Introduction

For a long time, the high-intensity exploitation and utilization of coal resources has led to surface subsidence and solid waste accumulation, which has seriously affected the coordinated development of ecological environment and resource exploitation in mining areas1,2,3,4,5,6,7. In order to promote the sustainable development of coal resources, filling mining is regarded as an important strategy, aiming to solve the pollution problem caused by the waste of coal power base to the biological environment, and promote the reduction, disaster reduction and resource utilization of solid waste4. Grouting and filling in caving zone is a kind of technical means to strengthen broken rock by injecting pre-prepared filling slurry into the gap of broken gangue in caving zone, so as to achieve the goal of controlling roof movement and surface subsidence. This method can be operated in parallel with coal mining at the face, effectively balancing production safety, solid waste treatment and economic benefits.

At present, scholars related to slurry flow characteristics have carried out a large number of studies5,6,7. Zhu Lei et al.8 established a gangue accumulation model in caving zone in the dirt hill on the ground of Daze Coal Mine, and used gangue slurry for grouting and filling to study the seepage and diffusion characteristics of slurry and the precipitation rules at different locations. Using fly ash and mine water as filling materials, Jiang N9 studied the flow characteristics of pulverized coal ash slurry through laboratory experiments and theoretical analysis. Wang Kai10 studied the influence of the crack opening of micro-cracks on the percolation process of cement grout through experiments, and the results showed that cement grout was more likely to percolate at the entrance of cracks. Li Shucai et al.11,12,13 established a 3D front theoretical model of cement slurry considering deep percolation effect, and modeled and analyzed parameters such as permeability coefficient and cement slurry seepage velocity. Qin Pengfei et al.14 designed an orthogonal test to study the effects of water-cement ratio, pore ratio, grouting pressure and other factors on diffusion radius. Fang Kai et al.15 put forward a slurry diffusion model considering filtration effect under the condition of spherical diffusion. Zhang Yu et al.16 adopted the indoor grouting experiment method to simulate the single-hole grouting of the foundation reinforced by goaf grouting.

The mechanical behavior of crushed gangue under the pressure of overlying rock and its internal void structure have a leading effect on grouting filling in caving zone. In order to further study the deformation characteristics of crushed gangue under pressure conditions, many scholars use self-developed compaction test equipment to systematically study the compaction characteristics of broken bulk rock. Miao Xiexing et al.17 independently designed a comprehensive coal (rock) block compaction test scheme and equipment, and systematically studied the crushing and compaction characteristics of coal (rock) blocks in Yanzhou mining area. Zhang Jixiong et al.18 conducted a systematic study on the compaction time-related performance of granular gangue as a direct backfill. Ma Zhanguo et al.19 summarized the variation rules of parameters such as lateral strain, axial strain and elastic modulus of loose gangue during the compression process through experiments, and analyzed the deformation mechanism during the compression process. Li Meng et al.20 studied the creep compression characteristics of crushed gangue with different lithology under step load. Hu Bingnan et al.21 designed 23 groups of schemes under the conditions of large vessel, coarse particle size and high load to simulate the compression process of crushed gangue and study the relationship among compression rate, lateral pressure, gangue particle size and gangue grading. Su Chengdong et al.22 combined the self-developed device with the rock mechanics test system to conduct compaction tests, obtained the stress–strain relationship in the compaction process of sandstone, sandy mudstone and mudstone gravel, and analyzed the influence of rock strength, particle size of crushed rock and test stress on compaction characteristics. Jiang Ning et al.23 developed a large-size deformation and seepage test system of broken rock by themselves, and analyzed the influence of lithology, stress and particle grade on the dry and wet cycle bearing deformation characteristics of broken gangue through the test. Xiao Meng et al.24 established a numerical model by 3D scanning gangue particle morphology and introducing PFC, and deeply analyzed the influence of particle grading and loading rate on the compaction characteristics and side pressure coefficient of crushed gangue. Wang Wen et al.25 analyzed the effects of gangue particle size gradation, axial stress and water content on gangue compactness through experiments. Zhang Tianjun et al.26 conducted compaction tests on fractured rock mass with different particle gradations by means of graded loading, and analyzed the changes in the internal structure of fractured rock mass with different gradations. Li Meng et al.27 tested the instantaneous and creep compressive strength characteristics of crushed gangue, and analyzed the structural morphological evolution characteristics of compacted deformation particles of filled gangue. Li Junmeng28 believed that the deformation resistance of gangue particles was the key to controlling overburden movement and surface settlement. Zhang Pengfei et al.29 conducted an in-depth study on the spatio-temporal evolution of goaf roof settlement and the bearing characteristics of gangue area by combining the methods of field measurement and numerical simulation.

With the high-intensity exploitation of coal mine resources, problems such as water accumulation and ground subsidence in coal mine goafs are becoming increasingly serious, which has brought great pressure on the safety and environment of the mining area. As an effective solution, coal gangue backfilling and slurry grouting technology are of great significance in controlling ground subsidence, improving resource recovery and environmental management.

At present, although there has been some research attention to the technology of backfilling in goaf areas, there are still many technical difficulties in terms of slurry flow characteristics, compaction and crushing mechanism of coal gangue particles, and long-term bearing capacity of the backfill body. For example, the existing technology lacks in-depth exploration of the interaction between coal gangue of different particle size grading and slurry, and the penetration and diffusion laws in the process of slurry injection still do not form a clear theoretical framework. Through this study, we aim to fill these technical gaps and propose more optimized backfilling schemes.

The goal of this study is to solve the current technical deficiencies in terms of slurry flow and diffusion, backfill body stability, and bearing capacity through experimental research on the flow characteristics, compaction performance, and bearing capacity of coal gangue and fly ash slurry at different particle size gradings. Our research provides theoretical basis and practical guidance for optimizing the selection of backfill materials, improving the backfill effect, and ensuring the ground stability in mining areas.

The flow characteristics of slurry and the evolution characteristics of the internal void of crushed gangue under the influence of mining are one of the important factors affecting the grouting filling effect in caving zone. Previous studies have failed to form a clear understanding of the flow characteristics of fly ash slurry in gangue granules, the bearing capacity of gangue-slurry backfill formed after slurry infiltration, and the internal structure change of backfill after pressure. In this paper, the above problems are systematically studied through the fluidity test of the slurry of gangue loose fly ash and the limited load compression test before and after the grouting of gangue loose.

Test summary

Test materials

The coal gangue and fly ash used in the test were taken from Ningdong coal power Base in Ningxia Hui Autonomous Region, and the fly ash materials in the test were ready to use and did not need to be pre-treated. According to the literature, the average uniaxial compressive strength of coal gangue is 34.19 MPa30. The coal gangue is broken by PEX jaw crusher with large particle size, and then according to different particle size (5–10, 10–15, 15–20, 20–30 mm).Stage by stage screening.

In order to eliminate the influence of moisture content on the characteristics of waste, fly ash and crushed coal gangue were dried in a DHG-9030A electric hot air oven, with a drying temperature of 40℃ and a drying time of 48 h. After drying, they were allowed to cool down for 6 h. The final test materials prepared are shown in Fig. 1.

Fig. 1
figure 1

Experimental materials.

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Test scheme and device

Fly ash slurry fluidity test scheme

In practice, the bulk accumulation form of gangue is very complicated. In order to fit the actual situation as much as possible, multiple continuous gradations and single particle size groups are set in this paper. According to literature review, Talbol grading theory has been widely used in the optimization design of mineral grading31. Talbol hierarchical coordination theory, that is, the maximum density curve n power formula:

$$P = 100 \times \left( \frac{d}{D} \right)^{n}$$
(1)

where, P is the passage percentage of each particle size of gangue granular,%; d is the particle diameter of gangue at all levels, mm; D is the maximum particle size of gangue, mm; n is the grading coefficient.

Talbol grading theory can be used to calculate different particle size grading schemes with the same maximum particle size and different coefficient. According to the maximum particle size D = 30 mm, and the grading coefficient n = 0.3, 0.4, 0.5, 0.6, 0.7, 5 continuous grading groups were set, and 4 groups of single particle size were selected (5–10 mm, 10–15 mm, 15–20 mm, 20–30 mm), as shown in Table 1 for specific schemes. Before the test, the gangue is gradually put into a beaker with a capacity of 1L according to the design proportion. The experimental instrument is fixed in accordance with the funnel, beaker and test frame from top to bottom, so that the grouting mouth under the funnel is close to the top of the crushed gangue. The specific diagram is shown in Fig. 2. At present, the main purpose of caving zone grouting filling in Ningdong mining area is to absorb coal-based waste, which has low requirements for surface settlement control. At the same time, in order to simplify the test process and facilitate operation, no cement is added to the prepared fly ash grout, and three gradients of 60%, 65% and 70% concentration are set.

Table 1 Experimental plan for gangue particle size gradation.
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Fig. 2
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Loading of gangue material and grouting method.

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Backfill bearing performance test scheme

The limited compression experiment of gangue is designed, which is divided into two parts: limited instantaneous compaction and limited creep compaction, and the bearing capacity of gangue bulk and gangue-slurry filling body is compared and analyzed. The size distribution of gangue required by the experiment is 5–30 mm, and the grading scheme is in 9 groups as shown in Table 1. In this experiment, 170 mm × 125 mm(height × diameter) steel cylinder was used to control the quality of waste rock loading. The rock mechanics servo system transfers the pressure to the gangue in the steel cylinder through the indenter, records the stress and strain changes during the loading process, and monitors the crushing of the gangue through the acoustic emission monitoring system. Figure 3 shows the tools and equipment required for the test.

Fig. 3
figure 3

Instruments required for the experiment.

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In order to reduce the friction resistance of the inner wall of the cylinder, lubricating oil was applied on the wall of the cylinder before each group of tests, and the dirt was evenly mixed and loaded into the steel drum in different times, and the total loading was controlled to be 2400 g. The axial loading peak of the rock servo system is set at 15 MPa and the loading rate is constant at 0.1kn/s. The instantaneous compaction and creep compaction characteristic tests are carried out. The instantaneous compaction test stops when the stress reaches the set value, and the creep compaction test keeps the peak stress load for 6 h after the peak stress reaches 15 MPa.

Test results of fly ash slurry diffusion characteristics

Space measurement of gangue injection

Different graded gangues were put into a beaker with capacity of 1L, and water was poured into the beaker to a 1L scale line. The amount of water injected was recorded and the effective porosity was calculated32. The effective porosity refers to the ratio of the volume of interconnected pores (except closed pores) in the loose gravel body to the total volume of the broken gangue body :

$$n_{e} = \frac{{V_{e} }}{V}$$
(2)

Theoretical grouting space:

$$V_{0} = V - \frac{m}{\rho }$$
(3)

theoretical porosity:

$$n_{0} = \frac{{V_{0} }}{V}$$
(4)

In the formula, Ve is the volume of intercon-nected pore (i.e., the grouting volume of water), cm3; V is the total volume of broken gangue body, take 1000 cm3; ne is the effective porosity; V0 is the the-oretically calculated grouting space, cm3; m is the mass of gangue, g; ρ is the density of gangue, g/cm3. Small pieces of gangue were weighed after drying, and the volume was obtained by drainage method, The density of coal gangue was calculated to be 2.583 g/cm3, The final calculation results are shown in Table 2.

Table 2 Theoretical Effective Porosity and Measured Effective Porosity under different ratios.
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Under the single particle size group of gangue, the porosity is proportional to the particle size, and the porosity of different particle sizes varies signif-icantly. The maximum effective porosity and measured effective porosity of gangue with particle size of 20–30 mm are 0.600 and 0.557 respectively. The porosity of the gangue with a particle size of 5–10 mm is smaller than that of the other large particle size gangue, which is 80.3%, 81.7% and 86.2% of the gangue with a particle size of 10–15 mm, 15–20 mm and 20–30 mm, respectively. The difference is large, indicating that the single particle size gangue with a particle size of 5–10 mm is the most compact.

Under the Talbol grading of gangue, the measured effective porosity and theoretical porosity are the smallest when the grading coefficient n = 0.5, which are 0.496 and 0.500, respectively, the gangue accumulation body is the most dense. Taking n = 0.5 as the dividing line, when n = 0.3–0.5, the porosity is inversely proportional to the gradation coefficient ; when n = 0.5–0.7, the porosity is proportional to the gradation coefficient. By comparison, the vari-ation trend of effective porosity and measured ef-fective porosity of gangue under different gradations is basically the same, which is shown as follows : single particle size 5–10 mm gangue < Talbol graded gangue < single particle size 10–15 mm < single particle size 15–20 mm < single particle size 20–30 mm gangue. Taking n = 0.5 as the dividing line, when n = 0.3–0.5, the porosity is inversely pro-portional to the gradation coefficient ; when n = 0.5–0.7, the porosity is proportional to the gradation coefficient.

Through comparison, the change trend of ef-fective porosity and measured effective porosity of gangue under different gradations is basically the same, which is as follows : single particle size 5–10 mm gangue < Talbol graded gangue < single particle size 10–15 mm < single particle size 15–20 mm < single particle size 20–30 mm gangue.

Fly ash slurry diffusion trace line

The fly ash slurry was prepared, the solid phase material was fly ash, and the liquid phase was water. The fly ash slurry with a concentration of 60%, 65%, and 70% was fully stirred for 10 min and injected into the accumulated gangue. The diameter of the grouting hole was 10 mm. The flow diffusion law was observed during grouting until the grouting could not be injected or the 1L scale line was filled. The slurry diffusion trace mainly includes transverse diffusion and longitudinal diffusion33,34. The transverse diffusion is the diffusion process of the upper surface of the gangue body, and the longitudinal diffusion is the network diffusion along the longitudinal through pores and the infiltration diffusion process along the cup wall direction. After the grouting is completed, the maximum longitudinal penetration diffusion distance is measured and the actual grouting amount of the fly ash slurry is counted. The experimental process is shown in Fig. 4.

Fig. 4
figure 4

Fly ash slurry diffusion trace (ω = 60%, n = 0.3).

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Diffusion process of fly ash slurry under single particle size group

When grouting with fly ash slurry with concentration of 60% and 65% for single particle size gangue, the diffusion law of the two is basically the same. The diffusion of slurry in the grouting process is mainly divided into three stages, as shown in Fig. 5:

Fig. 5
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Diffusion process of 60% concntration fly ash slurry (10–15 mm).

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The first stage (Fig. 5a) : When the slurry begins to contact the gangue accumulation body, due to the low concentration and good fluidity of the slurry, the slurry does not spread obviously in the horizontal direction, and takes the grouting mouth as the center to form a network. The longitudinal expansion flow channel, because the grouting space formed by the gangue accumulation is random and different, the flow speed and direction of the fly ash slurry when it diffuses in it also change frequently.

The second stage (Fig. 5b) : When the slurry diffuses to the bottom of the beaker along the longitudinal flow channel, it begins to diffuse to the edge of the beaker wall in the horizontal direction until the slurry fills the bottom of the beaker.

The third stage (Fig. 5c) : with the increase of slurry injection, the slurry continues to spread downward in a net shape, and the total height continues to rise upward along the cup wall. The lateral boundary of the overburden above the same layer of gangue further expands outward, and finally stops grouting at 1L scale. When the concentration of fly ash slurry is 70%, the diffusion law of single particle size 15–20 mm and 20–30 mm gangue grouting is the same, and the diffusion process is also divided into three stages, as shown in Fig. 6.

Fig. 6
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Diffusion process of 70% concentration fly ash slurry (15–20 mm).

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The first stage (Fig. 6a)): When the slurry begins to contact the gangue accumulation body, it first diffuses along the transverse direction, with the grouting mouth as the center, and the slurry forms an approximate circular or elliptical cover above the gangue body. After the slurry is continuously injected, the cover layer gradually expands to the cup wall.

The second stage (Fig. 6b)) : The slurry infiltrated along the cup wall and the longitudinal flow channel to form a network of longitudinal flow expansion channels, and continued to spread downward to the bottom of the beaker.

The third stage (Fig. 6c) : With the increase of slurry injection, the slurry continues to diffuse downward in a network, and finally stops grouting at the 1L scale.

However, when the slurry is not filled in the single particle size of 5–10 mm and 10–15 mm gangue, the slurry siltation occurs, which leads to the failure of injection and the phenomenon of ‘infiltration’.

The reason is that when the fly ash slurry penetrates and diffuses in the gangue accumulation body, the gangue particle skeleton will adsorb and filter the fly ash particles in the migration, resulting in the continuous retention and deposition of the fly ash particles, and finally the slurry diffusion channel is blocked.

Diffusion process of fly ash slurry under Talbol gradation

When the concentration of 60% and 65% fly ash slurry is used to grout the Talbol graded gangue dispersion, the slurry diffusion process under each gradation is basically the same as that of the single particle size group of 15–20 mm and 20–30 mm gangue concentration 70% fly ash slurry, as shown in Fig. 7.

Fig. 7
figure 7

Diffusion process of 65% concentration fly ash slurry (n = 0.5).

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When the concentration of 70% fly ash slurry is used for grouting, the transverse diffusion occurs first and then along a small amount of longitudinal through pores and the downward diffusion of the cup wall, and the slurry does not reach the bottom of the beaker. The phenomenon of ‘percolation’ occurs, which makes the slurry unable to continue to be injected, as shown in Fig. 8.

Fig. 8
figure 8

Diffusion process of 70% concentration fly ash slurry (n = 0.5).

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Analysis of experimental results of fly ash grouting under single particle size group

The maximum longitudinal penetration diffusion distance of fly ash slurry under single particle size gradation is statistically analyzed, as shown in Fig. 9.

Fig. 9
figure 9

Maximum vertical diffusion distance at different Concentrations under single particle size gradation.

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The fly ash slurry with a concentration of 60% can infiltrate to the bottom of the beaker by its own gravity in the gangue body of different particle sizes under no pressure. When the concentration of fly ash slurry is 65%, the maximum longitudinal infiltration diffusion distance in the 5–10 mm small particle size gangue is 125 mm, and the rest of the larger particle size can penetrate to the bottom of the beaker. When the concentration is 70%, the maximum longitudinal infiltration diffusion distances of 5–10 mm and 10–15 mm are 27 mm and 70 mm respectively, and the remaining large particle size gradation slurry can diffuse to the bottom of the beaker.

The injection amount of fly ash slurry with different concentrations in the single particle size experimental group was counted, and the results are shown in Fig. 10.

Fig. 10
figure 10

Grouting volume at various concentrations for single particle size gangue.

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When the particle size of gangue is the same : the greater the slurry concentration, the smaller the actual amount of fly ash slurry injected, and with the increase of particle size, the influence of concentration on grouting amount gradually decreases ; when 20–30 mm large particle size gangue is grouted, the effective porosity is large, so the grouting space is large, so that the influence of slurry concentration on the injection amount of fly ash slurry is weakened, and the grouting amount of fly ash with slurry concentration of 65% and 70% is small.

When the slurry concentration is the same : the greater the concentration, the greater the difference in the grouting amount of gangue of each particle size ; the larger the particle size is, the larger the effective porosity is, and the more the grouting amount is. When the slurry concentration is 65%, the maximum amount of fly ash injected into 20–30 mm gangue is 679.4 g.

Analysis of experimental results of fly ash grouting under Talbol gradation

The maximum longitudinal penetration diffusion distance of slurry under Talbol gradation is shown in Fig. 11. The fly ash slurry with a concentration of 60% and 65% can be freely infiltrated to the bottom of the beaker, and the maximum diffusion distance is the same. The fly ash slurry with a concentration of 70% has a certain degree of infiltration and cannot be filled. When the gradation coefficient n = 0.5, the maximum longitudinal penetration diffusion distance is 36 mm, which is the lowest point. When the gradation coefficient n increases from 0.3 to 0.5, the maximum longitudinal penetration diffusion distance decreases. When the gradation coefficient n increases from 0.5 to 0.7, the maximum diffusion distance increases, and the maximum difference of the maximum diffusion distance is 81 mm.

Fig. 11
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Maximum vertical diffusion distance at different concentrations under talbol gradation.

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The injection amount of fly ash slurry with different concentrations in gangue bulk with different gradation coefficients is counted, as shown in Fig. 12. When the gradation coefficient is the same, the variation law of grouting amount of fly ash slurry with different concentrations is as follows : concentration 60% grouting amount > concentration 65% grouting amount > concentration 70% grouting amount, among which the grouting amount of fly ash slurry with concentration 60% and 65% is smaller. The grouting amount of fly ash slurry under each gradation coefficient is divided by n = 0.5.When the gradation coefficient increases, the grouting amount increases in turn. When the gradation coefficient decreases, the grouting amount also increases in turn. When the grading coefficient is 0.5, the gangue accumulation is the most dense and the grouting amount is the smallest.

Fig. 12
figure 12

Injection volumes at different grading coefficients.

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Test results of bearing characteristics of gangue before and after grouting

Analysis of the results of the gangue granular compression test

Characteristics of instantaneous compression deformation of gangue granular

Figure 13 shows the stress–strain relationship curve during the instantaneous compression of gangue. During the test, the degree of compaction and breakage of gangue gradually increased, but the rate of strain increase showed a decreasing trend. With the increase of stress, the instantaneous compression process can be divided into compaction stage (0–2.5 MPa), crushing and re-compaction stage (2.5–7.0 MPa) and stable compaction stage (7.0–15.0 MPa). In the compaction stage, the small particle size gangue is subjected to axial pressure inside the steel cylinder, and a large range of slippage first fills the space between the particles of gangue, which limits the slippage space of the gangue with large particle size after compression, and causes the gangue with large particle size to break. In the crushing and recompaction stage, most of the void between gangue is filled and no longer occurs particle slip, and the remaining void gradually decreases with the increase of stress. In the stable compaction stage, the whole structure of gangue tends to be stable, and the stress–strain curve tends to be flat. Compressive deformation becomes more and more difficult, and the rate of strain increase decreases with the increase of stress, and finally tends to stabilize.

Fig. 13
figure 13

Different grading gangue stress–strain curves.

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In the stable compaction stage (7.0–15.0 MPa), although the stress continues to increase, the slope of the curve does not completely flatten, but instead shows some fluctuations. This indicates that, despite most of the voids being filled, there is still a small amount of deformation and interactions between particles. These fluctuations may be due to slight structural changes in localized areas or micro-damage occurring during the compaction process. As the stress increases further, the rate of deformation gradually slows down and eventually stabilizes.

In the compaction stage, the strain of single particle size graded gangue is smaller than that of Taibo graded gangue because the particle size is similar to slip, the particles will support each other, and the main phenomenon of compaction and breakage occurs. After the compaction stage, the strain of 10–15 mm, 15–20 mm and 20–30 mm graded gangue increases with the stress to exceed the Tybo continuous grading, indicating that the stability of single grain size graded gangue is weaker than that of continuous grading. In the case of Taibo grading, the maximum strain decreases with the decrease of voidage, and the proportion of gangue with large grain size significantly affects the maximum strain value when the porosity is similar, and the maximum strain is proportional to the amount of gangue with large grain size.

Instantaneous compression acoustic emission response characteristics of gangue granules

The AE frequency response range adopted by the acoustic emission monitoring system test is 100–400 kHz, and the sampling frequency is 1 MHz. Two sensors are used to collect acoustic emission signals, each sensor is equipped with a pre-amplifier, and the fixed threshold value is 40 dB to reduce the impact of noise on acoustic emission monitoring results. During the crushing process of gangue, the AE characteristics are collected. When the axial pressure is 15 MPa, the curves of strain, AE energy and accumulated energy of mixed gangue with different levels are shown in Fig. 14. Meanwhile, based on the AE energy characteristics of mixed gangue with different levels, the AE energy-accumulated energy curves are shown in Fig. 15. It can be seen from Figs. 14 and 15 that:

Fig. 14
figure 14

AE energy, cumulative energy, and strain–time curve during the compression process of gangue.

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Fig. 15
figure 15

AE energy and cumulative energy.

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  1. (1)

    The nine gangue grading schemes showed a similar overall trend in terms of energy change law, which could be divided into three stages: pre-loading, mid-loading and post-loading. In the early and late loading period, acoustic emission signals are limited, mainly because the pores between gangue are compacted by external pressure, and gangue is less broken, so the friction acoustic emission signals are mainly used, and the overall energy changes are relatively stable. In the middle stage of loading, due to the relative slide caused by the mutual extrusion of gangue, the crushing of gangue increases significantly. At this time, the acoustic emission signal is composed of friction type and crushing type, resulting in significant energy change. At the later stage of loading, the gangue eventually forms a relatively stable pressure entity under external pressure, and the main source of acoustic emission signals turns to friction type again, making the overall energy relatively stable.

  2. (2)

    According to AE energy, accumulated energy and strain–time curve, it can be seen that AE energy and accumulated energy change with the change of mixed gangue void ratio and the proportion of gangue with large particle size during Taibo continuous grading, and the change law is similar to the maximum strain of gangue, that is, mixed gangue void ratio is small, and AE energy distribution range is wide. When the proportion of gangue with similar void ratio and large particle size increases, both the maximum AE energy and cumulative AE energy increase. The small acoustic emission energy of small voids indicates that the crushing degree of gangue is low. When the proportion of large particle size gangue is large, the AE energy is high, indicating that the energy generated by the crushing of large particle size gangue is greater than that of small particle size gangue. In the case of single particle size, because the particle size is similar, the slip phenomenon of dirt in the early stage of loading is less, and the AE energy of AE is a combination of frictional AE and crushing AE, showing an increasing trend different from that of continuous gradation. With the increase of particle size, AE energy and accumulated energy showed a significant increase, and the decrease was more severe at the later stage of loading, indicating that AE energy of AE for single particle size was mainly from the crushing AE energy generated by gangue breaking, and again indicating that the energy generated by the crushing of gangue with large particle diameter was greater than that of small particle size.

  3. (3)

    The cumulative AE energies of the 9 gradations were 6.04 × 106, 10.67 × 106, 10.3 × 106, 17.6 × 106, 24.9 × 106, 4.65 × 106, 4.74 × 106, 29.2 × 106 and 35.7 × 106, respectively. These cumulative energies are related not only to the intensity of the strongest acoustic emissions, but also to the duration and total number of events. In each stage, the energy of the grade with large size gangue is higher than that of the grade with small size gangue. Especially in the gradation of large particle size, the AE parameters in each stage reach the highest level, which indicates that the AE activity of large particle size gangue is frequent when it is crushed under pressure.

Analysis of compression test results of gangue and slurry backfill

Instantaneous compression deformation characteristics of gangue and slurry backfill

In order to study the load-bearing capacity of gangue and slurry filling body after grouting in caving belt, the gangue with different levels in the cylinder was grouting filled, and the filling slurry was the pulverized coal ash slurry with a concentration of 65% prepared in advance. After filling, the cylinder was put into the drying box, and compression test was carried out after the slurry drying. Figure 16 shows the state of the filled sample in different stages of the test process.

Fig. 16
figure 16

Comparison before and after gangue grouting compression test.

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Figure 17 shows the stress–strain curve after grouting of different gangue gradations. It can be seen from Fig. 17 that in the test process, the deformation in the early stage is relatively fast while the deformation in the later stage is relatively slow. Therefore, the deformation can be divided into two stages, namely the crushing compaction stage when 0–4 MPa and the stable compaction stage when 4–15 MPa. In the crushing and compaction stage, the space between gangue particles is filled after grouting, which limits the slip of gangue particles. With the increase of pressure, particle size particles are broken first, and the gangue powder and small particle size particles generated after crushing are quickly compacted, resulting in the overall strain increase. In this stage, the structure of gangue slurry filling body continues to change. In the stable compaction stage, the strain increase degree gradually decreases, and the backfill structure gradually stabilizes without drastic deformation. The variation law of the maximum strain value of grouting compression is consistent with that of the condition without grouting, and the porosity and the proportion of gangue with large particle size affect the compaction. The reason is that after filling the grout and drying, a part of the filled void between the particles reappears as the water disappears, resulting in the pressure situation continuing to be affected by the void, resulting in a law consistent with that without grouting.

Fig. 17
figure 17

Stress–strain curves after grouting for gangue with different particle size distributions.

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The stress–strain curves of the compaction of granular gangue particles under Taibo grading were drawn before and after grouting, and compared with those before grouting were analyzed, as shown in Fig. 18.

Fig. 18
figure 18

Comparison before and after grouting under Talbol gradation.

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As can be seen from Fig. 18, when the maximum axial pressure loading is set to 15 MPa, the lateral compression axial strain of bulk gangue grouting with different Talbo grading coefficients is greatly reduced. The change rate of the strain of granular gangue before and after filling (the ratio of the difference of strain variation to the strain before filling) of each gradation coefficient n was calculated respectively. The strain variation rates of bulk gangue with grading coefficient n of 0.3, 0.4, 0.5, 0.6 and 0.7 after filling are 51.83%, 45.93%, 49.97% and 43.29%, respectively. The deformation resistance of the filling body after grouting is significantly improved. The ratio of strain change rate after gangue filling with grading coefficient n increases successively from 0.6, 0.4, 0.5, 0.3 and 0.7. The strain change rate of gangue fly ash consolidated body with grading n = 0.7 after grouting is the largest, the grouting effect is the best, and the compressive deformation resistance is the largest. The axial pressure data of 20% axial strain is taken from Fig. 18a, and the axial pressure before and after grouting is obtained as shown in Fig. 18b. It can be seen that when the same compressive deformation is generated, The load-carrying capacity of gangue and slurry filling body with different Tai-wave grading coefficients increased by 268.9%, 149.8%, 272.2%, 143.9%, and 343.5%. respectively compared with that before grouting, and the load-carrying capacity of bulk gangue pile after grouting and filling was greatly improved.

Acoustic emission response characteristics of slurry filling under instantaneous compression

Information is collected on the acoustic emission characteristics of gangue slurry backfill during the instantaneous compression and crushing process. When the axial pressure is 15 MPa, AE energy and cumulative energy curves of gangue slurry backfill with different gangue ratio schemes are shown in Fig. 19. The contrast curves of AE energy and accumulated energy of gangue fragments and backfill with different proportions are obtained, as shown in Fig. 20.

Fig. 19
figure 19

Instantaneous compression process AE energy and cumulative energy curve of gangue-slurry mixture.

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Fig. 20
figure 20

Comparison of AE energy and cumulative energy between gangue discrete particles and gangue-slurry mixture.

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  1. (1)

    There are some differences between the filling body and the gangue bulk in the overall trend of energy change. For continuous gradation, the whole can be divided into three stages: front, middle and back. The acoustic emission signals in the early and late loading stage are limited, while the acoustic emission signals in the middle loading stage are more frequent. In contrast, the single particle size ratio is different in the early stage of loading and the late stage of loading, with more acoustic emission signals in the early stage and less acoustic emission signals in the late stage. Unlike the acoustic emission characteristics of gangue bulk compression, the continuously graded backfill showed an upward trend in the early stage of loading and reached a peak in the middle stage, while the acoustic emission signal gradually decreased in the late stage, showing a slight fluctuation. In the single particle size ratio, the acoustic emission signal reaches the peak rapidly in the early stage of loading, but weakens rapidly in the late stage of loading, and the transformation gradually becomes stable.

  2. (2)

    According to the AE energy and cumulative energy curves, compared with the AE energy diagram of gangue scattered waste, AE energy distribution of gangue-slurry fill is more loose, and the acoustic emission signal at the beginning of loading is dominated by crushing AE and supplemented by friction AE, showing an upward trend. This indicates that the backfill is difficult to slip due to the gap being filled during loading, and the main source of the sound is the sound made by the pressure crushing of gangue, and gangue crushing does not occur all the time. During the test, friction AE signals are generated between fly ash particles, broken gangue particles, and pulverized coal ash particles and broken gangue due to the loading pressure. As a result, the distribution of acoustic emission energy of the backfill is more loose than that of the gangue bulk. The cumulative AE energy curve of the continuously graded backfill showed an upward trend before and during the middle period of loading, while the slope of the curve showed an obvious inflection point at the late stage of loading, and the increase rate slowed down. The cumulative energy curve of the single particle size ratio backfill showed an upward trend in the early stage of loading, and the increase potential slowed down in the late stage of loading, but there was no obvious inflection point. It shows that the internal structure of the filling body under continuous gradation is more stable in the late loading period, and it is not easy to have large gangue breakage and friction.

  3. (3)

    The cumulative AE energy of the backfill with 9 matching schemes is 1.57 × 106, 1.95 × 106, 2.41 × 106, 2.46 × 106, 3.67 × 106, 0.89 × 106, 1.04 × 106, 3.49 × 106, 4.59 × 106. According to the AE energy and cumulative energy diagram of the backfill, the AE energy peak value of the backfill is roughly the same as that of the gangue bulk in continuous gradation, and the AE energy peak difference between the backfill and the gangue bulk in single ratio increases with the increase of the particle size. This indicates that the major changes in the internal structure of continuously graded samples before and after grouting during the loading process are mainly due to gangue crushing, and the particle size of gangue with a single ratio is similar. As the particle size increases, the pore space of gangue particles becomes larger and more significant after grouting, so the AE energy peak value before and after grouting varies greatly with the increase of particle size. By comparing the accumulated energy of backfill and waste rock, it can be seen that there are obvious differences between them. The accumulated acoustic emission energy of backfill with different ratio schemes is significantly lower than that of gangue loose rock, indicating that the structure of the sample after grouting is more stable and better bearing performance is obtained.

Analysis of stress–strain relationship

During the compaction process of coal gangue, the relationship between stress and strain exhibits distinct nonlinear characteristics as external pressure increases. Coal gangue, being a particulate material, undergoes compaction in three main stages: the initial stage, the crushing and recompaction stage, and the stable compaction stage. Each stage exhibits different stress–strain behaviors and underlying physical mechanisms.

Initial stage: linear elastic deformation

In the initial stage of compaction, the relationship between stress and strain is nearly linear. During this stage, the material predominantly experiences elastic deformation, and the rate of pore reduction is relatively high. According to Hooke’s Law, the stress–strain relationship during the elastic stage can be represented by a linear equation:

$$\sigma = E \cdot \varepsilon$$
(5)

where σ is the stress, ε is the strain, and E is the modulus of elasticity. In this phase, the contact forces between coal gangue particles gradually increase, causing small displacements and relative sliding of the particles, leading to a reduction in pore space. As external pressure is applied, the gaps between the particles are rapidly filled, and the porosity decreases quickly.

Since the particles have not yet undergone significant fracturing or interaction, compaction is primarily governed by the elastic deformation of the particles. The deformation is relatively uniform, and most of the stress is used to overcome the voids between the particles. As a result, stress increases in a linear manner, and the compaction process is relatively swift.

Crushing and recompaction stage: emergence of nonlinear characteristics

As external pressure increases, the deformation of coal gangue particles transitions into the crushing and recompaction stage. During this phase, the contact forces between particles intensify, leading to the fracture of larger particles, generating more smaller fragments. These small fragments then fill the voids between the larger particles, further reducing porosity. This process causes the stress–strain curve to display clear nonlinear characteristics.

At this stage, the material’s deformation is no longer purely elastic; the combined effects of particle crushing and recompaction result in a nonlinear stress–strain relationship. During the crushing process, part of the energy is converted into friction between particles, particle fragmentation, and crack propagation, which results in a change in the rate of strain increase. Specifically, as larger particles break and small fragments fill the existing voids, the porosity decreases further, but due to the frictional forces between particles and the limitations imposed by the fracturing process, the rate of deformation gradually slows. Thus, despite the continuous increase in stress, the rate of strain increases more slowly.

The stress–strain curve during this stage typically exhibits a larger curvature, indicating more complex and nonlinear deformation. This can be described by the following nonlinear model:

$$\sigma = C \cdot \varepsilon^{n}$$
(6)

where C and n are constants, and when n > 1, the curve shows a pronounced nonlinear growth, reflecting changes in the material’s internal structure, especially due to the mechanical behaviors triggered by particle fracture. The rate of strain increase is significantly reduced, but it still continues to rise as the small particles generated from the fracture process fill more voids, thereby reducing porosity and further enhancing the compaction.

Stable compaction stage: strain rate approaching zero

After undergoing the initial compaction and crushing processes, as pressure continues to increase, the deformation of coal gangue gradually enters the stable compaction stage. At this point, porosity has significantly decreased, and the overall structure of the material becomes stable, making further compaction increasingly difficult. The stress–strain curve during this stage flattens, reflecting a gradual decrease in the rate of strain increase, which ultimately tends towards zero.

The compaction behavior during this phase can be described by the following model:

$$\sigma = E_{0} \cdot \varepsilon^{m}$$
(7)

where m approaches 0, indicating that the relationship between stress and strain becomes nearly flat. At this stage, while the stress continues to increase, the rate of deformation becomes very slow, signifying that the material’s overall structure is stabilizing and deformation is nearing saturation. The porosity has almost reached its minimum level, and the deformation is now primarily caused by the microscopic adjustments between particles and the further closing of micro-cracks.

At this point, since most of the voids have already been filled, the remaining small voids are further compacted, and the deformation of the material is mainly driven by microscopic adjustments between particles. As the reduction in pore space becomes increasingly difficult, the rate of strain increase gradually approaches zero, causing the stress–strain curve to flatten.

Pressure creep analysis of waste bulk and backfill

In order to further analyze the change of bearing capacity of the samples before and after grouting, the creep experiment was carried out by selecting the gangue bulk and the gangue slurry backfill with Taibo continuous grading schemes of 0.3, 0.5 and 0.7 respectively. The creep test process is as follows: the steel cylinder loaded with gangue bulk (or backfill) is placed in the rock mechanics servo test system, and the pressure is set to peak184.5 kn(15 MPa), the loading rate selected 0.1 kn/s, wait for pressure maintain peak pressure for 6 h after reaching 15 MPa.

Figure 21 shows the time strain curve during the creep test of gangue bulk and backfill before and after grouting. The loading process can be roughly divided into two stages, the stage of stress increase and the stage of stress stabilization. In the stage of stress increase, the axial strain of the gangue bulk increases rapidly, and most of the deformation of the sample is realized in this stage. When the pressure reaches 15 MPa, it enters the pressure stabilizing stage. In this stage, the increase of axial strain decreases significantly after the first turning point, and the axial strain changes gently after the second turning point, and the curve is approximately straight. The axial strain of the backfill also increases rapidly at the stress increase stage, and most of the deformation of the backfill sample is completed at this stage. However, at the stress stable stage, the axial strain does not change gently at the inflection point until the curve becomes straight. The creep compaction curve also shows that the filling body after grouting has better long-term stability and load-bearing performance.

Fig. 21
figure 21

Time-strain curve for creep and consolidation test of samples before and after grouting.

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Conclusion

In this paper, the slurry flow characteristics of 3 concentrations of fly ash slurry injected into gangue granules of 9 gradation schemes and the bearing performance of gangue before and after grouting are obtained through experiments, and the following conclusions are drawn:

  1. (1)

    When the slurry of single particle size gangue is grouting with 60% and 65% fly ash slurry, the slurry first spreads longitudinally and then transversally. When the fly ash slurry with a concentration of 70% is injected, horizontal diffusion occurs first in the gangue with a single particle diameter of 15–20 mm and 20–30 mm, and then longitudinal diffusion occurs. Slurry silting occurs in the gangue with a single particle size of 5–10 mm and 10–15 mm, resulting in the inability to inject and the phenomenon of “infiltration” occurs. When the slurry of Talbol graded gangue is grouting with 60% and 65% fly ash slurry, the slurry first diffuses horizontally and then longitudinally. When the concentration of 70% fly ash slurry is injected, the slurry first spreads laterally and then longitudinally, and the slurry appears “percolation” without method filling.

  2. (2)

    The amount of gangue grouting is related to the gangue particle size and slurry concentration. The larger the slurry concentration, the smaller the actual slurry amount of injected fly ash, and with the increase of particle size, the influence of the slurry concentration on the grouting amount gradually decreases. The larger the grain size of gangue is, the larger the effective porosity is and the larger the grouting amount is. When the grading coefficient n = 0.5, the gangue deposit is the most dense and the grouting amount is the least.

  3. (3)

    The compaction process of gangue bulk can be divided into compaction stage, breaking and re-compaction stage and stable compaction stage, while gangue slurry filling body can be divided into early crushing compaction stage and late stable compaction stage. With the increase of axial stress, the rate of strain increase gradually decreases and finally tends to be stable. When the same compressive deformation is generated, the bearing capacity of the filling body after grouting filling is greatly increased.

  4. (4)

    In the early and late loading stage, the waste bulk is mainly dominated by frictional acoustic emission signals, which are relatively limited. In the middle loading stage, the acoustic emission signals are composed of frictional and crushing types, and the acoustic emission signals are relatively significant. The acoustic emission activity is more frequent when the gangue with large particle size is crushed under compressive force. The loading process of gangue and slurry backfill can be divided into three stages: front, middle and back. The acoustic emission signal of backfill increases in the early and middle stages, and decreases in the later stages. The accumulated AE energy of the filling body is significantly lower than that of the gangue loose body, indicating that the structure of the sample after grouting is more stable and better load bearing performance is obtained.

  5. (5)

    The creep compaction test shows that the loading process of gangue bulk and backfill before and after grouting is divided into stress increasing stage and stress stabilizing stage, in which the axial strain of gangue bulk and backfill rapidly occurs in the stress increasing stage, but the axial strain changes gently when the backfill experiences the stress stabilizing stage. It shows that the backfill has better long-term stability and load-bearing performance after grouting.