*4.4. Control Technology and Application for Large Section Whole Coal Cavern Groups* 4.4.1. A Novel Stratified Reinforcement Ring Concept of the LBG

According to the stress distribution nephogram and curve of the cavern group, the surrounding rock of the roadway can be divided into internal and external bearing structures, as shown in Figure 12.

Different stress states in the broken zone, plastic zone, and elastic zone require different supporting strengths. The supporting effect should be coupled with the mechanical characteristics of the surrounding rock to effectively control the deformation and failure of the surrounding rock. A novel stratified reinforcement ring concept of "long cable-boltgrouting" (LBG) was proposed to control the deformation of a whole coal cavern group based on the obtained deformation mechanism. The specific surrounding rock control mechanism is as follows:

(1) The surrounding rock of the cavern is divided into the fracture zone, the plastic zone, and the elastic zone from a shallow part to a deep part. The damage degree decreases gradually, and the support strength required to achieve stability also decreases gradually. The coordinated support of an anchor bolt, a grouting anchor cable, and a long anchor cable can meet this requirement. In the shallow part of the surrounding rock, the supporting density demand of a bolt, a grouted anchor cable, and a long anchor cable is small, while the supporting strength demand is large. In the deep surrounding rock, the support density demand decreases successively, and the support intensity decreases correspondingly. By partitioning the surrounding rock and adding reinforcement, a stable bearing circle that is consistent with the surrounding rock can be formed. For the fracture zone (0–2.5 m), plastic zone (3–8 m), and elastic zone (beyond 8 m), the coordinated support technology of an anchor bolt (3 m), a grouting anchor cable (8.3 m), and a long anchor cable (15 m) has been proposed and adopted to connect the broken zone, the plastic zone, and the elastic zone. Three kinds of supporting equipment are arranged at intervals and form three stable bearing rings of 0–3 m, 3–8 m, and 8–15 m from the shallow to deep layer of the surrounding rock;


#### 4.4.2. Determine the Surrounding Rock Control Parameters of the Whole Coal Cavern Group

The surrounding rock control parameters of the whole coal cavern group are determined based on the deformation and failure characteristics of the whole coal cavern group and the above surrounding rock control theory. On the roof of whole coal cavern groups, the supporting configuration of a high-strength bolt with a high pre-tightening force and a high-strength anchor with a high pre-tightening force were determined. On the two sides and floor of the whole coal cavern group, the grouting was determined. These two supporting configurations, in both the roof and two sides, were applied in the large section gas storage cavern group. The specific parameters are as follows:

(1) Roof support parameters of the cavern group

On the roof of whole coal cavern groups, the supporting configuration of a highstrength bolt with a high pre-tightening force and a high-strength anchor with a high pre-tightening force were determined.

In the roof and two sides of the cavern, the anchor cable specifications are <sup>Φ</sup> <sup>22</sup> × 9300 mm2 and <sup>Φ</sup> <sup>22</sup> × 8300 mm2, and the spacing and row layout are 1600 × 1600 mm2 and1500 × 1600 mm2. The size of the bolt is <sup>Φ</sup> <sup>22</sup> × 2400 mm2 with high-strength ribbed steel. The size of the laying metal mesh is 100 × 100 mm2, and a WD 280-2.7 steel belt is adopted. The thickness of the concrete injection is 150 mm, and the roof is reinforced with a combined prestressed anchor cable. There are five combined anchor cables, each of which is prestressed with a dimension of 22 × 12,300 mm2 and a spacing and row spacing of 2200 × 3200 mm2. The supporting form of the roadway is shown in Figure 13;

**Figure 11.** A cavern displacement diagram. (**a**) Roof displacement of a low-negative pressure cavern; (**b**) roof displacement of a high-negative pressure cavern; (**c**) two-sided displacement of a low-negative pressure cavern; and (**d**) two-sided displacement of a high-negative pressure cavern.

**Figure 12.** A schematic diagram of the inner and outer bearing structures of the surrounding rock.

**Figure 13.** The roadway support layout plan.

#### (2) Grouting parameters

Three holes are arranged in each row of the cavern sections. The top row holes are 1.5 m away from the roof, the bottom row holes are 1.5 m away from the floor plate, and the middle holes are 3.3 m away from the floor plate. The holes are arranged in parallel. The diameter of the grouting hole is 42 mm, and the depth of the hole is 3 m. A seamless steel pipe is used as the grouting pipe. The hole sealing depth must be at least 1 m, and the hole sealing device is used to seal the hole. Shallow-hole grouting pressure is 2.0–4.0 MPa using a pneumatic single liquid grouting pump, and the diffusion radius is about 2 m.

#### **5. Verification and Application**

#### *5.1. Numerical Simulation Results of LBG Control Technology*

In order to study the control effect of the roadway surrounding rock with LBG technology and verify the rationality of the support scheme and parameters, a FLAC3D finite difference program was used to simulate the stress and deformation of the roadway surrounding rock. The stress and displacement nephograms are shown in Figures 14 and 15.

The stress and displacement nephograms (Figures 14 and 15) show that the LBG control technique can effectively improve the stress state of the surrounding rock in the chamber groups. The tensile stress of the surrounding rock is reduced, and surrounding rock control technology plays an active supporting effect. The maximum principal stress is reduced. The two-sided displacement and roof subsidence are reduced, and roadway deformation is completely controlled within the required range, which ensures the stability of the surrounding rock. It shows that LBG control technology is reasonable.

#### *5.2. Engineering Practice*

The supporting configurations in both the roof and sidewalls above were applied to the large section gas storage cavern group. In order to verify the rationality of the proposed surrounding rock control technology, a roof separation instrument and a borehole peeping instrument are arranged in the reinforcement section of the cavern to monitor the surface displacement change of the cavern. Eight measuring stations are arranged in the main body of the cavern (measuring stations 2–7) and the connecting crossover points of the cavern (measuring stations 1 and 8). The monitoring diagram and results are shown in

Figures 16 and 17. The field monitoring results and numerical simulation results on the maximum deformation of the surrounding rock are compared (Figures 18–20).

**Figure 14.** A stress cloud chart of LBG control technology. (**a**) A vertical stress cloud chart of LBG control technology; and (**b**) a maximum principal stress cloud chart of LBG control technology.

**Figure 15.** Displacement cloud chart of LBG control technology. (**a**) A horizontal displacement cloud chart of LBG control technology; and (**b**) a vertical displacement cloud chart of LBG control technology.

**Figure 16.** Layout of surface displacement measuring points and peep holes.

**Figure 17.** Surrounding rock deformation curve. (**a**) 1#; (**b**) 2#; (**c**) 3#; (**d**) 4#; (**e**) 5#; (**f**) 6#; (**g**) 7#; and (**h**) 8#.

**Figure 18.** Numerical simulation results versus the field monitoring results of the maximum twosided displacement.

**Figure 19.** Numerical simulation results versus the field monitoring results of the maximum roof subsidence.

**Figure 20.** Numerical simulation results versus the field monitoring results of the maximum bottom drum displacement.

For measuring stations numbers 2–7 located in the main position of the cavern, the deformation of the surrounding rock in the cavern can be divided into three stages over time. They are the rapid deformation stage, the slow deformation stage, and the stable deformation stage. Within 10 d after excavation, the cavern is at the stage of rapid deformation. The maximum roof subsidence, bottom drum displacement, and two-sided displacement are 31 mm, 24 mm, and 46 mm, respectively. On the 15th day, the support technology of LBG was applied, and the excavation cavern began to increase slowly, which belongs to the slow deformation stage. Roof subsidence, bottom drum displacement, and two-sided displacement are 54 mm, 52 mm, and 79 mm, respectively. The deformation of the surrounding rock is almost constant after 74–95 d of excavation, which is the stable stage of deformation.

For the No. 1 and No. 8 measuring stations at the intersection and the connection of the roadway, the roof subsidence of the surrounding rock and the two-sided displacement are 85 mm, 91 mm, 125 mm, and 130 mm, respectively, and the bottom drum displacements are 77 mm and 79 mm, respectively. This is because measuring stations 1 and 8 are located in the passageway, and the roadway deformation is large due to the influence of concentrated stress.

Meanwhile, Figures 16–18 show the field-measured results are in good agreement with the numerical results. The monitoring results show that the deformation and failure of the surrounding rock in the cavern are obviously reduced by using a LBG control scheme. According to the peeping results of the drilling hole, the failure depth of the surrounding rock is only within 2 m of the surrounding rock, and there were no large tensile and shear failure areas. An anchor bolt and cable can play a better anchoring role. The novel support technology of a LBG can ensure the long-term stability of the cavern. There is no stratification within 2–15 m of the surface of the cavern.
