The Influence of the Key Characteristics of Overburden Rock Structure on the Development Height of Water-Conducting Fracture in Yushenfu Coal Mine Area, China

Abstract

The destruction of shallow aquifers by water-conducting fractures of overlying strata caused by underground coal mining is the most representative form of mining-induced damage in the Yushenfu mining area. It has become an important factor restricting the green mining of coal in the Yushenfu mining area and even the ecological protection and high-quality development of the middle reaches of the Yellow River. As the key scientific problem of water-preserved coal mining, the scientific understanding of the development law and main influencing factors of water-conducting fractures in overlying strata has attracted great attention. Taking the geological occurrence characteristics of the main coal seam in Yushenfu mining area as the prototype, 24 different types of numerical models are constructed with the key characteristics of the overburden structure, such as the number of layers of sandstone in the overburden (sand layer coefficient) and the thickness ratio of sandstone and mudstone in the overburden (sand–mud ratio), as the main variables. By means of numerical simulation experiment and theoretical calculation, combined with field measurement and comparison, the influence of the key characteristics of overburden structure on the development height of water-conducting fracture is studied and revealed. It is proposed that the effective area for the study area to achieve water-preserved coal mining by using the height-limited mining method must conform to the coal seam overburden structure characteristics of “sand–mud ratio 6:4 and sand layer coefficient less than 70%” and “sand–mud ratio 8:2 and sand layer coefficient less than 80%”. The results not only enrich and deepen the research on the influence of geological factors and the law of controlling the development of water-flowing fractures in overlying strata, but also provide theoretical support for the precise protection of groundwater resources in the Yushenfu mining area in the middle reaches of the Yellow River.

1. Introduction

The energy endowment pattern of “lack of oil, less gas, and relatively rich coal” determines the ballast status of coal resources in China’s energy security [1,2]. In 2022, China’s total raw coal production will reach 4.56 billion tons, an increase of 9% [3]. Driven by China’s “dual-carbon goal” strategy, the energy status of coal resources has gradually changed from the main energy source to the main energy source [4], and the shift of coal production centers to the west is becoming more and more obvious. The Yellow River Basin, especially the middle reaches, has become an important continuation of the westward shift of China’s coal production center strategy due to its rich coal resources and superior mining conditions. At present, 9 of the 14 large coal bases planned and constructed in China are distributed in the Yellow River Basin, of which 6 are densely distributed in the middle reaches of the Yellow River [5]. By 2022, coal production in the Yellow River Basin will have reached 3.59 billion tons [6], accounting for 78.7% of the total coal production in the country. Among them, coal production in the middle reaches of the Yellow River has exceeded 1.77 billion tons, accounting for about 40% of the total coal production in the country [7]. However, the ecological environment in the middle reaches of the Yellow River is fragile, soil erosion is serious, and water resources are scarce [8]. The coal mining area in the middle reaches of the Yellow River, represented by the Yushenfu mining area, is also characterized by the coal–water spatial relationship of “upper and lower water coal” [9] and the ecological response characteristics of “underground shallow aquifer supporting the surface ecosystem” [10]. Coal mining often causes serious damage to groundwater resources and further deteriorates the regional ecological environment. According to the latest survey data [11,12], after large-scale mining in the Yushenfu mining area, the shallow groundwater level in the whole area decreased by 5–20 m, the number of springs decreased by 84%, and the flow of springs decreased by 70% compared with that before mining. The resulting surface river cutoff, vegetation degradation, soil desertification, and other phenomena are common. In October 2021, the “Outline of the Yellow River Basin Ecological Protection and High-quality Development Plan” issued by the Central Committee of the Communist Party of China and the State Council clearly requires that the whole Yellow River Basin should unswervingly implement the “four principles” of determining the city by water, determining the land by water, determining the people by water, and determining the production by water. The middle reaches of the Yellow River highlight the work of soil and water conservation and water resource protection [13]. Therefore, the contradiction between the large-scale mining of coal resources in the middle reaches of the Yellow River and the protection of water resources has become more acute under the constraints of national strategies.

Scientific understanding of the development law, degree, and main influencing factors of water-flowing fracture in overlying strata is the key to solving the sharp contradiction between coal resource exploitation and groundwater resource protection in the Yushenfu mining area. It has attracted the attention of relevant scholars at home and abroad and has achieved a number of valuable results. Because mining activities are the direct cause of the development of water-flowing fractures in the overlying strata of coal seams, the influence of mining factors on the development of water-flowing fractures in overlying strata of coal seams is first concerned. The relevant research results mainly focus on mining height, working face geometry, mining methods, and roof management methods. Wu Jianhong and colleagues [14] established two models of multivariate nonlinear regression and GA-BP neural network based on field-measured data and analyzed that mining thickness and inclined length of working face are the main controlling factors affecting the development height of water-conducting fractures. Palchik [15] revealed that the development height of water-flowing fracture is related to the thickness of coal seam mining through the combination of theoretical analysis and experiment. Singh and colleagues [16] used stratum monitoring technology to monitor the study area and obtained that the longwall mining method can effectively control the development height of a water-flowing fracture; Wang H. et al. [17] used the borehole television method to study the temporal and spatial evolution law of the water-conducting fractures generated during the mining process from the coal seam to the surface and obtained the multiple relationship between the development height of the water-conducting fractures and the mining height. Yin H. and colleagues [18] analyzed the influence of mining thickness and working width on the development height of water-flowing fracture and constructed a three-dimensional structure model of the development height of water-flowing fracture to visualize the spatial distribution of water-flowing fracture. Li Zhenhua and others [19] obtained that the development height of water-conducting fractures is proportional to the mining thickness by means of field measurement, indoor simulation, and theoretical analysis. With the development of a large number of monitoring works on the development of water-conducting fractures in coal seam overburden and the in-depth analysis of the measured data of the maximum development height, it is found that the development degree of water-conducting fractures in coal seam overburden will be completely different under the same mining factors and different geological factors. Therefore, the study of the influence of geological factors on the development of water-conducting fractures in overlying strata of coal seams has gradually become a research hotspot. At present, the relevant research results mainly focus on mining depth, faults, key strata, loose strata, the physical and mechanical properties of overlying strata, and so on. Ju Jinfeng and colleagues [20] believed that the mining depth is also the main controlling factor of the development height of water-conducting fractures; especially in the shallow coal seam mining area, it is not appropriate to use the “mining height multiple” to predict the height of water-conducting fractures. Huang Bingxiang and colleagues [21] used a similar material simulation test method to study the influence of small faults on the development height of mining-induced fractures in overlying strata and obtained that normal faults would increase the development height of water-conducting fractures, while reverse faults had little effect on the development height of water-conducting fractures. Wang F. et al. [22] established the mechanical model of the arch structure in the loose layer and deduced that the arch structure of the loose clay layer can effectively block the development of the water-conducting fracture zone and the formation mechanism of the arch structure. Wang Shuangming and colleagues [23] used numerical simulation software to study the influence of thick sandstone on the development characteristics and laws of water-conducting fractures and concluded that the position of thick sandstone is the main factor affecting the development height of water-conducting fractures. Based on the key stratum theory, Xu Jialin and colleagues [24,25] analyzed the influence of different key stratum positions on the development height of water-flowing fracture and determined the relationship between the critical height of the key stratum fracture and the mining height. Wang Xiaozhen and colleagues [26] believed that the development height of water-flowing fracture was affected by the key stratum structure and mining height at the same time, and under the control of the key stratum structure, the development height of water-flowing fracture showed a step-type mutation characteristic with the mining height. Wang Xu and colleagues [27] concluded through comparative analysis, multiple linear regression, multiple nonlinear regression, BP neural network, and other methods that the physical and mechanical properties of overlying rock have a great influence on the development height of water-flowing fracture and the greater the proportion of hard rock, the greater the development height of water-flowing fracture, and the overall logarithmic relationship. However, coal is a sedimentary mineral, and the layered structure of its overlying strata is not only a very significant and important geological factor but also a carrier for the upward propagation of underground mining activity effects [28]. Therefore, the shape structure and key characteristics of the overlying strata control the whole process of the development of the water-flowing fracture in the overlying strata of the coal seam and also determine the maximum development height of the water-flowing fracture. However, the current research results mainly focus on mining depth, faults, key strata, loose strata, the physical and mechanical properties of overlying strata, and so on. There are few reports on the influence of overburden structure on the development law of water-conducting fractures at home and abroad.

In view of this, this paper takes the Yushenfu mining area in the middle reaches of the Yellow River in China as the research area and takes the 2⁻² main coal seams and overburden strata in the area as the geological prototype. The numerical simulation test and the theoretical calculation of rock fragmentation are used to study and reveal the influence of the key characteristics of the overburden structure on the development height of water-conducting fractures. It not only has important scientific value for enriching and deepening the research on the influence of geological factors and controlling the development of water-conducting fractures in mining overburdens but also has important practical significance for supporting the protection of groundwater resources in the Yushenfu mining area in the middle reaches of the Yellow River.

2. Overview of the Study Area

2.1. Characteristics of Natural Geographical Environment

The Yushenfu mining area is located in the transition zone between the Loess Plateau and the Mu Us Desert in northern Shaanxi, China (see Figure 1). The terrain is high in the northwest and low in the southeast. The landform in the area can be divided into two types: aeolian sand beach and loess gully. It belongs to the semi-arid continental monsoon climate in the temperate zone. The maximum temperature is 35.9 °C, the minimum temperature is −8.5 °C, the average temperature is 8.1 °C, the average annual rainfall is 402.7 mm, and the evaporation is 1753.8–1978.7 mm. The soil in this area is mainly loessial soil and aeolian sandy soil. The soil structure is loose, the corrosion resistance is poor, the soil erosion and desertification are serious, and the surface vegetation is scarce. The vegetation coverage in the study area is only about 45%, and it is dominated by artificial herbs, and the ecological environment is very fragile [29].

2.2. Characteristics of Strata and Coal Seams

The main strata in the study area from old to new are the Upper Triassic Yongping Formation (T₃y), the Lower Jurassic Fuxian Formation (J₁f), the Middle Jurassic Yan’an Formation (J₂y), the Zhiluo Formation (J₂z), and the Anding Formation (J₂a). Lower Cretaceous Luohe Formation (K₁l), Neogene, and Quaternary. Yan’an Formation is the coal-bearing strata in the study area, generally containing 3–5 layers of mineable coal seams, of which 2⁻² coal seams are the main coal seams. The buried depth is between 180 and 316.36 m, the average buried depth is 300 m, the thickness is between 0.26 and 12.16 m, the average thickness is 6.5 m, the dip angle is less than 3°, and it is nearly horizontal. A large number of boreholes in the study area revealed that: First of all, the macroscopic layered structure of 2⁻² coal seam overburden is most widely distributed in the type of “sand layer–red soil layer–bedrock layer”. Secondly, the overlying bedrock of 2⁻² coal seam is mainly composed of sandstone and a small amount of mudstone interbedded structure, and there are two distinct sections of upper and lower characteristics, that is, the upper characteristic section is “sandy mudstone–fine sandstone” interbedded structure; the lower characteristic section is an “siltstone–fine grained sandstone” interbedded structure; the sand–mud ratio of the overlying bedrock of the 2⁻² coal seam is generally 60~80%, while the sandstone is dominant. The number of sandstone layers is at least five layers, up to 35 layers, and the thickness is generally between 3 and 35 m. There are four layers of sandy mudstone, and the thickness is generally between 10 and 30 m. Thirdly, sandy clay or laterite is developed on the overlying bedrock of 2⁻² coal seam, with an average thickness of 20 m, and is covered by modern aeolian sand with an average thickness of 30 m. The physical and hydraulic parameters of various rock and soil layers are shown in

Table 1. Physical and mechanical parameters of various rock and soil layers used in the numerical model.

Lithology	Saturated Uniaxial Compressive Strength/MPa	Elastic Modulus/MPa	Tensile Strength/MPa	Weight/KN/m3	Internal Friction Angle/°	Poisson’s Ratio	Cohesion/MPa	Porosity/%	Natural Moisture Content/Mpa	Permeability Coefficient
Sand–soil layer	0.72	23	0.03	17.2	37.0	0.31	0.07	0.38–0.47	11.9~17.3	4.58 × 10−4
Red soil layer	0.85	69	0.20	18.6	37.2	0.30	0.08	0.42	17.4~18.7	3.27 × 10−6
Sandy mudstone	30.4	2400	3.13	25.1	38.0	0.24	1.58	2.70	0.47	3.21 × 10−8
Fine-grained sandstone	36.1	6100	2.50	26.1	41.7	0.20	5.33	0.8–10.2	0.10–2.03	2.13 × 10−7
Silt sandstone	34.9	6300	2.45	25.5	37.7	0.21	6.06	0.4–9.34	0.22–2.26	4.32 × 10−6
2−2 coal seam	22.1	5100	0.24	13.6	38.5	0.22	5.11	-	-	-
Bottom plate	80.7	35,000	1.86	26.8	43.0	0.29	48.22	-	-	-

. There are aquifers in the upper and lower parts of the study area (see Figure 2), and the roof and floor of the coal seam are fine-grained sandstone and siltstone aquifuge. Under natural conditions, the aquifuge separates the coal seam from the aquifer, but there is a huge head difference under the condition of coal seam mining. The aquifuge will become a weak permeable layer or even a partially permeable layer, so that the groundwater in the aquifer will flow to the mine.

3. Research Methods and Process

3.1. Numerical Simulation Test (FLAC^3D6.0)

The perspective and core purpose of this paper is to reveal the influence of the layered structure of coal seam overburden on the development of water-flowing fractures in the process of large deformation of mining overburden. FLAC^3D6.0 is a three-dimensional fast Lagrangian analysis numerical software of continuous medium mechanics developed by the ITASCA company in the United States (Minneapolis, MN, USA). It is a numerical analysis method based on a three-dimensional explicit finite difference. FLAC^3D6.0 software of the finite difference method has outstanding advantages in simulating large deformation of rock strata and calculating the efficiency of a three-dimensional numerical simulation test. It has good convenience in model modification and real-time visualization. In addition, the deformation and damage of overlying strata in the process of coal seam mining are mainly caused by the renewal and change of overlying strata stress caused by mining. After the excavation of underground coal seams, the stress of surrounding rock in goaf is redistributed and renewed, which leads to the pressure relief deformation of overlying strata. When the deformation reaches the limit state, damage will occur. The mining failure of overlying rock is mainly caused by the concentration of local shear stress and tensile stress of rock mass. The Mohr–Coulomb yield criterion in FLAC^3D considers both shear yield and tensile yield of materials, which can better simulate the shear and tensile failure characteristics of rock and soil. Based on the above factors, this paper uses the finite difference method FLAC^3D software to carry out the numerical simulation study on the development of an overburden mining water-flowing fractured zone.

3.1.1. Numerical Model Construction

Taking the buried depth of 2⁻² coal seam in the study area, the macroscopic layered structure of the overburden, the thickness of the rock and soil layer, and the coal seam as the geological prototype, the numerical model is constructed in FLAC^3D software. Firstly, the geometric size parameters of the numerical model are designed. The total thickness of the model is 315 m, of which the thickness of the floor is 10 m, the thickness of the 2⁻² coal seam is 5 m, the total thickness of the bedrock is 250 m, and the total thickness of the loose layer is 50 m (including the thickness of the red soil layer is 20 m, and the thickness of the sand layer is 30 m). The width of the model is 300 m and the length is 1000 m. It is used to simulate the width and annual advance distance of the 2⁻² coal seam mining face in the study area. Secondly, according to the characteristics of the number of strata, lithology, and interbedded structure of the overlying bedrock section of the 2⁻² coal seam in the study area, the bedrock layered structure is designed as the upper and lower sections. The upper section is the “sandy mudstone–fine sandstone” interbedded structure, which is fixedly divided into 4 groups of cycles, and the lower section is the “siltstone–fine sandstone” interbedded structure. The number of cycles is set according to the geological prototype and research needs. Thirdly, the constraints of each boundary of the numerical model are set, specifically: the left and right boundaries are set as single constraint boundaries, and $u=0, v\ne 0, w\ne 0$ is taken. The front and rear boundaries are set as single constraint boundaries, taking $u\ne 0, v=0, w\ne 0$. The bottom boundary is set as the fully constrained boundary, taking $u=0, v=0, w=0$. The upper boundary is set as a free boundary and is not constrained, as shown in Figure 3.

3.1.2. Model Type Design

According to the research results of Song Shijie [28,30], the sand layer coefficient (defined as the ratio of the total number of sandstone layers to the total number of strata in the coal seam overburden) and the sand–mud ratio (defined as the ratio of the total thickness of sandstone to the total thickness of mudstone in the coal seam overburden) are the key characteristics of the coal seam overburden structure in the study area. Because this paper is to study the influence of the relationship between the number and thickness ratio of sandstone and mudstone in the overlying bedrock section on the water-flowing fracture rather than the morphological changes of the rock layer and the slope, the model is simplified according to the study area. The number of sandstone layers in the bedrock section of the numerical model is designed as 6 layers, 8 layers, 10 layers, 20 layers, 30 layers, and 40 layers. The number of sandy mudstone layers is designed as 4 layers, and the sand–mud ratios are designed as 6:4 and 8:2. The single layer thickness of fine sandstone and siltstone is treated as equal thickness, and the mining thickness is designed as 3 m and 5 m. After permutation and combination of the above variables, 24 different types of numerical models are obtained, as shown in

Table 2. Types of numerical models and characteristics of overlying strata structure.

Numbering	Deep Mining/m	Height Mining/m	Loose Layer Characteristics		Bedrock Layered Structure Characteristics
			Thickness of Sand Layer/m	Red Soil Layer Thickness/m	Total Number of Rock Strata/Layer	Sand–Mud Ratio	Sandstone Layer Coefficient/%	Sandstone		Sandy Mudstone
								Number of Layers/Layer	Thickness/ m	Number of Layers/Layer	Thickness/m
M1	300	5	30	20	10	6:4	60.0	6	25.00	4	25.00
M2	300	5	30	20	12	6:4	66.7	8	18.75	4	25.00
M3	300	5	30	20	14	6:4	71.4	10	15.00	4	25.00
M4	300	5	30	20	24	6:4	83.3	20	7.50	4	25.00
M5	300	5	30	20	34	6:4	88.2	30	5.00	4	25.00
M6	300	5	30	20	44	6:4	90.9	40	3.75	4	25.00
M7	300	3	30	20	10	6:4	60.0	6	25.00	4	25.00
M8	300	3	30	20	12	6:4	66.7	8	18.75	4	25.00
M9	300	3	30	20	14	6:4	71.4	10	15.00	4	25.00
M10	300	3	30	20	24	6:4	83.3	20	7.50	4	25.00
M11	300	3	30	20	34	6:4	88.2	30	5.00	4	25.00
M12	300	3	30	20	44	6:4	90.9	40	3.75	4	25.00
M13	300	5	30	20	10	8:2	60.0	6	33.33	4	12.50
M14	300	5	30	20	12	8:2	66.7	8	25.00	4	12.50
M15	300	5	30	20	14	8:2	71.4	10	20.00	4	12.50
M16	300	5	30	20	24	8:2	83.3	20	10.00	4	12.50
M17	300	5	30	20	34	8:2	88.2	30	6.67	4	12.50
M18	300	5	30	20	44	8:2	90.9	40	5.00	4	12.50
M19	300	3	30	20	10	8:2	60.0	6	33.33	4	12.50
M20	300	3	30	20	12	8:2	66.7	8	25.00	4	12.50
M21	300	3	30	20	14	8:2	71.4	10	20.00	4	12.50
M22	300	3	30	20	24	8:2	83.3	20	10.00	4	12.50
M23	300	3	30	20	34	8:2	88.2	30	6.67	4	12.50
M24	300	3	30	20	44	8:2	90.9	40	5.00	4	12.50

.

3.1.3. Extraction of Test Results

The excavation test of each numerical model is carried out step by step. The plastic zone method and the maximum principal stress method are used to identify and extract the development height of the water-conducting fracture of the overburden rock produced by each step of excavation. Until the full mining is reached, the maximum development height of the overburden water-flowing fracture is obtained.

3.2. Theoretical Calculation of Rock Expansion

3.2.1. Calculation Principle

The overlying strata of the coal seam will undergo a series of movements and deformations such as bending, cracking, breaking, and caving as the underground coal mining activities continue, and the volume will increase, which is called the swelling characteristics of the rock strata [31,32,33]. When the full mining is achieved, the volume of the underground mining space formed by the coal seam mining is basically equal to the sum of the volume of the rock mass in the overburden caving zone, the volume of the rock mass in the fracture zone, and the subsidence volume of the bending zone. On the main section of the subsidence basin, this law is expressed as (1) [34]:

$$M=\Delta h_k+\Delta h_l+\Delta h_d$$

(1)

where M—mining height, m; Δh_k—the vertical expansion of the rock mass in the overburden caving zone after compaction, m; Δh_l—the vertical expansion of the rock mass in the overburden fracture zone after compaction, m; and Δh_d—the subsidence of the bending zone, m.

Considering that the size of Δh_k is directly related to the height of the overlying rock fracture, the calculation method of the maximum development height of the overlying rock water-flowing fracture based on the theory of rock fragmentation and expansion proposed by Wei Jiangbo [35] can be used to calculate the maximum development height of the overlying rock water-flowing fracture of the coal seam in 24 numerical models, as shown in Equation (2).

$$H_l={\displaystyle \sum _{i=1}^k{h}_i K_{ci}}-(1-q)M$$

(2)

where q—coefficient of sinking; M—height mining, m; Δh_l—thickness of overlying i-th strata of coal seam, m; K_ci—residual bulking coefficient after mining failure of the i-th overlying strata of coal seam; and H_l—the maximum development height of water-conducting fracture in overburden rock, m.

3.2.2. Calculation Process

The main calculation steps are as follows:

The first step: Determine the subsidence coefficient q. According to a large number of survey and observation results [28,30], the coal mining subsidence in the study area is generally between 0.6 and 0.8, so the q of the study area is 0.7.

The second step: Determine the position of the holding layer. Based on the discriminant theory and method of the key stratum [24,25], it is considered that when the load (i.e., the maximum vertical stress) borne by the hard rock stratum $k$ is less than or equal to its ultimate shear strength, the rock stratum plays a controlling role in the upper rock stratum, which is the bearing stratum. The diagram of the position of the bearing layer with the change in sand–mud ratio and sand layer coefficient is shown in Figure 4.

The third step: Determine the height of the caving zone. According to a large number of measured caving ratio ranges in the study area (generally 4–10) [36], the height of the caving zone is determined.

The fourth step: Determine the $K\mathrm{c}$ of each rock stratum between the coal seam roof and the overlying bearing stratum. According to Wei Jiangbo [35], based on the theory of rock bulking, the residual bulking coefficients of the bottom of the caving zone, the top of the caving zone, and the bearing layer are taken as 1.05, 1.02, and 1.002, respectively [37], and the residual bulking coefficients of each rock layer between the bearing layer and the top of the caving zone are taken according to Formula (3) [38].

$$Kc(H)=a-\eta ln(H+b)$$

(3)

where K_C(H)—the residual expansion coefficient of rock mass after mining failure and compaction of overburden rock with high H from the coal seam; H—the distance between each rock stratum of the overlying rock and the roof of the coal seam; η—the attenuation coefficient of the residual expansion coefficient of the rock stratum is taken according to

Table 3. Position of bearing layer and correlation coefficient table.

Numbering	a	η	b	Numbering	a	η	b
M1	1.071~1.109	0.016~0.024	3.692~11.836	M13	1.057~1.073	0.011~0.015	1.788~4.705
M2	1.071~1.111	0.016~0.024	3.772~12.208	M14	1.054~1.067	0.010~0.013	1.420~3.616
M3	1.064~1.090	0.014~0.019	2.800~8.132	M15	1.056~1.071	0.011~0.014	1.659~4.315
M4	1.057~1.074	0.012~0.015	1.886~5.006	M16	1.053~1.065	0.010~0.012	1.312~3.309
M5	1.056~1.071	0.011~0.014	1.700~4.438	M17	1.052~1.063	0.010~0.012	1.190~2.973
M6	1.055~1.070	0.011~0.014	1.620~4.199	M18	1.051~1.062	0.010~0.012	1.137~2.829
M7	1.088~1.097	0.021~0.021	5.989~8.984	M19	1.063~1.091	0.014~0.020	2.611~8.016
M8	1.066~1.098	0.015~0.022	2.906~9.251	M20	1.055~1.071	0.011~0.014	1.561~4.266
M9	1.060~1.082	0.013~0.017	2.147~6.252	M21	1.052~1.066	0.010~0.013	1.250~3.312
M10	1.054~1.069	0.011~0.014	1.428~3.852	M22	1.050~1.061	0.009~0.012	0.977~2.521
M11	1.053~1.066	0.010~0.013	1.282~3.408	M23	1.048~1.059	0.009~0.011	0.882~2.256
M12	1.052~1.065	0.010~0.013	1.219~3.221	M24	1.048~1.058	0.009~0.011	0.841~2.143

; and a, b—all of them are the coefficients related to the occurrence characteristics of the stressed strata, and the values are taken according to Table 3.

4. Results and Analysis

4.1. Results

The maximum development height of water-flowing fracture in coal seam overburden of 24 numerical models is obtained by numerical simulation test method and rock fragmentation theory calculation method, respectively, and the results are shown in

Table 4. The maximum development height of water-flowing fracture in 24 numerical models of coal seam overburden.

Numbering	Numerical Simulation Test Results		Theoretical Results		Results Average Relative Error/%	Numbering	Numerical Simulation Test Results		Theoretical Results		Results Average Relative Error/%
	Maximum Height/m	Crack Production Ratio	Maximum Height/m	Crack Production Ratio			Maximum Height/m	Crack Production Ratio	Maximum Height/m	Crack Production Ratio
M1	78.5	15.7	82.7	16.5	3.1	M13	138.0	27.6	135.5	27.1	1.9
M2	97.6	19.5	81.6	16.3		M14	148.3	29.6	158.7	31.7
M3	99.1	19.8	97.7	19.5		M15	150.0	30.0	139.4	27.9
M4	117.5	23.5	128.1	25.6		M16	157.2	31.4	164.7	32.9
M5	128.4	25.7	138.4	27.7		M17	170.5	34.1	176.8	35.4
M6	133.2	26.6	147.9	29.6		M18	177.9	35.6	182.9	36.6
M7	48.0	16.0	57.1	19.0	10.6	M19	64.6	21.5	90.6	30.2	17.0
M8	74.1	24.7	81.5	27.2		M20	98.8	32.9	112.6	37.5
M9	99.0	33.0	97.6	32.5		M21	118.1	39.4	142.1	47.4
M10	118.0	39.3	127.8	42.6		M22	157.9	52.6	164.7	54.9
M11	129.0	43.0	138.4	46.1		M23	171.3	57.1	176.9	59.0
M12	134.2	44.7	147.5	49.2		M24	178.2	59.4	183.1	61.0

.

According to Table 4 the relative error rate of the maximum height of water-flowing fracture calculated by the two methods used in this paper is 1.9~17.0%, and the average absolute error rate is only 8.15%. It can be seen that the accuracy of the two results is high.

4.2. Analysis

4.2.1. The Influence of Sand Layer Coefficient on the Maximum Development Height of Water-Conducting Fracture

According to Table 4, when the mining height is 5 m and 3 m, the corresponding relationship curves between the sand layer coefficient of coal seam overburden and the maximum development height and fracture–mining ratio (N-MH-5m and N-MH-3m) of water-conducting fracture based on a numerical simulation test and the maximum development height and fracture–mining ratio (T-MH-5m and T-MH-5m) of water-conducting fracture based on the theory of rock fragmentation and expansion are drawn, as shown in Figure 5 and Figure 6.

The following can be seen from Table 4, Figure 5 and Figure 6:

(1)

Under the condition of a given sand–mud ratio, the maximum development height of a water-flowing fracture increases with the increase in the sand layer coefficient. That is to say, under the same buried depth, the more sandstone layers in the coal seam overburden, the greater the maximum development height of the water-flowing fracture.

The numerical simulation results show that when the “sand–mud ratio is 6:4 + mining height is 5 m”, the maximum development height of water-flowing fracture presents a three-stage change process of “rapid increase–stable–slow increase” with the change in sand layer coefficient. That is, when the sand layer coefficient is between 60~66.7%, the maximum development height of water-flowing fracture increases rapidly with the increase in sand layer coefficient, from 78.5 m to 97.6 m, with an average increase of 24.3%. When the sand layer coefficient is between 66.7% and 71.4%, the maximum development height of water-conducting fractures does not change much. When the sand layer coefficient is between 71.4% and 90.9%, the maximum development height of water-flowing fracture increases slowly with the increase in sand layer coefficient, from 99.1 m to 133.2 m, with an average increase of 10.5%. When the “sand–mud ratio is 6:4 + mining height is 3 m”, the maximum development height of water-conducting fracture shows a two-stage change process of “rapid increase–slow increase” with the change in sand layer coefficient; that is, when the sand layer coefficient is between 60% and 71.4%, the maximum development height of water-conducting fracture increases rapidly with the increase in sand layer coefficient, from 48.0 m to 99.0 m, with an average increase of 43.99%. When the sand layer coefficient is between 71.4% and 90.9%, the maximum development height of a water-flowing fracture increases slowly with the increase in the sand layer coefficient, from 99.0 m to 134.2 m, with an average increase of 10.85%. Under the two conditions of “sand–mud ratio of 6:4 + mining height of 5 m” and “sand–mud ratio of 6:4 + mining height of 3 m”, the distance between the bearing stratum and the coal seam increases from 75 m and 50 m to 135 m with the increase in sand layer coefficient, and the trend of the maximum development height of water-conducting fracture with the change in sand layer coefficient is consistent with the numerical simulation test results, which can be confirmed by each other. In particular, the two methods are highly consistent in identifying the maximum development height of water-conducting fractures when the “sand–mud ratio is 6:4 + mining height is 5 m”, and the average error rate is only 3.1%.

The numerical simulation results show that when the “sand–mud ratio is 8:2 + mining height is 5 m”, the maximum development height of water-flowing fracture increases slowly and evenly with the change in sand layer coefficient; that is, when the sand layer coefficient increases from 60% to 90.9%, the maximum development height of water-flowing fracture increases from 138 m to 177.9 m, with an average increase of 5.4%. When the “sand–mud ratio is 8:2 + mining height is 3 m”, the maximum development height of water-flowing fracture increases rapidly and evenly with the change in sand layer coefficient; that is, when the sand layer coefficient increases from 60% to 90.9%, the maximum development height of water-flowing fracture increases from 64.6 m to 178.2 m, with an average increase of 23.74%. Under the two conditions of “sand–mud ratio of 8:2 + mining height of 5 m” and “sand–mud ratio of 8:2 + mining height of 3 m”, the distance between the bearing stratum and the coal seam increases from 120 m and 79.2 m to 180 m with the increase in sand layer coefficient, and the trend of the maximum development height of water-conducting fracture with the change in sand layer coefficient is consistent with the numerical simulation test results, which can be confirmed by each other. In particular, the two methods are highly consistent in identifying the maximum development height of water-conducting fractures when the sand–mud ratio is 8:2 + mining height is 5 m, and the average error rate is only 1.9%.

It can be seen that the results of the theoretical calculation of rock fragmentation and the results of the numerical simulation test tend to be more consistent when the mining height and sand–mud ratio are large and tend to be more discrete when the mining height and sand–mud ratio are small. Based on the results of the two methods, combining the results of the two methods, the quantitative relationship between the maximum development height of water-conducting fractures and sand layer coefficient, sand–mud ratio, and mining thickness is shown in Equation (4).

$$\begin{array}{c}{H}_l=-11.76M^2-64.78x^2+0.07y^2+101.35M+319.50x-5.61y-251.04\hfill \\ R^2=0.818\hfill \end{array}$$

(4)

where H_l—the maximum development height of water-conducting fracture, m; x—sandstone layer coefficient, %; y—sand–mud ratio; and M—height mining, m.

(2)

Under the condition of the same sand–mud ratio and the sand layer coefficient greater than 67%, the increase in the sand layer coefficient will eliminate the difference of the positive effect of different mining heights on the maximum development height of the overburden water-flowing fracture.

The numerical simulation test results show that when the sand–mud ratio is 6:4 and the sand layer coefficient exceeds 71.4%, the maximum development height of the water-conducting fracture produced by 5 m and 3 m mining height is very small, only within 0.6 m. When the sand–mud ratio is 8:2 and the sand layer coefficient exceeds 83.3%, the maximum development height of water-conducting fractures produced by 5 m and 3 m mining heights is very small, only within 0.8 m. It can be seen that when the sand–mud ratio is 6:4 and the sand layer coefficient exceeds 72% or the sand–mud ratio is 8:2 and the sand layer coefficient exceeds 83%, the influence of mining height changes on the maximum development height of water-flowing fracture tends to disappear. The theoretical calculation results of rock fragmentation and expansion show that when the sand–mud ratio is 6:4 and the sand layer coefficient exceeds 66.7%, the maximum development height of water-conducting fractures produced by 5 m and 3 m mining heights is very small, only within 0.4 m; when the sand–mud ratio is 8:2 and the sand layer coefficient exceeds 83.3%, the maximum development height of water-conducting fractures produced by 5 m and 3 m mining heights is very small, only within 0.2 m. The results of the two methods are highly consistent, which shows that: First, under the same buried depth, the increase in the number of sandstone layers weakens the anti-interference ability of the overlying rock mass to the small mining height, so that when the sand layer coefficient exceeds 70~80%, the maximum development height of the water-flowing fracture caused by the small mining height is equivalent to the large mining height. Secondly, the overburden structure of coal seam in the study area should meet the conditions of “sand–mud ratio 6:4 and sand layer coefficient less than 70%” and “sand–mud ratio 8:2 and sand layer coefficient less than 80%” so as to achieve the purpose of water-preserved coal mining through the method of height-limited mining.

(3)

Under the condition of a fixed sand–mud ratio, the fracture–mining ratio will increase with the increase in the sand layer coefficient, and the increase in mining height will significantly reduce the increase in the fracture–mining ratio.

Both the numerical simulation test results and the theoretical calculation results of rock fragmentation and expansion show that when the “sand–mud ratio is 6:4 + mining height is 5 m or 3 m”, the fracture–mining ratio increases uniformly with the increase in sand layer coefficient. When the “sand–mud ratio is 8:2 + mining height is 5 m”, the fracture–mining ratio increases with the increase in sand layer coefficient, and the increase is not obvious. When the “sand–mud ratio is 8:2 + mining height is 3 m”, the fracture–mining ratio increases significantly with the increase in sand layer coefficient. When “the sand–mud ratio is 6:4 or 8:2 + mining height is 3 m”, the increase in sand layer coefficient leads to an average increase of about 24%. When “the ratio of sand to mud is 6:4 or 8:2 + mining height is 5 m”, the increase in sand layer coefficient leads to an average increase of about 8%, and the former is 3 times that of the latter.

4.2.2. The Influence of Sand–Mud Ratio on the Maximum Development Height of Water-Conducting Fracture

According to Table 4, when the mining height is 5 m and 3 m, the corresponding relationship curves between the sand–mud ratio of coal seam overburden and the maximum development height and fracture–mining ratio of water-conducting fracture based on numerical simulation tests (N-SMR-6:4 and N-SMR-8:2) and the maximum development height and fracture–mining ratio of water-conducting fracture based on the theory of rock fragmentation and expansion (T-SMR-6:4 and T-SMR-8:2) are drawn, as shown in Figure 7 and Figure 8.

The following can be seen from Table 4 and Figure 7 and Figure 8:

(1)

Under the condition of a fixed sand layer coefficient, the maximum development height of a water-conducting fracture increases with the increase in the sand–mud ratio. That is to say, under the same buried depth, the greater the total thickness of sandstone in the coal seam overburden is, the greater the maximum development height of the water-conducting fracture.

The results of the numerical simulation test show that under the conditions of “any sand layer coefficient + mining height 3 m” and “any sand layer coefficient + mining height 5 m”, with the sand–mud ratio increasing from 6:4 to 8:2, the maximum development height of water-flowing fracture increases by 16.6~44 m and 39.7~59.5 m, respectively, with an average increase of 31.1 m and 47.9 m; that is, for every 1% increase in the proportion of total thickness of sandstone in bedrock, the maximum development height of water-flowing fracture increases by 1.56 m and 2.40 m. The theoretical calculation results of rock layer expansion show that under the conditions “arbitrary sand layer coefficient + mining height 3 m” and “arbitrary sand layer coefficient + mining height 5 m”, as the sand–mud ratio increases from 6:4 to 8:2, the distance between the bearing layer and the coal seam increases by 40~76 m and 29.2~45 m, respectively, and the maximum development height of the water-conducting fracture increases by 31.1~44.5 m and 35.0~77.1 m, respectively, with an average increase of 36.7 m and 46.9 m. That is, for every 1% increase in the proportion of the total thickness of sandstone in the bedrock, the maximum development height of the water-conducting fracture increases by 1.84 m and 2.35 m, respectively. This is highly consistent with the results of the numerical simulation. The results of the two methods show that the increase in the sand–mud ratio of the overlying strata of the coal seam will amplify the positive effect of the increase in the mining height on the maximum development height of the water-conducting fracture of the overlying strata.

(2)

Large mining height will amplify the positive effect of the increase in sand–mud ratio on the maximum development height of water-conducting fractures.

The results of the numerical simulation test show that the maximum development height of water-flowing fracture in overlying strata of coal seam is 48~134.2 m under the condition of “sand–mud ratio of 6:4 + mining height of 3 m”, and the maximum development height of water-flowing fracture in overlying strata of coal seam is 64.6~178.2 m under the condition of “sand–mud ratio of 8:2 + mining height of 3 m”, which is 31.1 m higher than that of the former. Under the condition of “sand–mud ratio of 6:4 + mining height of 5 m”, the maximum development height of water-flowing fracture in overlying strata of coal seam is 78.5~133.2 m. Under the condition of “sand–mud ratio of 8:2 + mining height of 5 m”, the maximum development height of water-flowing fracture in overlying strata of coal seam is 138~177.9 m, which is 47.9 m higher than that of the former. The theoretical calculation results of rock fragmentation and expansion show that the maximum development height of water-conducting fractures in overlying strata of coal seam is 57.1~147.5 m under the condition of “sand–mud ratio of 6:4 + mining height of 3 m”, and the maximum development height of water-conducting fractures in overlying strata of coal seam is 90.6~183.1 m under the condition of “sand–mud ratio of 8:2 + mining height of 3 m”, which is 36.7 m higher than the former on average. Under the condition of “sand–mud ratio of 6:4 + mining height of 5 m”, the maximum development height of water-conducting fractures in overlying strata of coal seam is 82.7~147.9 m. Under the condition of “sand–mud ratio of 8:2 + mining height of 5 m”, the maximum development height of water-conducting fractures in overlying strata of coal seam is 135.5~182.9 m, which is 46.9 m higher than the former on average. It can be seen that the results obtained by the two methods are very close, which together show that after the mining height increases from 3 m to 5 m, the increase in the sand–mud ratio makes the average increase in the maximum development height of water-conducting fractures by about 51.0%.

(3)

Under the condition of any sand layer coefficient, the fracture–mining ratio will increase with the increase in sand–mud ratio, and the increase in mining height will have a certain inhibitory effect on the increase in fracture–mining ratio.

The results of the numerical simulation test show that the ratio of fracture to production is 16~44.7 under the condition of “sand–mud ratio of 6:4 + mining height of 3 m”, and the ratio of fracture to production is 21.5~59.4 under the condition of “sand–mud ratio of 8:2 + mining height of 3 m”, which is 10.37 higher than that of the former. Under the condition of “sand–mud ratio of 6:4 + mining height of 5 m”, the crack–production ratio is 15.7~26.6, and under the condition of “sand–mud ratio of 8:2 + mining height of 5 m”, the crack–production ratio is 27.6~35.6; the latter is 9.58 higher than the former on average. The theoretical calculation results of rock fragmentation expansion show that under the condition of “sand–mud ratio of 6:4 + mining height of 3 m”, the fracture–mining ratio is 19~49.2, and under the condition of “sand–mud ratio of 8:2 + mining height of 3 m”, the fracture–mining ratio is 30.2~61, which is an average increase of 12.23 compared with the former. Under the condition of “sand–mud ratio of 6:4 + mining height of 5 m”, the fracture–mining ratio is 16.5~29.6. Under the condition of “sand–mud ratio of 8:2 + mining height of 5 m”, the fracture–mining ratio is 27.1~36.6. The latter is 9.38 higher than the former on average. It can be seen that the results obtained by the two methods are very close, and together they show that after the mining height increases from 3 m to 5 m, the increase in the sand–mud ratio makes the fracture–mining ratio decrease by about 15.0% on average.

5. Example Verification and Mechanism Analysis

5.1. Example Verification

This paper collected the measured data of the ‘three-zone hole’ overburden structure and the observation hole of the water-conducting fracture development in the typical working face of the Yushenfu mining area and compared with the results obtained by the prediction method of the maximum height prediction model of the water-conducting fracture development established in this paper (see

Table 5. Comparison of the maximum development height of overburden water conduction fractures in typical working faces in Yushenfu mining area.

Name	Drilling Number	Thickness/m	The Ratio of Sandstone Thickness to Total Thickness/%	Sand Layer Coefficient/%	Theoretical Calculation Results/m	Measured Results/m	The Relative Error Rate Between Theoretical Calculation and Measured Results Is/%
Jinjitan coal mine	ZK1	5.4	92.67	87.17	122.88	109.72	11.99
Yu-shu-wan mine	ZK2	5	98.57	92.86	149.21	137.30	8.67
	ZK3	5	98.40	96.00	128.80	117.80	9.34
Hang lai bay coal mine	ZK4	4.49	95.74	91.30	113.88	114.38	−0.44
	ZK5	4.49	97.38	89.29	88.79	93.87	−5.41

).

From Table 5, based on the prediction model established in this paper, the relative error rate between the calculated maximum height of water-conducting fracture development and the measured value is −5.41–11.99%, and the average absolute error rate is only 7.17%. The accuracy of the prediction model is relatively high. Therefore, the constructed prediction model can provide a scientific reference for the study of the development of an overburden water-conducting fracture zone in the Yushenfu mining area and the guarantee of coal mine safety production.

5.2. Mechanism Analysis

(1)

As the most important mining factor affecting and controlling the development of water-conducting fractures in the overlying strata of coal seams, mining height has been recognized by scholars at home and abroad [14,15,16]. The goaf formed by mining height is actually the space of movement, deformation, and fracture of the coal seam overburden. The size of the mining height determines the size and size of this “space”, which also profoundly affects the degree of movement and deformation of the overlying strata of the coal seam. Generally speaking, the mining height has a significant positive effect on the development of water-conducting fractures in overlying strata; that is, the larger the mining height is, the larger the size of the goaf is, the larger the space for the movement and deformation of the overlying strata is, and the higher the development height of water-conducting fractures in overlying strata is [18,19]. Under the same mining depth and overburden structure conditions, the maximum development height of overburden water-conducting fractures caused by 5 m mining height is greater than that of 3 m mining height, and the former is 1.1~1.2 times of the latter on average, which not only verifies the positive effect of mining height on the development of overburden water-conducting fractures but also is consistent with the research results of Wang et al. [26] and Xun et al. [39].

(2)

As a typical geological occurrence factor of coal resources in Yushenfu mining area, the number of sandstone layers (sand layer coefficient) in overlying strata will also have an important impact on the development of water-flowing fractures in overlying strata of coal seams. Under the condition of equal burial depth, the larger the sand layer coefficient is, the more the number of sandstone layers in the overlying strata of the coal seam is, and the smaller the thickness of the single-layer sandstone is. This will produce the following two effects: First, the increase in the sand layer coefficient will not only lead to the increase in the number of sandstone layers in the overlying strata of the coal seam but also lead to the increase in the number of layers between the sandstones (the main structural plane or weak surface in the rock mass), which makes the overall stability and anti-interference ability of the overlying strata of the coal seam worse [28,30] and produces a higher degree of failure response under the influence of the same mining activities. In addition, the latest research results show that [40] when the sand layer coefficient is large, that is, the number of sandstone layers is large, and the sandstone of each layer will lead to significant differences in the size, shape, and arrangement of internal particles due to differences in deposition time and environment. The particles between the sandstone layers are difficult to deform synergistically and produce relative displacement when subjected to external forces, so that the cracks are more likely to expand rapidly along the particle boundary, resulting in a greater development height, as shown in Figure 9a; when the sand layer coefficient is small, that is, the number of sandstone layers is small, the thickness of each layer of sandstone is large, and the internal particles are relatively uniform and dense, so that under the same external force, it is easier to block and limit the generation and expansion of cracks, which in turn restricts the development of cracks in the rock layer, as shown in Figure 9b. Second, the increase in the sand layer coefficient will lead to the decrease in the thickness of the stressed rock layer (such as sandstone) in the overburden of the coal seam, which will lead to the deterioration of the mechanical properties of the stressed rock layer [41]. Under the same load and mining activities, it is more likely to crack, break, and eventually lose the ability to support the load of the overlying rock mass [42] and the ability to block the development of the underlying mining cracks [22]. In this case, the overlying strata of the coal seam will seek a new balance upward in the process of overall movement and deformation, that is, to determine the new stressed rock strata and to provide a larger space for the development of water-conducting fractures while completing the upward movement of the stressed rock strata in the entire overlying strata. The coupling superposition of the above two effects may be one of the main reasons for the larger sand layer coefficient of coal seam overburden and the larger maximum development height of water-conducting fractures.

(3)

The thickness ratio of sandstone and mudstone in overlying strata (sand–mud ratio), as another typical geological occurrence factor of coal resources in Yushenfu mining area, will also have an important impact on the development of water-conducting fractures in overlying strata. Under the condition of equal burial depth, the greater the sand–mud ratio, the greater the total thickness of the sandstone (i.e., hard rock) in the overlying rock of the coal seam, the stronger the overall rigidity of the overlying rock. According to the Griffith energy release rate criterion [43,44], according to the energy point of view of linear elastic fracture mechanics, the condition of fracture is that when the deformation energy released by crack propagation is equal to or greater than the energy required for crack propagation, the crack will expand unstably. The effect of sandstone accumulating the load potential energy of the overlying rock mass is stronger [45], and the energy released during the fracture is larger and more concentrated, which makes the development rate and degree of mining-induced fractures in the overlying rock significantly improved. This will also produce the following two effects: First, under the influence of the same mining activities, the greater the sand–mud ratio of the overlying strata of the coal seam, the greater the total thickness of the sandstone and the thickness of the single layer, the more energy accumulated in the mining process, and the stronger the damage to the overlying strata after the fracture. The result is mainly manifested in the rapid development and expansion of the water-conducting fractures of the overlying strata in a short period of time, which can simultaneously cut through multiple overlying strata and even directly reach the surface. The large-area “overhang” in some mines in the Yushenfu mining area is the best example [46]. Second, the increase in sand–mud ratio will lead to the decrease in the total thickness of mudstone (soft rock) in the overlying strata of coal seam, which not only weakens the inhibitory effect of mudstone on the development of water-conducting fractures [47], but also indirectly causes the position of the stressed rock strata in the overlying strata of coal seam to move up [48] and finally promotes the development of water-conducting fractures (see Figure 3). In addition, the majority of studies have shown that once the mudstone is broken and cracked, once it encounters water, it will show a very obvious phenomenon of breaking fracture closure or even pinch-out [37,49]. The decrease in the sand–mud ratio leads to an increase in the total thickness of the mudstone in the overlying rock of the coal seam, which greatly strengthens the above-mentioned effect, thus reducing the development height of the water-conducting fracture of the overlying rock (see Figure 9c). The coupling superposition of the above two effects may be one of the main reasons for the larger sand–mud ratio of overlying strata and the larger maximum development height of water-conducting fractures.

6. Conclusions

(1)

The maximum development height of the water-flowing fracture in the overlying strata of the Yushenfu Coal Mine will increase with the increase in the sand layer coefficient; that is, under the same buried depth, the more sandstone layers in the overlying strata of the coal seam, the greater the maximum development height of the water-flowing fracture. This effect will show different change processes under different sand–mud ratio and mining thickness combination conditions. When the sand layer coefficient is greater than 67%, the increase in the number of sand layers in the overlying rock will eliminate the difference in the positive effect of different mining thicknesses on the maximum development height of the overlying rock water-flowing fracture.

(2)

The maximum development height of water-conducting fractures in the overlying strata of the study area will increase with the increase in sand–mud ratio; that is, under the same buried depth, the greater the total thickness of sandstone in the overlying strata of the coal seam, the greater the maximum development height of water-conducting fractures. The two methods together show that when the sand–mud ratio increases by 1%, the maximum development height of the water-conducting fracture increases by 1.56–2.40 m in the range of 60–90.9%; that is, the increase in the sand–mud ratio of the overlying strata of the coal seam will amplify the positive effect of the increase in the mining thickness on the maximum development height of the water-conducting fracture of the overlying strata.

(3)

Based on sand layer coefficient, sand–mud ratio, and mining thickness, the prediction model of maximum development height of a water-conducting fracture is constructed. It is verified by an example that the average absolute error rate between the calculated results of the model and the measured values is 7.17%, which is in line with the actual situation.

(4)

Based on the numerical simulation test and the theoretical calculation results of rock fragmentation, it is proposed that the effective area of water-preserved coal mining in the study area can be achieved by using the height-limited mining method and must conform to the coal seam overburden structure characteristics of ‘sand–mud ratio 6:4 and sand layer coefficient less than 70%’ and ‘sand–mud ratio 8:2 and sand layer coefficient less than 80%’.