Simulation test on mixed water and sand inrush disaster induced by mining under the thin bedrock

Accepted Manuscript Simulation test on mixed water and sand inrush disaster induced by mining under the thin bedrock Weifeng Yang, Lu Jin, Xinquan Zhang PII:

S0950-4230(18)30284-5

DOI:

https://doi.org/10.1016/j.jlp.2018.11.007

Reference:

JLPP 3804

To appear in:

Journal of Loss Prevention in the Process Industries

Received Date: 27 March 2018 Revised Date:

8 October 2018

Accepted Date: 4 November 2018

Please cite this article as: Yang, W., Jin, L., Zhang, X., Simulation test on mixed water and sand inrush disaster induced by mining under the thin bedrock, Journal of Loss Prevention in the Process Industries (2018), doi: https://doi.org/10.1016/j.jlp.2018.11.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Simulation test on mixed water and sand inrush disaster

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induced by mining under the thin bedrock

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Weifeng Yang *, Lu Jin, Xinquan Zhang

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School of Resources and Geosciences, China University of Mining and Technology,

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Xuzhou, 221116, China

6 Weifeng Yang: [email protected]

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Lu Jin: [email protected]

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Xinquan Zhang: [email protected]

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*Corresponding author

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Weifeng Yang

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School of Resources and Geosciences, China University of Mining and Technology,

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Xuzhou, 221116, Jiangsu, China

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E-mail: [email protected]

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ABSTRACT: Coal seam mining under thin bedrock will make transmission fissure

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zone go through to the water-rich aquifer under the covering layer, which will cause

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water flooding and sand gushing in the working face. The test model of the mixed

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water and sand inrush transfer and inrush was designed and manufactured to simulate

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the startup, transfer and inrush process of the mixed water and sand inrush in the

ACCEPTED MANUSCRIPT overburden fracture channel. The characteristics of the mixed water and sand inrush in

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the fracture channels were researched. The variation characteristics of water pressure

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in different positions of the fracture channel were revealed through analyzing “\ /”

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type fracture form which is wide at the top and goes narrower down to the base, of

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various inclination angles under the water pressure of 0.06 MPa and 0.08 MPa. The

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results show that the water and sand inrush took place instantly in the mined area.

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Based on the characteristics of water pressure variations, changes in water pressure

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are divided into two phases: rising phase and lowering to stable phase. Under the

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other same conditions, when the fissure channel angle increases, the pore water

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pressure and its sudden drop will increase too, and the inrushing process will get more

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rapidly; the bigger the water pressure is, the pore water pressure increases more

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rapidly and violently. From this, the transfer characteristics and dynamic mechanism

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of the mixed water and sand inrush were explored.

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Key words: thin bedrocks; inrush disaster; mixed water and sand inrush; transfer

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characteristics

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1 Introduction

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Coal seam mining under thin bedrock will make transmission fissure zone go

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through to the water-rich aquifer under the covering layer, which will cause water

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flooding and sand gushing in the working face. Water filled in the mined-out area

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under thin bedrock and thick unconsolidated layer actually contain dual water sources

ACCEPTED MANUSCRIPT under natural and artificial conditions. The channels of water inrush are also

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composed of natural channels of water inrush, such as fracture and outcrop, and

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artificial channels of water inrush, such as water flowing fractured zone. Zhang (2005)

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introduced the hydrogeological conditions of coal mines and potential water inrush

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disasters from aquifers under coal seams, and then presented the water inrush

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mechanism. Sui et al. (2007) carried out the experimental study on the critical

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hydraulic gradient and pore water pressure variation of water and sand inrush for

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cracks in coal mining near unconsolidated soil layers. Yang et al. (2011, 2012, and

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2016) explored the breaking mechanism of thin bedrock and the mechanism of water

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and sand inrush in single uniform fracture through a similar material model test and

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water sand mixing inrush migration and gushing simulation experiment. A fuzzy

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comprehensive estimation model is developed to judge mine water and sand inrush

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caused by underground mining. Through numerical models for the roof fracture and

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seepage development rule, Yao et al. (2012) analyzed the changes in fracture zone,

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stress, water pressure and seepage vector with the advancement of working face. The

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case studies (Dash et al., 2016) on lessons learnt from Indian inundation disasters

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required more attention to maintaining safety by analyzing the past three disasters due

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to water inrush in coal mines in Indian including Chasnala coal mine disaster, the

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greatest disaster in India’s mining industry. Gao and Sui (2017) presented an

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experimental investigation on quicksand through an orifice and obtained the velocity

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of the sand inrush and velocity distributions. In addition, some case studies on water

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and mud inrush from tunnels have also been carried out (Zhao et al., 2013; Li et al.,

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2017; Jiang et al., 2017; Zhang et al., 2018). The influencing factors of transfer and inrush of the mixed water and sand inrush

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include water and sand source, inrush channel and dynamic source. The interaction of

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these three factors is the internal mechanism leading to the transfer and inrush of the

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mixed water and sand inrush. With the advancing of the working face, the cracks in

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the overburden strata are formed, but due to different force forms and positions in

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different areas, the specific fracture shapes will be different. After coal mining, the

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supporting force of the coal suddenly disappears, and the load on the overburden

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strata completely acts on the roof. When rock formations collapse, the rock masses

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near the unconsolidated layers are subjected to flexural tension, and the fracture is of

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the type “\ /” wide at the top and narrow at the bottom.

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In this paper, the experiments were carried out through a self-designed and

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manufactured model of mixed water and sand inrush process. In view of the “\ /”

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shaped fracture, wide at the top and narrow at the bottom, we simulated the start,

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transfer and inrush process of the mixed water and sand inrush in the inrush channel

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of overburden strata during mining. By setting up the models of different water

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pressure and different angles of fracture channel, the geological information of the

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transfer and inrush of mixed water and sand inrush was quantitatively studied, and the

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curves of water pressure were obtained at different locations in the inrush channels.

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The inrush mechanism of the mixed water and sand inrush induced by coal mining

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sand inrush and making decisions in mining under the thin bedrock and thick

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unconsolidated layers.

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2 Design and Monitoring of Experimental Model

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2.1 Design of test equipment

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The test equipment mainly consists of a water pressure device, a storage device

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of the mixed water and sand inrush, a channel device of mixed water and sand inrush

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transfer, a pressure sensor system, an orifice device of water and sand inrush, an

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automatic control and recording device, a computer control device and a voltage

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overload protection device etc.

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The geological prototype of test model design is based on a working face of thin

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bedrock in Jining Taiping Coal Mine of Shandong province, China. The permeability

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coefficient of Quaternary bottom aquifer is 0.428 m/d, considering the inrush channel,

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20 ~ 30 cm wide and 5 ~ 20 m long, will cause a groundwater level falling funnel

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with a diameter of not less than 120 m. The test is designed according to the

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geometric proportion of n=200 and the influence radius can reach to about 0.3 m.

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Therefore, a test tank with a diameter of 0.6m is used as the storage tank of a

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simulated water-bearing sand reservoir. The fracture channel setup is based on a

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cylindrical test barrel with a diameter of 0.3 m and a length of 1.5 m. The fractured

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rock is set inside the barrel body, that is, the channel simulation fracture, and the

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fracture width can be adjusted freely to simulate the channel with the width of

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ACCEPTED MANUSCRIPT different fractures. At the bottom, the valve is set for the outlet of the sand inrush. In

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the experiment, the “\ /” type fracture form which is wide at the top and goes narrowly

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down to the base with a length of 1.5 m is made by using concrete in the simulated

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fracture. Three water head supply pipeline interfaces are installed on the side of the

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test tank. The test equipment is shown in Fig. 1.

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(a)

(b)

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1—Collection box of water and sand; 2—Quick opening valve; 3—Plugging joint; 4—Sensor cable; 5—Pore

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water pressure sensor; 6—Fractured rock; 7—Clay filling; 8—Fracture channel; 9—Water-bearing sand layer; 10

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—Hydraulic recharge; W① ~W③—Test point of water pressure1~3

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Fig. 1. Experimental equipment.

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2.2 Design and Monitoring of Model Scheme Considering the change of pore water pressure during the inrush of mixed water

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and sand inrush under different initial water heads and angles of fracture channel, the

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fracture shape adopts “\ /” type with a width of 3 cm at the top and 1 cm at the bottom.

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The inner surface of the fracture channel is smooth. The specific model scheme is

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shown in Table 1.

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Table 1 Parameters of model test. Water pressure (MPa)

Fracture angle (°)

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0.06

30, 60, and 90

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30, 60, and 90

The inrush channel in the test barrel can be adjusted to different angles by the

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hydraulic cylinder provided outside the barrel to achieve the inclination angle of the

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channel. The test first laid an inrush channel in the barrel body, and the fracture

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channel was made from two pieces of concrete plate, and adopted “\ /” type with a

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width of 3 cm at the top and 1 cm width at the bottom. Then the bottom valve is

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closed, the aquifer sand layer is laid directly in the test tank, and the short tube set is

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used in the upper part of the test tank as the water supply channel. The positions of the

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sensors are arranged in sequence according to the axial length of the channel. There

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are 3 water pressure sensors in total, with a spacing of 0.5m. One of the water

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pressure sensors is set at the position of the water inrush outlet to measure the change

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of water pressure along the axial length of the channel. After the water pressure

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sensors are installed and sand layers are laid, a layer of canvas and plastic cloth is laid

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on the top to simulate the upper aquifuge, and then the load is applied to the

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designated load by the pressure pump at the upper end of the soil. The aquifer at the bottom of the unconsolidated layer is composed of medium

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sand, and contains a small amount of gravel. The grain composition of the medium

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sand used in the model test is shown in Fig. 2.

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Fig. 2. Grading curve of medium sand.

The pore water pressure sensors for monitoring are selected as BSY2D type,

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produced by Xi'an Lanhua Sensor Factory of China, 32 mm in diameter, 55 mm in

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length, ranging from 0 to 100 kPa, with an accuracy grade of 0.5. Each pore water

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pressure sensor is calibrated before the test. The sensor signal acquisition is collected

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by the KSD series data display instrument into the computer, its parameters are shown

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in Table 2. The hydraulic pressurizing device is mainly composed of a high pressure

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pump device, the flow rate is 5 L/min, the power is 5.5 kW, and the maximum output

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pressure is 5 MPa.

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Table 2 Parameters of Digital Display Instrument Parameters

Indicator values

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Voltage

0~5V DC

Measurement resolution

1/60000

Basic error

±0.2%F⋅S

Number of channels

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Measurement control cycle

Number of channels ×0.2s

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Maximum channel speed

3 Variation of pore water pressure in the channel of mixed water and

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sand inrush transfer

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The water pressure change of the model 1 is continuously monitored by the

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sensors during the transfer process of the mixed water and sand inrush is shown in Fig.

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3.

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(a) Inclination angle 30°

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(b) Inclination angle 60°

(c) Inclination angle 90°

Fig. 3. Water pressure change curve of model 1.

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In Fig. 3, during the whole process of the mixed water and sand inrush, the water

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pressure generally shows a sharp decline after rising to an extreme value, and finally

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tends to be stable. According to the pore water pressure monitoring curves in fracture

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instantly when the water flowing fractured zone leads to the bottom aquifer after coal

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seam mining. Before the opening of the fracture channel, the pore water pressure at

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the monitoring point P3 increased fastest, and the extreme value was also the largest.

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P1 was the minimum. Once there is a water and sand inrush, the water pressure in the

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entire fracture channel decreases sharply and then stabilizes at a certain level over

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time until the end of the inrush. Among them, the stability value of P3 is the largest,

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and P1 is the smallest, which is mainly due to the narrow fracture in the location of P3,

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and the water pressure is more easily accumulated and increased here. During the

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inrush process, the flow of water is blocked because it is narrow here.

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In addition, during the stable stage of mixed water and sand inrush transfer, the

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signal of water pressure fluctuates strongly. Mainly because the lower part of the

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fracture channel is narrow and sand particles selected are not of the same size and

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containing a small amount of silt, the silty sand will easily disintegrate, so a certain

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amount of sand bodies will accumulate near the inrush opening, and the hydraulic

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gradient will increase. Hydraulic gradient, when reaching its critical value, will cause

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the disintegrated and accumulated sand particles to flow, and water and sand inrush

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will continue. Besides, at the beginning of water and sand inrush, the amount of sand

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inrush is greater than draining sand through the outlet. Some sand accumulates at the

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outlet of inrush, and blocks the channel of inrush, resulting in poor drainage

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conditions. At this time, the hydraulic gradient at the place where the sand

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ACCEPTED MANUSCRIPT accumulates is smaller than the critical hydraulic gradient of sand inrush but greater

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than the critical value of the seepage, that is, no more inrush of sand, but seepage

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failure occurs. It is because the seepage and the suffosion have taken away the sand at

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the outlet of inrush, and drainage has begun to open. When the hydraulic gradient

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reaches the critical hydraulic gradient of sand inrush, sand inrush will occur again,

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followed by seepage destruction. It is just like this that the cyclical process of “sand

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inrush — seepage — sand inrush again” occurs continuously. As a result, when the

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fracture channel is opened, the water head pressure drops rapidly, and it will a long

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time before the process of sand inrush stops, which corresponds to the violent

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fluctuation of the water pressure signal in the stable phase during water and sand

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inrush. This will lead to large-scale inrush damage of mixed water and sand flow

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during mining.

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In mixed water and sand inrush process, each measuring point is different from

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the position of the outlet of inrush. The water level near the inrush outlet is high,

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while that far from the inrush outlet is low. The water pressure shows a certain

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hydraulic gradient property. It is this kind of hydraulic gradient that causes the flow of

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mixed water and sand to burst out along the fractured channel. The value of the water

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pressure along the channel gradually increases, mainly because the increase of the

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water pressure gradient in the aquifer overcomes part of the flow resistance loss.

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Taking 90° as an example, point P1 is near the bottom aquifer, and the maximum

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value of monitored water pressure is 61 kPa. Point P2 is located in the middle of the

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Point P3 is close to the outlet of water inrush, and the maximum value of the

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monitored water pressure is 67 kPa. Taking into account that the initial water head is

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60 kPa, the monitored water pressure basically conforms to the maximum value of the

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water pressure at each measuring point and satisfies Darcy’s law.

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Model 1 compared the different angles of fracture channel, and took monitoring

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points P1 and P3 as examples. The water pressure changes during the inrush of water

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and sand are shown in Fig. 4-5.

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Fig. 4. Water pressure curve at different angles of model 1 (monitoring point P1).

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Fig. 5. Water pressure curve at different angles of model 1 (monitoring point P3).

Point P1 is located at the top of the fracture channel, near the overlying loose

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aquifer, and the monitored water pressure is closer to the change in water pressure in

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the aquifer. As shown in Fig. 4, at 30°, the maximum of water pressure at the

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measuring point P1 is 58 kPa, and then it is reduced to 39 kPa after the fracture

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channel opens with a reduction of 19 kPa; in the case of 60°, the maximum value is

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60 kPa, and then it drops to 38 kPa after the fracture channel is opened with a

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decrease of 22 kPa; in the case of 90 °, there is a sharp drop from the maximum 63

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kPa to 40 kPa with the largest decline of 23 kPa. This shows that in the same initial

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head of the bottom aquifer, with the increase of the angle of the fracture channel, the

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water pressure increases in the upper part of the fracture channel increases. The

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maximum value of the water pressure also increases, and when the fracture channel is

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opened, the sudden drop in the water pressure also increases.

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Point P3 is located at the end of the fracture channel near the outlet of water

ACCEPTED MANUSCRIPT inrush. Once the fracture channel is opened, P3 first detects the sudden drop in water

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pressure. In Fig. 3, at different angles, the sudden drop in water pressure at point P3 is

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the fastest, followed by that at P2, and that at P1 is the slowest, but the delay time is

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shorter than 5 s. As shown in Fig. 5, with the increase of the angle of the fracture

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channel, the increase in water pressure at the bottom of the fracture channel and near

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the outlet of water inrush is greater than that in other locations, and the maximum of

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water pressure is also increased. When the fracture channel is opened, the magnitude

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of the sudden drop in water pressure also increases.

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In Fig. 3, the curve of the water pressure generally includes two stages: the rising

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stage and dropping sharply to stable stage, changing the test pressure and increasing

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the water pressure from 0.06 MPa to 0.08 MPa. The test angles are 30°, 60°, and 90°,

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respectively. Other conditions are unchanged. The test results are shown in Fig. 6.

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Comparing Fig. 3 and Fig. 6, it can be seen that when the angle of the fracture

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channel is the same, the water pressure in each measuring point increases

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correspondingly with the increase of the initial water head pressure and shows a

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certain linear property. The 0.08 MPa increase of the water pressure at each measuring

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point is also greater than the water pressure the 0.06 MPa increase in the curves.

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When the fracture channel is opened, the magnitude of 0.08 MPa sudden drop in

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water pressure at each measuring point is also greater than that of the water pressure,

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which is 0.06 MPa. During the test, when the fracture channel is opened, more water

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and sand mixture gush in the model 2, but taking less time. It can be seen that the

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ACCEPTED MANUSCRIPT greater the water pressure in the aquifer, the faster the increase of the water pressure

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in the rising stage of the fracture channel, the sharper the drop in the water pressure

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during inrush, the more violent the mixed water and sand inrush. Therefore, mine

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drainage and water head pressure reduction of the aquifer can slow down the transfer

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and inrush of the mixed water and sand inrush.

(a) Water pressure curve of model 2 (inclination angle 30°)

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(b) Water pressure curves at rising stage of model 2 (inclination angle 60°)

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(c) Water pressure curves at inrush stage model 2 (inclination angle 90°) Fig. 6. Water pressure change curve of model 2.

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At the same time, the relationship between sand inrush rate and time at the

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overflow outlet of the channel obtained from the test observation shows that the sand

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content of the gushing material decreases with time. At the beginning, there is a large

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short duration. With the passage of test time and continuous completion of the

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seepage deformation and failure, the sand content of the gushing material gradually

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decreases. Finally, after the collapse doline is formed, the sand content in the gushing

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material gradually declines to zero, and the gushing process of mixed water and sand

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inrush is completed.

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4 Conclusions

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The test model of the mixed water and sand inrush transfer is designed and

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manufactured to simulate the transfer and inrush process of the mixed water and sand

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inrush in the “\ /” type fracture channel which is wide at the top and goes narrower

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down to the base. The variation characteristics of water pressure in different positions

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of the fracture channel were revealed based on the variations of inclination angle of

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the fracture channel under the water pressure of 0.06MPa and 0.08MPa. The

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characteristics of the mixed water and sand inrush in the fracture channels were

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researched.

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(1) During the whole process of the mixed water and sand inrush, the water

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pressure generally shows a sharp decline after rising to an extreme value, and finally

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tends to be stable. The curve of the water pressure generally includes two stages: the

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rising stage and dropping sharply to stable stage.

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(2) Under the condition of initial water head pressure of 0.06 MPa, the water

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pressure increases with the increase of the angle of the fracture channel, the amplitude

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of the sudden drop increases after it is opened, when the sudden inrush process is

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rapid, and the amount of water and sand mixture is large. (3) In the case of the same angle of the fractured channel, when the initial water

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pressure increases to 0.08 MPa, the water pressure increases rapidly, and the inrush

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process becomes more violent. Therefore, mine drainage and water head pressure

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decline of the aquifer can be used as a feasible method for alleviating sudden inrush

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of mixed water and sand flow.

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(4) In the inrush process of mixed water and sands flow, the water pressure

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shows a certain hydraulic gradient property. It is this kind of hydraulic gradient that

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causes the flow of mixed water and sand to burst out along the fractured channel. The

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value of the water pressure along the channel gradually increases, mainly because the

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increase of the water pressure gradient in the aquifer overcomes part of the flow

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resistance loss.

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Acknowledgments

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Financial support for this work is provided by the National Key R&D Program

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of China (2017YFC0804101), the Fundamental Research Funds for the Central

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Universities (2017ZDPY11), the National Natural Science Foundation of China

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(40802076), the Science and Technology Planning Project of Jiangxi Provincial

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Education Department (GJJ150601) and the Doctoral Science Foundation of ECUT

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(DHBK2015101), all of which are gratefully acknowledged. We also would like to

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express our acknowledgments to editors and the anonymous reviewers who had gave

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us useful comments to help improved the earlier version of the manuscript.

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Mech. Min. 42 (3), 350-360.

Zhao Y., Li P.F., Tian S.M., 2013. Prevention and treatment technologies of railway tunnel water

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346

Zhang J.C., 2005. Investigations of water inrushes from aquifers under coal seams. Int. J. Rock

inrush and mud gushing in China. J. Rock Mech. Geotech. Eng. 5 (6), 468-477.

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ACCEPTED MANUSCRIPT Topic: Simulation test on mixed water and sand inrush disaster induced by mining under the thin bedrock

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Highlights

(1) The test model of the mixed water and sand inrush process was designed and

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manufactured.

(2) The mixed water and sand inrush process in the “\ /” type fracture channel

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was simulated.

(3) The variation characteristics of water pressure in different position of the fracture channel were revealed through analyzing “\ /” type fracture form of various

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inclination angles of the channel under the water pressure of 0.06 MPa and 0.08 MPa.

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(4) The measures to prevent water and sand inrush disasters were proposed.