J. Cent. South Univ. (2013) 20: 821−829 DOI: 10.1007/s11771­013­1553­4

Mechanism of wet shotcrete interacting with rock in support systems

LI Li(李莉), WU Ai­xiang(吴爱祥), WANG Yi­ming(王贻明), HAN Bin(韩斌), WANG Hong­jiang(王洪江), WANG Chun­lai(王春来)

School of Civil & Environmental Engineering, University of Science & Technology of Beijing, Beijing 100083, China

© Central South University Press and Springer­Verlag Berlin Heidelberg 2013

Abstract: In order to have a good understanding of the behavior of wet shotcrete as a support element interacting with the rock mass, mechanism of wet shotcrete interacting with rock in support systems was analyzed through theoretical, numerical study and analytical analysis. A new model of distribution of rock stress state after wet shotcrete was applied, which includes shotcrete layer, composite layer, strengthening layer, plastic layer and elastic layer, and a full illustration of the rock mass stress state was given after shotcrete interacting with rock mass. At the same time, numerical analysis with FLAC gives a stress distribution along the monitor line, respectively, at the sidewall and roof of the tunnel. The displacement obviously decreases with the depth of rock, the tangential stress for tunnel supported by shotcrete is lower than that without shotcrete, and radial stress for tunnel supported by shotcrete is higher than that without shotcrete. It has been demonstrated by AIRY’S stress function, which gives a reasonable solution. Finally, the application of wet shotcrete in Jinfeng Gold Mine shows that the displacement of tunnel decreases obviously in sidewall and roof.

Key words: wet shotcrete; support systems; mechanism; simulation; distribution of stress

Foundation item: Project(50934002) supported by the National Natural Science Foundation of China Received date: 2011–12–28; Accepted date: 2012–04–28 Corresponding author: LI Li, PhD Candidate; Tel: +86–10–62334680; E­mail: skylily616@163.com

1 Introduction

Shotcrete has been widely used for rock supporting in civil and mining engineering for a long time. Resin bolt combined with wet shotcrete reinforces rock mass through restraining the deformation within the rock masses. Shotcrete completely coats the rock surface, locks in rock fragments, reduces rock movement, prevents loosening around the excavation, and promotes overall stability. In order to improve the shotcrete design in mining, it is necessary to have a good understanding of the behavior of wet shotcrete as a support element which interacts with rock mass and transfers load to bolts evenly.

Shotcrete is defined as pneumatically applied to mortar or concrete, projected at high velocity [1]. Until the last decade, the dry process had been the most common method of application, however, the wet shotcrete rapidly gains popularity in mining. In the wet process, the shotcrete components and the water are mixed (usually in a truck mounted mixer) before delivery into a positive displacement pumping unit which then delivers the mixture hydraulically to the nozzle where air is added to project the material onto the rock surface. Main advantages are listed as follows: 1) Low rebound, typically about 5%–15% [2]; 2) Little dust,

environmental friendly; 3) Easy achieving of designed mixture and sprayed thickness; 4) Better mechanic performance and smaller strength deviation; 5) Speed operation and highly mechanized equipment; 6) Wide application for perpetual or temporary support.

There can be no doubt that the overwhelming trend is toward wet shotcrete, due to performance and overall cost considerations. In Sweden, researches on shotcrete as a means of support for hard rock started in 1973 [3]. When applied as an arch or support ring [4–5], shotcrete can also provide a support reaction pressure. In argillaceous rocks, shotcrete reduces slaking and retains the original rock­mass moisture conditions. While our knowledge of how shotcrete works, what its role is in a support system and how it can be applied to prevent large deformations is still incomplete. Because of the complicated geological conditions, shotcrete design is still based on empirical and observational methods, often combined with simplified analytical analysis.

The aim of this work was to analyze the mechanism of wet shotcrete interacting with rock mass as a part of support system. The mechanism of wet shotcrete combined with resin bolts or independently in field was analyzed. A new model of surrounding rock stress distribution was presented, in which rock strength changed with the solidification of wet shotcrete. Wet shotcrete plays an important role in reducing displacement

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and stress around the tunnel, which had been confirmed by numerical analysis and analytical study.

2 Rock instability modes near excavation

Shotcrete is used for supporting underground mine where two primary instability modes must be dealt with: gravity­induced wedge­type failures and stress­induced rock fracturing in a gradual (yielding) or violent (rock burst) manner [6−7]. What happened to the rock near excavation, i.e. rock instability modes, is the premise of understanding the roles of shotcrete. Figure 1 shows the rock instability images from Jinfeng Gold Mine, China.

Fig. 1 Rock mass instability modes: (a) Broken rock; (b) Tunnel working face collapse

The rock­mass behaviors or the possible instability modes for an underground excavation depend upon the magnitude of stress relative to the rock­mass strength and structure. Figure 1(a) shows broken rock falling from the roof supported by mesh and split bolts. Figure 1(b) shows the tunnel working face collapse caused by large ground pressure or mining­reduced stress. For above instability modes, an excavated tunnel should adopt appropriate support ways which can survive imposed loads and deformations. Consequently, before using shotcrete, other support components must be considered, such as rock bolts or grouted rebar and mesh, which can be seen as an integrated support system that includes

shotcrete as only one support component [8]. No matter what kind of instability modes are displayed, shotcrete must be the first choice to reinforce the rock mass after tunnel excavation. Why the shotcrete can perform well in supporting systems is explained as follows.

3 Mechanism of wet shotcrete in supporting

According to the different modes of rock instability, specified rock reinforcement should be applied to the rock mass in order to achieve safe working environment in the underground mining. Support systems are commonly composed of various components: bolts, mesh, cable and shotcrete. Bolts and cables are used to hold rock in place or to reinforce the rock mass, mesh is used to retain loose or broken rock, and shotcrete performs a combination of these functions. It holds by adhesion, strengthens the rock by preventing relative movement at the shotcrete/rock interface, and acts as a “supermesh” by providing a stiff retaining component with substantial bending or flexural capacity.

In China, the common method of rock support in underground mining is rock bolts combined with shotcrete, such as Jinfeng Gold Mine and Jinchuan Mine, China. How the shotcrete controls potential unstable rock blocks is clearly shown in Fig. 2. The wet shotcrete performs four types of functions typically in the rock bolts and shotcrete support system [9–11]: 1) Retaining broken or unstable rock; 2) Filling the fracture of rock; 3) Sealing the surrounding wall to prevent weathering or slaking; 4) Reinforcing the rock and control deformation.

Fig. 2Mechanisms of shotcrete interacting with rock mass

1) Retention When combined and connected to proper holding

elements (rock bolts and cable), shotcrete provides a stiffness and high­capacity retaining function. In well­joint or blocky rock mass, shotcrete prevents raveling between bolt heads, locks key block in­place, and helps transfer the load onto bolts. In this case, shotcrete works as a high­capacity, very tight mesh (super mesh). However, unlike mesh, it prevents relative shear of rock blocks, creating a major stable effect.

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2) Filling Because of good liquidity of wet shotcrete slurry,

mortar and fine materials are highly­rapidly jetted into cracks and joint of rock mass by applying sprayed machine. It develops wedge­effect function through filling the fracture or joint of rock mass, and greatly improves the integrity of rock mass.

3) Sealing Wet shotcrete layer can perform good adhesion with

the rock surface, which could seal the rock surface totally and effectively prevent rocks from connecting with air and water, aiming at protecting rocks susceptible to rock­mass degradation, such as weathering or slaking, corrosion of mesh and bolts, avoiding weakening the strength and self­supporting capacity in moisture condition of underground mine. Another benefit is to reduce wall roughness, showing more beautiful smooth wall rock.

4) Reinforcement Wet shotcrete, as a surface­support component,

does not directly reinforce the rock. However, if we define the effect of reinforcement as strengthening the broken rock, then shotcrete has an equivalent effect. It is able to form a conjunct loading mechanical system with the rock mass because of high adhesive force, transferring various types of stress in combination with the surrounding rock. In addition, thin layer of wet shotcrete, with a certain flexibility, especially with mesh or steel fiber or plastic fiber, makes compatible deformation with the surrounding rock to discharge the ground pressure.

4 Model of surrounding rock stress supported by wet shotcrete

The stability of rock mass in tunnel is determined by three factors, that is, the stress of surrounding rock, rock strength, and support systems. How to reduce the surrounding rock stress and enhance the rock strength, what support system can be chosen is the guarantee of stability of tunnel. The support elements play a significant role in maintaining the integrity of rock mass and the stability of tunnel. Wet shotcrete conducts above four mechanisms in support systems and changes the distribution of surrounding rock stress. A new model of stress distribution of surrounding rock has been developed, and the support resistance of wet shotcrete has been taken into account.

Before excavation of tunnel, the rock mass stress is usually in the elastic state. After excavation, the in site stress near a drift or tunnel may change with time, surrounding rock stress redistribution or concentration may lead to rock failure if rock stress exceeds the limit of

rock strength. At this moment, the rock mass stress changes into plastic state. This is caused by transferring stress from rock surface to the inner of rock mass. When the stress of surrounding rock is obviously higher than rock strength, rock failure or deformation can easily happen, and this area is called broken zone. Researches [12–13] have shown that rock radial stress and tangential stress are both zero near the surface of tunnel before supporting, with the original three dimensions stress changing into two. Figure 3 shows the distribution of stress in tunnel after shotcrete supporting. Compared to Fig. 3, No. 2 and No. 3 areas are broken zone, No. 4 is plastic zone, and No. 5 is elastic zone. No. 4 is the main load­bearing area, where the stress of rock is higher than original ground stress.

Fig. 3 Distribution of stress in tunnel after shotcrete supporting

When wet shotcrete was applied directly to the surface of tunnel as soon as it is excavated, with the solidification of shotcrete, it can provide a proper support resistance combined with resin bolts. Figure 3 illustrates the distribution of surrounding rock stress, which is different from the situation of non­supporting. Initial broken zone is replaced by shotcrete layer (No. 1 area), composite layer (No. 2) and strengthening layer (No. 3), whose roles distinctly vary with the depth of fracture. The stress state of rock mass near excavation is changed greatly due to the shotcrete reinforcement function, which can be applied about 1.5 MPa support resistance onto the tunnel surface. Figure 3 shows the trend of rock radial stress and tangential stress. The peak of

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load­bearing shift to strengthen layer (No. 3), which means that rock mass stress state is close to original stress, and the integrity of rock mass gets better due to the adhesion of wet shotcrete.

Shotcrete layer (No. 1), acting as a support ring or arch, could transfer loads to other holding elements such as rock bolts or cables, and seal the cover of rock in order to protect rock from weathering, bulking or corrosion for bolts. Composite layer (No. 2) includes the boundary of inner shotcrete and rock surface, high bonding capacity of shotcrete is attained together, and open joint and fractures are filled with shotcrete. In good quality of rock, shotcrete can be designed as an integrated support system that includes shotcrete as only component. Strength layer (No. 3) performs the key role in support system, with high load­bearing capacity. This layer typically functions as a stiff arch based on the bolts reinforcement. In the plastic layer (No. 4), where the stress of rock is obviously higher than original stress, the boundary of plastic layer and elastic layer recovers three dimensions. Non­elastic areas are the result of support elements interacting with rock mass. Elastic layer (No. 5) is relatively safe area, and rock mass is basically the same as the virgin rock, maintaining the original rock mechanics.

5 Numerical analysis

A simplified opening geometry with dimensions similar to those of the drift under the depth of 300 m at Jinfeng Gold Mine was used in the analysis. Software FLAC was used to simulate the wet shotcrete interacting with the rock, which is a famous software used in mining engineering. The modeling is a typical semi­arch cross section, the radius of drift is 2.4 m, and the height is 4.8 m, supported by cable and/or shotcrete. In order to minimize the boundaries effects, the domain is more than ten times the size of the cavity, with 60 m in length and 60 m in width and 100×100 grids, as shown in Fig. 4(a). Boundary condition is shown in Fig. 4(b). Stresses about 6 MPa are applied at the top of model. The parameters of rock mass, cable and shotcrete are listed in Table 1, and tunnel dimensions are described, as shown in Fig. 4.

The stress distributions of tunnel supported by shotcrete combined with cable and just only cable supporting are compared. Figures 5(a) and (b) show the radial and tangential stresses, respectively, along the vertical and horizon monitor lines. The results indicate the correction of new model. There is a new discovery about the stress distribution with the depth of tunnel. Figure 6 gives a clear trend of stress distribution along the monitor line. Four parts are similar to the new model. As a whole, the tangential stress for tunnel supported by

Fig. 4 Modeling of drift: (a) Grid model and support; (b) Fixed boundary

Table 1 Parameters used for analysis Type Parameter Value

Density/(kg⋅m –3 ) 2 776 Bulk modulus/GPa 1.49 Shear modulus/GPa 1.31

Friction/(°) 35 Cohesion/kN 1 600

Rock

Tension/MPa 2.15 Density/(kg⋅m –3 ) 7 500 Diameter/mm 25.4 Modulus/GPa 98.6

Ultimate tensile capacity/MN 0.548 Grout compressive strength/MPa 20

Grout shear modulus/GPa 9

Cable

Length/m 2.4 Density/(g⋅m −3 ) 2 100

Elastic modulus/GPa 20 Thickness/mm 80

Cross­sectional area/m 2 0.15 Poisson ratio 0.2

Axial compressire yield strength/MPa 40

Shotcrete

Axial peak tensile yield strength/MPa 4

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Fig. 5 Comparison of stress distribution along monitor line supported by shotcrete & cable and cable only: (a) Radial stress along tunnel from face to depth; (b) Tangential stress along tunnel from face to depth

Fig. 6 Stress distribution along monitor line: (a) Stress distribution on sidewall; (b) Stress distribution on roof

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shotcrete is lower than that without shotcrete, and radial stress for tunnel supported by shotcrete is higher than that without shotcrete. Both of them are close to original ground stress of 6 MPa. However, the stresses near the tunnel surface are evenly distributed along the arch cross section, keeping in a lower level at a distance about 3 m from the surface. This part is just supporting effecting zone where cables are applied both on the sidewall or roof. Lower stress around the tunnel is beneficial to the stability surrounding rock, which is the result of two dimension stress changing into three dimension one. Because of the support element, rock stresses transfer from near the surface to the depth, improving surrounding rock stability to some extent. Wet shotcrete plays an important role in maintaining the stability of rock, as shown in Fig. 7. The maximum axial force is

approximately 1 831 kN. The main load­bearing area is around the peak­point as the interface of plastic and elastic areas. The displacement and deformation of rock decrease as well, as shown in Fig. 8.

6 Mechanical analysis of tunnel applied shotcrete

Several elastic solutions [14–16] exist for calculating displacements and stresses in the ground caused by tunnelling. These solutions typically use either MICHELL’s or AIRY’s stress functions in conjunction with the equilibrium equations to solve the stress and displacement fields around tunnel. In this work, solutions based on AIRY’s stress function have been adopted.

Figure 9 shows the problem geometry, where P0

Fig. 7Axial force of shotcrete and cables

Fig. 8 Displacement along monitor line

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denotes the initial vertical stresses in the ground, and horizontal stress is kP0. For the solution, the initial area in site stress field is separated into a hydrostatic component, and a deviatoric component. As detailed below, it is assumed that the shotcrete has elastic modulus E1, Poission ratio ν1, cross­sectional area A1, radius R1, moment of inertia I1, and joints with rotational stiffness Kθ. The shotcrete is surrounded by a thick annulus of disturbance rock that is mathematically treated as a cylinder with elastic modulus E2, Poission ratio ν2, and inner and outer radii R2 and R3. Shotcrete and plastic cylinders are embedded in elastic ground with elastic modulus Eg and Poisson ratio νg. Plane strain conditions are assumed.

Fig. 9 Problem geometry–jointed double liners system

It is generally accepted that tunnel construction relieves in site stress in the ground, reducing the boundary around the circumference of the tunnel at r=R3

until new boundary stresses are reached that are in equilibrium with the shotcrete reactions. Figure 10 defines the internal reactions required for equilibrium. For the hydrostatic stress component (see Fig. 10(a)), the outer and inner shotcrete reactions at r=R3 and R2 are in the radial direction and denoted by σ1 H and σ2 H , respectively. For the deviatoric component (see Fig. 10(b)), the liner reactions in the radial and tangential direction at r=R3 are denoted by σ1 D cos(2θ) and τ2 sin(2θ). The reaction stresses are equal and opposite at each interface.

Radial and tangential displacements of a continuous shell are related to the applied external stresses by the following differential equations [17]:

2 2

c

d d d d r v u R

D θ τ θ θ

+ = − (1)

4 2 2 f

r 2 4 2 c c

D d d d ( 2 ) d D d d v u u R u u

D R σ

θ θ θ + + + + = (2)

where Dc=E1A1(1–v1 2 ), Df=E1I1(1–v1 2 ). In accordance with Ref. [18], the following equations can be derived from Eqs. (1) and (2) with the radial and tangential displacements of the inner lining related to the reaction stress at r=R2 (e.g. σr=σ1 H +σ2 D and rθ τ = 2 sin(2 ) τ θ ).

4 4 H D 1 1 1 2

f c f

D 1 s 1

( ) cos(2 ) 9 2 D

( ) cos(2 ) 2

R R u D R D

C

τ σ σ θ

τ σ θ

= + + + +

+

4 4 2 D 1 1

f f c ( ( ) ) sin(2 ) 18 36 4 R R R v D D D

σ τ θ = − + +

where π 3

D s 1

0 cos(2 )sin

6

i

i

i i R C k

θ

θ θ σ θ θ

<

<

= − ∑ is a term that

Fig. 10 Reactive stresses: (a) Hydrostatic component j; (b) Deviatoric component

accounts for the influence of liner joints on the radial lining displacement [18] and R is the radius to the centerline of the inner lining. The moment and thrust in the inner liner lining are

2 D D 1

1 ( ) cos(2 ) 3 2 R M

τ σ θ = − +

D D 1 1 ( 2 ) cos(2 )

3 R T σ τ θ = − −

7 Application of shotcrete in Jinfeng Gold Mine

Shotcrete forms a “skin” and provides area coverage to the inner surface of a drift. Reinforced shotcrete has very good load­bearing capacity, even at small deformations. It is obviously stronger and tougher and absorbs more energy than mesh, which has been demonstrated by field trail in Jinfeng Gold Mine, China [19]. Figure 11 has clearly shown the different effects of wet shotcrete combined with resin bolts and mesh combined with split bolts.

Fig. 11 Different effects of mesh combined with split bolts (a) and shotcrete combined with resin bolts (b)

Results from field trials of shotcrete in Jinfeng Gold Mine, China, with shotcrete as a support element and as a substitute for screen, show that the back deformations were considerably reduced when shotcrete was used. But in very poor ground condition [20] or encountering large mining­induced stress, some shotcrete failures happened

occasionally. Compared with mesh and split bolts support system, shotcrete combined with resin bolts or shotcrete only has gained very practical application. This support method has been used for five years, which develops into an irreplaceable role in reinforcing rock mass. Experience from Jinfeng Gold Mine will be useful to our underground mine support.

8 Conclusions

1) Four types of roles are revealed: Retaining broken or unstable rock between bolt heads; Filling fracture or joint of rock mass developing wedge­effect; Sealing the surrounding wall to prevent weathering or slaking; Reinforcing the rock mass and control deformation.

2) A new model of distribution of rock stress state after wet shotcrete has been developed, which includes shotcrete layer, composite layer, strengthening layer, plastic layer and elastic layer. The strength of rock masses is obviously improved due to the function of shotcrete which promotes the tunnel overall stability.

3) Numerical simulation results confirm the correction of the new model, The tendency of radial and tangential stress is close to original ground stress. Importantly, shotcrete plays a great role near the surface of tunnel where the stress is effectively transferred from the surface to the depth, and keeps a low level demonstrated by simulation and field test.

4) The mechanical effect of shotcrete is presented by adopting AIRY’s stress function for in­plane displacement, moments and thrusts in shotcrete layer, which demonstrates the roles of shotcrete displayed during the supporting system.

5) Shotcrete develops mechanical function and has perfect application effect with less displacement and deformation and even stress distribution.

Acknowledgements

A lot of experiments have been done from the year of 2005 by HAN bin, WANG Yi­ming and WANG Shao­yong, and the valuable comments by them are also greatly appreciated.

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