**4. Determination of the Heights of "Three Zones"**

#### *4.1. Longwall Mining*

In a longwall mine, several interconnected roadways are developed as passageways (often called gate roads) for operators, equipment, coal transport, and ventilation. These roadways split the coal seam into longwall panels, which are typically 100–300 m wide and 1000–5000 m long. A shearer is mounted along the width of each panel, i.e., the longwall face, and cuts coal in 0.6–1.2 m thick slices from the seam as it moves along the face. Chain pillars are usually left between panels, and the roadways on the two sides of a pillar are connected by crosscuts as shown in Figure 9.

**Figure 9.** Longwall panel layout.

Longwall mining has been widely used in the United States, Australia, Poland and India because of its significant advantages, e.g., continuous mining, high productivity, high efficiency and high seam recovery rate etc. It is also extensively used by coal mines in Northwest China, where the coal seams are typically thick and shallow [25]. However, high intensity longwall mining leads to severe fracturing in the overlying strata to reach the aquifer, which cause groundwater to infiltrate to longwall working face along the fractures and surface vegetation to die, as shown in Figure 10.

After the coal seam is extracted, the overburden tends to collapse, forming goaf (gob) behind face. Generally, the goaf can be divided into three zones based on the degree of damage: caved zone, fractured zone, and continuous deformation zone as shown in Figure 10 [39,40]. The caved zone and fractured zone are known as the WCFZ as they provide the main paths for groundwater flow [41].

**Figure 10.** The three zones in overlying strata and groundwater inrush resulting from longwall mining.

#### *4.2. Height of Caved Zone*

The ratio of caved zone height to mining height in Yu-Shen mining area was measured and found between 4.12 and 6.38 as describe in Table 2. According to the measured data, the equation of calculating the height of caved zone is obtained:

$$H\_k = \frac{100M}{0.6M + 14.1} \tag{3}$$

where, *Hk* is the height of caved zone (m) and *M* is the effective mining height (m).


**Table 2.** Field observation of caved zone height and WCFZ height in Yu-Shen mining area.

The height of the caved zone near each borehole was calculated using Equation (3). The results obtained is plotted showing the caved zone height in an area as shown in Figure 11.

**Figure 11.** Caved zone height contour (Xi'an geodetic coordinate system 1980; in km).

#### *4.3. Height of WCFZ*

To determine the height of WCFZ, four main methods can be used: field measurement, numerical modeling, empirical equation, and physical modeling. As it is difficult to obtain accurate results by each single method, a combination of two or more of them is usually used.

#### (1) Field measurements

The location of top boundary and height of WCFZ induced by mining of each main seam in Yu-Shen mining area were determined through observation of: drilling fluid loss combined with engineering geological logging of rock cores, packer testing, borehole television logging, and geophysical logging [42,43]. The measured heights of WCFZ detail are given in Table 2.

The average measured ratio of WCFZ height to mining height in the study area was 24.73. Overall, the ratio decreases with the increase of mining height.

#### (2) Numerical calculation

The thickness of the main coal seam in the Yu-Shen mining area varies between 0.3–12.4 m, while the mining height obtained according to the measured height of WCFZ varies from 3.5 to 5.5 m. In order to thoroughly analyze the height of WCFZ under different stratigraphic structures and mining heights, the universal distinct element code (UDEC) was employed and four numerical calculation models were developed. In these numerical models, the height of WCFZ under different mining heights was calculated for stratigraphic structures of sand-soil-bedrock, sand-bedrock, bedrock and soil-bedrock respectively. It is noted that, for the stratigraphic structure of burnt rock, the burial depth of coal seam is less than 150 m, and the mining-induced WCFZ will generally develop and reach the surface. Therefore, it is not necessary to simulate the height of the WCFZ of the burnt rock.

Based on the stratigraphic column of the Yu-Shen mining area, numerical calculation models were developed to simulate the height of WCFZ based on different boreholes. The model parameters were continuously optimized until a good agreement of the numerical results and the field measured results was reached. Taking the typical sand-soil-bedrock stratigraphic structure as example, similar lithologic strata were merged and 14 strata were identified from bottom to top according to the Y5 borehole in Yushuwan Coal Mine. The numerical model with dimensions of 800 m × 302 m was constructed based on reducing the boundary effect and supercritical mining as shown in Figure 12. The left and right sides of the model were fixed in the X direction, and the bottom boundary of the model was fixed in the Y direction. Mohr-Coulomb model was used during the calculation. The WCFZ height for 5 m mining height is calculated and continuously adjusting parameters according to the numerical results, the optimum model parameters which can result in the measured results were obtained, and they are shown in Tables 3 and 4.

**Figure 12.** Numerical simulation model of sand-soil-bedrock stratigraphic structure.


**Table 3.** Rock physical and mechanical parameters (block).

**Table 4.** Rock physical and mechanical parameters (contact).


The WCFZ of 5 m mining height is shown in Figure 13. The optimized model parameters were used, the heights of WCFZ zone under different mining heights were calculated for 4 different stratigraphic structures, and the results are shown in Table 5.

**Figure 13.** WCFZ of 5 m mining height of sand-soil-bedrock stratigraphic structure.

**Table 5.** Numerical simulation results of WCFZ height and the ratio of WCFZ height to mining height.


#### (3) Calculation equation

The equation for WCFZ height calculation was obtained by regression analysis using field measured data under medium thickness seam mining condition in eastern China [31,32]. However, due to the change of coal occurrence, geological and mining conditions, the conventional equation is not suitable for use in the Yu-Shen mining area [44]. Therefore, it is necessary to derive an equation which suits the regional geology based on the measured heights of WCFZ combined with the numerical results [30,33,45–47].

Based on field observations and the numerical simulation results shown in Tables 2 and 5 respectively, four equations for predicting the height of WCFZ were obtained by regression analysis using different stratigraphic structures in Yu-Shen mining area:

$$\begin{cases} \begin{array}{llll} H\_d = 21.75M + 28.28 & \mathbf{R}^2 = 0.99 & (\text{Sand} - \text{soil} - \text{bedrock}) \\ H\_d = 22.2M + 37.13 & \mathbf{R}^2 = 0.97 & (\text{Sand} - \text{bedrock}) \\ H\_d = 16.7M + 30.8 & \mathbf{R}^2 = 0.97 & (\text{Bedrock}) \\ H\_d = 21.97M + 28.42 & \mathbf{R}^2 = 0.98 & (\text{Soil} - \text{bedrock}) \end{array} \end{cases} \tag{4}$$

where, *Hd* is the height of WCFZ; *M* is the mining height.

Using Equation (4), the heights of WCFZ at different borehole positions were calculated. The WCFZ height is shown in Figure 14.

**Figure 14.** WCFZ height contour (Xi'an geodetic coordinate system 1980; in km).

#### *4.4. Thickness of Protective Zone*

A protective zone overlying the WCFZ can help to prevent water in the overlying aquifer from flowing into coal faces [31,32]. A water-resisting index based on rock strength was proposed for the quantitative assessment of the water-resisting property of this zone [48]. The more impermeable the protective zone, the higher the index and the harder the rock in the protective zone, the lower the index. If there is a continuous clay layer greater than 3 m thick separating the WCFZ from the overlying aquifer, it can effectively prevent the downward percolation of water from the aquifer. The amount of water loss, even if only relatively small can cause considerable ecological damage, so the acceptable the safety factor was set at 4. This means that for a clay layer to be an effective protective zone, its thickness should be at least 12 m. If water-resisting index of clay is 1, the water-resisting index of the bedrock in the Yu-Shen mining area would be 0.4 for bedrock of sandstone, mudstone and sandy mudstone. Therefore, with the absence of clay, the thickness of bedrock should be at least 30 m to be an effective protective zone. The method for determining the thickness of protective zone can be expressed as follows:

$$H\_b = \begin{cases} 12 & \text{( $H\_t \ge 12$ )}\\ 30 - 1.5H\_t & \text{( $0 \le H\_t < 12$ )} \end{cases} \tag{5}$$

where, *Hb* is the thickness of protective zone (m) and *Ht* is thickness of clay layer (m).

Using Equation (5), the thicknesses of protective zone at different boreholes in the Yu-Shen mining area were determined, and the results can be observed in the contour as shown in Figure 15.

**Figure 15.** Protection zone thickness contour (Xi'an geodetic coordinate system 1980; in km).

## **5. Zoning Based on Coal Mining's Impact Level on Groundwater**

There are five main seams in the Yu-Shen mining area and their thicknesses vary widely. The overall overburden thickness increases gradually from east to west, but regional increase or decrease occurs as a result of change in sedimentary environment or denudation. As the roof strata are relatively unstable, mining activities can easily induce the development of "three zones" (caved zone, WCFZ and protective zone) in the vertical direction. These factors determine that the loss of aquifer water during mining varies spatially and different WCCM method should be implemented according to local conditions. Based on measured bedrock thickness, the spatial relationship between caved zone and WCFZ induced by mining, and the thickness of protective zone, coal mining's impact level on groundwater was classified using the standard summarized in Table 6. Then the study area was divided into zones of different impact levels according to the results, as illustrated in Figure 16:



**Table 6.** Classification standard for coal mining's impact level on groundwater.

**Figure 16.** Zoning the extent of coal mining impact on groundwater. (Xi'an geodetic coordinate system 1980; in km).

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## **6. Zoning Based on Applicability of WCCM Methods**

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A system of WCCM methods mainly including restricted mining height, (partial) backfill, and/or narrow strip mining has been established after years of research and practice [49]. Prior to carrying out field practice under site specific geological conditions, the applicability of these methods needs to be considered. The Yu-Shen mining area was then partitioned into zones that are suitable for different WCCM methods.

#### *6.1. Maximum (Theoretical) Allowable Mining Height*

According to the classification standard for coal mining's impact level on groundwater. If the thickness of soil strata plus the thickness of bedrock is greater than the height of WCFZ plus the height of protective zone, then the WCCM can be realized. Subtracting the necessary thickness of protective zone from the total thickness of soil and bedrock yields the height of WCFZ. The theoretical allowable mining height at each longwall face, *Mc*, can be calculated according to Equation (4). The theoretical allowable mining height obtained was then compared with the actual coal thickness, *Mm*. Furthermore, the maximum allowable mining height (MAMH), namely the maximum mining height allowed by WCCM, can be derived using Equation (4) under the four different stratigraphic structures:

(1) The equation of MAMH for sand-soil-bedrock overburden is as follows:

$$M\_{\mathcal{C}} = \begin{cases} \frac{(H + H\_l - 12) - 28.28}{21.75} & \text{( $H\_l \ge 12$ )}\\ \frac{[H + H\_l - (30 - 1.5H\_l)] - 28.28}{21.75} & \text{( $0 \le H\_l < 12$ )} \end{cases} \tag{6}$$

(2) The equation of MAMH for sand-bedrock overburden is as follows:

$$M\_{\xi} = \begin{cases} \frac{(H + H\_l - 12) - 37.13}{22.2} & \text{( $H\_l \ge 12$ )}\\ \frac{[H + H\_l - (30 - 1.5H\_l)] - 37.13}{22.2} & \text{( $0 \le H\_l < 12$ )} \end{cases} \tag{7}$$

(3) The equation of MAMH for bedrock overburden is as follows:

$$M\_{\mathcal{L}} = \begin{cases} \frac{(H + H\_l - 12) - 30.8}{16 \mathcal{I}} & \text{( $H\_l \ge 12$ )}\\ \frac{[H + H\_l - (30 - 1.5H\_l)] - 30.8}{16 \mathcal{I}} & \text{( $0 \le H\_l < 12$ )} \end{cases} \tag{8}$$

(4) The equation of MAMH for soil-bedrock overburden is as follows:

$$M\_{\mathcal{C}} = \begin{cases} \frac{(H + H\_l - 12) - 28.42}{21.97} & (H\_l \ge 12) \\ \frac{[H + H\_l - (30 - 1.5H\_l)] - 28.42}{21.97} & (0 \le H\_l < 12) \end{cases} \tag{9}$$

where, *Mc* is the maximum allowable mining height (m); *H* is bedrock thickness (m); *Ht* is soil thickness (m).

The MAMH contour calculated from Equations (6) to (9), indicating its distribution across Yu-Shen mining area is shown Figure 17.

**Figure 17.** Maximum (theoretical) allowable mining height contour. (Xi'an geodetic coordinate system 1980; in km).

#### *6.2. Height-Restricted Mining*

In height-restricted mining areas, the mining height is limited for the purpose of reducing overburden displacement and thereby the height of WCFZ. This method can be easily implemented in thick coal seams without extra cost. In addition, the roof over the goaf will collapse completely, thus avoiding the problem of coal pillar failure as occurred in other methods. However, as the coal seam is partially extracted, the recovery rate is obviously lower when using this method.

In height-restricted mining, WCCM is achieved by reducing the actual mining height. Theoretically, it is applicable throughout the study area. Considering the mine production, profit, mining equipment layout and other relevant factors, this method should be more suitable for zones where the maximum allowable mining height (*Mc*) is equal or greater than 2 m. The distribution of zones to which height-restricted mining is applicable is shown in Figure 18a.

**Figure 18.** Applicability partition of water conservation coal mining method (Xi'an geodetic coordinate system 1980; in km): (**a**) Height-restricted mining application area; (**b**) Backfill mining application area; (**c**) Partial backfill mining application area; (**d**) Narrow strip mining application area.

#### *6.3. Backfill Mining*

In backfill mining, goaf is refilled with backfill concurrently with mining to support the roof, so to control the overburden movement and deformation, the height of WCFZ is therefore reduced. In this way, aquifers are protected from being disturbed by mining activities and to achieve water conservation. In theory, this method allows all coal resources in an area to be extracted. It is recognized as an ideal approach to protecting water resources from coal mining-induced damage and is an important component of green mining technology [50]. However, the disadvantages of backfill mining, such as higher cost and large backfill requirement, have restricted its wide applications in coal mines.

Considering the time required for the filling body to reach the designed strength and the value of roof displacement before working face is filled. It is generally accepted in the mining industry that the maximum backfill mining height is 4.0 m. The inherent compressibility properties of backfill materials, it is impossible to achieve 100% filling ratio. Considering a filling ratio of 75%, backfill mining is applicable to zones where *Mc* ≥ (1 − 75%) *Mm*, and the corresponding minimum backfill height is (*Mm* − *Mc*) / 75%. The zones which are suitable for the use of backfill mining in Yu-Shen mining area is shown in Figure 18b.

#### *6.4. Partial Backfill Mining*

In the Yu-Shen mining area, the height of WCFZ at normal areas of a longwall face is about 10–15% less than that at the open-off cut and the location of first main roof weighting (after 2 to 3 periodic weighting events), as shown in Figure 10. According to the development characteristics of WCFZ, backfill can be carried out around the open-off cut and near the location of first main roof weighting, so to reduce regional stress concentration in the roof and its damage to overburden. As longwall face advances, the consequence of backfill gradually weakens and the stress in the roof will increase again. The partially backfilling at the two above mentioned positions, the roof would collapse more gently than normal face without backfilling, and the roof movement would be in the overall deformation stage.

Partial backfill is able to effectively reduce the heights of WCFZ of the open-off cut zone and the first weighting zone effectively. However, its effect on controlling the development of water conductive fractures is limited when compared with other WCCM methods. Therefore, this method can be used as a supplement to other WCCM methods or adopted in regions with lower water loss risk, such as areas where the height of WCFZ is greater than bedrock thickness but within 15% of difference.

Therefore, when the sand-soil-bedrock overburden satisfies Equation (10), partial backfill mining should be employed:

$$\frac{21.75(M\_m - M\_c) + 28.28}{21.75M\_m + 28.28} \le 15\% \tag{10}$$

When the sand-bedrock overburden satisfies Equation (11), partial backfill mining should be employed:

$$\frac{22.2(M\_{\rm m} - M\_{\rm c}) + 37.13}{22.2M\_{\rm m} + 37.13} \le 15\% \tag{11}$$

When the bedrock overburden satisfies Equation (12), partial backfill mining should be employed:

$$\frac{16.7(M\_m - M\_c) + 30.8}{16.7M\_m + 30.8} \le 15\% \tag{12}$$

When the soil-bedrock overburden satisfies Equation (13), partial backfill mining should be employed:

$$\frac{21.97(M\_m - M\_c) + 28.42}{21.97M\_m + 28.42} \le 15\% \tag{13}$$

where, *Mc* denotes the maximum allowable mining height (m); *Mm* is coal thickness (m).

Figure 18c shows the locations suitable for partial backfill mining in Yu-Shen mining area.

#### *6.5. Narrow Strip Mining*

In narrow strip mining, the block of coal to be mined is divided into strips. After one strip is extracted, the adjacent strip will be left in place to support the overlying strata and the strip next to it will be extracted (similar to drift and pillar mining), as shown in Figure 19. The overburden will thus displace and deform more mildly and evenly than it in the full-seam extraction, maintaining the structural stability of aquicludes during mining, and thereby protecting water resources. However, this method achieves WCCM at the cost of low recovery rate, which is similar to that of height-restricted mining. This is also the main reason restricting its use for water conservation.

In addition, as the coal pillars left to support overburden may fail due to weathering after years of extraction, narrow strip mining is only suitable under certain geological conditions. The recovery of coal pillars and the associated safety issues within the goaf are still challenging issues when using narrow strip mining. Therefore, narrow strip mining can be adopted at low-production coal mines with thin seams as long as local conditions permit. Taken into account the current technological and economic conditions and equipment available in the Yu-Shen mining area, narrow strip mining should be a suitable WCCM method for mines with annual production below 0.9 Mt/a (million tons every year). Figure 18d shows the zones suitable for this method to use within the Yu-Shen mining area. It is worth noting that room and pillar mining has been employed by some local coal mines for many years. This method has two major weaknesses, low recovery rate and non-standard face layout.

**Figure 19.** Sketch map of narrow strip mining.

Despite being relatively old-style, narrow strip mining could be an option for some small and medium sized mines to repace the room and pillar mining, whcih can increase recovery rate by over 20% while also protecting groundwater [7]. This would be particularly useful in coal areas where longwall panels are difficult to design due to adverse geology for example.
