The Supporting Technology Research in Large Section Inclined Shaft with Fracture and Soft Surrounding Rock

This paper is based on the large section inclined shaft crossing goaf of Pingdingshan No.6 Mine as engineering background, and aimed at solving the difficult supporting problem of fractured surrounding rock. After establishing and calculating the mechanical models of U-steel and inverted arch, the support’s vertical reaction force (N1) and horizontal counterforce (X1) are determined as 180.96 KN and 48.12 KN, while the maximum bending stress (σmax) and ultimate bearing capacity of the inverted arch are obtained as 375.59 Mpa and 0.27 Mpa. It shows that the deformation of surrounding rock is well controlled by the supporting structure. The numerical simulation model is built by using the software FLAC3D to analyze the stability of surrounding rock after supporting. The results suggest that the deformation of roof, floor and sides is reduced by 17%, 23% and 71% respectively after supporting with U-steel in the inclined shaft, and the accuracy of results has been verified by a field experiment. Therefore, the “U-steel+ pouring concrete + inverted arch + backwall grouting” technology can effectively control the damage of surrounding rock and improve the stability of surrounding rock.


INTRODUCTION
With the increasing number of high yield and high efficiency working face in the mine in china, more and more the roadways require a large cross section (Zhang, 2009) [1].Unstable and broken surrounding rock has become an important characteristic of a long abandoned goaf.In mining engineering field, it is quite difficult to support a large section roadway crossing a goaf.Therefore, the mining scientific community has been working to solve supporting problems of large section roadway with fracture surrounding rock.For instance, academician He M.C. believes that secondary supporting is the key parts of the deep roadway supporting, reasonable supporting effects have been achieved through theoretical deducting secondary supporting time and applying large rigidity and high strength supporting technology in key positions (He, 2008) [2].Wang H.W. proposed "U-steel yieldable support + anchor-net-spray with high strength and high prestressed " supporting scheme which applied in China Tiefa coal mine, which provides satisfactory supporting effects (Wang, 2012) [3].Fang X.Q.applied a new supporting technology of "U type steel support + bolt-injection-anchorage cable" in China Xuehu coal mine's roadway with fracture surrounding rock (Fang, 2012) [4].According to the condition of broken surrounding rock in Yaoqiao coal mine roadway, Huang X.X.used a *Address correspondence to this author at the School of Energy ＆Safety Engineering, Hunan University of Science ＆Technology, Xiangtan, China; 411201; Tel: ++86-0731-58290040; E-mail: pengwenqing@163.comcreative supporting technology, which involves two steps: the first step is to side clearing after pregrouting, while the second step is to bolt-shotcrete (Huang, 2010) [5].Dalgic (2000), Anagnostou (1993) and R. Yoshinaka (1997) et al have performed extensive research in the weak surrounding roadway supporting, and proposed the joint supporting theory [6][7][8][9][10].The deformation mechanism of large section tunnels has been extensively investigated in recent years too.For example, Lee (2006) explored tunnel stability and arching effects [11].Huang (2013) and Fraldi (2011) concluded that collapse is the main failure mode in tunnels at a specific depth [12,13].Mollon (2010) and Wonga (2012) conducted an analysis of the passive failure mechanism of tunnel faces [14,15].Corkum (2007) and Chang (2007) determined that the failure mechanism of rocks around deep buried tunnels [16,17].
Despite recent advancements in large section roadway failure and support technology research, the supporting technology research in large section roadway remains challenging.In this paper, the large section inclined shaft of Pingdingshan No. 6 Mine is taken as the research instance, mechanical model of U-steel and inverted arch is established by structural mechanics to analyze the stability of U-steel structure.The numerical simulation model was established by the software FLAC3D to analyze the stability of surrounding rock after supporting.The results have certain reference significance to similar roadway supporting.

ENGINEERING SITUATION
The length of the inclined shaft is 1575 m, and the length of the inclined shaft in goaf is 117m.As shown in Fig. (1), the size of the inclined shaft section is 6.33 m*4.665 m (width*height), while the net sectional area is 23.1 sq.As shown in Fig. (2).Detection recorder YTJ20 is used to detect the scope of fracture zone in this paper, which the surrounding rock was damaged.X ray diffraction experiment is applied to analyze mineral composition of surrounding rock.As shown in Fig. (3), there are a lot of clay minerals, such as kaolinite and montmorillonite in surrounding rock, which shows that the surrounding rock is geological typical soft rock.Therefore, the roof accident will probably happen in the inclined shaft after an unreasonable support technology is applied in the inclined shaft with fracture and soft surrounding rock.

Mechanical Model of U-steel
According to the interaction relationship between U-steel supporting and surrounding rock of roadway, the mechanical models of U-steel were established.As shown in Fig. (4a), there are four constraint forces in this mechanical model, which belongs to one-order statically determinate structure.
The load of arch crown is q 1, and the load of column base is q 2. The positions of a and d are simplified to fixed hinge supports.

Calculation of the Support's Vertical Reaction Force
For d-bearing, because , the support vertical counterforce of a-bearing can be calculated by

Calculation of the Support's Horizontal Counterforce
The internal force was solved through the principle of force method in structural mechanics, the right-hand support changed to movable hinge support, then the structure is a basic static structure, and the redundant constraint was replaced by an unknown force X 1 .As shown in Fig. (4b), horizontal counterforce of a-bearing R 2 is obviously equal to X 1 .According to the denatured coordination condition, for abearing, there is no displacement in the X 1 direction, consequently the typical equation can be listed by the principle of force method.
Where, X 1 is an unknown force instead of the redundant constraint; Δ 1 is displacement of the simplified basic system; Δ 11 is the displacement produced by unknown force; Δ 1p is the displacement produced by load; δ 11 is the displacement along the X 1 direction.
Δ 11 andΔ 1p were solved by static structure theory.Due to structural with bending deformation mainly, effects of axial force and shear force on the displacement are very small, then: Where, M p is the bending moment of the support, when the basic structure affected by the only force (q 1 ) ; 1 M is the bending moment of the support under only force (X 1 ), and X 1 =1; S is the length along the axial bracket; E is the elastic modulus of the support; I is the moment of inertia of crosssectional neutral axis.
After calculation, the formula (4) can be obtained.
) 2 According to the formula (2) and formula (4), the support's vertical reaction force can be calculated by formula (5).

Calculation of the Support's Bending Stress
The bending moment of the U-steel structure can be calculated by formula (6) Through the formula derivation, .In order to ensure that the structure is not damaged, the maximum bending stress must satisfy formula (7).
[ ] Where, Mmax is the maximum bending moment of the U-steel structure; W is the Section modulus of bending; [σ] is the permissible bending normal stress.
When the roadway's buried depth is deeper (Z > 5 a), and surrounding rock of roadway is broken, Terzaghi's ground pressure theory believe that the roadway's ground pressure should be calculated by formula (8).
Where, is the top pressure collection degree, and c q is the lateral pressure collection degree, KN/m 2 ; γ is the average volume force of the overlying strata, KN/m 3 ; a is the radius of roadway section, m; b is the height of load, and q m; λ is the lateral stress coefficient; φ is the angle of internal friction.
For completely broken and has not been chemical erosion of rock, Terzaghi's put forward the point of view that the height of load should be calculated by formula (9).b=1.10(2a+H) (9) Where, H is the height of roadway.
According to the characteristics of the inclined shaft, We can get the following conditions, a=3.77 m, H =5.07 m, γ=23 KN/m 3 , φ=30°.After calculation, we can obtain q d is 0.32 Mpa, q c is 0.15 Mpa.
In this paper, the mechanical model of U-steel was simplified to that the load on arch ring come from q d , and the load on sides come from q c .Because the width of the U-steel is 0.15m, q 1 =0.15q d =48 KN/m, q 2 =0.15q c =22.5 KN/m.For U 36 support, when h=1.3 m and r=3.77m, then its' permissible bending normal stress equal to 520 Mpa ([σ]=520 MPa), W=137 cm 3 .According to the formula (1) and formula (5), we can get the following data, the support's vertical reaction force (N 1 ) is 180.96KN, the support's horizontal counterforce (X 1 ) is 48.12 KN, the maximum bending stress (σ max ) is 375.59Mpa, because σ max <[σ], the U 36 support can remain stable.

Stability Analysis of Inverted Arch
According to the roadway with fracture surrounding rock, it is easy to cause serious floor heave if the supporting scheme is not reasonable due to the transfer of broken rock mass from two sides transfer to the floor under the action of stress.Floor heave can be controlled by U -steel inverted arch and supplemented by bolt.While mechanical model of inverted arch, as shown in Fig. (6), is established by the inclined shaft's floor supporting (Fig. 5).According to the mechanical equilibrium of inverted arch, the formula ( 9) can be established.
Where, L X is the width of the roadway, L x =6.33 m; L Z is the width of roadway along the long axis direction, L z = 0.4 m; q 1 is the vertical stress on the inverted arch, q 1 =48 KN/m; T is the vertical tension of floor bolting; P 0 is the ultimate bearing capacity of the inverted arch.Lx T p 0 p y q 1 q 1

Fig. (6). The mechanical model of inverted arch.
There are three bolts on floor supporting, the bolts' diameter is 22 mm, and the length is 2.4 m, the vertical tension of floor bolting is 600 kN.So the ultimate bearing capacity of the inverted arch can be calculated by formula (9).According to the experience of roadway floor support, when the reaction force equal to 0.2 Mpa, stability of surrounding rock can be achieved.Therefore, it is a feasible design.

Simulation Scheme
In order to contrastive analysis the effect of different supporting schemes, as shown in Fig. (7), two simulation models are established by FLAC 3D .Two kinds of supporting schemes are compared in this simulation, as shown in Table 1.

Analysis of the Simulation Results
As shown in Fig. (8), Fig. (9) and Fig. (10), in scheme 1, the subsidence displacement of roof is 321.4 mm, the amount of floor deformation is 55.1 mm, the amount of sides deformation is 175.7 mm. in scheme 2, the subsidence displacement of roof is 267.5 mm, the amount of floor deformation is 42.6 mm, the amount of sides deformation is 50.2 mm.Compared with the results of scheme 1 and scheme 2, the deformation of roof, floor and sides is reduced  by 17%, 23% and 71% respectively, which suggests that the scheme 2 provides a more stable support effect for the inclined shaft.

Support Parameters
According to the theoretical analysis and numerical simulation, and combining with practical application, the supporting scheme "U-steel+ pouring concrete + inverted arch + backwall grouting" is applied in the inclined shaft with fracture and soft surrounding rock.The first step of construction is lay the metal net(the diameter is 4 mm, the grid is 40×40 mm) on the roadway.The second step is to erect U 36 -steel with inverted arch, while the spacing of U36steel is 500mm, with three bolts on floor supporting, which has 22 mm diameter and 2.4 m length.The third step is to pour concrete, which intensity of the concrete is C30, with 500 mm thickness.The last step is to backwall grouting after the initial set of concrete with 1 m depth of the grouting hole, 2000 mm spacing and 3000 mm array pitch.The supporting arrangement in the inclined shaft is shown in Fig. (11).

Supporting Effect
In order to obtain deformation law of surrounding rock and supporting effect after using the new support scheme, field observation is conducted.As shown in Fig. (12), the convergence between roof and floor is 60 mm, and the convergence of two sides is 43 mm.The deformation of surrounding rock was large in the early stage, while decreased obviously in the late stage, and tended to be stable after 60 days.As shown in Fig. (13), the damage of surrounding rock has been controlled effectively after supporting in Pingdingshan No.6 Mine.After supporting in Pingdingshan No.6 Mine, the damage of surrounding rock has been controlled effectively, and the stability of surrounding rock has been improved.
maximum bending moment of the U-steel structure can be obtained by Fig. (7).The simulation model.

Fig. ( 12 ).( 3 )
Fig. (12).Displacement curves of surrounding rock.CONCLUSION(1) According to the mechanical model of U-steel, the following data is achieved, the support's vertical reaction force (N 1 ) and horizontal counterforce (X 1 ) are 180.96KN and 48.12 KN respectively, while the maximum