Deformation mechanism and excavation process of large span intersection within deep soft rock roadway

M INING SCIENCE AND TECHNOLOGY Mining Science and Technology 20 (2010) 0028–0034

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Deformation mechanism and excavation process of large span intersection within deep soft rock roadway LI Guofeng*, HE Manchao, ZHANG Guofeng, TAO Zhigang Institute of Geotechnical Engineering, China University of Mining & Technology, Beijing 100083, China Abstract: The FLAC3D software was used to simulate and analyze numerically the displacement, stress and plastic zone distribution characteristics of a large span intersection in a deep soft rock roadway after the surrounding rock was excavated. Our simulation results show that there are two kinds of dominating factors affecting roadway stability at points of intersection, one is the angle between horizontal stress and axial direction of the roadway and the other are the angles at the points of intersection. These results are based on a study we carried out as follows: first, we analyzed the failure mechanism of a large span intersection and then we built a mechanical model of a rock pillar at one of the points of intersection. At the end of this analysis, we obtained the failure characteristics of the critical parts on the large span intersection. Given these failure characteristics, we proposed a new supporting method, i.e., a Double-Bolt Control Technology (DBCT). By way of numerical simulation, DBCT can effectively control the deformation of the surrounding rock at the points of intersection in roadways. Keywords: deep; soft rock; junction; numerical simulation

1

Introduction

The increase both in energy demand and mining depth has coincided with a gradual decline in shallow resources. Mines at both home and abroad are gradually forced to engage in deep exploitation. With the increase in mining depth and the effect of high ground stress, the problems of soft rock are becoming increasingly more serious. The degree of destruction of underground chambers has increased and the damage inflicted on the surrounding rock in large span intersections is sufficiently serious and difficult to support in light of current concerns about the environment on safety or financially[1-4]. The condition of stress in the surrounding rock from the tunneling of the laneway is complex due to the concentration of this stress. Often, the problem is lack of money, but also production is endangered owing to a lack of concern for safety[5-16]. Therefore, we must control the stability of the surrounding rock while the laneway is under construction. It is an urgent problem that must be solved quickly.

a terrain laneway at a depth of 755 to 761 m. The #2 yard is a laneway which passes through the terrain and connects with the #3 level track diphead. The #2 yard is located under the II1 coal seam at a vertical distance to the II 1 coal seam of 6~12 m. The lithological characteristics of the #2 yard are silty mudstones and medium sandstone. Because the laneway is located near the synclinal axis and deeply embedded, the ground stress is enormous and the wall rock is much crushed and therefore the lithologic is poor. The laneway we studied includes the #2 yard and the #1 intersection in the #3 level diphead tracks. The #2 yard includes the #2 intersection, as well as passages one, two and three of the #2 yard. The overall length of the laneway is 140 m. The plane layout of the laneway is shown in Fig. 1 and its profile in Fig. 2. According to our in situ investigation, the 1RKRUL]RQWDOWUDFNGLSKHDG SRLQWVRILQWHUVHFWLRQ

2 Geological conditions in engineering laneway LQWHUPHGLDOWH VHFWLRQLQ\DUG

The #3 level diphead track in the #5 mine in Hebi is Received 12 May 2009; accepted 13 September 2009 *Corresponding author. Tel: 86 10 62331294 E-mail address: [email protected] doi: 10.1016/S1674-5264(09)60156-3

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Fig. 1

#1 and #2 intersections and roadway plane chart of the #5 coal mine

LI Guofeng et al

Deformation mechanism and excavation process of …

laneway has the shape of a stalk and a round arch. Its dimensions are as follows: 1) track diphead: 3800 mm wide×3400 mm high; 2) the #1 and #2 interme6WUDWXP V\VWHP

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29

diate sections in the #2 yard: 3800 mm wide×3460 mm high; 3) the #1 and #2 points of intersection: 7700 mm wide×5200 mm high.

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Fig. 2 Profile strata of the #5 coal mine

3

Numerical simulation analysis

3.1 Geological engineering model In order to understand the conditions after the excavation of the large span intersection and by further study of the deformation mechanism at the two points of intersection, the distribution of stresses and strains of the #2 yard are simulated. A 3D model was established by two software packages: ANSYS and FLAC3D to simulate the dynamic state after the laneway was excavated. The model was composed of a large number of tetrahedral elements, including 23953 nodes (approximate 24 thousand) and 130729 elements (approximate 131 thousand). The width of the model (x direction) is 56 m, its length (y direction) 70 m and its height (z direction) 30 m. The horizontal displacements of the model at both sides were fixed as zero, as was its bottom displacement. The upper surface of the model is a stress boundary. In order to simulate the dead weight of the rock mass above the laneway, we added a load of 17.64 MPa on top of the model. The material failed to meet the Mohr-Coulomb strength criterion. Table 1 shows the physical and mechanical parameters of the rock mass. The lateral stress coefficient (horizontal direction) is 0.85, the load is 15 MPa and shown in Figs. 3 and 4.

Medium sand rock stratum Mud stone rock stratum Interbed between sand rock and mud stone

Fig. 3

Fig. 4

Geologcal engineering model

Roadway model of #1 and #2 points of intersection in the #2 yard

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Table 1

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Physical and mechanical parameters of engineering rock mass in the Hebi #5 coal mine

Name of lithologic

Bulk density (kg/m3)

Bulk modulus (GPa)

Shear modulus (GPa)

Tensile strength (MPa)

Cohesive strength (MPa)

Internal friction angle (°)

1

Medium sand

2640

3.6

3.1

2.6

3.0

36

2

Mud stone

2230

1.2

2.6

0.133

0.52

23

3

Sandy mud

2550

1.4

2.3

1.56

2.0

30

No.

Mark

3.2 Numerical simulation analysis 3.2.1 Analysis of the distribution of the displacement field Fig. 5 shows the distribution of the displacement fields, based on the numerical simulation results, after the laneway had been excavated.

9.2937×10–1 to 1.0000×10–2 1.0000×10–2 to 2.0000×10–2 2.0000×10–2 to 3.0000×10–2 3.0000×10–2 to 4.0000×10–2 4.0000×10–2 to 5.0000×10–2 5.0000×10–2 to 6.0000×10–2 6.0000×10–2 to 7.0000×10–2 7.0000×10–2 to 8.0000×10–2 8.0000×10–2 to 9.0000×10–2 9.0000×10–2 to 9.0147×10–2 Interval=1.0×10–2

From the displacement fields, we can see that the displacement distribution between the roof and floor at the #1 point of intersection is larger than that point #2 and its displacement value is also larger than that of the #2 point, demonstrated by deformation monitoring data measured later. Furthermore, the displacement of the NBZ (the rock pillar in the laneway junctions) of the #1 point of intersection is clearly larger than that of point #2, as is the range of the longitudinal displacement of the #1 point of intersection. From this analysis, we have learnt that the distribution range of the displacement field of the surrounding rock along the track diphead is clearly larger than that of the horizontal yard. 3.2.2 Distribution rule of stress field From the results of our numerical analysis, the distributions of the stress fields of the bottom, roof and sides of the laneway are shown in Fig. 6.

(a) Displacement of intersection roof

3.8095×10–1 to 2.5000×10–2 2.5000×10–2 to 5.0000×10–2 5.0000×10–2 to 7.5000×10–2 7.5000×10–2 to 1.0000×10–1 1.0000×10–1 to 1.2500×10–1 1.2500×10–1 to 1.5000×10–1 1.5000×10–1 to 1.7500×10–1 1.7500×10–1 to 2.0000×10–1 2.0000×10–1 to 2.1392×10–1 Interval=2.5×10–2

1.8828×107 to 1.8000×107 1.8000×107 to 1.6000×107 1.6000×107 to 1.4000×107 1.4000×107 to 1.2000×107 1.2000×107 to 1.0000×107 1.0000×107 to 8.0000×106 8.0000×106 to 6.0000×106 6.0000×106 to 4.0000×106 4.0000×106 to 2.0000×106 2.0000×106 to 1.7912×106 Interval=2.0×106

(a) SXX stress distribution of junction roof

(b) Displacement at intersection sides

1.7679×10–4 to 2.0000×10–2 2.0000×10–2 to 4.0000×10–2 4.0000×10–2 to 6.0000×10–2 6.0000×10–2 to 8.0000×10–2 8.0000×10–2 to 1.0000×10–1 1.0000×10–1 to 1.2000×10–1 1.2000×10–1 to 1.3256×10–1 Interval=2.0×10–2

(c) Displacement of intersection floor

Fig. 5

Distribution of the displacement fields

1.5121×107 to 1.4000×107 1.4000×107 to 1.2000×107 1.2000×107 to 1.0000×107 1.0000×107 to 8.0000×106 8.0000×106 to 6.0000×106 6.0000×106 to 4.0000×106 4.0000×106 to 2.0000×106 2.0000×106 to 3.4012×105 Interval=2.0×106

(b) SYY stress distribution of junction roof

LI Guofeng et al

Deformation mechanism and excavation process of …

2.4863×107 to 2.2500×107 2.2500×107 to 2.0000×107 2.0000×107 to 1.7500×107 1.7500×107 to 1.5000×107 1.5000×107 to 1.2500×107 1.2500×107 to 1.0000×107 1.0000×107 to 7.5000×106 7.5000×106 to 5.0000×106 5.0000×106 to 2.5000×106 2.5000×106 to 0 0 to 4.3023×105 Interval=2.5×106

31

1.8828×107 to 1.8000×107 1.8000×107 to 1.6000×107 1.6000×107 to 1.4000×107 1.4000×107 to 1.2000×107 1.2000×107 to 1.0000×107 1.0000×107 to 8.0000×106 8.0000×106 to 6.0000×106 6.0000×106 to 4.0000×106 4.0000×106 to 2.0000×106 2.0000×106 to 1.7912×106 Interval=2.0×106

(c) SZZ stress distribution at roof of intersection

(g) SXX stress distribution at floor of intersection

1.8638×107 to 1.7500×107 1.7500×107 to 1.5000×107 1.5000×107 to 1.2500×107 1.2500×107 to 1.0000×107 1.0000×107 to 7.5000×106 7.5000×106 to 5.0000×106 5.0000×106 to 2.5000×106 2.5000×106 to 0 0 to 1.7826×106 Interval=2.5×106

1.5121×107 to 1.4000×107 1.4000×107 to 1.2000×107 1.2000×107 to 1.0000×107 1.0000×107 to 8.0000×106 8.0000×106 to 6.0000×106 6.0000×106 to 4.0000×106 4.0000×106 to 2.0000×106 2.0000×106 to 3.4012×105 Interval=2.0×106

(h) SYY stress distribution at floor of intersection

(d) SXX stress distribution at side of intersection

2.4863×107 to 2.2500×107 2.2500×107 to 2.0000×107 2.0000×107 to 1.7500×107 1.7500×107 to 1.5000×107 1.5000×107 to 1.2500×107 1.2500×107 to 1.0000×107 1.0000×107 to 7.5000×106 7.5000×106 to 5.0000×106 5.0000×106 to 2.5000×106 2.5000×106 to 0 0 to 4.3023×105 Interval=2.5×106

1.6216×107 to 1.6000×107 1.6000×107 to 1.4000×107 1.4000×107 to 1.2000×107 1.2000×107 to 1.0000×107 1.0000×107 to 8.0000×106 8.0000×106 to 6.0000×106 6.0000×106 to 4.0000×106 4.0000×106 to 2.0000×106 2.0000×106 to 0 0 to 1.1359×106 Interval=2.0×106

(i) SZZ stress distribution at floor of intersection (e) SYY stress distribution at side of intersection

2.5858×107 to 2.5000×107 2.5000×107 to 2.0000×107 2.0000×107 to 1.5000×107 1.5000×107 to 1.0000×107 1.0000×107 to 5.0000×106 5.0000×106 to 0 0 to 2.7418×106 Interval=5.0×106

(f) SZZ stress distribution at side of intersection

Fig. 6

Distributions of the stress fields of the bottom, roof and sides of the laneway

From the distribution of stress fields, we can see that the range of stress relaxation at the #1 point of intersection is wider than that of #2, i.e., the radius of the arc at the #1 point of intersection is clearly larger than that at #2. There is a wide range of tensile stress concentrated at the #1 point of intersection; its maximum tensile stress is several times larger than that of the rock. As well, the tensile stress at the #1 point of intersection is larger than that at point #2, as is the distribution range. The angle of the #1 point of intersection is 30°, but 60° at point #2. From the distribution of stress, we can see that the effect of the stress superimposed on the #1 point of intersection is

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Mining Science and Technology

greater than that on point #2. From the stress distribution range of the roof of the laneway, we can conclude that the effect of the stress superimposed between #1 and #2 points of intersection is not obvious, i.e., the interactions of the stresses are not clear and there is no disturbance as a consequence of the excavation. Therefore, the stress relaxation radius of the

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track diphead is larger than that of the horizontal #2 yard and the balancing of stresses in the surrounding rock migrates to deeper areas. 3.2.3 Rules of distribution of the plastic zone From the results of our numerical analysis, the distributions of the plastic zone of the bottom, roof and sides of the laneway are shown in Fig. 7.

None Shear-n shear-p Shear-n shear-p tension-p Shear-p Shear-p tension-p Tension-n shear-p tension-p

(a) Roof

Fig. 7

(b) Side

Distributions of the plastic zone of the bottom, roof and sides of the laneway

From the distribution of the plastic zone, we can conclude that the number of units in the tension and shear region of destruction of both sides of track diphead is greater than that of the horizontal yard. For the roof and bottom of the laneway, the region of the plastic distribution at the #1 point is larger than that at point #2. The number of units destroyed at the #1 NBZ is greater than that at point #2. 3.2.4 Analysis of key part Given our analysis, we can conclude that there are two main factors that affect the stability of laneway at the points of intersection, i.e., 1) the angle between the horizontal stress and the axial direction of the laneway and 2) the angle between the two points of intersection. Both factors have a large effect on the deformation and failure, for both points of intersection are different from all other points. In addition, when designing the support of the diphead laneway, we should reinforce the tensile and shearing strength of the support body. Especially, the #1 point of intersection is the key part in this project, because it may affect the stability of the laneway; hence, we must consider it carefully during the design and construction.

4 4.1

(c) Floor

Analysis of deformation mechanism at points of intersection Failure mechanism of points of intersection of large span laneway

The combinatorial arch theory suggests that when pre-stressed anchor bars are installed in a fracture zone of surrounding rock in laneways[15], coneshaped stress distributions will appear at both ends of

the roof bolt. If the anchor rod group is arranged around the periphery of the roadway, and so long as the space between anchor rods is small enough, the cone-shaped stress distribution formed by every anchor rod will alternate with each other and form a uniform compression zone, i.e., a compressed arch, which can bear the radial load from the fractured rock above. The rock in the compressed arch bears the pressure of the radial and tangential directions, which is in a three-dimensional stress state. This increases the strength of the surrounding rock and at the same time its sustaining power is also increased, as shown in Fig. 8[7]. H QJ UD QJH F L W UD QH DV ]R (O VWLF Q D O R L 3 V HV SU P &R

Fig. 8

Sketch of a bolt combination arch

Therefore, the key for the anchor rod support lies in acquiring greater thickness and strength of the compressed arch. However, it is difficult to enhance the thickness of the compressed arch owing to the length of the anchor rod, which is generally 1.6 to 2.5 m long, limiting its range of support. If the section of the laneway supported by bolts and wire mesh is overly large, it will make the ratio (r/t) between diameter r and thickness t of the compression arch too

LI Guofeng et al

Deformation mechanism and excavation process of …

33

large and its supporting power will be inadequate. Usually, the span of a single laneway section is very small, so that the ratio of r/t can meet the support requirements. However, for deep points of intersection and even for small sections of a main laneway and small branch entries, without the support of a strong middle rock pillar, the main laneway and the branch entries will become one integrated unit, with the result that the radius of r/t will be too large and the roof of the middle rock pillar will subside. Therefore, it is important to keep the support power of a middle rock pillar at the points of intersection and enhance the support on the local part of the laneway.

tersection is increased several times owing to the damage inflicted on the rock of the middle pillar by the blast action. The rock in this section will become cataclastic. Cataclastic rocks will form major perforation cracks under the action of vertical pressure. These cracks bisect the intact rocks and form them into unattached rock pillars which increase the ratio between the length and the width of each rock pillar and reduce the stability of the rock mass before it can reach its optimal strength. Therefore, it is important to reinforce the key positions at the points of intersection to limit the extent of major vertical cracks.

4.2 Pressure cracks and lateral swell of middle rock pillar

5 Reinforcement principle of double-control anchor rod

The middle rock pillar is considered the major component of the midspan support in large span laneways and provides the elastic support for the roof of laneways, when we take into account the interaction between the support system and the deformation of the surrounding rock. The mechanical model of a middle rock pillar at a point of intersection is shown in Fig. 9. &UHVWVODE

2[QRVH

Fig. 9

Mechanical model of medial rock pillar at laneway intersection

The depth of fissure zone around the point of in-

Z Y X

2.2309×106 to 2.0000×106 2.0000×106 to 1.7500×106 1.7500×106 to 1.5000×106 1.5000×106 to 1.2500×106 1.2500×106 to 1.0000×106 1.0000×106 to 7.5000×105 7.5000×105 to 5.0000×105 5.0000×105 to 2.5000×105 2.5000×105 to 0 0 to 9.8844×104 Interval=2.5×105

(a) Before applying DCAS

Fig. 10

6

The principle of a Double-Control Anchor Rod Support (referred to as DCAS) is to increase the horizontal side restrictions and improve the stress state of the middle rock pillar. The double-control anchor rod is obtained from the anchor rods of commonly deformed steel bars. The double-control anchor rods are installed in the two sides of the middle rock pillar when it is formed and pre-stress is applied in order to improve the horizontal constraint stress and increase the three-dimensional stress state. Given our numerical simulation results, we conclude that the DCAS can increase the supporting capacity of middle rock pillars and benefits the entire stability of a laneway. Fig. 10 shows the horizontal stress distribution of a middle rock pillar before and after the support of a double-control anchor rod. In addition, the entire strength and the punching resistance of middle rock pillars are increased by anchor rods, improving the stress condition of an entire laneway.

Z Y X

1.7939×106 to 1.7500×106 1.7500×106 to 1.5000×106 1.5000×106 to 1.2500×106 1.2500×106 to 1.0000×106 1.0000×106 to 7.5000×105 7.5000×105 to 5.0000×105 5.0000×105 to 2.5000×105 2.5000×105 to 0 0 to 2.5000×105 2.5000×105 to 5.0000×105 5.0000×105 to 5.0000×105 2.5000×105 to 7.5000×105 7.5000×105 to 1.0000×105 1.0000×105 to 1.1411×105 Interval=2.5×105

(b) After applying DCAS

Comparison of horizontal stress in middle rock pillar before and after application of double-control anchor rod support

Conclusions

1) According to our numerical simulation of the excavation process of the points of intersection of a deep and large span laneway, two major factors which impact the stability of the points of intersection were determined, i.e., the angle between the horizontal stress and the laneway axial direction and the angle

between the points of intersection with the laneway. These two major factors play an important role in the deformation damage at points of intersection. 2) Given the analysis of a failure mechanism, a mechanical model of points of intersection was established and the key position and the failure characteristics of these points were obtained. 3) The DCAS countermeasure for points of intersection in a deep large span laneway has been pro-

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Mining Science and Technology

posed and from our numerical simulation results, we conclude that a double-control anchor rod can control this key position effectively at the points of intersection.

[7]

Acknowledgements

[8]

Financial supports for this work, provided by the Major Program of the National Natural Science Foundation of China (No.50490270), the National Basic Research Program of China (973) (No. 2006CB202200) and the Innovation Term Project of Ministry of Education of China (No.IRT0656), are gratefully acknowledged.

[9]

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