-situ rock stress state. In roadway side, when α =0° , the maximum principal stress has symmetrical distribution on both sides of roadway. With the increase of α , the maximum principal stress begins to reverse, and the extent of torsion first increases then decreases. When α =90°, the distribution of maximum principal stress recovers symmetrical. （2）Stress distribution of surrounding rock influenced by different span-depth ratios:

= = = = （a） f 4 （b） f 2 （c） f 1 （d） f 0.5 Figure 4: Partial enlarged drawings of the maximum principal stress distribution under different span-depth ratios

The figure 4 shows the maximum principal stress distribution of surrounding rock under different span-depth ratios, which can be figured:

The maximum principal stress value on the surface of roadway roof and floor is small. With the distance to the surface of roof and floor further, the maximum principal stress will increase ① gradually, reaching a maximum value then gradually decreasing and eventually being stable.

As the span-depth ratio decreasing, only the height of roadway enlarged, tensile stress in the roof and floor gradually decreases, while the compressive stress increases, but the range is very ② small. At the same time, the distance between the maximum of maximum principal stress and the surface of the roof-floor increases. At first, the maximum principal stress on the surface of roadway side is small. With the distance to side increasing, the maximum principal stress increases rapidly and ultimately remains stable. When the span-depth ratio decreases, the maximum principal stress in roadway side changes obviously, and stress-relaxed area increased.

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= = = = （a） f 4 （b） f 2 （c） f 1 （d） f 0.5 Figure 5: Partial enlarged drawings of vertical stress distribution under different span- depth ratios

Figure 5 shows the vertical stress distribution of surrounding rock under different span-depth ratios. From the figure it can be found that:

Vertical stress around the roadway becomes small after excavation, and the phenomenon of stress concentration will appear within the scope of 3-5 m on two sides. When stress concentration ① area reaches to peak, the vertical stress begins to decrease gradually, finally tending to be stable and being in original rock stress state. ②With the decrease of span-depth ratio, vertical stress of surrounding rock in roof and floor changes slowly, while the range of stress concentration in roadway side largens fast.

Plastic Zone of Surrounding Rock of Roadway （1）Plastic zone of surrounding rock influenced by the angle between tunnel axis and the maximum horizontal principal stress:

o o o o （a）α =0 （b）α =30 （c）α =60 （d）α =90 Figure 6: Contours of plastic zone under different angles

500 f =4 480 f =2 460 f =1 440 f =0.5 420 400 380 360

/ mm 340 h

∆ 320 300 280 260 240 220 200 0 20 40 60 80 100 / ° a Figure 7: Roof-to-floor convergence under different angles

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Figure 6 shows the distribution of plastic zone when the angle between maximum principal o o stress and roadway axis changes from 0 to 90 . Blue represents elastic zone, whereas yellow and pink are plastic zone. Figure 7 shows the roof-to-floor convergence under different angles. From the figures it can be found that:

The plastic zone of semicircle arch roadway surrounding rock is mostly distributed in two sides and the upper corner section, which is mainly ring arranged, while the fracture area is mainly ① in the roof and floor. With the increase of α , the plastic zone develops gradually above the upper corner, and the depth of destroyed roof and floor deepens. Meanwhile, the plastic zone in two sides grows faster, ② finally it is similar to rectangular distribution. As the whole plastic zone of surrounding rock expands, part of the destruction form will change.

（2）Plastic zone of surrounding rock influenced by different span-depth ratios:

= = = = （a） f 4 （b） f 2 （c） f 1 （d） f 0.5 Figure 8: Contours of plastic zone under different span-depth ratios

Figure 8 shows the plastic zone distribution of surrounding rock under different span-depth ratios. It can be seen from the figure: With the decrease of span-depth ratio, the depth of plastic zone of surrounding rock in roadway side increases, and the lower part is more obvious. However, the depth of roadway roof ① decreases, and the change of plastic zone in floor is not noticeable, the whole approaching standard rectangle. Finally, part of plastic zone destruction form change. As the span-depth decreases and α increases, roof-to-floor convergence has increased to different extents. The span-depth ratio has little influence on roof-to-floor convergence and the ② change is gentle. When α is small, the impact on roof-to-floor convergence is not obvious. while

α is greater than 30° , the roof-to-floor convergence increases evidently. But when the distance to roadway roof-floor is far, the displacement of surrounding rock will decrease gradually and eventually become stable.

CONCLUSIONS （1）The angle between tunnel axis and the maximum horizontal principal stress α has great influence on the stability of roadway surrounding rock. With the increase of α, the vertical stress

Vol. 19 [2014], Bund. J 2334 concentration degree and scope in two sides as well as the maximum principal stress in roof-floor and the roadway sides increase. The deformation of roadway increases too, and the change in the vicinity of 45° is the most obvious. Therefore, taking the horizontal stress into consideration in the process of roadway arrangement, according to the mine deployment and layout of working face, choose the reasonable orientation of roadway axis. （2）Under the same condition of ground stress and surrounding rock, transverse dimensions of roadway have great influence on the stability of surrounding rock. The increase of roadway span- depth ratio can improve the stress condition of surrounding rock and reduce the plastic zone and deformation. （3）To strengthen the stability of surrounding rock and reduce support costs, the roadway section shape and the direction of axis should be firstly considered in the development and layout of roadway.

ACKNOWLEDGEMENTS

Financial support for this work, provided by the National Natural Science Foundation of China (No.50974115) and the National Basic Research Program of China (973 Program) (No.2013CB227900) and the Programme of Introducing Talents of Discipline to Universities (B07028) are gratefully acknowledged.

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