Deformation and Subsidence prediction on Surface of Yuzhou mined-out areas along Middle Route Project of South-to-North Water Diversion, China

2.1 Mining and Geology

The Yuzhou main canal goes through 4 mined-out areas, namely, Xinfeng, Guocun, Gongmao and Fuli mined-out area, as shown in Figure 1 and Table 1. The mined-out areas are horizontally 3.11 km long. Most of the mined-out areas have one layer, a few areas have two layers, and each coal seam is about 1m thick. Most of mined-out areas were formed by small-scale coal mining after 1990s, and the mining has stopped since 2002. The mined-out areas are generally 100-269m deep, and a mobile basin occurs on the ground which is over 1700 m long in the WE direction and about 40m wide in the SN direction.

Figure 1

Schematic layout of mined-out areas along Yuzhou section of the Diversion Project.

The study area is located in the southwest of North China Craton [14]. The terrain is generally flat, with above sea level of 123-145m in the transition zone between the alluvial apron and the plain. The coal measure strata are the Permian Shanxi formation and Shihezi formation with attitude of 195^∘ < 14^∘.

The coal measure strata are monoclinic. The main faults in the area were formed before Quaternary, and Quaternary faults are not well developed. The largest fault is the Hutoushan Fault, which controls coalfield distribution. The fault is a normal fault with attitude of about 30^∘ < 70^∘, but the fault plane is undulated. The faults are covered by loess, with fault throw of 117-427m.

The groundwater is 4-8m deep, and its flow direction is in compliance with slope of the terrain. Permian mudstone, shale, and Quaternary clay are good aquifuges, and there is no hydraulic connection between phreatic water and mines.

2.2 Characteristic and mechanism of Surface Deformation

After underground mining, the collapse, fracture and deformation of overlying rock mass happened due to the geological environment disruptions such as strata loss and stress field change, which then resulted in the subsidence, collapse fissures on the ground surface. There are both large and small coal mines in Yuzhou mining area. In the small mines, the fifth and sixth coal measures of upper Shihezi Formation formed in Permian were mined. The mining sequence and spaces did not match the specification requirements. Therefore, the movement and deformation on surface varied greatly, and there were many ground fissures in the mined-out area, as listed in Table 2. Most ground fissures are less than 5m deep, a few centimeters to more than ten centimeters wide, and less than 200m long, with strike of WE.

As we know, before mining, the rock mass was stable, and there were no any deformation or failure in the area. After mining, especially disordered mining or mining without support, initial in-situ stress field was destroyed, which caused some deformations and failures such as bending, breaking and caving. If the mining coal seam was shallow, or the deformation and failure of the wall rock was serious, they would influence ground surface, and caused subsidence, crack, tilt and horizontal displacement. With the enlargement of mined-out area, the deformation and failure of ground surface would develop into a subsidence trough.

Occurrence, development and results of deformation and failure in mined-out area were controlled by mechanical properties and structure of upper strata. At the same time, position and occurrence of coal bed, depth, thickness and method of mining also influenced the deformation and failure.

Upper strata are soft rock with hard rock, and their strikes are generally along EW direction, with south dip and dip angle of 12-19^∘. The deformation of upper rock mass was a funnel-shaped subsidence from mining position to ground, and the deformed rock mass can be divided three zones: caving zone, fault zone, and bend zone, as shown in Figure 2.

Figure 2

Model of deformation and failure in the mined-out area. 1-caving zone; 2-fault zone; 3-bend zone; 4- subsidence trough; 5-ground fissure.

Most ground fissures are found on the outer margin of the subsidence trough, and they were parallel to boundary of the mined-out area. The scale (including width, depth and length) of the ground fissures are closely related to the type, thickness, component, physical and mechanical properties of topsoil. Generally, when the surface tensile deformation value exceeds 6 to 10mm/m in large plastic clayey soil, the ground fissures would occur around the subsidence trough. In addition, the fractures will happen when the tension deformation on the ground surface is 2-3mm in sandy clay, clay sand or rock. The range of subsidence trough and ground fissures is larger than underground mined-out area.

The area of ground subsidence increases with depth of mining, and the depth of subsidence decreases with depth of mining.

3.1 Method of subsidence prediction

The probability integral method used to predict the subsidence is based on the random medium theory. The rock mass is considered to consist of a large number of loose granular media. During the movement of rock mass, its continuity was destroyed, and the original connection between the media units changes. The units separate and move relative to each other. The influence of the whole mining on rock strata and surface is equal to the sum of the influence of each unit mining on strata and surface. According to the random medium theory, the surface subsidence basin caused by unit mining is the normal distribution, and is consistent with the probability density distribution. The probability integration method considers the overburden subsidence due to the unit mining as a random event to describe the possibility of settlement and settlement of rock mass [15, 16]. The probability integral method has become one of the most mature and widely used methods in China, and it is used in this study [17, 18, 19].

Figure 3 shows the comparison between the ideal sinking curve and actual sinking curve. The W^′(x) represents the ideal limit settlement curve, and the W (x) denotes the actual subsidence curve.

Figure 3

Comparison between ideal and actual subsidence curves.

The ideal limit subsidence curve is expressed as

where W^′(x) is the ideal limit subsidence curve, q^′ is the limiting subsidence coefficient, obviously q < q^′ ≤ 1, m is the goaf thickness, α is the dip angle of coal seam, W^′₀ = q^′m cos α is the maximum settlement, r is the influence radius, r=H/tanβ, β is the influence angle of coal seam,x is the length of goaf, and erf is the probability integral function.

The actual surface subsidence curve is

where W(x) is the actual subsidence curve, q is the current surface subsidence coefficient, and s is the offset of inflection point.

In the limit condition, the distribution curve of goaf surface subsidence is W^′(x), and q^′ in Formula (1) has the limit value of 1. The ultimate residual subsidence curve W₀(x) is

The maximum residual settlement is the maximum of W_e(x), which can be solved by Formula (3). Figure 2 shows that the most value point of W(x) is at the right side of the S. Assuming that the maximum point of W^′(x) is at the point x0, and the point of the maximum value of W(x) is x₀+S. According to Formula (3), the maximum value of W(x) is between x₀ and x₀+S. The results of surface subsidence distribution function are listed in Table 3, which shows that, from the point x0 to the point x₀+S, the W^′(x) value decreases while the value of W(x) increases. Therefore, W_e(x) reaches the maximum value at the point x₀, which is shown by

where η is the reduction factor which can be determined in Table 3 by letting x/r=1-S/r.

The first order reciprocal and two reciprocal of the curve W(x) are the tilt curve i _x and the curvature curve K _x, respectively. U_x is the horizontal displacement curve, and ε_x is the horizontal displacement curve. According to the literature [20], the maximum values are

where b is the horizontal movement coefficient,

where W₀ is the maximum residual settlement of actual settlement curve.

The substitute of maximum residual settlement value into equations (5) - (8) will result in the corresponding residual deformation.

3.2 Deformation monitoring

In order to correctly evaluate the stability of the mined-out areas and their influence on the section of the Diversion Project and other buildings, a high precision deformation observation network (including surface leveling and horizontal deformation observation) was installed along the channel (Figure 1).

The observation network covers 4 mined-out areas, namely, Xinfeng, Guocun, Gongmao and Fuli mined-out areas.

Deformation observation sections are set up every 100 to 200m along the center line of the channel. Each observation section has three deformation monitoring points, one in the center of the canal and two at two sides 50-150m from the canal center. Therefore, there are 33 deformation monitoring points in total. In addition, there are 3 vertical working bench marks and 3 horizontal working bench marks (Figure 1).

The datum network should be rechecked every six months, and the points should be observed every a month. If there were special factors, time interval could vary.

3.3 Prediction parameters

The prediction parameters generally include the coal seam thickness, subsidence coefficient, horizontal movement coefficient, tangent of major influence angle, uphill inflection point offset, downhill inflection point offset and influence propagation angle coefficient. According to the code for building, water body, railway and main well roadway pillar retention and pressure coal mining [19] and measured data, the parameters are listed in Table 4.

The subsidence coefficient is calculated with the following formula (9). Because the prediction is the residual displacement deformation of the mined-out areas, and the mined-out areas were grouted to reduce subsidence, the subsidence coefficient is relatively small.

where, q₁ is the surface residual subsidence coefficient after grouting, q is the surface subsidence coefficient (q = 0.88 here according to Code for coal mining under buildings, water-bodies and railways [19]), and q₂ is the effect of grouting.

3.4 Prediction results

The ground tilt prediction results along strike are shown in Figure 4(a), with maximum negative tilt of -0.8 mm/m at the northwest corner of Guocun mined-out area and maximum positive tilt of 0.6 mm/m at the southwest and north of Guocun mined-out area and the North and southwest of the Xinfeng mined-out area, which is under the channel.

Figure 4

Tilt distribution.

Figure 5

Surface curvature distribution.

The ground tilt prediction results along dip direction are shown in Figure 4(b), with maximum negative tilt of −0.9 mm/m at the northwest corner of Guocun mined-out area and maximum positive tilt of 0.6 mm/m at southwest and north of Guocun mined-out area, which is just under the channel.

The predicted results of surface curvature are shown in Figure 5, with maximum negative curvature of −0.08 mm/m² and maximum positive curvature of +0.08mm/m² at the northwest corner of Guocun mined-out area, which is just under the channel.

The predicted results of horizontal surface deformation are shown in Figure 6, with maximum negative horizontal deformation of −0.9 mm/m and maximum positive horizontal deformation of 0.9 mm/m at the northwest corner of Guocun mined-out area, which is just under the channel.

Figure 6

Distribution of horizontal deformation.

According to the code for building, water body, railway and main well roadway pillar retention and pressure coal mining [19], the allowable deformation of ground surface is: tilt i = ±3mm/m, curvature K=+0.2×10⁻³/m, horizontal deformation ε = +2mm/m. Therefore, the channel will not be destroyed in terms of the ground deformation.

The prediction results of subsidence for the surface, −50m, −100m and −150m are shown in Figure 7. It can be found from Table 4 that the subsidence increases with depth. The position with maximum subsidence of Guocun and Xinfeng mined-out areas are under the channel. However, the distance from the position with maximum subsidence of Fuli mined-out area to the channel is far.

Figure 7

Contour of subsidence.