1 Introduction

With the depletion of the shallow coal resources, the exploitation of the coal resources gradually develops to the deep, and the corresponding engineering geological conditions are increasingly complex (Zhang 2005), resulting in the maintenance difficulty of the surrounding rock support of the large section roadways and chambers, and the heavy workload of the repair, which not only brings great safety risks, but also seriously hinders the production efficiency. Under the deep environment, the ground stress of the roadway increases correspondingly, and the original rock stress has a great influence on the stability of the surrounding rock of the roadway (Ma and Hou 1995). The surrounding rock of the deep coal roadway forms engineering soft rock under the strong original rock stress (He and Sun 2004), and the serious floor heave is one of the development problems.

Scholars at home and abroad have done a lot of research on the control of deep roadway and chamber surrounding rock. In terms of new support means, the paper puts forward concrete-filled steel tube support (Liu et al. 2017a, b), strong bolt support (Wu et al. 2017), extendable bolt (Wang et al. 2016), constant resistance large deformation bolt (cable) (He et al. 2014), etc., which has been applied in the complex conditions of soft rock large deformation roadway and chamber (Hao et al. 2016; Zhang et al. 2016a), dynamic pressure affected roadway (Zhang et al. 2016b), chamber group reinforcement (Kang et al. 2011). In terms of the concept of surrounding rock reinforcement in weak roadway, scholars at home and abroad put forward support ideas such as coupling support (He et al. 2013), key part support (Sun and He 2005), composite support (Wei et al. 2013; Liu et al. 2017a, b), and collaborative support (Long et al. 2012). Zhang (2016) put forward the support principle of “two high and one large” in view of the large deformation of the fault broken coal rock roadway. Zheng et al. (2018) designed a composite structure, including concrete antiarches and bolts to control the floor heave. Guo et al. (2017) put forward the comprehensive treatment methods of the grouting bolt combined with reinforcing sidewalls and corners for the floor heave. Yan et al. (2016) proposed that “advance grouting, shotcreting and anchoring grouting stage reinforcement is the foundation support, and then the key parts of anchor cable reinforcement and surrounding rock grouting technology are implemented”. Meng et al. (2017) put forward the technical scheme of “primary steel mesh spraying + reinforced concrete lining + secondary grouting” combined support for the difficult problem of roadway passing through faults under complex geological conditions. Song et al. (2018) used the theory of ultra-low friction effect of rock mass and the theory of dynamic load stress wave to study the influence of dynamic load on fault fracture zone, and put forward the corresponding safety measures of roadway passing through fault. Wang et al. (2019) put forward a comprehensive repair and support scheme, which optimizes the full section support with the technology of bolt mesh cable spraying + concrete filled steel tube support and strengthens the floor support with the reinforced concrete bottom beam. The above research results can provide important reference for the surrounding rock control of roadway and chamber under the deep complex conditions, but due to the complexity of the engineering geological conditions, the engineering analogy method often has certain limitations, which must be analyzed for the specific roadway and chamber, and there are relatively few researches on the combination of support and improvement of the rock structure of the bottom plate of the roadway.

Based on this, this paper studies the deformation mechanism of the soft rock roadway floor by the methods of engineering geological condition analysis, numerical simulation and field test, puts forward the joint reinforcement plan of “prestressed grouting anchor cable + grouting anchor bolt + grouting behind the wall + floor reconstruction”, develops a new type of floor drilling rig, and carries out field monitoring of mine pressure.

2 Deformation Characteristics of Soft Rock Roadway Floor

2.1 Project Overview

The No.11021 working face is located in the western part of the Zhaogu No. 2 Mine. The east side of the lower trough of the No.11021 working face is the inner section of the unearthed No. 11021 working face, the west side is the upper trough of the No.11041 working face, the north is the contact lane of the No.11021 working face, and the south is the F18 fault. The layout of No. 11021 working face is shown in Fig. 1.

Fig. 1
figure 1

Schematic diagram of mining engineering in No. 11021 working face

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The average thickness is 5.8 m of No. 21# coal seam, and the pseudo roof mainly consists of mudstone and carbonaceous mudstone with a thickness less than 0.5 m. The thickness of direct roof is generally 1–6.5 m, mainly mudstone roof. The main roof is coarse, medium and fine sandstone with an average thickness of 7.46 m. The floor is mainly composed of mudstone and sandy mudstone, with an average of 12.84 m. The floor is easy to soften and swell with water. The cracks in the floor are developed with obvious horizontal bedding, the cracks in the floor are developed, and there is old empty water in the tunnel. The conditions of coal seam roof and floor are shown in Table 1.

Table 1 Coal seam roof and floor
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2.2 Deformation Characteristics of Soft Rock Roadway

After the excavation of the rock mass, the initial deformation rate of the lower channel in the inner section of the No.11021 working face is very large. In a short period of time (generally 3–7 days), the deformation can reach 500 mm, and the deformation in some areas can reach 1000 mm, resulting in the condition that the roadway is excavated before and repaired after excavation, the concrete is cracked, the support is damaged seriously, and the roadway has to be expanded and reinforced again, and the damage form of the roadway is shown in Fig. 2. According to the analysis of engineering geological conditions, the reasons for the instability of the inner section of No.11021 working face are as follows:

  1. 1.

    The geological structure and stress environment are complex. The fault is developed near the mining roadway, the buried depth of the roadway is about 700 m, the horizontal stress is 1.2–1.5 times of the vertical stress, the roadway around the mining roadway is crisscross, and the supporting stress between the roadways overlaps with each other, resulting in a large supporting pressure of the rock pillar.

  2. 2.

    The fracture of surrounding rock is developed. In the structural area, the surrounding rock is tensioned and pulled by faults, with developed fissures, broken surrounding rock and poor self-stability.

  3. 3.

    The lithology of surrounding rock is poor and cemented. The lower channel of the inner section of No.11021 working face is mainly composed of mudstone and sandy mudstone. It is greatly affected by water. After encountering water, it will expand and muddy. It has strong softening, sharp reduction of strength and loss of bearing capacity.

  4. 4.

    Rheology of surrounding rock of roadway. The initial support form of roadway surrounding rock is bolting and shotcreting, but due to the untimely shotcreting, the surrounding rock is weathered and even loose, which is easy to cause bolt failure and instability of support structure. Under the action of high stress and fault structural stress, the surrounding rock of roadway changes.

  5. 5.

    The strength of the original support system is low. Although the inner section of No. 11021 working face adopts strong anchoring technology, the surrounding rock has poor anchoring ability, resulting in uneven stress on the support and easy to be broken by each.

Fig. 2
figure 2

Bottom drum of lower groove in inner section of 11021 working face

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3 Soft Rock Roadway Control Mechanism and Support Design

Based on the above analysis, and considering the requirements of simple construction technology and operability, the bottom drum treatment and support scheme is determined as “prestressed grouting anchor cable + grouting anchor bolt + grouting behind the wall + base plate reconstruction”.

3.1 Overall Foundation Reinforcement

The overall reinforcement of roadway foundation adopts full section anchor mesh cable support, as shown in Fig. 3.

  1. 1.

    Roof bolt specification: Φ20 × 2400 mm, row spacing of 800 × 900 mm, bolt tray of δ10 × 150 × 150 mm, used together with steel ladder, the length of steel ladder is 4160 mm, row spacing of 900 mm.

  2. 2.

    Specification of side anchor cable: Φ17.8 × 4250 mm, row spacing of 900 × 900 mm, tray of 2860 mm long steel ladder and δ12 × 200 × 200 mm steel plate.

  3. 3.

    Specification of anchorage cable of channel steel beam: Φ21.6 × 8250 mm, spacing between rows of 1300 × 1800 mm and 1500 × 1800 mm, tray of 4200 mm and 3300 mm long 16# channel steel beam and δ12 × 120 × 120 mm, δ12 × 80 × 80 mm steel plate.

  4. 4.

    The top anchor bolt, steel ladder and metal mesh support are closely following the working face; the lag head-on distance of two anchor cables on the upper part of the side shall not exceed two rows; the lag head-on distance of two anchor cables on the lower part of the side shall not be less than five rows, and shall not exceed eight rows.

  5. 5.

    The metal mesh shall be welded with Φ6 mm steel bars, with mesh size of 1000 × 1900 mm and overlapping length of not less than 100 mm. It shall be bound firmly with 14# galvanized wire, with top four corners and side corners.

Fig. 3
figure 3

Cross section of roadway support

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3.2 High Preload Reinforcement Requirements

The anchoring force of all anchor bolts in the tunnel construction is 70 kN, and the bolt torque is not less than 300 N m, the prestressed force of Φ21.6 mm anchor cable is not less than 350 kN, the prestressed force of Φ17.8 mm anchor cable is not less than 150 kN.

3.3 Bottom Plate Self Drilling Grouting Anchor Bolt Scheme

Self-drilling grouting anchor has the characteristics of lengthening, self-anchoring, self-drilling, active support, grouting and flexible support. It can be used for advanced support of roadways, radial support, slope reinforcement, roadbed reinforcement, river bank reinforcement, arch angle reinforcement, and roadway disaster rectification projects, as shown in Fig. 4.

Fig. 4
figure 4

Self drilling anchor

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The self-drilling anchor consists of four parts: drill bit (anchor head), rod body, fastening device and accessory device, the role of drill bit is drilling and anchoring. The rod body is composed of three parts: drill pipe when drilling, anchor bolt when anchoring, and passage when grouting. At the same time, pre tightening force can be exerted by fastening device. Therefore, the research and development and improvement of the floor bolt equipment provide technical support for the smooth implementation of the deep grouting construction method of the floor. The parameters of grouting anchor bolt in the bottom plate are as follows:

The length of the grouting anchor is set to 4000 mm. At the same time, four small outlets are added to the depth range of 2–4 m of the anchor, and three per row are arranged with a row spacing of 2000 mm, as shown in Fig. 5. The details are as follows: 1# grouting anchor is arranged at 1800 mm to the left of the centerline, and is vertically angled 10°, and the hole depth is 4200 mm. 2# grouting anchor is arranged at 100 mm to the left of the centerline, and the vertical bottom plate is set, and the hole depth is 4200 mm. 3# grouting anchor is placed 500 mm to the right of the center line, tied vertically 30°, and the hole depth is 4200 mm.

Fig. 5
figure 5

Cross section of self drilling grouting bolt support of bottom plate

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3.4 Prestressed Grouting Anchor Cable Support at the Side

Two rows of reinforced channel beam anchor cables are arranged at the lower side of the roadway. The specification of common anchor cable is Φ17.8 mm × 4250 mm, and the specification of grouting anchor cable is Φ22 × 4250 mm. The common anchor cable and grouting anchor cable are arranged alternately. The spacing between the anchor cables is 900 mm, the length of common anchor cable is 1500 mm, and the length of grouting anchor cable is 1000 mm.

Two rows of reinforced anchor cables are arranged on the upper side of the roadway. The specification of ordinary anchor cable is Φ17.8 mm × 4250 mm, and the specification of grouting anchor cable is Φ22 × 4250 mm. The ordinary anchor cable and grouting anchor cable are arranged alternately. The spacing between the anchor cables is 900 mm, and the anchorage length of the anchor cable is the same as that of the lower side.

3.5 Development of Drilling Construction Equipment for Bottom Plate

At present, the size of the construction machines and tools for floor drilling is large in size and the construction conditions are high. For example, hydraulic pipes, electric hydraulic pumps, and power distribution equipment need to be laid at the construction site, and there must be a sufficient section of the roadway. It is forbidden to use live equipment in the return airway. Existing equipment cannot meet the requirements of small aisle space, return airway construction, and the simultaneous deployment of multiple machines in the aisle. Therefore, this paper has developed a floor drilling construction equipment that can solve the above problems. The drilling machine uses the three-dimensional frame to fix the two-way propulsion cylinder and the hand-held pneumatic drilling machine on the same axis, and uses the two-way propulsion cylinder to push up and down, without any external force. It can reduce labor intensity and increase drilling accuracy and depth, as shown in Fig. 6.

Fig. 6
figure 6

Schematic diagram of floor drilling machine

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Specific usage: Through the external high-pressure air flow, the air flow is divided into three ways to provide power for the air drill, the upper supporting air cylinder and the pushing and pulling air cylinder. First, open the upper supporting hand rotary valve to fix the overall support. Close the supporting cut-off valve, open the hand rotary valve of air drill, adjust the forward and reverse direction of the drilling machine as required, and start the drilling machine for work. Turn the push pull-out hand rotary valve to control the cylinder to slide up and down. Finally, close the air drill, respectively push the pull-out cylinder, and pull the cylinder back.

The main innovations and advantages are: fixed by powerful cylinder, enhanced thrust and better stability. Smaller unit volume, smaller construction area compared with large-scale drilling machine, it can adapt to smaller space area, and can construct multiple units at the same time, greatly improving drilling construction efficiency. The whole structure of the drilling frame is simple and the quality is light, so it can be handled manually.

4 Numerical Simulation Study

4.1 Simulation Analysis Under Different Stress Conditions

4.1.1 Model Establishment

Based on the engineering background of No. 11021 working face of Zhaogu No. 2 mine, a 3D model with the dimension of X × Y × Z = 50 m × 5 m × 37 m is established by using FLAC3D numerical simulation software. The model is divided into 37,000 units and 45,450 nodes, as shown in Fig. 7. Displacement constraints are applied to the left, right, front, back and bottom boundaries of the model, and uniform load is applied to the top of the model, which is the weight of the overlying strata. According to the field geological survey and rock mechanics test results, considering the scale effect of rock, the rock mechanics parameters used in the simulation calculation are shown in Table 2.

Fig. 7
figure 7

Three-dimensional model

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Table 2 Mechanical parameters of coal and rock mass
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4.1.2 Results Analysis

In order to study the impact of stress changes on the surrounding rock failure characteristics, different stress states are assigned by controlling the depth of the coal seam. The depths are set to 600 m, 700 m, 800 m, and 900 m respectively. The overburden rock volume is calculated at 25 kN/m2, the vertical stress applied to the upper part of the model is 14.5 MPa, 17 MPa, 19.5 MPa, and 22 MPa, and the horizontal stress is set to 1.2 times the vertical stress.

  1. 1.

    Stress analysis

Figure 8 shows the distribution of vertical stress field under different burial depths (600 m, 700 m, 800 m, 900 m).

Fig. 8
figure 8

Cloud chart of vertical stress distribution under different burial depth

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After the roadway is excavated, there is a stress reduction area in the roof and floor of the roadway, and the stress concentration is formed in the depth of the side. The stress state of the surrounding rock is mainly compressive stress. Only in a certain depth range of the roadway floor, there is tensile stress, and the existence of tensile stress is the direct factor leading to floor heave. It can be seen from Fig. 6 that the stress is mainly concentrated in the range of 1.5–3 m from the roadway side, and the maximum stress concentration coefficient is 1.26 when the depth is 600 m, the maximum stress concentration factor is 1.43 at 700 m depth, the maximum stress concentration factor is 1.56 at 800 m depth, the maximum stress concentration factor is 1.71 at 900 m depth. It can be seen that with the increase of the buried depth, the stress concentration in the roadway side increases. With the increase of the buried depth, the stress of the surrounding rock of the two sides and the roof and floor increases in varying degrees, and the peak value of the stress gradually transfers to the depth, and the bearing capacity of the surrounding rock transfers to the deep surrounding rock, thus causing a large range of damage areas.

  1. 2.

    Displacement analysis

The distribution law of vertical displacement field under different burial depth is shown in Fig. 9.

Fig. 9
figure 9

Distribution law of vertical displacement field under different burial depth

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As can be seen from Fig. 9, after the roadway is excavated, the roof moves downward and the floor bulges upward. Under the condition of no support, the maximum roof subsidence is 130 mm when the buried depth is 600 m, and the maximum floor heave is 86 mm. The maximum roof subsidence is 140 mm when the buried depth is 700 m, and the maximum floor heave is 112 mm. The maximum roof subsidence is 215 mm when the buried depth is 800 m, and the maximum floor heave is 133 mm. The maximum roof subsidence is 258 mm when the buried depth is 900 m, and the maximum floor heave is 160 mm. It shows that with the increase of buried depth, the displacement range of surrounding rock increases and the maximum displacement increases nonlinearly.

  1. 3.

    Plastic zone analysis

Figure 10 shows the range of plastic zone after tunnel excavation under four kinds of buried depth conditions. In the figure, none indicates that there is no tension shear failure of the unit, shear-p indicates that the unit has shear failure, shear-p indicates that the unit is shear failure, tension-p indicates that the unit has tensile failure, and tension-n indicates that the unit is tensile failure.

Fig. 10
figure 10

Cloud diagram of plastic distribution of surrounding rock of roadway with different buried depth

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It can be seen from Fig. 10 that with the increase of burial depth, the plastic failure of the surrounding rock of the roadway gradually expands, and the failure mode is mainly shear failure. Due to the shear expansion effect of the coal and rock mass, there is a little tensile failure on the roadway surface. When the buried depth is 600 m, the plastic damage range of the two sides of the roadway and the floor is about 2 m, and the roof is about 1.5 m; when the buried depth is 700 m, the plastic damage range of the two sides of the roadway is about 2.5 m, the roof is about 2 m, and the floor is about 3 m; when the buried depth is 800 m, the plastic damage range of the two sides of the roadway is about 4 m, the roof and the floor are about 3.5 m, and the plastic area at the top and bottom corner increases obviously; when the buried depth is 900 m, the plastic damage range of the two sides of the roadway is about 2 m, and the floor is about 3 m The damage range is about 5 m, and the top and bottom plates are about 4.5 m. Therefore, with the increase of in situ stress, the plastic range of the surrounding rock of the tunnel increases nonlinearly.

4.2 Numerical Simulation Analysis of Different Reinforcement Methods

4.2.1 Model Establishment

In order to study the effect of the original support, the anchor grouting support of the bottom plate and the cooperative reinforcement support of the side and the bottom in the bottom drum treatment, the FLAC3D numerical simulation software is used to establish the three-dimensional model as shown in Fig. 7. The displacement constraints are imposed on the left, right, front, back and bottom boundaries of the model, the uniformly distributed load is applied on the top of the model, and the buried depth of the No. 11021 working face is about 700 m, then the load of 17.5 MPa is applied on the top of the model, the side pressure coefficient is taken as 1.2, the mechanical performance parameters of the coal and rock mass are shown in Table 2, and the parameters of the support components are shown in Table 3. The three-dimensional model of different reinforcement methods is shown in Fig. 11.

Table 3 Parameters of cable structural unit
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Fig. 11
figure 11

Three dimensional model under different reinforcement methods

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4.2.2 Result Analysis

  1. 1.

    Stress analysis

The distribution law of vertical stress field in unreinforced mode is shown in Fig. 12.

Fig. 12
figure 12

Stress field distribution under different support conditions

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It can be seen from Fig. 12 that under the original support state, the concentrated stress distribution in the two sides of the roadway is obvious, and the maximum stress concentration coefficient is 1.48. Under the condition of reinforcement of the side and bottom, the stress concentration area of the side disappears basically, and the influence of mining activities on the original rock stress is concentrated in a small area. Without the effect of the concentration stress, the coal and rock mass in the depth of the surrounding rock remains intact, which proves that the reinforcement of the side and bottom enhances the bearing capacity of the surrounding rock of the roadway.

  1. 2.

    Displacement analysis

The distribution of vertical displacement field under different reinforcement methods is shown in Fig. 13.

Fig. 13
figure 13

Vertical displacement distribution under different support conditions

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It can be seen from Fig. 13 that under the original support state, the maximum roof subsidence is 120 mm, and the floor heave is 86 mm; under the anchor injection support state, the maximum roof subsidence is 83 mm, and the floor heave is 8 mm, and the area where the floor heave occurs is greatly reduced; under the condition of the synergetic reinforcement support of the top and bottom, the maximum roof subsidence is 65 mm, and the floor heave is only 4 mm, and the deep surrounding rock is basically in a stable state without displacement change.

  1. 3.

    Plastic zone analysis

The distribution law of plastic zone in surrounding rock of roadway under different reinforcement methods is shown in Fig. 14.

Fig. 14
figure 14

Plastic zone distribution law under different support conditions

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It can be seen from the analysis in Fig. 14 that under the original support state, the floor without support is in the state of free surface, and the plastic failure depth of the floor under the action of in situ stress is about 4 m, which is obviously larger than that of the roof and the side under the support state. Compared with the support without support, the plastic failure depth of the base plate is obviously reduced, which is mainly the failure of the coal body of the base plate within 2 m. In this case, the surrounding rock of the roadway forms a unified bearing body, the plastic failure area becomes smaller, and the surrounding rock of the roadway is more stable.

5 On-Site Monitoring

In the tail entry of inner section of No. 11021 working face, the displacement observation station is arranged by cross point arrangement method, as shown in Fig. 15.

Fig. 15
figure 15

Displacement observation station

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In this figure, AB represents the horizontal displacement, and CD represents the roof to floor convergence. Four groups of stations are arranged every 10 m, among which 1# and 4# stations are non-bottom grouting anchor control areas, 2# and 3# stations are bottom grouting anchor control areas. And draw the curve of roadway deformation with time as shown in Fig. 16.

Fig. 16
figure 16

Relationship curve of roadway deformation with time

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It can be seen from Fig. 16a that the two side displacement approaches of tunnel 1#–4# measuring station in about 50 days are 318 mm, 269 mm, 231 mm and 345 mm respectively. It can be seen from Fig. 16b that the bottom heave of tunnel 1#–4# measuring station in about 50 days are 338 mm, 142 mm, 224 mm and 361 mm respectively.

It can be seen that the displacement and bottom heave of two sides in the control area (2# and 3# stations) of the bottom grouting anchor are significantly smaller than those in the non-control area (1# and 4# stations), the displacement of two sides in three stations is about 67% of that in four stations, and the bottom heave of two stations is about 39% of that in four stations. The overall reinforcement effect of the roadway is good, the surface of the roadway is smooth, and there is no crack of concrete and bolt.

6 Conclusion

  1. 1.

    It is considered that the floor heave of the soft rock roadway is the result of the joint action of the surrounding rock fracture development, poor lithology, cementation, low strength of the original support and high ground stress.

  2. 2.

    The distribution characteristics of stress, displacement and plastic zone of surrounding rock in soft rock roadway under different stress and support mode are analyzed by numerical simulation.

  3. 3.

    The grouting bolt is used in the treatment of roadway floor, and a new type of floor drilling equipment is developed.

  4. 4.

    The combined control technology of “prestressed grouting anchor cable + grouting anchor bolt + grouting behind the wall + floor reconstruction” is proposed to effectively control the occurrence of roadway floor.