Introduction

Energy sources which were indispensable input for economic growth and social development1 could be divided into conventional and non-conventional energy sources2. Among them, fossil fuels always be used as the most consumed conventional sources3 and have been used as the main energy source4,5. Although coal was one of the most resources of greenhouse gas, it was still one of the main sources of fossil fuels for industrial development all around the world. Longwall top coal caving method was used extensively in China to extract a large amount of coal resources because of its advantages of high production, high efficiency, and lower development rate6. Nevertheless, a lower recovery rate of this method should not be ignored7. The gob-side entry retaining technology was proposed by researchers to improve the recovery rate of the longwall top coal caving technology. The haulage roadway of the previous mining face was be reused as the return airway of the subsequent working face in the gob-side entry retaining engineering which had been widely used in China8. The gob-side entry retaining technology reduced the roadway tunneling and the coal pillar remaining, avoided producing an island mining face and eliminated the stress concentration of the coal pillar from the upper and lower sections. Besides, it had realized mining face’s Y type ventilation and solved the problems of transfinite gas and accumulation at corners of mining face9.

At present, there were many achievements in the research on the law of the gob-side entry surrounding rock activities and the supporting technology in the roadway10, which had laid a rich theoretical foundation for the stability of filling body and the control of surrounding rock of the roadway. A combination method of hydraulic fracturing and snake-shaped energy-absorbing rockbolts was also verified to be effective for improving the stability of the gob-side roadway11. An innovative gateroad layout and high-strength cable-rockbolt support method were introduced by Zhang12. These methods could improve the stability of the filling wall in highly gassy longwall top coal caving based on gob-side entry retaining. With the working face mining, the key block formed by periodic fracture along the main roof of the roadway would turn to the side of the gob and sink in rotation along with the pivot point treating which as the axis and subsidence value near the wall side was much higher than that of the integrated coal13.

In the process, to avoid rupture and instability, it was necessary for the filling body to have “deform and yielding” appropriately to adapt to the roof sinking14,15. In order to achieve this goal, the way of constructing compound filling body was used, in general. Xie16 adopted flexible-formwork concrete wall in gob-side entry retention engineering, the optimal width of the flexible-formwork concrete wall was determined by theoretical calculation and numerical simulation. A corresponding control technology was proposed to control the convergence of surrounding rock of gob-side entry retention. Du17 established a gob-side entry retaining mechanical model, the bag filling material was investigated through experiments. The deformation control effect of retention roadway was good during field tests. Wang18 used the reserved top coal as a soft medium to improve the stress environment of the filling body. These attempts provide a new idea for further improving the force environment of the backfill and enhancing the synergistic deformation ability of the filling body and roof.

Inorganic filling materials were widely used in the gob-side entry due to their advantages of fast coagulation, early strength, flexible strength and easy construction. Cao19 proposed a new underground backfilling materials using enzyme-induced calcium carbonate precipitation technology. Liu20 used an inorganic material and analyzed the scientific location, drilling technology and other issues of underground grouting cutoff curtain wall construction. Ngo21 developed a novel carbonated CO2-fly ash-based backfill material to co-dispose CO2 and fly ash in the mine goaf as negative carbon backfill materials. But the inorganic filling body also faced the problem of weak “deformation-yielding” and the ability of synergistic deformation with roof, which led to filling body break seriously, even the phenomenon of instability at the beginning of the roof movement. So Li22 studied the deterioration characteristics of filling body combinations and surrounding rock in excavating technology without coal pillars and modified the intrinsic model using the stress-strain relationship. Huang23 analyzed the the overburden movement patterns of strip filling mining by theoretical methods, numerical simulations and similarity experiments. Wu24 researched the creep characteristics and potential deformation patterns of gangue filling body.

As mentioned above, the current research on the gob-side filling body mainly consisted of cement-based paste material25, concrete26,27 and gangue bricks28, and the research emphasis was focused more on the material characteristics test and the construction of the mechanical model of roadway support, etc. However, the mechanical behavior of the composite filling body has not been fully explored. In particular, the load-bearing deformation characteristics, the failure mode of the composite filling body constructed by filling material and effects on rock mass stress were not studied thoroughly.

This paper aimed to study the mechanical behavior of the “multi-layer” filling body constructed by two kinds of filling material. For this purpose, a foamed expansion filling material(FFM) was invented to overcome the stiffness failure of conventional filling material(CFM). A mechanical test was conducted to analyze the properties of FFM and CFM. Subsequently, a “multi-layer” filling body was designed by theory calculation method. In the meantime, FLAC3D software was employed to analyze the loading and deformation characteristics of the “multi-layer” filling body, failure mode and the effect of stress readjustment of roadway surrounding rock. The validity of the “multi-layer” filling body was verified and the mechanical behaviors and bearing mechanism of the “multi-layer” filling body was discussed by field monitoring.

Engineering background and problem analysis

The Chengzhuang coal mine, which is located in the Jincheng mining area, China was selected for the case study. Longwall mining face 4311 was used in this study. The mining face 4311 is 210 m wide and 1320 m long, and the average buried depth is 480 m, using a fully mechanized top coal caving method with an average thickness of the coal seam was 6.3 m. The immediate roof of the mining face is mudstone layer with an average thickness of 2.45 m, the basic roof is sandy mudstone layer with an average thickness of 10.10 m, the immediate bottom is sandy mudstone layer with an average thickness of 11.45 m, and the basic bottom is fine sandstone layer with an average thickness of 5.20 m. Two intake air roadways and one return air roadway were arranged. The shape of the roadway cross-section was rectangular, sized 5 m×3 m (height×width), and was arranged along the roof and the floor of roadway is the 3.3 m thick coal. The mine location and mining face layout were shown in Fig. 1.

Fig. 1
figure 1

Mine location and mining face layout of the test site.

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In order to solve the problem of gas accumulation at the corner of mining face and increase the resource recovery rate, the technology of gob-side entry retaining was conducted in the intake airway on one side of the mining face(the position of section A-A in Fig. 1). After the mining face pushed, the support structure inside the roadway, the backfill body beside the gob and the coal pillar would work together to jointly bear the rotational movement of the overlying rock layer. Traditionally, concrete or filling materials are used to arrange the filling bodies along the edge of the goaf, as shown in the Fig. 2(a). The gob-side filling body solidified only by conventional filling materials(CFM) demonstrated high stiffness and low compressibility.When the working face of this section was under recovery, the roof at the junction of its side and the mining face of the next section were broken to form an arc triangular plate (key block B). Under the influence of a mining operation, the arc triangular plate deduced the whole process of breaking, instability, and rotary subsidence. The immediate roof and filling body were not enough to control this sinking trend and could only passively withstand the “given deformation” imposed by the arc triangular plate29. Thus, the traditional gob-side filling body solidified only by conventional filling materials(CFM) would significant damage and deformation due to the intense mine pressure caused by the rotation of the overlying rock layer. In the process, to avoid filling the body’s instability caused by fracture, it shall adapt to roof sinking by “deformation-yielding”. Thus, the gob-side filling body was designed as a “multi-layer” structure which divided into upper and lower parts. The upper part consolidated by foamed expansion filling material(FFM) deformed and yielded to achieve the effect of pressure relief when the overlying rock rotated and sank. The lower part was formed by conventional filling materials(CFM) to provide sufficient support strength and maintain structural stability. This “multi-layer” gob-side filling body have the following bearing characteristics: its characteristics of fast coagulation and early strength at the beginning of loading could be able to cut off the roof with a certain thickness timely after the roof cutting resistance is reached; compression yielding characteristics of medium load; high strength at the later stage, to ensure the stability of roof hinged structure before reuse of roadway30.The “multi-layer” gob-side filling body structure shown in the Fig. 2(b).

Fig. 2
figure 2

Mechanical model of the surrounding rock structure and gob-side entry retaining gateway:(a) Traditional filling body; (b) “multi-layer” filling body.

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Design of “multi-layer” gob-side filling body

The mechanical charactristics of samples

According to the design concept of the above-mentioned “multi-layer” gob-side filling body structure, the filling body is divided into the upper part composed of FFM and the lower part composed of CFM. Among them, FFM should have foaming and expansion properties to enable the upper part of filling body to play a role in reducing pressure when the overlying rock rotated and sank. Thus, a new filling material added polypropylene fibers and foaming agents was tested to improve the toughness and compressibility of the filling body. The final material ratio scheme was selected with a water-cement ratio of 1.0:1, a foaming agent dosage of 0.03%, a fiber dosage of 0.2%, and an early strength agent dosage of 0.5% after a large number of material proportioning experiments. Standard cylindrical samples with diameter d = 50 mm and height l = 100 mm were made by CFM and FFM respectively to determine the mechanical properties of them. The uniaxial compression test was carried out on the RMT-301 rock mechanics test machine after the sample had been maintained for 28 days(See Fig. 3).

Fig. 3
figure 3

The test instrument and specimen.

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The failure mode and stress-strain curves of samples were selected to analyze the differences in mechanical properties of the two materials, as shown in Figs. 4 and 5.

Fig. 4
figure 4

Failure patterns of specimens under uniaxial compression. (a) CFM; (b) FFM.

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According to Fig. 4, CFM samples mainly suffered from longitudinal splitting failure under uniaxial compression, accompanied by local spalling. The samples showed relatively obvious brittle failure characteristics and typical tensile failure modes. FFM samples were mainly shearing failure with tensile crack. The brittleness of the sample was weakened which showed tensile and shear failure mixed failure mode.

Fig. 5
figure 5

Stress-strain curves for specimens under uniaxial compression.

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Besides, it could be seen from Fig. 5 that the stress-strain curves of CFM and FFM samples were quite different. During the compaction stage (oa or o’a’), the degree of concavity under the curve was significantly higher than that of FFM sample because there were many small pores in CFM sample. In the elastic phase (ab or a’b’), the average modulus of FFM sample was smaller than CFM sample, and there would be a large compression deformation under the same load. When gone into the yield phase (bc or b’c’), the curve slowed down and the slope dropped, the internal microcracks of the sample gradually expanded and penetrated, the specimen produced irreversible plastic deformation, but the curve slope corresponding to CFM sample was higher than that of FFM sample, which showed that the former had a higher axial stress under the same strain. The peak strength of CFM sample was Rc1=12.94 MPa, and the corresponding peak strength of FFM sample was Rc2=9.86 MPa, but the former corresponded to the peak strain εc1 = 4.8 × 10−3, while the latter’s peak strain εc2 = 6.72 × 10−3. The peak strain of FFM sample increased by about 28.6% on the basis of CFM sample. This was because that FFM had higher expansion rate and contained a large number of bubbles inside. The deformation of bubbles caused a relatively high peak strain of the sample when it suffered a compressive load. After entered the post-failure stage (cd or c’d’), the stress drop rate of FFM sample was far lower than CFM sample, and the corresponding residual strength was higher than CFM sample. The residual strength of the FFM sample was increased by about 31.9% relative to the CFM sample. This was mainly because polypropylene fibers played the role of one-villa or net-type three-villa in the interior of FFM sample and could share some external loads. In other words, when subjected to axial compression, the FFM sample could provide a larger space for compression deformation due to the large number of bubbles inside. Because of the added fiber material inside, the fiber material played a very good role in resisting deformation when local deformation occurred. This means that under the same axial pressure conditions, FFM specimens could withstand a greater degree of compressive deformation and have a greater residual deformation capacity.

In addition, extra triaxial compression test were executed to confirm the elastic modulus E, Poisson’s ratio υ, internal friction angle \(\varphi\) and cohesion c. The results were shown in Table 1, E and \(\varphi\) of the FFM sample were less than the CFM sample, while c and µ were greater than the CFM sample.

Table 1 Mechanical parameter of the two kinds of filling materials.
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The experimental results showed that the CFM sample had more obvious brittleness than the FFM sample. Although the CFM sample had high strength, it had poor compressibility and was prone to brittle fracture under compression. The FFM sample, although its strength was lower than the former, it had higher compressibility. In order to better compare the brittleness of the two specimens, the brittleness index proposed by Luan31 and Xia32 were adopted for calculation. The expressions were shown as Eq. (1) and Eq. (2) separately.

$$\:B=E/\upsilon\:$$
(1)

where B is the brittleness index; E is the elastic modulus(GPa); υ is the Poisson’s ratio.

$$\:B=({\sigma\:}_{p}-{\sigma\:}_{r})/({\varepsilon\:}_{p}-{\varepsilon\:}_{r})+({\sigma\:}_{p}-{\sigma\:}_{r})({\varepsilon\:}_{r}-{\varepsilon\:}_{p})/\left({\sigma\:}_{p}{\varepsilon\:}_{p}\right)$$
(2)

where σp is the stress value at peak point; εp is the strain value at peak point; σr is the stress value at the starting point of residual strength; εr is the stress value at the starting point of residual strength.

Substituting the corresponding parameters in Fig. 5; Table 1 into Eq. (1), the brittleness index of CFM sample is 15.5, the brittleness index of FFM sample is 8.94. Besides, substituting the corresponding parameters in Fig. 5; Table 1 into Eq. (2), it could be obtained that the brittleness index of CFM sample is 1.87, the brittleness index of FFM sample is 0.84. It could be seen that the brittleness of FFM sample is significantly lower than that of CFM sample. Therefore, according to the respective mechanical characteristics of the two kinds of materials, and combined with the bearing characteristics of the support body for the gob-side entry, the “multi-layer” gob-side filling body could be made up by the upper part composed of FFM and the lower part composed of CFM to better adapt to roof sinking by “deformation-yielding”.

The microstructure of samples

The microstructure of the consolidated samples with CFM or FFM was tested by scanning electron microscopy(SEM). The microscopic morphologies of the two groups of samples at different magnifications are shown in Figs. 6 and 7.

Fig. 6
figure 6

The microscopic morphology of CFM sample. (a) Magnify 50 times;(b) magnify 100 times;(c) magnify 1000 times;(d) magnify 4000 times.

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Fig. 7
figure 7

The microscopic morphology of FFM sample. (a) Magnify 50 times; (b) magnify 100 times; (c) magnify 1000 times; (d) magnify 4000 times.

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By comparing the microscopic morphologies of the solidified bodies in Figs. 6 and 7, it could be seen that a large quantity of needle-like ettringites could be seen in both samples, especially under high magnification conditions. In conditions of low magnification, such as 50 times or 100 times, the surface of CFM sample was relatively smooth and uniform, but densely distributed small-diameter pores and fibers interspersed around them could be seen clearly in FFM sample. It was manifested that the densely distributed small pores inside FFM sample increased the compressible space and achieved the purpose of pressure reduction, and the fibers improved the toughness and integrity of the consolidated body. This formed a very good correspondence with the above-mentioned stress-strain characteristics of the FFM sample.

Calculation method of stratified thickness

According to masonry beam theory, the broken main roof hinges with surrounding rock mass to form an equalizer similar to masonry structure after the gangue was stabilized in the gob33,34. Combined with the actual working conditions of 4311 working face, the load-bearing structural mechanics model of the “multi-layer” filling body is constructed (See Fig. 2(b)).

If the overburden pressure is applied uniformly on the main roof, the given deformation S applied to the overburden rock during the sinking of the main roof can be expressed as Eq. (3).

$$S=\frac{{\left( {2{x_0}+2b+a} \right)\Delta S}}{{2L}}$$
(3)

where x0 is the distance between the fracture point of the key block and the coal seam (m); b is the roadway width (m); a is the width of the filling body (m); L is the fracture length of the key block (m); ΔS is the key block in the maximum subsidence of the gob-side (m), which can be obtained by Eq. (4).

$$\Delta S=m+{h_3}(1 - K)$$
(4)

where K is the residual crushing coefficient of the immediate roof, h3 is the immediate roof height (m), and m is the mining height of the working face (m). According to the literature35, the key block fracture length L was expressed as Eq. (5).

$$L={l^{\prime}}\left( {\sqrt {{{{{(l^{\prime}}/D)}^2}+1.5}} - {l^\prime}/D} \right)$$
(5)

where l′ is periodic weighting interval of working face (m); D is the working face length (m).

It was known that the mining face x0 = 6.9 m, D = 210 m, l′ = 30 m, K = 1.13, m = 6.3 m, b = 3 m, a = 2 m, h3 = 38.4 m, we could get the maximum compression amount of the filling body as 433 mm. Here, the most extreme case was considered that the lower part of the filling body did not occur any compressive deformation, while all the deformation was caused by the upper part of the filling body. Under the action of the top load, the upper part completely destroyed the compression layer. So, it could be determine that the height of the upper layer was 0.5 m and that of the lower layer was 2.5 m.

Numerical simulation and analysis

Numerical simulation scheme

In order to study the bearing and deformation characteristics of the “multi-layer” filling body and its influence on the stress distribution of roadway surrounding rocks. Numerical analysis was performed using FLAC3D. According to the principle of symmetry, the model size was set as 145 m × 120 m × 100 m (length × width × height). Based on the buried depth of the longwall mining face and the height of the simulation model, 9.3 MPa vertical stress needs to be applied to the upper boundary of the model. Horizontal displacement constraint was applied to the left and right boundary. Vertical displacement constraint was set to the bottom boundary, and the lateral pressure coefficient was selected as 0.5. The Mohr-coulomb constitutive model was adopted for coal seam, filling body and other rock formations in the model. The rock masses were assumed to be homogeneous, and the anisotropy of it was neglected. Each advancement of the mining face was excavated in 3 m cycles, 40 times in total. The size of the filling body was 6 m×2 m×3 m, which was filled by following the excavation of the working face. The physical and mechanical parameters of the materials used in the numerical analysis were listed in Table 2.

Table 2 Physical-mechanical parameters of the roof and floor rock layers.
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The haulage roadway of 4311 working face was adopted the combined support method of anchor net rope + steel ladder beam, the filling body was reinforced with the bolt. At the same time of back mining, the roof and the coal rib within a certain range of the advance face were reinforced by cable bolts. The specification and arrangement of bolt (cable) were shown in Fig. 8(a)～(c). Cable element was used to simulate bolt (cable) support in the model, and the surface protection effect of metal mesh was not considered. The partial sketch of the numerical model was shown in Fig. 8(d). As shown in the model of Fig. 8, the upper edge of the bottom coal was sloping rather than having an uniform thickness. The reason was that the mining roadways on both sides of the working face were excavated along the roof rock strata of the coal seam. This tunneling method led to a certain thickness of bottom coal remaining. At the two ends of the mining face, it was necessary to gradually transition to the normal mining height of the working face within the range of 30 ~ 40 m.

Fig. 8
figure 8

Roadway support scheme and numerical model diagram. (a) Support pattern of the filling body; (b) support pattern of the tunnel roof; (c) support pattern of the solid coal rib; (d) the partial sketch of the numerical model.

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The calculations were carried out in two schemes and the descriptions of filling bodies in two schemes were listed in Table 3.

Table 3 Descriptions of filling bodies in two schemes.
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Analysis of simulation results

The stress and deformation characteristics of filling body

After numerical calculation, a monitoring point was taken at a height of 1.0 m in the filling bodies in two schemes and the stress datas of the monitoring point were extracted to compare. The variation curve of horizontal and vertical stress at the monitoring point in two schemes with the excavation of the mining face was drawn in Fig. 9.

Fig. 9
figure 9

Vertical stress curves of the monitoring point in two schemes with the mining face advancing.

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According to Fig. 9, the variation trend of filling body’s bearing characteristics under the two schemes were basically similar. However, the horizontal and vertical stress values of the “multi-layer” filling body in scheme 2 were significantly less than those of the conventional filling body in scheme 1. The “multi-layer” filling body in scheme 2 reduced its stress concentration through stress transfer and deformation-yielding. Take the working face excavated 24.0 m as an example, the vertical stress was 21.2 MPa and the horizontal stress was 4.7 MPa in scheme 1. However, in scheme 2, the vertical stress was 17.6 MPa and the horizontal stress was 3.2 MPa, which was about 17.0% and 31.9% lower than that of scheme 1. It could be indicated that the surrounding rocks formed a stable balanced structure, the stress would transfer to the lower part of the “multi-layer” filling body through the upper part after the working face exploitation. the upper part solidified by FFM deformed and yielded to achieve the effect of pressure relief when the overlying rock rotated and sank because of its foaming and expansion characteristics. Further, the load on the filling body and the vertical stress of the roof strata could be reduced in scheme 2.

Additionally, the horizontal displacement graph of the filling bodies in the two schemes were plotted in Fig. 10(a). And more intuitive curves about the horizontal displacement of the two filling bodies at different heights were plotted in Fig. 10(b). A positive horizontal displacement indicated that the filling body at the corresponding position was compressed in the horizontal direction, while a negative value indicated that the filling body at the corresponding position was bulging in the horizontal direction.

Fig. 10
figure 10

The horizontal deformation curves of the filling bodies in the two schemes. (a) The horizontal displacement graph; (b) the horizontal displacement at different heights.

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As shown in Fig. 10(a) and Fig. 10(b), the filling body deformations were “spindle-shaped”, which indicated that the horizontal displacement of the “multi-layer” filling body in scheme 2 within the height range of 2.0 m～3.0 m was significantly greater than that of the filling body constructed only by CFM in scheme 1. However, the horizontal displacement of the “multi-layer” filling body in scheme 2 within the height range of 0.0 m～2.0 m was smaller than that of he filling body formed only by CFM in scheme 1. It was suggested that much more deformation was consumed owning to the high compression ratio of FFM from a given deformation of the roof, the deformation of the lower part in the “multi-layer” filling body was reduced. Therefore, there existed a neutral surface with zero displacement inside the filling body, which was consistent with the results of the literature36. The wall displacement in a certain range around the neutral surface was small, the crack degree was low and the integrity was high.

Failure patterns and characteristics of filling body

The evolution process of the plastic zone of filling bodies in two schemes were drawn in Figs. 11 and 12 respectively. It could be seen that the propagation path of the failure area of the filling bodies was basically same under the two schemes, but the failure forms were quite different. With the increase of roof load, the yield area of the filling body gradually extended from the upper and lower end of one side of the gob to the middle part of the surface of the body and the inner part of the wall. At the same time, plastic failure also appeared at the shallow part of the filling body-left, and shear failure was the main failure form. In terms of failure form, the failure area and depth of the filling body constructed by a single CFM was larger. The damaged area was mainly concentrated in the middle of the wall, and the overall damage form was v-shaped. While, the damaged area of the “multi-layer” filling body mainly concentrated in the upper part formed by FFM and on its shallow surface. Besides, it was obvious that the major part to bear the loading of the filling body constructed only by CFM was approximately hourglass-like. The middle part of the filling body was almost cut through by the plastic zone, the plastic zone area reached 56%, and the stability of the filling body was poor in scheme 1. However, the damaged area inside the “multi-layer” filling body was relatively smaller. The major part of the “multi-layer” filling body to bear the loading was approximately rectangular, the plastic area was almost 40%, and the stability of the filling body was higher in scheme 2.

Fig. 11
figure 11

Evolution process of the plastic zone of filling body in scheme 1. (a) The excavation length of the working face was 20 m; (b) 50 m; (c) 100 m.

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Fig. 12
figure 12

Evolution process of the plastic zone of “multi-layer” filling body in scheme 2. (a) The excavation length of the working face was 20 m; (b) 50 m; (c) 100 m.

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Implementation effect and discussion

Filling system

The filling system for filling materials was shown in Fig. 13. Slunry A and B were made at the preparation station. The preparation station was located in the intake airwary to avoid impact on the coal transportation. The slurry A and B were separately pumped by double-liquid grouting pump in the same proportion and then mixed near the filling body. The filling speed should not be too fast to prevent the filling body from bulging out due to insufficient solidification time.

Fig. 13
figure 13

The filling system for filling materials: (a) Layout of the filling system for high-water materials; (b) Schematic of the filling process for high-water materials.

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“Multi-layer” filling body monitoring

Two monitoring stations had been set up with a certain distance between them. A vertical drilling in the filling body on the roadway side was carried out in each filling bodies. Each borehole was equipped a multi-point displacement meter which contained three displacement basis points. The burial depth of displacement basis points were 0.5 m, 1.0 m and 1.5 m, respectively. The two displacement meters were identified as SJD-1 and SJD-2, respectively. The final bed-separation volume and bed-separation percentage of the filling body were listed in Table 4. In Table 4, the bed-separation percentage was the percentage of the ratio between the bed-separation volume in the base monitoring area and the total bed-separation volume.

Table 4 Lateral separation of the filling body.
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In order to display intuitively the transverse bed-separation characteristics of the filling body, the monitoring data of SJD-1 was extracted to plot the curve about the amount of bed-separation volume and the percentage of that with the transverse depth of the filling body, as shown in Fig. 14. It could be seen that the bed-separation volume of the “multi-layer” filling body reached 178 mm in area I (0～0.5 m), accounting for 86% of the total volume of bed-separation. The bed-separation volume was 17 mm and 12 mm respectively in area II (0.5～1.0 m) and area III (1.0～1.5 m), accounting for 8.2% and 5.8% of the total volume of bed-separation, respectively. The regional fracture characteristics of “multi-layer” filling body was obvious and consistent with the simulated results above.

Fig. 14
figure 14

Schematic diagram of separation of “multi-layers” filling body in scheme 2.

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Roadway surrounding rock deformation monitoring

Five monitoring sections separated by 50 m were arranged to determine the convergence value of rib-to-rib and roof-to-floor. The convergence curves of the rib-to-rib and roof-to-floor during mining were shown in Fig. 15.

Fig. 15
figure 15

Convergence curves of the rib-to-rib and roof-to-floor.

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According to Fig. 15, the deformation of the roadway surrounding rock could be divided into three stages during mining (stage I, stage II, and stage III). In the initial phase (Stage I), roof subsidence, floor heave and filling body deformation were the main characteristics because that the immediate roof was cut off. Nevertheless, the deformation value of coal rib was small. The initial phase of roof movement was started at approximately 30 m behind the working face. The transitional stage (stage II) was located at approximately 30～150 m behind the working face. The main characteristics of this transitional stage was the twice breakage of main roof. The thickness of the immediate roof was two large(reached 38 m) to that it would take overmuch time to entirely cut off the immediate roof. The gob would be almost filled and the distance between the filling gangue and main roof was small. The main roof would break until the subsidence of the roof reaches the critical value. As a consequence, there existed hysteresis for the surrounding rock activity near the roadways in the condition of a thicker immediate roof. The main roof would be cut off along the edge of the filling body until 50～60 m behind the mining face. The deformation velocity of the “multi-layer” filling body increased rapidly and reached 55 mm/d. Then, a temporary stable structure composed by the broken rock block and the main roof was formed. This temporary stable stage was located at approximately 80～100 m behind the working face. Subsequently, the main roof occurred secondary breakage as the gangue in the gob was compacted gradually. The position of the secondary breakage was located at approximately 120 m behind the mining face. More loads were borne by the coal due to its higher strength compared with the backfill body, and so, the deformation velocity of the coal rib increased rapidly and reached 45 mm/d. Subsequently, the deformation velocity of the coal rib decreased gradually. The roof gradually stabilized to form stabilization stage(stage Ⅲ) at 150 m behind the mining face. The roof subsidence parallel and floor heave slightly were the min characteristics for this stage. A stable hinged structure was gradually established between the broken rock blocks above the filling body. The gob-side entry was located at the stress-decreased zone, and so, the surrounding rock movement became slow.

The convergence values and the cross-section shrinkage rates of roadway surrounding rocks were shown in Fig. 16(a) and Fig. 16(b). The final rib-to-rib convergence of the roadway under the influence of mining was 350 mm～490 mm. The final roof-to-floor convergence was 290 mm～470 mm. The shrinkage rate of the roadway cross-section was 20%～30%, which could meet the requirement of working face mining subsequently. Based on the monitoring results of surrounding rock deformation during mining, grouting and anchor could be adopted to improve the stability of the coal rib during 80～100 m behind the mining face because of the rapid deformation of the coal body.

Fig. 16
figure 16

Monitoring results of the roadway and surrounding rock deformation: (a) Section shrinkage of the roadway. (b) Surrounding rock deformation.

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Discussion

It was usually accepted that the function of gob-side filling body support should meet the requirement of both large compressibility and high bearing capacity. Based on this analysis, the composite wall composed by CFM and FFM satisfied the requirement mentioned above. In addition to the common characteristics, such as fast coagulation, large compressibility, and early strength, some interesting phenomena had been discovered.

When the self-weight of the main roof rock and the additional load caused by its rotation subsidence acts on the “multi-layer” high-water material filling body, the upper part of “multi-layer” filling body formed by FFM used its compressible deformation to absorb and transfer a certain amount of roof pressure, so as to promote the high stress to transfer to the coal side and reduce the vertical stress of roadway roof. Thus, the stress concentration of filling body and its own bearing environment were optimized. The deformation amount of the lower part of the “multi-layer” filling body was reduced to avoid the severe rupture of the filling body.

Under the roof load, the downward shrinkage and yielding of the wall and plastic yielding firstly appeared inside the upper part of the “multi-layer” filling body with low stiffness and high compressibility. The roof pressure was transferred to the coal seam, and the load on the filling body could be reduced. Thus, in the process of forming a stable large structure by the roof, the lower part of the “multi-layer” filling body with high stiffness and low compressibility was always in a relatively low load environment. Therefore, the large-scale yielding destroy of the “multi-layer” filling body was avoided and rectangle bearing body with higher stability was finally formed.

Although the surrounding rock activity was still violent for the gob-side entry retaining with the composite wall in the work face with large mining height, the deformation value and stability cycle of the surrounding rock were both smaller compared with the uniform wall. The role of the composite wall remained to be ascertained. Overall, a set of mechanical behaviors of composite filling body constructed by CFM and FFM were analyzed. However, some important issues were remain not touched. In the years to come, the long-term performance of the composite filling body, the difference of mechanical behaviors of the “multi-layer” filling body with different thickness of FFM, and stability control technology of the “multi-layer” filling body in the fully-mechanized top-coal caving face could be studied.

Conclusions

In this paper, a foamed expansion filling material(FFM) was prepared to overcome the stiffness failure of conventional filling material(FFM). Then, the “multi-layer” filling body was designed and its mechanical characteristics was studied by theory calculation, numerical simulation and filed monitoring. Some results were drawn as following:

(1) A tensile failure mode was presented under uniaxial compression of CFM sample with obvious brittleness characteristics. After modification, FFM sample showed a weakened brittleness and enhanced compressibility, and presented a tensile and shear failure mode under uniaxial compression. The average modulus, internal friction angle, compressive strength and falling speed of pressure after the stress peak of the FFM sample were less than that of CFM sample, but the peak strain, cohesion, residual strength and the Poisson ratio were all larger than that of CFM sample.

(2) A “multi-layer” gob-side filling body structure made up by the upper part composed by FFM and the lower part formed of CFM to better adapt to roof sinking by “deformation-yielding” was designed. Through theory calculation, it was determined that the height of the upper layer was 0.5 m and that of the lower layer was 2.5 m in the “multi-layer” gob-side filling body.

(3) Numerical simulation results showed that the failure form of the filling body constructed only by CFM was in the shape of “V”, and the bearing area inside the filling body was approximately hourglass-like. While the damaged area of the “multi-layer” filling body composed by CFM and FFM mainly concentrated inside the upper part and the surface of the lower part in the filling body. The bearing area inside the “multi-layer” filling body was approximately rectangular.

(4) The practical results showed that the upper part of “multi-layer” filling body formed by FFM had high compressible deformation property to absorb and transfer roof pressure. It could rapidly increased resistance, timely supported the roof and cut off the immediate roof with a certain thickness; transferred high stress, reduced the stress concentration of roof pressure and self downward shrank and yielded to improve the synergistic deformation ability of the filling body and roof. Through the application of the “multi-layer” filling body for gob-side entry retaining gateway, the stability of filling body had been improved significantly. The average shrinkage rate of roadway cross-section was 20%～30%, which was able to satisfy the requirement of the subsequently mining.

Consequently, the results of this study showed that the “multi-layer” filling body constructed by CFM and FFM could improve the stability itself and optimize the stress environment of the surrounding rock in the gob-side entry. However, a limitation of this study was that the effects of the FFM height on the mechanical behaviors of the composite structure were not discussed. Additionally, the long-term performance, stability control technology of the “multi-layer” filling body were also should not be neglected. Solving the problem mentioned above remains to be another future enhancement of the proposed study.