In western China coal mines, the mining-induced caving zone is regarded as a main pathway for water and sand inrush mixture hazards. The paper experimentally studied the flow behavior and the mechanism of water and sand mixture through mining-induced caving zones. Transport experiments are performed by using a laboratory-scale model, and the caving zone is modelled by using different sizes of glass beads. Four different sand sizes are used for the sand layer. The test results reveal that the mass flow rate of sand and water mixture increases with the increase of the initial water head. And an equation is proposed for the mass flow rate of sand and water mixtures that correctly reproduces the data for all the conditions. In addition, the sudden decreases in water head loss is monitored at the commencement of the water and sand flow, which would result in a large number of sand particles that rapidly start up and make the kinetic energy transfer from water to sand.

1. Introduction

With exhaustion of shallow coal resources in eastern China, the coal mining activities in western China has been raised, which result in consequent increment of geohazards. One of the major challenges need to be addressed is the water and sand mixture inrush [16]. As the main pathway for gas, water, and sand, the flow behavior of the geological fractures and faults has been studied by using theoretical analysis, numerical simulations, and in situ and laboratory tests [712]. Contrary to this, little research has been carried out to study the flow behavior of the mining-induced caving zone, especially for the sand and water mixture. Nevertheless, the discharge of mixture has often been neglected in the previous studies. However, it is frequently reported that the water along with sand could flow into the working face through the rock fragmentations in the caving zone to induce water and sand gushing hazards in underground engineering [1316], as shown in Figure 1. Note that the caving zone could directly contact with the unconsolidated aquifer due to the thin bedrock. This kind of water and sand inrush is especially common in western China due to its typical geological characteristics and the high-intensity underground mining [17, 18]. Therefore, it is necessary to understand the flow behavior of water-sand mixtures in caving zone.

Obviously, the flow behavior of the water and sand mixture in the caving zone is quite different from that of water. The process of water and sand mixture through the caving zone involves the conveying of two-phase mixture of sand and water through porous medium, since the caving zone in the mining is full of broken rock mass, which generally regards as a kind of porous medium [1921]. In this paper, the movement of water and sand mixture in the rock fragments in the caving zone can be simplified to that in a cylinder. All the experiments were conducted by using laboratory-scale packed columns under well-controlled hydraulic conditions. Sands with three different grain sizes were employed as the model particles. The porous medium was packed with glass beads, which is assumed to be the simplest representation of an ideal caving zone at the laboratory scale. Then, a series of experiments are performed to examine the behaviors of flow process under various experimental conditions. The results are helpful for the control and treatment of the water and sand inrush hazards.

2. Experimental Materials and Method

2.1. Experimental Setup and Materials

The experiments described in this paper are performed in two transparent cylinder cells with the same diameter of 200 mm, as shown in Figure 2. The laboratory experimental setup in this study is modified based on that used to study the transport behavior of sand/mud/water mixture in Liang et al. [22]. The height of the upper cylinder is 1200 mm; the bottom one is 300 mm. The same-sized glass beads were placed into the bottom cylinder in layers, with the diameters of 25 mm, 21 mm, and 16 mm, respectively. The porosity of the mining-induced caving zone usually ranges from 0.3 to 0.45. The glass beads were closely packed at a porosity of approximately 0.48 in order to reduce the difference between the ideal and real broken rock mass in the porosity. The bottom layer of glass beads was fixed in place with chicken wire. The control value was placed above the top of the porous medium, so that the sand and water would instantaneously flow into the porous medium of glass beads when the control value was open after the start of the experiment. The upper transparent cylinder was filled with sand and water up to a height of and , respectively. The value of in the paper was set as 500 mm, while the was set to different values, such as 550, 600, 700, 800, 9000, and 1000 mm. A bucket was used to collect the outflow from the porous medium, in which the discharge of mixture of sand and water was measured at intervals of 1 s with the electronic scale.

The sand used in these experiments is natural river sand sampled from Xuzhou city in Jiangsu Province, China. The particle size has a significant effect on the dynamic properties of soil [23, 24]; the grain size distribution curves in the paper are shown in Figure 3. Note that the grain size of sand that easily undergoes sand and water mixture inrush usually ranges from 0.005 to 2 mm. Therefore, narrow-screened sand with four different particle sizes was used in this study: 0.1-0.25 mm, 0.25-0.5 mm, 0.5-1.0 mm, and 1.0-2.0 mm, respectively. The majority of the grains of sand are subangular in shape. The sand was washed and then sieved in order to reduce the influence of clay. In the preparation of the sand samples, the dry bulk density was maintained as 1.57 g/cm3. The amount of sand required was determined so as to correspond to this dry bulk density.

Water pressure transducers were placed at a distance of 130, 330, and 430 mm from the bottom surface of water and sand column to monitor the fluid pressure distribution during sand-water mixture inrush.

2.2. Experimental Procedure

After the porous medium and sand column were packed, water was then gradually injected into the upper transparent cylinder until reaching the specified height. Subsequently, the flow control valve was opened, and discharged sand and water mixture whose quantity was recorded by using the electronic scale then flowed into the bucket. After no more sand and water flowed out, the transparent cylinder was washed to remove any residual sand particles. The experimental procedure was then repeated by changing the size of the porous medium and sand column. Each experiment was repeated at least three in order to minimize any possible errors.

3. Results and Discussion

3.1. Mass Flow Rate

Figure 4 shows the variations in the quantity of accumulated discharged sand and water mixture which falls from the porous medium. Note that with the increase in the height of initial water head, the total discharge mass increases and the duration of water and sand mixture inrush decreased. In addition, the discharge curves can also be approximately divided into three stages. At the first stage, there is no, or very little, sand and water mixture flow from the porous medium. With time, the cumulative mass of sand and water gradually increases. In this stage, the total discharged mass is increasing linearly with time, which means that the flow rate of water and sand is almost constant. Finally, the total mass shows different responses depending on the initial height of water head at the ending stage. At higher water head, i.e., 900 mm, there is an abrupt increase in the total discharged mass due to the sudden release of water after the sand particles totally flow from the porous medium. However, the amount of total discharged mass mixture does not significantly increase when the water head is 600 mm, and the total discharged mass is relatively small compared to that in the higher water head. That may be because there is still some sand left in the porous medium after the completion of water and sand mixture inrush.

Since the water sand mixture per unit time is almost constant in the second stage, then the slope rate of the second stage is considered as the mass flow rate of water and sand mixture, which is denoted as . The is about 0.41, 0.55, and 0.72 kg/s for , 700, and 900 mm, respectively, which is conducted under the condition of  mm and -0.5 mm.

3.2. Mass Flow Rate under Different Conditions

As expected, the mass flow rate is related to the initial water head, the size of sand particles, and porous medium. A series of experiments were carried out to study the influence of these factors on the mass flow rate; the results are shown in Figure 5. Figure 5(a) shows the influence of initial height of water head on the mass flow rate. Note that mass flow rates increase with the increase of initial water heads, while the magnitude of the increase in the mass flow rate for was greater than that for . For example, when the initial infiltration heads increase from 600 mm to 1000 mm, the mass flow rate for substantially increases from 0.41 kg/s to 0.93 kg/s, and the mass flow rates for only increases from 0.16 kg/s to 0.36 kg/s. Figure 5(b) shows the relationship between the mass flow rate and average diameter of the sand particles. It can be observed that the lager the glass beads, the higher the discharge rate. This behavior is attributed to the fact that the larger diameter of glass beads led to quite large interstices of the pores. Interestingly, the discharge rate increases linearly with increasing diameter of sand particles. This is in contrast with prior work with dry sand flow, where the discharge rate decreases with the increasing size of grains [25], which can be explained by the fact that the mechanics of the dry sand flow is quite different from that of the sand water mixture flow. In addition, there was only a slight difference in the mass flow rate when the sand particle is small. With the increase of the sand grain size, the influence of the interstices between porous medium has increased, which implied that the blocking effect of porous medium has obviously increased.

In this study, a modified equation to fit the rate of mass discharge of the sand and water () is developed as follows: where is the apparent density; is the acceleration of gravity; is the diameter of the glass beads; is the average diameter of the particles; and and are constant values that are related to the shape and roughness of the sand particles and glass beads. In this experiment, is approximately 1.57 g/cm3, , and . The predicted discharge rate with the use of Equation (1) is compared with that obtained from the tests, as shown in the dashed curves in Figure 5. It is observed that there is an excellent agreement between the experimental results and predicted values.

3.3. Water Pressure

The height variations of water head vs. time for and and the corresponding schematic during the sand and water inrush are illustrated in Figure 6. Note that there is an obviously decrease in the water head as soon as the water and sand inrush starts. After the sudden reduction, the height of water head regains to constant value until the sand column is lower than the transducers. Then, the height of the water head linearly decreases with the time after the transducer is lower than the sand column. With the water column passing through transducers, the water pressure goes to zero. In addition, the changes of the water head are related with the position of transducers. The water head loss increases with the increase in the position of transducers. At the point of transducers No. 3, the water head at time of is even lower than zero, which indicated that the water head loss is much greater than the initial water head, while there is no significant difference in water head for transducers No. 1.

The phenomena of negative pore water pressure have been reported in the dewatering well seepage [26]. When there is water seepage from the sand layer, the water would exert force on the sand particles, which is directly proportional to the water head loss; the well-known equation for seepage force () is written as where is the water loss between the samples, is the distance travelled by the water, and is the unit weight of water. At the commencement of the water and sand flow, the sudden increased water head loss results in the rapid increase in seepage force that is exerted on the sand particles. On other hand, the increased seepage force and gravity could contribute to a large number of sand particles rapidly starting up; then, the kinetic energy is transferred from water to sand. That may be the main reason why water and sand mixture inrush hazard in coal mining usually contains large volumes of sand with a sand particle concentration. The sand and water mixture inrush hazard in coal mining refers to water along with a large volumes of sand burst into the panel, which can be regarded as a kind of granular flow. This kind of negative water pressure behavior has been observed in granular dynamics research [27, 28], while the mechanism of such behavior in granular dynamics research was not entirely understood; the sudden reduced water pressure did have a significant negative on the sand and water mixture inrush, which we will consider in the future.

3.4. Discussion

In this paper, the experimental work is not intended to simulate any realistic project; thus, the caving zone full of broken mass rock has been idealized into a homogeneous porous medium in order to reduce the possible difficulties. However, the voids in the simplified homogeneous porous medium are continuous. There is no immobile domain or in a homogeneous porous medium compared with the caving zone packed with broken mass rock. In practice, the absence of an immobile domain would greatly reduce the possibility of blocking the caving zone during sand inrush. In addition, the effect of clay particles was not considered in this study, which has already confirmed that the location, content, and composition of clay particles also influence the solid-water mixture inrush [11, 29, 30]. In future studies, the properties of these factors would be considered.

The experimental results have not been combined with the actual project, but the findings from this study have engineering implications. During coal mining in eastern China, pumping water to reduce the water head has been widely used to prevent water or sand inrush accidents. However, this measure could drastically worsen the fragile ecological environment of western China. Note that reducing the broken rock mass in the caving zone can also effectively prevent water and sand mixture inrush, which can help to migrate geohazards. In some cases, the aggregates or sands can therefore be poured into the caving zone to control the sand inrush to the panel in the coal mines.

4. Conclusion

A series of scaled physical experiments have been carried out to study the mass flow rate and water pressure under different conditions, including the initial height of water head () and the size of glass beads and sand (). The following conclusions are made based on the experimental results: (1)The total discharge mixture mass flow rate increases with the initial height of the water head. Also, the increasing initial height of the water head would result in earlier completion of water and sand inrush(2)High initial water head and large size of glass beads facilitated discharge of water and sand mixture. The mass flow rate increase in a directly proportional manner increases in the sand size for the same diameter of glass beads. The relationship between mass flow rate and these factors can be expressed by using the modified equation(3)When the water and sand mixture starts, the monitored water head suddenly decreases, then returns to a stable value. The sudden increase in water head loss would result in a large number of sand particles rapidly starting up, and the kinetic energy is transferred from water to sand

Data Availability

All data included in this study are available upon request by contact with the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.


The authors would like to acknowledge the financial support from the National Natural Science Foundation of China under Grant No. 41902283 and Henan Institution of Higher Education key scientific research project No. 22A170013.