1 Introduction

With the deepening of China's national strategy of developing the western region, the main production area of China's coal resources has shifted from the eastern to western regions such as Inner Mongolia and Xinjiang. The coal measures in western China are dominated by Jurassic and Cretaceous soft rocks, typically characterized by low strength and poor cementation. In its natural state, the soft rock structure is relatively compact and complete (Zhang et al. 2022, 2023a; Zhao et al. 2020). After roadway excavation, the surrounding rock deformation is large, and the deformation rate is fast. After superimposing the influence of the high humidity environment of groundwater and the stress transmission of adjacent chambers, the surrounding rock is very easy to soften and expand, and the bearing capacity of the anchorage structure deteriorates rapidly, resulting in frequent occurrence of large deformation disasters in the roadway, which poses a serious security threat to the efficient mining of coal resources in western China (Li et al. 2023; Ren et al. 2023; Xiong et al. 2023; Zhang et al. 2023b).

After the roadway excavation, it will be exposed to a humid environment, and the water infiltration will lead to the expansion and deformation of surrounding rock. The adjacent chambers transmit the stress of the surrounding rock, induce more significant expansion pressure and accelerate the deterioration of mechanical properties of the surrounding rock, which has become a typical problem of joint action of the water environment and stress field (Montero et al. 2021; Feng et al. 2018; Song et al. 2021; Cao et al. 2014; Guo et al. 2015). Scholars have studied the physical and mechanical properties of swelling soft rock by combining engineering geology and rock mechanics experimental analysis. Yang et al. (2017) used X-ray diffraction and electron microscope scanning to conduct microscopic analysis of soft rock, and discussed the relationship between soft rock expansion phenomenon and various factors through laboratory tests. Environmental humidity, soaking time, ion concentration in water and stress state would lead to the disintegration of swelling rock, affecting its stability. Togashi et al. (2021a) found that the mineral composition and microstructure of swelling soft rock would undergo physical and chemical changes under the action of water. After considering the effects of seepage, chemical action and temperature on soft rock, various water–rock interaction models have been proposed, which further deepen the understanding of the expansion mechanism of water–rock interaction of soft rock. Yang et al. (2015) conducted a similar material simulation test of swelling soft rocks, found the expansion of soft rock is a complex process of physical and chemical changes, and obtained the migration rule of groundwater in the surrounding rock.

According to the above analysis, weakly cemented soft rock has low strength and is easily affected by the groundwater environment, making excavation and maintaining weakly cemented roadways more difficult. In addition, due to the ≤ 30 m distance between adjacent soft rock chambers, such as water tanks, electromechanical chambers, water pump rooms, and refuge chambers, the mutual influence during excavation is significant. Therefore, a reasonable support method is of great engineering significance for the stability of weakly cemented roadways in adjacent chambers (Cao et al. 2013; Yang et al. 2021; Arora and Gutierrez 2021). Theocharis and Roux (2020) proposed a nonlinear dynamic support design concept, providing a new approach for supporting weakly cemented soft rock roadways. Kang et al. (2015) discussed the distribution law of surrounding rock stress and plastic zone under the action of humidity and obtained the failure mechanism and support method of soft rock roadway. Meng et al. (2014) carried out the numerical simulation analysis of soft rock roadway and established the theoretical constitutive model of swelling soft rock based on the theory of humidity stress field. Togashi et al. (2021b) analyzed the deformation characteristics of surrounding rock under the disturbance effect of weakly cemented roadway excavation near the chambers. They found that the disturbance still exists when the distance between soft rock roadways is 5 times larger than the roadway diameter.

Based on the above research, research on weakly cemented roadways in adjacent chambers primarily focuses on a single roadway and lacks theoretical experience. Moreover, the traditional support technologies for soft rock roadway ("anchoring + shotcrete + anchor mesh + grouting") have problems such as low deformation control force and large internal force of the structure in the later period, which often lead to "excavation–support–destruction–repair–redestruction", resulting in high maintenance costs (Mojtaba et al. 2018; Yokota et al. 2019; Wang et al. 2022; Yin et al. 2022, 2021). The "U-shaped steel shed + filling the flexible material behind shed" can form a supporting structure according to the section shape of the roadway, which not only has the timely support function of the shaped steel support and greater bearing capacity but also overcomes the shortcomings of the rigid support of the shaped steel support and leaves a certain space for surrounding rock deformation. The fully enclosed combined support method of "U-shaped steel sheds closure roof and sides + filling flexible material behind the sheds + laying reinforcement mesh and concrete in floor + strengthening key parts" with U-shaped steel sheds as the main structure is conducive to exerting the self-supporting capacity of the surrounding rock and fully releasing the concentrated stress of the surrounding rock, providing technical support for the long-term stability of the roadway in western China.

2 Background of engineering

2.1 Conditions of engineering geology

Hongqingliang coal mine is located north of the Ordos Plateau in China and at the northern edge of the Dongsheng coalfield. The terrain elevation difference in the coal mine is large, which is the geomorphic feature of a strongly eroded and denuded plateau. The structural form of the coal bearing strata is generally a monoclinal structure inclined to the southwest, with a dip angle of 200–300° and a stratigraphic dip angle < 5°. Broad and gentle undulations are developed locally. No large-scale folds and faults are found, and no traces of magmatic activity exist. The Hongqingliang coal mine mainly mines 2 # and 3 # coal. The burial depth of 2 # coal is about 420 m, while that of 3 # coal is about 450 m. The working face adopts strike long-wall retreating comprehensive mechanized mining method, and the roof of the goaf is managed by natural caving method. The development roadway in 2# coal is excavated from 3 # coal to 2 # coal. The roadway is firstly driven horizontally, then driven at an angle of 5° 40′, and finally driven horizontally. The total length of the development roadway in 2# coal is about 572 m, with a horizontal section of 370 m, including 112 m for the initial horizontal section, 258 m for the final horizontal section, and 202 m for the inclined section.

The surrounding rock of the development roadway in 2# coal is mainly argillaceous swelling soft rock, which has strong expansibility and disintegration after encountering water. In addition, under the influence of the surrounding rock stress transmitted from the chambers in 3# coal, the development roadway in 2# coal has experienced large deformation. The roof subsidence of the roadway reached 500-800 mm, and the floor deformation reached 400-600 mm, which seriously affected the safe use of the roadway and the efficient production of coal. Given the large deformation of surrounding rock in the development roadway in 2# coal, the support method of "anchor bolt + anchor cable + anchor mesh + shotcrete" is adopted, but the deformation of surrounding rock cannot be controlled. Some sections of the development roadway in 2# coal can no longer meet the needs of transportation and ventilation, and there is a threat of large-scale collapse. It is urgent to propose a support method for swelling soft rock suitable for adjacent chambers (Fig. 1).

Fig. 1
figure 1

Distribution and damage of roadway in 2# coal

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2.2 Test of physical and mechanical property

In order to reveal the physical and mechanical properties of argillaceous swelling soft rock, rock cores were drilled in the deformation section of the development roadway in 2# coal, and rocks in corresponding layers were collected at the heading for relevant tests.

2.2.1 X-ray diffraction analysis

The surrounding rock of the deformation section of the development roadway in 2# coal is mainly mudstone, siltstone and sandy mudstone. As shown in Fig. 2, rocks of 3 lithologies in corresponding layers are selected from the drilled cores for X-ray diffraction test, and their mineral composition and content are analyzed. The mineral composition content is summarized in Table 1. In order to analyze the physical and mechanical properties of the surrounding rock of the development roadway in 2# coal and study its physical and mechanical properties, samples were taken at the head of the roadway excavation. The rock at this location is in-situ and is less affected by mining action.

Fig. 2
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Diffraction pattern of the rock samples

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Table 1 Mineral composition content of three kinds of rocks (%)
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Clay mineral is the main mineral of clay rock, with extremely fine particle size, mainly including kaolinite, illite, montmorillonite and vermiculite groups. The diffraction analysis shows that the clay mineral content in mudstone, sandy mudstone and siltstone is 52.20%, 47.43% and 39.51%, respectively. Among them, montmorillonite has the strongest water absorption and expansion ability among clay minerals. After absorbing water, it can expand and exceed the original volume several times. The montmorillonite content in the three kinds of rocks is 12.01%, 7.16% and 3.06%, respectively. Due to the low strength of swelling soft rock itself, it is very easy to cause crushing expansion. More than 45% clay content aggravates the expansion deformation of surrounding rock.

2.2.2 Microscopic observation

The meso-structure of minerals determines the macro properties of rocks, and different minerals will show different morphological characteristics under the microscope. To study the influence of the micro-structure of the soft rock on the stability of the roadway, after sampling, the mudstone, siltstone and sandy mudstone were observed under a 1000-fold microscope.

The mudstone is mixed with illite and montmorillonite after being magnified by 1000 times, had a pore size of about 10 μm. The siltstone has loose structure, and cracks appear after being magnified by 1000 times, which indicates that the weathering degree is serious and swelling deformation has occurred. After the sandy mudstone is magnified by 1000 times, obvious massive kaolinite and spherical chlorite are found, and the pore size is about 4–16 μm (Fig. 3).

Fig. 3
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Microscopic observation pictures of rocks with different lithology

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2.2.3 Disintegration test

Water inrush and seepage are at different levels in the development roadway of 2# coal. The swelling soft rock will change greatly under different humidity conditions, resulting in hydration expansion, crushing expansion and strength reduction. Therefore, water plays an essential role in weakening the swelling soft rock. The disintegration resistance of rock is one of the important means to test the strength parameters of rock under the influence of water (Fig. 4). After sampling, a disintegration resistance test is performed for mudstone, siltstone and sandy mudstone. Table 2 shows the statistical results after the test.

Fig. 4
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Failure form of mudstone with different water content

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Table 2 Mineral composition content of three kinds of rocks
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According to Table 2, the disintegration resistance of mudstone is poor, followed by sandy mudstone and siltstone. When the roof or floor of the roadway is dominated by mudstone, the hydraulic action causes the mudstone to collapse, expand, crack, and reduce its strength, resulting in severe roof subsidence and floor heave.

2.2.4 Strength test

Strength parameters are the key indicators of a rock’s ability to maintain its stability and resist external deformation, and also has an important reference for numerical simulation of engineering models. Table 3 shows the strength test results of mudstone, siltstone and sandy mudstone. The strength parameters of siltstone, mudstone and sandy mudstone show prominent decreasing characteristics. The compressive strength, tensile strength, cohesion and other indicators of swelling soft rock are relatively low, and plastic deformation and tensile-shear failure are easy to occur under complex geostress conditions. When physical and chemical reactions occur with water, the strength indicators will drop significantly, further increasing the deformation of surrounding rock (Fig. 5).

Table 3 Physico-mechanical properties of different rock
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Fig. 5
figure 5

Determination of physical and mechanical properties of rock

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2.2.5 Initial ground stress

As shown in Fig. 1, the in situ stress was tested by the borehole sleeve core stress relief method. Borehole 1# is located on the north side of the development roadway in 2# coal, 67 m away from the western opening, and the test section is 13 m away from the orifice. Borehole 2# is located on the south side of the development roadway in 2# coal, 86 m away from the western opening, and the test section is 15.8 m away from the orifice.

As shown in Table 4, the vertical stresses of boreholes 1# and 2# are 10.24 MPa and 10.19 MPa, respectively. The horizontal stresses of boreholes 1# and 2# are 26.11 MPa and 24.89 MPa, respectively. Additionally, the range of K is 3.06–3.44, which is likely to cause shear failure of the surrounding rock. The maximum horizontal principal stress direction is mainly NE, and the included angle with the axis of the development roadway in 2# coal is approximately 53°. From the above analysis, the development roadway in 2# coal is greatly affected by the initial ground stress, and its self-stabilization ability is poor.

Table 4 Test results of ground stress
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3 Analysis of instability mechanism

The stress distribution model of the surrounding rock affected by the excavation of adjacent chambers is established in Fig. 6. Assume that the model is a completely elastic homogeneous material, the thickness can be ignored, the elastic modulus is E, and Poisson's ratio is μ. The model’s top, bottom, left and right sides are applied with a uniform load. Rectangular chambers numbered 1-n are randomly arranged in the model. The vertical ground pressure is Qv, the horizontal ground pressure is Qh, and Qh = λ·Qv (λ is the horizontal stress coefficient). The water environment affects the swelling soft rock roadway, which causes the surrounding rock around the roadway to produce swelling deformation and swelling pressure. Assume that the roadway radius is ra, the radius of the water environment affected area is rb, and the humidity of the roadway water environment affected area is RH.

Fig. 6
figure 6

Stress distribution model of swelling soft rock roadway adjacent to chambers

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Only considering elastic deformation, the mechanical response of swelling soft rock roadway is the superposition of the surrounding rock stress field and the swelling deformation stress field of soft rock affected by the water environment. Ignoring the influence of time, the stress field after superposition is only related to the spatial position of the swelling soft rock roadway.

According to the theory of elastic displacement potential, the stress of surrounding rock due to water infiltration can be calculated by formula (1).

$$\begin{aligned} \sigma_{{{{r}}1}} & = - 2G\frac{1}{r}\frac{{{{d}}\phi }}{dr} = - \frac{2GK}{{\ln \beta }}\left( {2\ln \frac{b}{r} + 1} \right) \\ \sigma_{\theta 1} & = - 2G\frac{{{{d}}^{{2}} \phi }}{{dr^{2} }} = \frac{2GK}{{{{r}}\ln \beta }}\left( {2\ln \frac{b}{r} + 1} \right) \\ \end{aligned}$$
(1)

Formula (1) is a function of the radial coordinate r. In formula (1), G = E/(1–2·μ), ϕ = K·r2·(lnrb/r + 1)/lnβ,

K = (1 + μ)·α·RH/(4−4μ), β = ra/rb. E is the elastic modulus of swelling soft rock, μ is Poisson's ratio, and α is the expansion coefficient of swelling soft rock.

Considering the effect of the water environment, the radial stress at the boundary is 0. In order to achieve this condition, it is necessary to further solve the stress solution obtained by applying a stress boundary with the same magnitude and opposite direction as the above radial stress at the boundary of the affected area. The axisymmetric ring bears internal and external pressure, which can be calculated by formula (2).

$$\begin{aligned} \sigma_{{{{r}}2}} & = \frac{1}{{{{b}}^{2} - a^{2} }}\left[ {a^{2} p_{a} \left( {1 - \frac{{b^{2} }}{{r^{2} }}} \right) - b^{2} p_{b} \left( {1 - \frac{{a^{2} }}{{r^{2} }}} \right)} \right] \\ \sigma_{\theta 2} & = \frac{1}{{{{b}}^{2} - a^{2} }}\left[ {a^{2} p_{a} \left( {1 + \frac{{b^{2} }}{{r^{2} }}} \right) - b^{2} p_{b} \left( {1 + \frac{{a^{2} }}{{r^{2} }}} \right)} \right] \\ \end{aligned}$$
(2)

In formula (2), Pa = −2GK(2lnβ + 1)/lnβ, Pb = −2GK/lnβ. In combination with formula (1) and (2), the expansion stress caused by water infiltration can be calculated with formula (3).

$$\begin{aligned} \sigma_{r} & = \sigma_{{{{r}}1}} + \sigma_{{{{r}}2}} = - 4GK\left( {\frac{{\ln \frac{b}{r}}}{{\ln \frac{b}{a}}} - \frac{{\frac{{b^{2} }}{{r^{2} }} - 1}}{{\frac{{b^{2} }}{{a^{2} }} - 1}}} \right) \\ \sigma_{\theta } & = \sigma_{\theta 1} + \sigma_{\theta 2} = - 4GK\left( {\frac{{\ln \frac{b}{r} - 1}}{{\ln \frac{b}{a}}} - \frac{{\frac{{b^{2} }}{{r^{2} }} + 1}}{{\frac{{b^{2} }}{{a^{2} }} - 1}}} \right) \\ \end{aligned}$$
(3)

The stress distribution problem of the roadway adjacent to the chambers, is decomposed into n single chamber problems, and the analytical solution of its stress field is obtained by superposition. Finally, the stress distribution characteristics of the surrounding rock inside the model are obtained. The stress at any position of the model can be calculated by formula (4).

$$\left. \begin{aligned} \sigma_{r} & = \frac{q}{2}\left[ {1 - \frac{{a^{2} }}{{r^{2} }}} \right] + \frac{q}{2}\left[ {1 + \frac{{3a^{4} }}{{r^{4} }} - \frac{{4a^{2} }}{{r^{2} }}} \right]\cos 2\theta \\ \sigma_{\theta } & = \frac{q}{2}\left[ {1 + \frac{{a^{2} }}{{r^{2} }}} \right] - \frac{q}{2}\left[ {1 + 3\frac{{a^{4} }}{{r^{4} }}} \right]\cos 2\theta \\ \tau_{r\theta } & = - \frac{q}{2}\sin 2\theta \left[ {1 - \frac{{a^{2} }}{{r^{2} }}} \right]\;\left[ {1 + 3\frac{{a^{2} }}{{r^{2} }}} \right] \\ \end{aligned} \right\}$$
(4)

In order to calculate the stress of surrounding rock, the polar coordinate system is transformed into the rectangular coordinate system.

$$\left. \begin{aligned} \sigma_{x} & = q - \frac{{3qa^{2} }}{{2r^{2} }}\cos 2\theta - q\cos 4\theta \left[ {\frac{{a^{2} }}{{r^{2} }} - \frac{{3a^{4} }}{{2r^{4} }}} \right] \\ \sigma_{y} & = - \frac{1}{2} \times \frac{{qa^{2} }}{{r^{2} }}\cos 2\theta + \frac{1}{2}q\cos 4\theta \left[ {\frac{{2a^{2} }}{{r^{2} }} - \frac{{3a^{4} }}{{r^{4} }}} \right] \\ \end{aligned} \right\}$$
(5)

The development roadway of 2# coal is mainly affected by the chambers of No. 1, No. 2, No. 3 and No. 4 in 3# coal. The mechanical model established is shown in Fig. 7a, the horizontal stress of the surrounding rock is shown in Fig. 7b, the vertical stress is shown in Fig. 7c, and the vertical stress at different positions of the development roadway of 2# coal is shown in Fig. 7d.

Fig. 7
figure 7

Analysis model for the influence of the chambers

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The vertical stress increment of the development roadway in 2# coal before and after the excavation of the chambers in 3# coal is 4 MPa, and the horizontal stress increment is 9 MPa, indicating that the surrounding rock stress affected by the excavation of the chambers can be transmitted to adjacent roadways. Furthermore, the transmission distance of vertical stress in the vertical direction is 28–48 m, which is 7–12 times the height of the chamber. The critical failure stress of weakly cemented surrounding rock after stress redistribution can be calculated according to formula (6).

$$C_{0} \left( {P_{0} + \frac{{\sigma_{c} }}{{K_{1} }} + \frac{{kK_{2} p^{\prime}}}{{k_{p}^{2} - 1}}} \right) - \frac{{2kp^{\prime} + 2(\sigma_{c} - \sigma_{c}^{*} )}}{{k_{p}^{2} - 1}} - \frac{{\sigma_{c}^{*} }}{{K_{1} }} = 0$$
(6)
$$C_{0} = \frac{2}{{K_{2} }}\left( {\frac{kp^{\prime}}{{kp^{\prime} - \sigma_{c} - \sigma_{c}^{*} }}} \right)^{{K_{1} /2}}$$
(7)

In formula (6), kp is the triaxial stress coefficient, and kp = (1 + sinφ)/(1-sinφ). σc* is the uniaxial compressive residual strength of the surrounding rock. K1 is the calculation parameter, and K1 = kp—1. K2 is the calculation parameter, and K2 = kp + 1. p′ is the calculation parameter, and p′ = (1 + μ)/2∙[2p0sinφ + (1-sinφ)∙σc]. k is the strain weakening coefficient, and k = 1.

The vertical stress during roadway failure calculated according to formula (6) is 13.1 MPa, which is lower than the average adjusted stress 13.2–14.2 MPa affected by the excavation of the chambers. The calculation results indicate that the surrounding rock stress transmitted to the weakly cemented roadway after the excavation of the chambers is superimposed with the redistributed surrounding rock stress after the roadway excavation. The superimposed surrounding rock stress exceeds the failure stress of the weakly cemented roadway, leading to the instability of the roadways.

4 Stability control technology of weakly cemented roadway

The superposition of the surrounding rock stress concentration and the original rock stress caused by the excavation of the chambers has reached the critical stress of the surrounding rock fracture, leading to roadway failure. In order to control the deformation of surrounding rock effectively and ensure the safe use of the roadway, the support structure needs to have considerable support strength. In addition, the support structure can deform properly while providing support resistance, and the above performance can be maintained for a long time.

To verify the influence of the space between the U-shaped steel shed and the surrounding rock filled with flexible materials on the deformation control of swelling soft rock, numerical simulation was carried out in combination with the site engineering and geological conditions, and the deformation control effects of the surrounding rock, under the two working conditions of filled pebbles and unfilled pebbles, were compared and analyzed. The numerical model was established in CAD, meshed by ANSYS software, and imported into FLAC3D for calculation. As shown in Fig. 8, the model size was 50 m × 50 m × 30 m. A vertical stress of 10 MPa was applied to the top boundary of the model, a horizontal stress of 25 MPa was applied to both the left and right boundaries, and the bottom boundary was the displacement boundary. According to the geological engineering conditions of the coal mine, the dip angle of the rock stratum of the numerical model is set to 0°, the rock stratum is set to be homogeneous and the thickness is uniform. The elastic–plastic M-C model is selected as the constitutive relation of the numerical model. The parameters of the M-C model mainly include the shear modulus, bulk modulus, density, cohesion, internal friction angle and tensile strength. Table 5 lists the physical and mechanical parameters of the rock stratum required for calculating the numerical model.

Fig. 8
figure 8

Schematic diagram of numerical calculation model

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Table 5 Physical and mechanical parameters of rock
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The calculation results of the numerical simulation are shown in Figs. 9 and 10. The failure mode of the roadway before and after filling between the U-shaped steel shed and surrounding rock is similar. The floor is a tensile failure, the two sides are a shear failure, and the roof is a combination of tensile and shear failure.

Fig. 9
figure 9

Without filling between U-shaped steel shed and surrounding rock

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Fig. 10
figure 10

Filling between U-shaped steel shed and surrounding rock

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Before cobble filling between the U-shaped steel shed and surrounding rock, the distance between the concentration area of horizontal and vertical stress of the surrounding rock and U-shaped steel shed is approximately 0. Moreover, the maximum vertical stress of a U-shaped steel shed is 3.48 MPa, the maximum horizontal stress is 1.12 MPa, and the distribution length of a U-shaped steel shed with axial force > 170 kN is 5.35 m. After cobbles are filled between the U-shaped steel shed and surrounding rock, the distance between the concentrated area of horizontal and vertical stress of the surrounding rock and U-shaped steel shed is 0.2 times the roadway width. Additionally, the maximum vertical stress is 1.79 MPa, the maximum horizontal stress is 0.85 MPa, and the distribution length of U-shaped steel axial force > 200 kN is 3.13 m.

The above calculation results show that filling cobbles between a U-shaped steel shed and surrounding rock improves the bearing environment of a U-shaped steel shed. Under the same geological conditions, when there is no cobble filling, the surrounding rock stress distribution of the roadway is uneven, especially at the foot of the steel shed. After cobble filling, the stress of surrounding rock is more uniform, the integrity of the support structure is better, and the stress intensity near the free surface of the surrounding rock is lower than that before filling.

Therefore, by filling pebbles between the U-shaped steel shed and the surrounding rock, a rigid-flexible composite support system is formed to achieve the flexible release of the surrounding rock deformation, which can enhance the bearing capacity of the support structure. The proposed support technology includes the primary support of anchor bolt + anchor cable + initial shotcrete and the secondary support of U-shaped steel shed + filling flexible materials behind the shed + control of key parts, as shown in Table 6.

Table 6 Support mode of swelling soft rock
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In comprehensive consideration of the functional requirements and construction costs of the development roadway in 2# coal, the roadway section height is determined to be 4.1 m and the width as 4.6 m. The primary and secondary support schemes are shown in Fig. 11.

Fig. 11
figure 11

Step by step combined support mode

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The radius of semi-circular arch is 2.3 m, and the height of the straight wall is 1.8 m. Before installing the U-shaped steel shed, it is necessary to clean the broken rocks thoroughly between the U-shaped steel shed and the surrounding rock to prevent uneven stress on the U-shaped steel shed, and to leave enough space for filling between the U-shaped steel shed and the surrounding rock. According to the size of the development roadway in 2# coal, the filling thickness of cobbles on the roof is 1.5 m, and the filling thickness on both sides is 0.3–0.5 m.

The on-site support construction process of the development roadway in 2# coal is shown in Fig. 12. The U-shaped steel shed spacing is determined to be 1 m. The U-shaped steel sheds are fixed by channel steel tie rods to strengthen the stability of the supports along the axis of the roadway. The surface of U-shaped steel sheds is supported by shotcrete, and the thickness of the shotcrete is about 100 mm. The floor is made of steel net + 12# I-steel + concrete, the steel mesh is welded by steel bars with a diameter of 6.5 mm, the steel mesh size is 1200 × 2000 mm, and the concrete thickness is about 300 mm. Lay Φ12 round steel welded mesh and Φ6.5 × 100 × 100 mm round steel welded mesh in turn between the two supports. Reinforced mesh is made of round steel with a diameter of 12 mm, mesh size is 100 × 100 mm, and steel mesh size is 1200 × 2000 mm.

Fig. 12
figure 12

Flow chart of support process for swelling soft rock roadway

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In order to detect the support effect, four monitoring stations A, B, C and D are arranged in the development roadway of 2# coal (Fig. 1). The displacement of the roadway surface is observed by the cross-point method, and the bolt force is monitored by the bolt dynamometer. The cross method is used to monitor the surface displacement of the surrounding rock of the roadway, and the base points are set on the two sides, the roof and the floor of the roadway, respectively. One monitoring section is set for each measuring point, and a set of monitoring base points are set on the roof, floor and two sides for each monitoring section, as shown in Fig. 13a. The structure of the MJ-40 bolt dynamometer is selected, as shown in Fig. 13b. When installing the bolt dynamometer, put the dynamometer between the bolt tray and the nut at the end of the outer bolt. After tightening the nut, apply a pre-tightening force to the bolt, and record the monitoring data of the bolt dynamometer.

Fig. 13
figure 13

Supporting effect monitoring method and equipment

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As shown in Fig. 14, the surrounding rock deformation is 0 during the monitoring time. The minimum force monitoring of the bolt is 19 kN, while the maximum value is 24 kN, and the overall force value fluctuates around 22 kN. The monitoring data of the bolt stress gauge fluctuates around a specific value and is relatively stable, indicating that the U-shaped steel-rod support plays an important role in supporting the surrounding rock of the roadway. After the roadway in the damaged area is supported by U-shaped steel, the section is complete, and the surrounding rock deformation is effectively controlled.

Fig. 14
figure 14

Schematic diagram of roadway support effect

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5 Discussion

In order to compare the influence of support parameters on the control effect of the soft surrounding rock, numerical simulation was conducted to study three support schemes with U-shaped steel shed spacing of 1.0 m, 1.5 m, and 2.0 m, respectively. The failure characteristics of soft surrounding rock and plastic zone are analysed and the reasonable support parameters are determined. It was determined that the dimensions, boundary conditions, unit division, and other parameters of the numerical model are the same as those in Fig. 8. The spacing between sheds is set at 1.0 m, 1.5 m, and 2 m, respectively, and the stress and plastic zone corresponding to these three spacing schemes were calculated.

As shown in Fig. 15, under different support schemes with different shed spacing, the distribution characteristics of vertical and horizontal stresses in the surrounding rock are similar, with stress concentration occurring at the roof and floor of the roadway. However, the stress distribution varies with the increase range of shed spacing. Among them, when the shed spacing is 1.0 m, compared to the 1.5 m and 2.0 m shed spacing, the vertical stress distribution range has been reduced by 15% and 60%, and the horizontal stress distribution range has been reduced by 20% and 90%, respectively. Reducing the spacing between sheds can utilize the load-bearing capacity of U-shaped steel, and the effect of reducing the stress impact range between 1.0 and 1.5 m sheds is within 20%.

Fig. 15
figure 15

Calculation results of stress and plastic zone

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After using U-shaped steel shed support, the distribution range of the plastic zone in the roadway is reduced entirely. When the spacing between sheds is 1.0 m, the plastic deformation of the surrounding rock is concentrated at the edge of the U-shaped steel. When the spacing between sheds is 1.5 m, the plastic zone is concentrated on the roof of the roadway, with a plastic zone range of 0.2 m. When the spacing between sheds is 2.0 m, the plastic zone is mainly distributed on the roof and floor, and the range of the plastic zone increases to 0.5 m. Shortening the spacing between sheds can reduce the plastic zone area around U-shaped steel, and the effect of reducing the plastic zone area with a spacing of 1.0 m and 1.5 m is similar.

Based on the displacement of the roof, floor and sides, the stress, and the damage situation of the plastic zone, a comprehensive analysis shows that a 1.0 m shed distance is more effective in controlling the deformation of the surrounding rock of the roadway compared to a 1.5 m and 2.0 m shed distance. However, the support effects of a 1.0 m shed distance and a 1.5 m shed distance are not significantly different, and a 1.5 m shed distance saves significant economic costs compared to a 1.0 m shed distance. Under the condition of ensuring the reliability of roadway support, considering the actual economic cost of the mine, 1.5 m shed spacing is used to support the development roadway in 2# coal.

6 Conclusion

  1. 1.

    Due to the influence of the surrounding rock stress transmitted after the excavation of adjacent roadways, the vertical stress of the expansive soft rock roadway can increase to 128.6%-133.1% of the initial ground stress, and can exceed the critical stress for the failure of the expansive soft rock surrounding rock, leading to the instability of the roadway.

  2. 2.

    The surrounding rock stress will be transferred to the swelling soft rock roadway after the excavation of the adjacent to chambers. The vertical stress is mainly transmitted to the vertical direction, and the transmission distance can reach 7–12 times of the chamber height. The horizontal stress is mainly transmitted to the horizontal direction, and the transmission distance can reach 3–6 times of the chamber width.

  3. 3.

    The monitoring results for 60 days show that the deformation of the surrounding rock is 0, indicating that the fully enclosed composite support method with U-shaped steel shed as the main structure can effectively control the deformation of the expansive soft rock roadway, providing a powerful supplement for the support of the expansive soft rock roadway.