1 Introduction

Coal is a vital resource supporting China's economic development, with billions of tons of coal exploited from underground every year (Zhu et al. 2017; Guo et al. 2019a). Large-scale longwall mining of underground coal resources inevitably causes severe damage such as surface subsidence, water loss, infrastructure damage, and others (Palchik 2020; Zhang et al. 2021a; Gao et al. 2021). In addition, the large number of waste tailings such as gangue and fly ash generated by coal mining and power generation occupy land resources and cause severe damage to the ecological environment (Wu et al. 2020). Reducing mining-induced damage and recycling waste tailings is an inevitable choice for promoting clean production in mining areas (Helinski et al. 2011; Yin et al. 2020).

Cemented paste backfilling (CPB) has attracted considerable interest due to its high safety, environmental friendliness, and recyclability of waste tailings (Nasir and Fall 2010; Qi et al. 2018; Zhao et al. 2020). It uses a paste material composed of mill tailings (with proportion around 75–85%), binder (with proportion around 3–7%), and water to replace the ore bodies (Yilmaz et al. 2009; Sun et al. 2018; Fang and Fall 2019; Zhao et al. 2022b). Usually, paste material produced on the surface is transported to the underground goaf by pumping and/or gravity. With the support of backfilling structure, the ore body can be mined safely. Compared to hydraulic and rock backfilling, the CPB has obvious advantages, including the recovery and utilization of mining by-products such as tailings, effective control of ground deformation and failure, better working environment, higher resource recovery, etc. (Benzaazoua et al. 2008; Yilmaz 2011, 2018; Wang et al. 2021). Currently, the CPB is widely applied in the exploitation of remaining coal resources beneath existing surface structures and water bodies and has become a crucial technology for clean and sustainable production of resource depletion mines (Yin et al. 2020; Zhao et al. 2022a; Sari et al. 2023).

The key to recovering remaining coal resources by CPB is to reduce disturbance and damage to strata, so the load-bearing capacity and support functions of the backfilling body are crucial. Research on the mechanical properties and stability of the backfilling body has found that fineness of tailings, cement content, curing age and temperature, water content and filling interval time affect the compressive strength of CPB (Nasir and Fall 2010; Qi et al. 2018; Wang et al. 2019b; Qiu et al. 2020). Cement content and curing time of the backfill slurry also have a significant effect on the mechanical characteristics and failure mode of the backfilling body (Raffaldi et al. 2019; Fang and Fall 2020; Wang and Li 2022).

The types of fill materials and their proportions also significantly impact the strength and performance of the backfilling body (Qiu et al. 2020; Wu et al. 2020; Cavusoglu et al. 2021; Hong et al. 2023; Sari et al. 2023). Many studies have analyzed the effects on setting time, strength and bearing stability of different combinations of tailings type (e.g., red mud, fly ash, gangue, construction demolition waste) and cement type (e.g., Portland cement, sodium silicate, arsenic-containing biohydrometallurgy waste, sulphoaluminate cement), providing a theoretical basis for the selection of filling materials (Liu et al. 2019; Wang et al. 2019c; Chen et al. 2020; Jafari and Grabinsky 2021; Zhao et al. 2022a).

The effect of ground control is the most direct performance to measure the success of backfill mining, which mainly refers to the control of movement, deformation, and damage of surrounding rock, floor strata, and overlying strata caused by coal seam extraction (Peng 2015). It is closely related to the time matching of backfilling and mining, backfilled ratio, the backfilling body strength, and the backfilling process (Luan et al. 2017; Deng et al. 2017; Qi et al. 2018; Du et al. 2022). The CPB can be roughly categorized into partial backfilling and complete backfilling. Partial backfilling mainly includes strip and pier-column backfilling, which usually reduces surrounding rock deformation by about 85% compared to longwall mining (Kostecki and Spearing 2015; Zhu et al. 2017; Wang et al. 2019a; Chang et al. 2021). However, the process of partial backfilling is complex, and the bearing capacity, stability, and ground control of the backfill body are worse than that of complete backfilling. Therefore, complete backfilling is more widely used (Fall and Nasir 2010; Liu et al. 2022; Yilmaz 2011; Zhang et al. 2021b). And the effects of backfilling body’s strength, backfilled ratio, and mining parameters (mining height, mining depth, etc.) on the deformation of surrounding rock, movement of overlying strata, and surface subsidence are discussed (Lv et al. 2022; Wen et al. 2022; Zhu et al. 2022).

Combined with the research on mechanical properties, material ratio, stability, and ground control of the backfilling body by many scholars, using CPB underground can improve the recovery rate of resources and achieve the coordinated development of resource mining and environmental protection. However, CPB mining will inevitably lead to a certain degree of strata deformation. Initially, the surrounding rock will be deformed due to many factors, such as the strength and compression of the backfilling body, the support stress of the backfilling shield, the backfilled ratio, and others. Second, after the backfilling shields are moved forward, the large roof area is in a state of no support when the backfilling body does not form the support strength, and the roof will subside, even break, and then spread to the surface. This seriously impacts the CPB mining of coal seams under villages, roads, and other buildings and above-confined aquifers. In order to ensure the strength and stability of backfilling body, improve the backfilled ratio and the effectiveness of ground control, it is necessary to optimize the backfilling process, which is crucial to safely and efficiently mining the remaining coal resources.

In this study, an innovative L-shaped interval cemented paste backfilling (LCPB) technology is proposed. The technical principle of the developed technology is outlined, and theoretical analyses and engineering applications are performed to explore the critical parameters and ground control characteristics of the LCPB mining technology. The characteristics and technical advantages are discussed, solving traditional CPB deficiencies.

2 Geological and mining conditions

Zhucun coal mine is located north of the Jiaozuo coalfield in Henan Province, China, as shown in Fig. 1. The coal seam mined in the mine is coal seam 1−5 of the Carboniferous Taiyuan Formation. Decades of mining have been exhausting the No. 1−5 coal seam. Mining the remaining coal resources of the coal seam with complex geological conditions has become an inevitable choice for the mine's sustainable development.

Fig. 1
figure 1

Location of Zhucun coal mine and layout of LCPB mining panel

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The panel of 54,002 of the No. 1−5 coal seam is used as a representative site in this study. The strike and dip length of the panel are 865 m and 108 m, respectively. The average coal seam depth and thickness of the panel are 200.2 m and 1.5 m, respectively. The dip angle of the coal seam in the panel is 5°. Based on the fault distribution revealed during roadway excavation, the panel is estimated to contain three groups of fault fracture zones. The panel's surface is situated near the industrial square, and the main buildings are breeding farms, family areas, and machinery plants, which seriously affect panel mining. In addition, an aquifer of limestone A2 at the panel's floor has a thickness of 6.7 m and a distance of 18.5 m from the coal seam, which poses a severe threat to panel mining. As a result of fault fracture, water pressure, and ground pressure, large water inrush accidents can occur. The mining data of other panels indicate that the maximum water inrush from the floor can reach 10 m3/min.

To sum up, the geological and mining conditions of the panel of 54,002 are complex. The mining is subject to multiple constraints of fault structures, surface buildings, and floor-confined aquifers and does not meet the conditions for longwall mining. Therefore, LCPB mining is selected to recover the coal resources of the panel. Figure 2 shows the partial geological column and lithology parameters of strata.

Fig. 2
figure 2

The partial geological column and lithology parameters of strata

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3 The LCPB mining technology

3.1 Technical principle

There are two kinds of traditional CPB: roadway retained backfilling and roadway non-retained backfilling. Generally, the roadway retained backfilling is widely used, and its process is:

  1. (1)

    Set filling zone. Three barrier walls are set in the air-inlet roadway, air-outlet roadway, and working face to form a closed space with the previous backfilling body.

  2. (2)

    Backfilling operation. The paste is filled into the closed space through the #1–#n filling holes set along the barrier wall in the working face.

  3. (3)

    Remove the barrier wall and retain roadways. After the backfilling body reaches the design strength, the barrier wall is removed, the goaf is filled and the roadways are retained. The retained roadway can be used for the mining of adjacent panels, reducing the tunneling cost and relieving the tension between mining and roadway excavation.

The LCPB is an innovative filling technology proposed by improving the filling process of the traditional roadway retained CPB, as shown in Fig. 3. The difference between LCPB and traditional CPB is that:

  1. (1)

    Filling method. The overall one-time filling of the goaf is modified to interval multiple filling.

  2. (2)

    Set partition zones and L-shaped filling zones. In order to perform interval multiple filling of the goaf, several certain width partition zones and filling zones are set up along the dip within the current backfill step of the goaf. The filling zones of the current backfill step (the vertical part of the L-shaped) are combined with the partition zones of the previous backfill step (the horizontal part of the L-shaped) to form several L-shaped filling zones.

  3. (3)

    Roof support during the backfilling period. Hydraulic props are set in the partition zone to support the roof before the L-shaped backfilling body reaches the design strength.

  4. (4)

    Backfilling operation. Fill the L-shaped filling zone in sequence, forming several L-shaped backfilling bodies to support the roof strata. As the advance of the working face, the L-shaped filling zone and partition zone are sequentially installed to complete the goaf filling and retain the roadway.

Fig. 3
figure 3

Schematic of the LCPB process. Note: Ls is the backfill step distance; Wb is the width of the L-shaped filling zone (L-shaped backfilling body); Wp is the width of the partition zone; 1-1, …, 1-4 are the interval filling sequence within the first backfilling step; 6-1, …, 6-4 are the interval filling sequence within the sixt backfilling step

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3.2 Theoretical analysis of critical technical parameters

3.2.1 Mechanical model of L-shaped backfilling body

The coal pillar bears the overburden load when the coal seam is not filled. After filling, the overburden load is transferred to the backfilling body. The backfilling body's bearing capacity mainly depends on geological factors such as the number and thickness of strata and physical and mechanical parameters such as the elastic modulus of each layer and the backfilling body's strength. During the LCPB mining, the backfilling body mechanical model is developed along the dip of the panel, as illustrated in Fig. 4, with the following assumptions: (1) The overburden load on the backfilling body is uniformly distributed; (2) The backfilling body is a homogeneous continuum and in a stable state; (3) The study is conducted under the plane stress condition.

Fig. 4
figure 4

Mechanical model of the L-shaped backfilling body. Note: Wb is the width of the L-shaped filling zone (L-shaped backfilling body); Wp is the width of the partition zone; P is the support stress of backfilling body; σb is the allowable strength of backfilling body; qb is the uniform load on the backfilling body; hi is the thickness of stratum i

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During normal mining, the backfilling body size is generally regular. Based on the effective area theory, each backfilling body has equal strength and bears the same load. Therefore, it can be assumed that the overburden loads above the partition zone with width Wp are shifted to the backfilling body with width Wb during the LCPB mining. Consequently, the support stress P of the L-shaped backfilling body can be determined using Eq. (1):

$$P = q_{{\text{b}}} \left( {1 + \frac{{W_{{\text{p}}} }}{{W_{{\text{b}}} }}} \right)$$
(1)

where (Guo et al. 2019b)

$$q_{{\text{b}}} = \frac{{E_{1} h_{1}^{3} \left( {\gamma_{1} h_{1} + \gamma_{2} h_{2} + \cdots + \gamma_{i} h_{i} } \right)}}{{E_{1} h_{1}^{3} + E_{2} h_{2}^{3} + \cdots + E_{i} h_{i}^{3} }}$$
(2)

where Wb is the width of the L-shaped filling zone; Wp is the width of the partition zone; qb is the uniform load on the backfilling body; Ei is the elastic modulus of stratum i above the coal seam; hi is the thickness of stratum i; γi is the bulk density of stratum i; i is the number of overlying stratum from the immediate roof of the coal seam to the surface.

Since the fracture and looseness of the backfilling body edge will reduce the bearing area, it is necessary to consider a certain safety factor k, ranging from 1.5 to 2.0. Hence, the support stress P of the backfilling body can be expressed as:

$$P = kq_{{\text{b}}} \left( {1 + \frac{{W_{{\text{p}}} }}{{W_{{\text{b}}} }}} \right)$$
(3)

3.2.2 Strength of backfilling body

Theoretically, the stability calculation of an L-shaped backfilling body is similar to that of a coal pillar in strip mining. Generally, the coal's joints and fissures are developed, and the integrity is poor. The deformation and failure under stress are typical of brittle materials. The paste backfilling material is compact, with good integrity and no joint fissures. The deformation and failure under stress are typical plastic brittle materials. The paste backfill’s plastic characteristics contribute to the backfilling body’s stability. Hence, based on the Bieniawski formula (Putri 2020) for calculating coal pillar stability, the allowable strength σb of the backfilling body under a uniaxial stress state can be described as follows:

$$\sigma_{{\text{b}}} = \frac{{kq_{{\text{b}}} \left( {1 + {{{\text{W}}_{{\text{p}}} } \mathord{\left/ {\vphantom {{{\text{W}}_{{\text{p}}} } {{\text{W}}_{{\text{b}}} }}} \right. \kern-0pt} {{\text{W}}_{{\text{b}}} }}} \right)}}{{\sqrt {D/0.9} \left( {0.64 + 0.36L_{{\text{s}}} /m} \right)^{1.4} }}$$
(4)

where D is the diameter of the test piece; Ls is the backfill step distance; m is the mining height of the panel.

The allowable strength of backfilling body can be derived using Eq. (4), and in practical application, the strength of backfilling body should be greater than this value.

3.2.3 Width of L-shaped filling zone

A reasonable size design guarantees the L-shaped backfilling body's stability, which is vital for the safe recovery of coal resources and control of surrounding rock. Based on the theory of elastic core zone and yield zone of coal pillar proposed by British scholar A. H. Wilson, using Wilson’s empirical formula, the yield zone of L-shaped backfilling body is set as Y, so the minimum width of L-shaped backfilling body (L-shaped filling zone) Wb in the LCPB mining should meet Wb > 2Y, as shown in Fig. 5a. The value of Y is determined as follows (Wilson 1983):

$$Y = 0.00492\,{\text{mH}}$$
(5)

then

$$W_{{\text{b}}} > 0.00984\,{\text{mH}}$$
(6)

where H is the mining depth of the coal seam.

Fig. 5
figure 5

L-shaped interval cemented paste backfilling: a L-shaped filling zone and partition zone; b Mechanical model of roof key stratum; and c Mechanical model of floor key stratum

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The load on the L-shaped backfilling body is P, and the ultimate strength of the backfilling body is σu. Based on the ultimate strength theory, to ensure the stability of the backfilling body, the safety factor S should not be less than 1.5 times, i.e., satisfy the following requirements:

$$S = \frac{{\sigma_{{\text{u}}} }}{P} \ge 1.5$$
(7)

Therefore, the width of the L-shaped filling zone Wb can be expressed as follows:

$$W_{{\text{b}}} \ge \frac{{1.5kq_{{\text{b}}} W_{{\text{p}}} }}{{\sigma_{{\text{u}}} - 1.5kq_{{\text{b}}} }}$$
(8)

The width of the L-shaped filling zone Wb in the LCPB mining can be determined from Eqs. (6) and (8). During backfill mining, the width Wb can be appropriately increased or reduced depending on the surrounding rock deformation and the backfilling body's stress. When affected by surface buildings and floor-confined aquifers, the width of the L-shaped filling zone should be raised as much as possible to reduce the width of the partition zone.

3.2.4 Width of partition zone

Based on the design principle of backfilling body, to prevent the failure of surrounding rock, it must be ensured that the width of partition zone Wp (as shown in Fig. 5a) is less than the critical breaking span of the key stratum in the roof and floor (Ju and Xu 2015; Li et al. 2018).

  1. (1)

    Critical breaking span of the roof key stratum (Lr)

The mechanical model of the roof key stratum above the partition zone is shown in Fig. 5b. The exact solution of an elastic rectangular thin plate clamped around under a uniform load is provided. However, the exact solution is very complex for mining engineering applications, and more importantly, the bending moment is an implicit function. Therefore, the modified solution of Marcus is employed to obtain the maximum bending moment Mx of the roof key stratum (elastic rectangular thin plate) under the boundary condition of fixed supports around the plate (Qian et al. 2010):

$$M_{x} = q_{{\text{r}}} L_{{\text{r}}}^{2} \frac{{\left( {1 - \mu^{2} } \right)\left( {1 + \mu \lambda^{2} } \right)}}{{12\left( {1 + \lambda^{4} } \right)}}$$
(9)

where qr is the load on the roof key stratum; Lr is the critical breaking span of the roof key stratum; μ is Poisson's ratio of the key stratum; λ is the geometric shape coefficient of goaf, λ = Lr/Ls, Ls is the backfill step distance.

The relationship between the bending moment Mx and stress σx of the roof key stratum is:

$$\sigma_{x} = z\frac{{12M_{x} }}{{h_{{\text{r}}}^{3} }}$$
(10)

where hr is the thickness of the roof key stratum; z is a variable with a value range of − hr/2 to + hr/2.

The maximum stress of the roof key stratum occurs at the upper and lower interfaces, i.e., z =  ± hr/2, so it can be derived that:

$$\left( {\sigma_{x} } \right)_{{z = \pm \frac{{h_{{\text{r}}} }}{2}}} = \pm \frac{{6M_{x} }}{{h_{{\text{r}}}^{2} }}$$
(11)

where the positive value is the tensile stress that occurs at the lower interface of the stratum; the negative value is compressive stress that occurs at the upper interface of the stratum.

Then, the critical breaking span of the roof key stratum Lr can be expressed as follows:

$$L_{{\text{r}}} = h_{{\text{r}}} \sqrt {\frac{{2\sigma_{{\text{r}}} \left( {1 + \lambda^{4} } \right)}}{{q_{{\text{r}}} \left( {1 - \mu^{2} } \right)\left( {1 + \mu \lambda^{2} } \right)}}}$$
(12)

where σr is the critical tensile strength of the roof key stratum, which takes the tensile stress value at the lower interface of the stratum.

Considering the influence of the backfill step distance Ls on the breaking span, small variables μλ2 are ignored. Under the condition of four-side fixed support, the critical breaking span of the roof key stratum Lr is determined as follows:

$$L_{{\text{r}}} = \left\{ {\begin{array}{*{20}c} {L_{{\text{s}}} \sqrt[4]{{\frac{{L_{m}^{2} }}{{L_{{\text{s}}}^{2} - L_{m}^{2} }}}}} & {\left( {L_{m} < L_{{\text{s}}} < \sqrt 2 L_{m} } \right)} \\ {\frac{{\sqrt 2 L_{{\text{s}}} }}{{2L_{m} }}\sqrt {L_{{\text{s}}}^{2} - \sqrt {L_{{\text{s}}}^{4} - 4L_{m}^{4} } } } & {\left( {L_{{\text{s}}} \ge \sqrt 2 L_{m} } \right)} \\ \end{array} } \right.$$
(13)

where Lm is the criterion number of breaking span, \(L_{m} = h_{{\text{r}}} \sqrt {\frac{{2\sigma_{{\text{r}}} }}{{q_{{\text{r}}} \left( {1 - \mu^{2} } \right)}}}\).

  1. (2)

    Critical breaking span of the floor key stratum (Lf)

The mechanical model of the floor key stratum below the partition zone is shown in Fig. 5c. Without a fault structure, water inrush from the floor is essentially the failure of the key floor stratum. The engineering simplified algorithm of Marcus is still adopted, and the critical breaking span of the floor key stratum Lf can be described as follows (Li et al. 1995):

$$L_{{\text{f}}} = h_{{\text{f}}} \sqrt {\frac{{2\sigma_{{\text{f}}} }}{{q_{{\text{f}}} }}} \sqrt {\frac{1}{12f(\phi )}}$$
(14)

where f(ϕ) is the maximum principal moment coefficient of the plate; σf is the ultimate tensile strength of the floor key stratum; hf is the thickness of the floor key stratum; qf is the load on the floor key stratum, qf = qw–qa, and qw is the water pressure of aquifer, and qa is the load concentration of aquiclude.

Set \(l = h_{{\text{f}}} \sqrt {2\sigma_{{\text{f}}} } ,\quad W\left( \phi \right) = \sqrt {\frac{1}{12f\left( \phi \right)}} ,\quad Q = \sqrt {\frac{1}{{q_{{\text{w}}} - q_{{\text{a}}} }}}\), then

$$L_{{\text{f}}} = l \cdot Q \cdot W\left( \phi \right)$$
(15)

where l is the breaking span criterion number of floor key stratum under unit load; W(ϕ) is the influence coefficient; Q is the dimensionless correction coefficient of the criterion number l.

Therefore, the critical breaking span of the floor key stratum Lf affected by water pressure expressed by span criterion number l and its correction factor Q can be determined as follows:

$$L_{{\text{f}}} = \left\{ {\begin{array}{*{20}c} {L_{{\text{s}}} \sqrt {\frac{{ - \mu L_{{\text{s}}}^{2} + \sqrt {\mu^{2} L_{{\text{s}}}^{4} + 4\left( {L_{{\text{s}}} Ql} \right)^{2} - 4\left( {Ql} \right)^{4} } }}{{2\left( {L_{{\text{s}}}^{2} - l^{2} Q^{2} } \right)}}} } & {\left( {L_{{\text{s}}} < \sqrt {2\left( {\sqrt 2 - \mu } \right)} Ql} \right)} \\ {lQL_{{\text{s}}} \sqrt {\frac{{\left( {2\mu^{2} + 1} \right)L_{{\text{s}}}^{2} - 2\mu L_{{\text{s}}}^{2} Q^{2} - \sqrt {L_{{\text{s}}}^{4} + 4\mu \left( {L_{{\text{s}}} Ql} \right)^{2} - 4\left( {Ql} \right)^{4} } }}{{\left( {\mu^{2} L_{{\text{s}}}^{2} - l^{2} Q^{2} } \right)\left( {\mu L_{{\text{s}}}^{2} - 2l^{2} Q^{2} + \sqrt {L_{{\text{s}}}^{4} + 4\mu \left( {L_{{\text{s}}} Ql} \right)^{2} - 4\left( {Ql} \right)^{4} } } \right)}}} } & {\left( {L_{{\text{s}}} \ge \sqrt {2\left( {\sqrt 2 - \mu } \right)} Ql} \right)} \\ \end{array} } \right.$$
(16)

Therefore, the critical breaking span of the key stratum in the roof and floor can be obtained from Eqs. (13) and (16), respectively, and the width of partition zone Wp should be less than the smaller value of the two.

3.2.5 Backfilled ratio

The research indicated that the compressibility of backfilling body, roof subsidence before backfilling, and the defective distance of the roof-contact of the backfilling body are the main factors for controlling the surrounding rock deformation and surface subsidence of backfill mining. In order to facilitate analysis, these factors are transformed uniformly into backfilled ratio, which is the crucial index reflecting the filling effect. The higher the backfilled ratio, the larger the space occupied by the filling materials and the more influential the control of surrounding rock and surface subsidence. However, the higher the backfilled ratio is, the greater the corresponding filling material consumption, workload, and cost are. Therefore, the reasonable selection of backfilled ratio is crucial. Based on the definition of limited coal height mining (Sun et al. 2017), the backfilled ratio η can be expressed as follows:

$$\eta \ge 1 - \frac{\varepsilon H}{{1.52m\delta \varphi \tan \beta \cos \alpha }}$$
(17)

where ε is the maximum allowable horizontal deformation value of the ground surface at the building site; δ is the displacement factor; φ is the subsidence factor; tanβ is the tangent of the primary influence angle; α is the dip angle of the coal seam.

4 Application results of ground control

4.1 The LCPB implementation

The system of LCPB is constructed in the Zhucun coal mine, and the LCPB mining technology is applied in the panel of 54,002 according to the following procedure, as shown in Fig. 6.

Fig. 6
figure 6

The procedure of LCPB mining in engineering application

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Based on the lithology parameters of strata in Fig. 2, the relevant parameters of LCPB mining were calculated using the above theoretical formula, and the parameter values in field application are listed in Table 1.

Table 1 Parameter values of the LCPB mining in field application
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The on-site application of the LCPB mining is shown in Fig. 7. During the LCPB mining, the backfilled ratio was around 92% based on the statistics of the amount of coal extracted and the amount of filling paste. And the effectiveness of ground control at various strata levels (surrounding rock level, floor strata level, and overlying strata level) was monitored.

Fig. 7
figure 7

The LCPB mining in the panel of 54,002 of Zhucun coal mine: a The LCPB ground station; b The LCPB operating system; c Roadway; d Backfilling operation; e Partition zone; and f Retained roadway

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4.2 Ground control at surrounding rock level

Monitoring was conducted on the ground control effect of surrounding rock level by establishing observation stations. The established observation stations include: (1) Compression observation stations of backfilling body, A1–A4, with two observation points per station; (2) Stress observation stations of backfilling body, S1, and S2; (3) Support stress of hydraulic prop and roof-to-floor convergence observation stations in working face, B1–B3, with three observation points per station; (4) Roof-to-floor convergence observation stations in the air-inlet roadway, C1–C6, with two observation points at each station; (5) Roof-to-floor convergence observation stations in the air-outlet roadway, D1–D5, with two observation points at each station. The layout of the observation stations in the panel is depicted in Fig. 8.

Fig. 8
figure 8

Layout of observation stations

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4.2.1 Compression and stress of the backfilling body

In order to analyze the stability of the backfilling body and the ground pressure appearance law of the LCPB mining, monitoring devices are placed in the backfilling body to observe the compression and stress of the backfilling body, as demonstrated in Fig. 8.

The A1–A4 observation stations monitor the compression of the backfilling body, and each station contains two observation points. The A1 station is situated near the fault, while the rest stations are located in the intact roof section. By averaging the values of two observation points at A1–A4 stations, the compression of the backfilling body is shown in Fig. 9a. Stress monitoring devices are arranged on the S1 and S2 observation lines in the backfilling body. Based on the monitored data of the device, Fig. 9b reveals the obtained support stress of the backfilling body.

Fig. 9
figure 9

Backfilling body monitoring: a Compression; and b Support stress

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Figure 9a shows that when the roof stratum is intact, the compression of the backfilling body is minimal, with a maximum value of 5 mm, and about 80% of the compression is accomplished within 20 m after filling. The fault significantly influences the backfilling body's compression. The backfilling body's compression is largest near the fault, reaching 23 mm, 4.6 times that of the backfilling body in the intact roof section.

The compression characteristics of the backfilling body indicate that: (1) The LCPB mining can achieve a high backfilled ratio. When the roof subsidence is very small, the backfilling body can play a supporting role. (2) The integrity of the roof near the fault is poor. Affected by mining, the stress concentration occurs, resulting in intense roof activity and a large amount of compression of the backfilling body. Therefore, when geological structures such as faults are encountered during backfill mining, measures such as strengthening support, improving filling quality and roof-contact rate, and reducing backfill step distance must be implemented to ensure the stability and bearing capacity of the backfilling body.

Figure 9b shows that in the time range from the initial filling stage to 60 m when the working face is advanced, the support stress of the backfilling body increases linearly, reaching 3.6 MPa, which is 90% of the maximum support stress of the backfilling body. Subsequently, the support stress of backfilling body is gentle and stable, with stress of about 4 MPa. There is no sudden change in the entire process of the backfilling body's support stress, indicating no prominent ground pressure appearance in the LCPB mining.

4.2.2 Support stress of hydraulic props in working face

Support stress of the hydraulic prop of backfilling shield is the most direct reflection of the ground pressure appearance during LCPB mining. The adequate support stress of hydraulic prop can prevent roof strata from subsidence and maintain the integrity of the immediate roof. Figure 8 illustrates that three stations B1–B3 are placed in the working face, with three observation points at each station. The support stress of hydraulic props is observed during the mining. Averaging the observed stress values yielded the average support stress of hydraulic props at different distances from the coal wall, as shown in Fig. 10.

Fig. 10
figure 10

Support stress of hydraulic props in working face

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Figure 10 reveals that along the advancing direction (strike) of the working face, the support stress of the hydraulic prop generally decreases, obviously increases, slightly reduces, and finally tends to be stable. The change of hydraulic prop support stress is closely related to the LCPB mining process: (1) Mining causes loosening of surrounding rock, reducing the prop's initial support stress; (2) As the support area of the prop increases, the roof rock begins to deform, and the prop's support pressure increases; (3) Filling of goaf and bearing of backfilling body, reducing prop's support pressure; (4) The stiffness of the backfilling body and surrounding rock gradually equilibrium, the prop’s support stress tends to stabilize.

During the LCPB mining, there is no sudden increase of hydraulic prop’s support stress and no prominent ground pressure appearance. The hydraulic prop's support stress plays the role of active support, which can effectively prevent the subsidence and damage of the roof strata. However, it is necessary to increase the support stress of the hydraulic prop during the obvious stage of roof deformation and reduce the roof subsidence.

4.2.3 Roof-to-floor convergence of surrounding rock

  1. (1)

    Roof-to-floor convergence of working face

Figure 8 depicts the layout of the observation stations for the roof-to-floor convergence of the working face, whereas Fig. 11 illustrates the data of the observation points of each station.

Fig. 11
figure 11

Roof-to-floor convergence of working face: a Upper station; b Middle station; and c Lower station

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Figure 11 displays that: (1) The LCPB mining has a good control effect on the surrounding rock, and the maximum value of roof-to-floor convergence of working face before backfilling is 58 mm. The velocity of roof-to-floor convergence has undergone a process of small, large, and small, which is consistent with the change process of the support stress of hydraulic prop. (2) From the analysis of the overall data of the observation stations, along the dip of the panel, the roof-to-floor convergence of the upper station is the largest, the middle station takes the second place, and the lower station is the smallest. And the same law of roof-to-floor convergence is also reflected in the upper, middle, and lower observation points in the observation station.

In addition, the roof-to-floor convergence characteristics of the working face indicate: (1) With the distance between the observation point and the coal wall grows, the roof strata subside and deform affected by abutment pressure, and the surrounding rock is in a state of continuous movement. When the observation point is far away from the coal wall (close to the backfilling body), the velocity of roof-to-floor convergence decreases due to the backfilling body's support. (2) The influence of the dip angle of the panel leads to a high compactness and roof-contact rate of the lower backfilling body, resulting in the roof-to-floor convergence of the lower observation point/station is less than that of the upper. Therefore, strengthening the roof support during the mining process, and increasing the backfilling body's compactness and roof-contact rate can reduce roof-to-floor convergence.

  1. (2)

    Roof-to-floor convergence of roadway

During the mining, the air-inlet and air-outlet roadways are entirely retained by the LCPB mining. Observation stations are positioned in roadways to monitor the roof-to-floor convergence, as shown in Fig. 8. Observation stations C1-C6 are spaced 25 m apart along the air-inlet roadway. Observation stations D1-D5 are installed in the air-outlet roadway with a spacing of 25 m. Each station includes two observation points. The average of the observed data of two observation points of each station are obtained, and the roof-to-floor convergence and its velocity are determined, as shown in Figs. 12 and 13.

Fig. 12
figure 12

Air-inlet roadway observation station: a Roof-to-floor convergence; and b Roof-to-floor convergence velocity

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Fig. 13
figure 13

Air-outlet roadway observation station: a Roof-to-floor convergence; and b Roof-to-floor convergence velocity

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Figures 12 and 13 show that the roof-to-floor convergence of the roadway has the following characteristics:

  1. (1)

    The surrounding rock in the range of more than 20 m in front of the working face is stable. Within the range of 5–20 m in front of the working face, the roof-to-floor convergence gradually increase affected by the abutment pressure. Starting from 5 m in front of the working face, the roof-to-floor convergence and its velocity increase significantly, and the velocity reaches the peak at about 10 m behind the working face. The maximum convergence velocity of the air-inlet and the air-outlet roadway is about 32 and 38.5 mm/d, respectively. The velocity of roof-to-floor convergence gradually decreases from 20 m behind the working face. Until about 60 m behind the working face, the roof-to-floor convergence tends to be stable.

  2. (2)

    The abutment pressure affects the roadway with fast convergence velocity within 10 m in front and 15 m behind the working face. The roof-to-floor convergence within this range accounts for 80% of the total. The total roof-to-floor convergence of the air-inlet roadway is 208–239 mm and that of the air-outlet roadway is 234–259 mm.

The roof-to-floor convergence characteristics of the roadway indicate that:

  1. (1)

    Affected by the abutment pressure, the roof strata subsidence sharply before being balanced. Subsequently, the backfilling body has high stiffness and strength after solidification, which can bear part of the load of the roof strata, thereby reducing the load on the roadway so that the roadway deformation gradually decreases and tends to be stable.

  2. (2)

    The low compressibility of the backfilling body (less than 1%) reduces the free space of the roof after filling, and the roof-to-floor convergence drops significantly.

  3. (3)

    The main period of roof-to-floor convergence occurs from the beginning of mining to the time when the backfilling body reaches the support strength. Adequate roof support during this period can significantly control the deformation of surrounding rock. Affected by the dip angle, the compactness and roof-contact rate of the lower backfilling body of the working face are higher than those of the upper, as is its supporting effect on the roof. Therefore, the key measures to control roadway deformation are strengthening roadway support and improving the backfill body's compactness and roof-contact rate.

In conclusion, at the surrounding rock level, the total compression of the backfilling body in LCPB mining is very small (about 5 mm in intact roof section), indicating that LCPB mining has a high backfilled ratio and the backfilling body can play a supporting role when the roof subsidence is very small. This resulted in no prominent ground pressure appearance in the LCPB mining. The support stress of the hydraulic prop and velocity of roof-to-floor convergence of the working face has undergone a process of small, large, and small in LCPB mining. The maximum value of roof-to-floor convergence of the working face and roadway is 58 mm and 259 mm, respectively. Roadways of the panel have been effectively preserved.

4.3 Ground control at floor strata level

Coal seam mining can cause damage to the floor strata, and the depth of the floor damage directly affects the secure mining of coal seams above confined aquifers. The method of single borehole constant water pressure (0.15–0.2 MPa) injects water into the floor borehole in the air-inlet roadway. The amount of water injection can represent the fracture development and connectivity of the floor rock stratum. This approach has the advantages of simple installation, less engineering work, and accurate results.

  1. (1)

    Borehole parameters

There are eight test boreholes scattered within 15 m, as shown in Fig. 14. Borehole #8 is set as the test boreholes center; the left side is negative, and the right side is positive. Boreholes #1 through #5 are in the intact floor area, while boreholes #6 through #8 are near the fault. Borehole parameters are listed in Table 2.

Fig. 14
figure 14

Layout of water injection boreholes

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Table 2 Technical parameters of water injection boreholes
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  1. (2)

    Test data

Based on the measured data, the borehole water injection/gushing curve is obtained, as illustrated in Fig. 15.

Fig. 15
figure 15

Water injection/gushing of boreholes: a #1 and #3; b #2, #4 and #5; c #6 and #7; and d #8

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Figure 15a, b shows the water injection into boreholes #1 through #5. When the borehole is 50 m away from the working face, the water injection per minute of the borehole is less than 0.2 L, and the water in the borehole remains for a long time, indicating that the primary fractures of the rock stratum are not developed and are unaffected by the LCPB mining. As the advance of the working face, the water injection of boreholes #1, #3–#5 is not significantly changed, while the water injection of borehole #2 is considerably altered and experiences a process of stable, sharp increase, and attenuation. When the working face is 5 m (before mining) away from borehole #2, the water injection volume increases 23 times that of the stationary period. Then, as the working face gradually recedes, the water injection volume of borehole #2 gradually decreases, indicating that the floor stratum damage depth reaches 5 m.

Figure 15c, d shows the water injection/gushing of boreholes #6–#8. Boreholes #6 through #8 are affected by faults, and their water injection/gushing amount varies greatly. The water injection/gushing change of borehole #6 is the most obvious. Under the water injection pressure of 0.15–0.2 MPa, the borehole first gushes water and then injects water. When it is far from the working face, water gushes outward from the borehole. When the mining face exceeds 2.5 m of the borehole, water is injected into the borehole. When the mining face exceeds 18 m of the borehole, the water injection volume reaches the maximum and then decreases gradually as the advance of the working face. This indicates that the floor stratum damage depth near the fault reaches 10 m.

The quartz sandstone at the bottom of borehole #7 is constantly saturated with water, and there is a small amount of water gushing when the valve is opened, which is related to the rich water content of the quartz sandstone. As there is still some distance between the borehole and limestone A2, the borehole's water gushing and water pressure are small, indicating that the floor strata damage depth near the fault does not exceed 12 m and the LCPB mining does not activate the fault.

Borehole #8 is the nearest to limestone A2. The maximum water gushing of this borehole is 7.38 m3/h, and the maximum water gushing pressure is 0.4 MPa, similar to the water pressure of limestone A2. The water gushing from the borehole increases as the advance of the working face. However, the water gushing suddenly decreases when the mining face exceeds 14 m of the borehole. Once the mining face exceeds 20 m of the borehole, the water gushing drops to 1.6 m3/h, and the water pressure is 0.15 MPa. The coal seam floor is 18.5 m away from limestone A2. Borehole #8, with a vertical depth of 14 m, has water gushing, while borehole #7, with a vertical depth of 12 m, has no large water gushing, indicating that the secondary lift height of limestone A2 confined water is about 5 m.

To sum up, at the floor strata level, the maximum damage depth when the floor is intact is less than 4 m of the LCPB mining, the maximum damage depth of the floor at the fault affected area is 10–12 m and the secondary lift height of confined water near the fault is about 5 m. The LCPB mining can realize secure mining above the confined aquifer, but drainage depressurization measures still need to be taken when mining in the area where the distance between the coal seam and the confined aquifer is small or near the fault.

4.4 Ground control at overlying strata level

The state of overlying strata affected by LCPB mining can be reflected by the characteristics of surface movement and deformation. Therefore, surface movement and deformation have been monitored and analyzed.

  1. (1)

    Observation of surface movement and deformation

The observation station of surface movement and deformation is arranged on the panel's surface, as shown in Fig. 16. The Z1–Z25 observation points are arranged along the strike, with a spacing of 20 m; The Q1-Q19 observation points are arranged along the dip, with a spacing of 20 m.

Fig. 16
figure 16

Observation station layout of surface movement and deformation

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During the LCPB mining of the panel, the strike observation station was measured 25 times, and the dip observation station was measured 17 times. The subsidence curve of the surface observation point is shown in Fig. 17.

Fig. 17
figure 17

The subsidence curve of the surface observation point: a Along the strike; and b Along the dip

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Figure 17 shows that after the surface deformation of the LCPB mining is stable, the maximum surface subsidence value of the strike observation point is 128 mm, and the surface subsidence is characterized by basin, which is critical mining. The maximum surface subsidence value of the dip observation point is 128 mm, and the surface subsidence is funnel-shaped, which is subcritical mining.

The surface inclination and curvature along strike and dip are shown in Fig. 18.

Fig. 18
figure 18

Surface inclination and curvature: a Along the strike; and b Along the dip

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Figure 18 shows that once the surface deformation of the LCPB mining is stable, the maximum values (absolute values) of inclination and curvature along the strike are 0.950 mm/m and 0.018 mm/m2, respectively, and that along the dip are 1.750 mm/m and 0.060 mm/m2, respectively.

  1. (2)

    The parameters of surface movement and deformation

The surface movement and deformation parameters mainly include the angle of draw, the angle of critical deformation, the angle of the outmost crack and the subsidence factor, which can be calculated on the major cross-section of the surface subsidence basin when the mining size of the panel reaches a critical state. They are the primary evaluation indicators for analyzing the impact of coal seam mining on the surface and protecting surface buildings. The strike direction of the panel is critical mining state, so the surface movement and deformation parameters of the LCPB mining are as follows:

  1. (1)

    The angle of draw. The angle between the line connecting the edge on the major cross-section of subsidence basin (point subsidence 10 mm) and the edge of the panel and the vertical line at the edge of the panel is the angle of draw. The place where the surface subsidence is 10 mm is located between observation points Z5 and Z6. Taking observation point Z5 for calculation, the average mining depth of the panel is about 200.2 m. Therefore, the obtained draw angle along the strike is about 11.3° of the LCPB mining.

  2. (2)

    The angle of critical deformation. The angle between the line connecting the critical deformation point on the major cross-section of subsidence basin and the edge of the panel and the vertical line at the edge of the panel is the angle of critical deformation. The critical deformation refers to the surface inclination value of 3 mm/m or the surface curvature of 0.2 mm/m2. Since the parameter values obtained in the observation are less than the above indicators, there is no angle of critical deformation parameter of the LCPB mining.

  3. (3)

    The angle of the outmost crack. The angle between the line connecting the point of the outermost crack on the major cross-section of subsidence basin and the edge of the panel and the vertical line at the edge of the panel is the angle of outmost crack. Through observation on the surface, there is no crack, so there is no angle of the outmost crack parameter of the LCPB mining.

  4. (4)

    The subsidence factor. The ratio of maximum subsidence on the major cross-section of subsidence basin to mining height is called the subsidence factor, which applies only to cases involving horizontal or near horizontal coal seams. The dip angle of the coal seam in the panel is 5°, and it is a nearly horizontal coal seam with a mining thickness of 1.5 m. The maximum surface subsidence is 128 mm. Therefore, the subsidence factor is 0.085 for LCPB mining.

In conclusion, at the overlying strata level, the maximum inclination and curvature of the surface are 1.75 mm/m and 0.06 mm/m2, respectively. The calculated draw angle is 11.3°, the subsidence factor is 0.085, and there are no parameter values of the angle of critical deformation and the angle of the outmost crack. The ground surface deformation of the panel is reduced to be less than that allowed in the first level of the building damage (the inclination and curvature are 3 mm/m and 0.2 mm/m2, respectively). Therefore, LCPB mining effectively controls surface subsidence and deformation and can safely recover coal resources beneath existing surface buildings.

5 Discussion

One of the characteristics of traditional CPB mining is the overall one-time filling of the goaf, which can achieve good ground control effect when geological conditions are suitable. However, the geological conditions of underground mining vary significantly, and the ground control effect of traditional CPB mining is poor when filling areas with faults, folds and large strata dip angles. The other panel in the mine adopts traditional CPB mining, which has the following shortcomings during the backfilling mining process:

  1. (1)

    Filling leakage accidents occur frequently. The paste slurry flows during the filling process. When the working face has a large dip angle, many flowing fillings will exert significant pressure on the filling barrier wall at the lowest place, causing filling leakage accidents due to the pressure bearing or sealing problems of the barrier wall very likely.

  2. (2)

    Defective roof-contact of backfilling body. The setting time of the paste is short. When the filling range is large, the overall fluidity of the paste decreases as the filling time increases, resulting in defective roof contact of backfilling body. The situation is even more complicated when the panel is mined at a negative angle.

  3. (3)

    Poor adaptability to geological structures. The underground mining geological conditions are complex; faults, folds, and other geological structures occur frequently, and the overall filling adaptability is poor, resulting in a large amount of defective roof-contact. In addition, it is difficult to construct a barrier wall in the geological structure section; the sealing tightness is poor, and filling leakage accidents are easy to occur.

  4. (4)

    Support equipment is unrecyclable. During filling, the exposed area of the roof is large. In order to prevent the roof from sinking excessively or collapsing, support measures must be implemented in the area to be filled. The support equipment and materials cannot be recycled.

  5. (5)

    Many derivative issues. The defective roof-contact of the backfilling body results in substantial roof subsidence, surface subsidence, and ground pressure appearance. Mining and filling are complicated and inefficient when mining in geological structure sections. In addition, after the filling leakage accident, it is challenging to clean the semi-solidification and solidification filling paste, adjust the dislocated backfilling shields, and refill, which seriously affects mining.

Given the shortcomings of traditional CPB, it can only be applied under certain geological and mining conditions. Firstly, it requires simple occurrence conditions of strata without geological structures such as large faults and folds. Secondly, its application needs small gob filling ranges and dip angles of coal seams, which restrains the ability of the pressure caused by the filled slurry to damage barrier walls. Thirdly, when the traditional CPB is adopted, the panel should be mined at a positive angle as much as possible. If it is mined at a negative angle, the angle and filling range should not be too large; otherwise, a large defective roof-contact area may come into being. Finally, the traditional CPB could be used when immediate roof is thick and hard, because in this case, immediate roof will not be damaged or subsided significantly without support during filling period.

In order to solve the problems of traditional CPB mining, LCPB mining technology is proposed. The main characteristics of LCPB are embodied in the filling process and ground control. Regarding the filling process, the overall one-time filling is modified to interval multiple filling, with partition zone between backfilling bodies, which will be filled together during the next filling step, forming multiple L-shaped backfilling bodies to support the roof. Regarding ground control, the LCPB mining can achieve a high backfilled ratio, and hydraulic props can be installed in the partition zone to ensure that the roof can be effectively supported before the backfilling body reaches the support strength, thereby reducing the deformation and damage of surrounding rock and achieving better ground control effects. Compared to the traditional CPB, the LCPB has the following advantages:

  1. (1)

    Prevent filling leakage accidents. The goaf is divided into several L-shaped filling zones, and the entire one-time filling is also divided into multiple filling, reducing the lateral pressure of the backfilling paste on the barrier wall and effectively preventing the paste slurry leakage.

  2. (2)

    Strong adaptability to geological structures. Underground mining and backfilling frequently occur in areas with particular geological structures, such as faults and folds. The flexible layout of partition zones can facilitate the installation of the barrier wall, making the backfill mining more adaptable to geological structures. When mining with a negative dip angle, a filling can be carried out from a high place in the partition zone to improve filling efficiency.

  3. (3)

    Improve the backfilled ratio. The reduction of single filling volume enables the paste in the L-shaped filling zone to be filled with strong fluidity, effectively improving the roof-contact rate of the backfilling body. In addition, the flexible layout of partition zones can effectively prevent paste slurry leakage accidents and significantly enhance the roof-contact rate of backfilling body in geological structure sections.

  4. (4)

    High security and low cost. The roof support provided by hydraulic props in the partition zone ensures the safety of personnel and the backfilling operation. The hydraulic props in the partition zone can then be re-used after the backfilled body reaches its design strength to reduce the backfill mining cost.

  5. (5)

    Better ground control effects. By setting support equipment in the partition zone, roof subsidence can be effectively reduced, and roof fracture and damage can be prevented during backfilling and before the backfilling body reaches its support strength. In addition, after the backfilling body reaches the support strength, a higher backfilled ratio can also effectively prevent the deformation and damage of surrounding rock, achieving better ground control effects.

In conclusion, LCPB is adaptable to underground geological and mining conditions. Specifically, it can be applied to filling of coal seam mining areas with complex geological structures including faults and folds, and those with large or negative angles. It can also be used to fill large goafs, goafs in areas with weak immediate roof which prone to damage and subside, and areas with high requirements for ground control of coal seam mining.

6 Conclusions

  1. (1)

    The LCPB, an innovative filling method proposed to improve the filling process of the traditional CPB, supports coal seam roofs by filling several L-shaped backfilling bodies along the working face with partition zones with a certain width reserved between these bodies. Compared to the traditional CPB, it prevents filling leakage accidents, has a strong adaptability to geological structures, enhances the backfilled ratio, conduct ground control more efficiently, and is highly safe and cost-effective.

  2. (2)

    The mechanical model of the backfilling body was established. Critical parameters of the LCPB, including the strength of the backfilling body, the width of the L-shaped filling zone and the partition zone, and the backfilled ratio, were analyzed theoretically. Additionally, the parameters were determined under specific mining geological conditions, providing a theoretical and technical basis for the LCPB mining.

  3. (3)

    The field application of the LCPB in ground control was analyzed, and the following beneficial results were yielded:

    1. (1)

      At the surrounding rock level, the surrounding rock deforms insignificantly during the LCPB mining, and no prominent ground pressure appears. The maximum values of roof-to-floor convergence of the working face and roadways are 58 mm and 259 mm, respectively. The LCPB mining can effectively control the deformation of surrounding rock and achieve roadway retention.

    2. (2)

      At the floor strata level, the LCPB mining causes minimal damage to floor strata, with a depth of less than 4 m when the floor is intact, and a depth of 10–12 m when it is affected by faults. Meanwhile, the secondary lift height of confined water near faults is about 5 m. With the LCPB, secure mining can be realized above a confined aquifer when the distance between the coal seam and the confined aquifer meets the aforementioned requirements.

    3. (3)

      At the overlying strata level, the surface subsidence factor of the LCPB mining is 0.085, and the maximum inclination and curvature of the surface are 1.75 mm/m and 0.06 mm/m2, respectively. The calculated draw angle is 11.3°, and parameter values of the angles of critical deformation and outmost crack do not exist. The LCPB mining controls surface movement and deformation effectively, reducing surface deformation of the panel to less than that allowed in the first level of the building damage (inclination and curvature of 3 mm/m and 0.2 mm/m2, respectively).