Experimental Study on the Mud-Water Inrush Characteristics through Rock Fractures

Zhao, YanLin; Zhang, LianYang

doi:https://doi.org/10.1155/2018/2060974

Advances in Civil Engineering

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Damage and Fracture Behavior of Rock

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Research Article | Open Access

Volume 2018 | Article ID 2060974 | https://doi.org/10.1155/2018/2060974

Experimental Study on the Mud-Water Inrush Characteristics through Rock Fractures

YanLin Zhao^1,2and LianYang Zhang³

Academic Editor: Venu G. M. Annamdas

Received31 Aug 2017

Accepted23 Apr 2018

Published13 Jun 2018

Abstract

Fractured rock masses, which are widely distributed in the south of China, have an impartible relationship with mud-water inrush. In order to study the change laws of mud viscosity under different moisture content, flow property, and mechanism of mud-water inrush in fractured rock masses, a NDJ-8S rotary viscometer and a simulation model are used to carry out a series of experimental studies. Tests of mud viscosity show that mud viscosity depends on moisture content, and the relationship between viscosity and moisture content was proposed. Based on mud viscosity results, physical model tests’ results indicated that the process of mud-water rush can be divided into four stages: that is, the preparation stage of mud-water mixture burst, the seepage period of mud-water mixture inrush, the nonstable period of mud-water mixture outburst, and the stable period of mud-water mixture outburst. The excavation disturbance, high water pressure, and effect of erosion and corrosion of water were identified as key factors which lead to mud-water inrush in fractured rock masses

1. Introduction

Plenty of underground engineering constructions such as railway tunnel and coal extractions are threatened by various kinds of water inrush and mud gushing in China [1–4]. Research methods of theoretical analysis, laboratory experiments, field investigations, and numerical simulations have always been performed by many scholars and experts to discover the mechanisms and influential factors of water inrush and mud burst in fractured rock masses [5–8]. Accordingly, numerous prevention and treatment technologies were proposed. Physical tests and site investigations of mud-water inrush show that the main factors, related to water-mud burst, are the supply of water, pressure of water, geological structure, engineering excavation, and so on [9–11]. On one hand, the theoretical analysis and numerical simulations also come up with some effective and practical ways to prevent and treat mud-water burst disasters [12–14]. On the other hand, for setting risk countermeasure, eliminating and reducing the adverse influence of risk, many ways of risk evaluation and prediction about mud-water burst in fractured rock masses were raised to disclose potential risk factors [15–18].

In recent years, there have been more than 100 cases of mud-water burst observed in the southwest mountain, especially in Karst areas, such as Yunnan, Guizhou, and Sichuan, causing serious losses of human life and property and deterioration of construction conditions [19–22]. The fractured rock masses in Karst zones as a complex and widespread geological conditions in China, it is extremely easy to induce geologic hazard especially mud-water gushing when the underground engineering goes through it [23–25]. At present, the research on mud-water outburst mechanism is mainly concentrated on reaction of water-rich area, damage of floor strata, and prevention and treatment [26, 27] in addition to some technical solutions for special conditions that depended on different engineering backgrounds. Although many studies on fluid-solid coupling response in coal mass and fractured rock mass have been performed, indicating that the permeability characteristics of coal rock mass are closely related with their mechanical behavior [28–30], the previous studies did not research mud viscosity, pressure of mud-water outburst, and flow laws of mud-water on rock fractures.

Essentially, solid clay cannot flow, only when mixed with water, it can be converted from solid state into fluid state with different viscosity; at the same time, under the pressure of mud-water which exceeds ultimate bearing capacity of rock pillars between Karst zones and tunnel excavation face, mud inrush and water outburst accidents are easily observed. So in this paper, the change laws of mud viscosity under different moisture content have been studied. The study establishes a similar material simulation model called the test device of mud-water burst in confined Karst to experimentally research the flow characteristics of mud on channels under different mud-water pressures. Through laboratory experiments, the process of mud-water outburst in Karst areas was performed many times through which we can deeply understand the comprehension of mud gushing in Karst areas and understand the improving and enriching mechanisms of the mud burst in Karst areas.

2. Tests of Mud Viscosity

2.1. Soil Sample and Experiment Equipment

Selected soil in these tests is widespread in the south of China called red clay; the amount of montmorillonite is 60–65% which is characterized by easily dispersed and high rate of making mud, and the chemical equation of montmorillonite is (Al_1.67Mg_0.33)[Si₄O₁₀][OH]₂·nH₂O. The major chemical components of red clay are SiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, and so on, and natural moisture content is 6–10%. Firstly, the soil sample is dispersed and exposed to sun for 7 days (Figure 1); during the period of exposure, a wooden stick was used to crush the clay particles successively and then dried under the temperature of 105°C–110°C for 24 h, and the standard soil sieve is used to select three different particle sizes which are 5–10 mm, 2.5∼5 mm, and below 2.5 mm. According to the method of earthwork test GB/T50123-1999, using the digital display measurement instrument of soil liquid-plastic limit to measure soil moisture content under liquid and plastic limit stages, the measurement results are obtained, and these results show that the moisture content of the plastic limit is 19.8% and liquid limit is 30.2%. The measurement instrument of mud viscosity is the NDJ-8S rotary viscometer as shown in Figure 2.

2.2. Tests’ Design and Results Discussion

Three different kinds of particle size clay are divided into four groups: the particle size of 5–10 mm is short for coarse particle clay, 2.5∼5 mm is short for medium particle clay, below 2.5 mm is short for fine particle clay and mixed particle size clay. After three mud viscosity experiments on each group were conducted, the mean was calculated. Moreover, the beakers and clay samples of each group were labeled as follows: #1, #2, and #3 for coarse clay; #4, #5, and #6 for medium clay; #7, #8, and #9 for fine clay; and #10, #11, and #12 for mixed clay. According to the national standard GB/T20973-2007, mud viscosity tests were performed. Table 1 shows the experimental parameter and design of the soil sample.

Based on the tests’ results (Figure 3), the changes of mud viscosity can be divided into three stages: that is, the mud viscosity declines rapidly at the stage of ab, the decline is slow at the stage of bc, and the decline is kept unchanged at the stage after the point c.

As is shown in Figure 3, when the moisture content is constant, mud viscosity does not vary with particle size, and it means that particle size has little ever no influence on mud viscosity which depends on moisture content completely.

At the stage of ab, the water in the mud is no longer in the form of combined water, but it is in the form of free and gravity water which can dissolve and erode some soluble bonding material between particles; furthermore, it also has some physical-chemical effects on clay particles, such as lubrication and softening. These poor effects will make the spacing increased between particles, and the chance of interaction reduces, bonding force decreases, and the soil skeleton becomes loose. Meanwhile, at the stage of ab, the viscosity is extremely sensitive to moisture content.

At the stage of bc, mud was in a state of flux completely; moreover, the direction and speed of motion of particles were controlled by water. During the process of mud flow, there is no drag force between particles, but for those which cannot dissolve, during the process of flowing, the sorting phenomenon occurred, which leads to rearrangement and sedimentation of particles and changes mesostructure and mechanical properties of particles. It is the sorting phenomenon caused by water that has large effects on the soil skeleton. But at this stage of bc, mud viscosity is less sensitive to moisture content than the stage of ab.

At the stage after the point c, the viscosity of the mud is tending to be the viscosity of pure water 5.46 MPa·s, and the bonding force and the interaction between the particles in the mud can even be ignored. It is evident that when the moisture content remains the same, the change of viscosity does not vary with the particle size.

The foregoing analysis shows that the mud viscosity does not depend on the particle size but depends on moisture content. Meanwhile, the varying tendencies of each viscosity curve are consistent, no matter how complex the process of the interactions about water and soil is. For further investigation of the relationship between the viscosity and moisture content, the mean method was applied to process each mud viscosity curve of different particle sizes, and then, MATLAB software was used to perform data fitting (Figure 4).

As is shown in Figure 4, at the stage of ab, when the moisture content is below 45%, viscosity will decline rapidly from 10462 MPa·s to 796 MPa·s which is caused by the increase of moisture content in mud; at the stage of bc, when the moisture content is 45%–56%, due to the action of the water, the bonding strength of the mud particle was deteriorative, aerosols were increased, and mud fluidity was enhanced, but with the increase of moisture content in mud, the rate of decline of viscosity slowed significantly; when the moisture content was above 56%, viscosity slowed down extremely as the moisture content of mud increased; finally, change in viscosity remained unchanged (5.46 MPa·s). Mathematical models of the relationship between moisture content and viscosity were obtained by fitting experimental data:where is the moisture content and is the viscosity.

3. Tests of Mud-Water Inrush

3.1. Equipment and Method

A test device of mud-water inrush through rock fractures was designed to study mud-water inrush characteristics though rock fractures. The skeleton of mud-water inrush through rock fractures is shown in Figure 5, and the test device of mud-water inrush through rock fractures is shown in Figure 6. As shown in Figure 5, the box system in the test device can be divided into three parts: water tank, mud tank, and upper tank, and the size of each part, in turn, is 300 mm × 300 mm × 100 mm, 300 mm × 300 mm × 400 mm, and 300 mm × 300 mm × 800 mm, respectively. The inside of the upper tank was stacked with cement bricks with the size of 70 mm × 70 mm × 70 mm as shown in Figure 7. Mud with a moisture content of 19.8%–30.2% was filled between bricks which were piled up in the upper tank. After mud consolidation, mud and bricks are formed as a whole. The space among the bricks can be considered as rock fractures. The mud tank was used to store mud. In this laboratory mud-water outburst tests, mud with a moisture content of 52% was chosen as mud-water outburst tests’ material, and the mud-water pressure values were set to 0.5, 0.8, 1.0, 1.2, and 1.5 MPa, respectively. At the same time, the mud tank and upper tank are connected through the porous steel plate. On one hand, the porous steel plate can bear the load from all cement bricks which were accumulated in the upper tank; on the other hand, holes in the porous steel plate can allow mud stored in the mud tank to pass through and enter into the upper tank under the water pressure from the water tank. There is a pressure conversion piston between the mud tank and the water tank. The pressure regulator system is made up of an air compressor, a servo pump, and a stabilizer system. During the process of tests, the water pressure in the stabilizer system was monitored in real time by the servo pump; in addition, another important function of the servo pump is to ensure the presence of adequate water in the stabilizer system. If the water pressure in the stabilizer system is below the preset value which was captured by the servo pump, then the air compressor works, until the water pressure reaches the preset value. According to the field investigation of engineering geological conditions in Karst zones, the mud with a moisture content of 19.8%–30.2% is used as the fillings in the mud tank.

Experimental principles can be summarized as follows: the space among cement bricks (70 mm × 70 mm × 70 mm) to stack each other (11 rows and 4 columns) was regarded as the rock fractures. Meanwhile, the space among bricks was filled with hard plastic clay; cement bricks, which were stacked upon each other, were used to simulate the rock skeleton. After the hard plastic mud was consolidated, the fractured rock mass which was composed of bricks and clay was formed in the upper tank. The fracture widths in the horizontal and vertical directions are about 7 mm and 3 mm, respectively. The water tank is connected to the external stabilizer system; moreover, the pressure and quantity of water in the external stabilizer system were provided by the air compressor and servo pump which were controlled by the computer. Slurry with a moisture content of 52% in the mud tank could pass through the porous steel plate into the top box under water pressure; meanwhile, the physical state of consolidated clay would change from solid to mobile liquid which can flow through the fracture.

3.2. Results and Discussion

Based on the observation and calculation from the physical modeling, the curves of the quality of mud-water mixture and time under different water pressures are obtained (Figure 8 and Table 2).

(a)

(b)

(c)

(d)

(e)

Based on the test results that were described above (Figure 8), the research results indicate that the changes in the mud-water mixture inrush can be divided into four stages: (1) The preparation stage of mud-water mixture burst (OA curve segment): at this stage, seepage channels of mud-water mixture were not formed, and thus, there was no mud-water mixture collected at the outlet of the test apparatus. Meanwhile, hard plastic clay, which was filled in the fissure, developed into soft-plastic and fluid-plastic states under the effect of washout and erosion of mud-water mixture gradually. This kind of the evolution stage of hard plastic clay from immobility to mobility was named as the preparation stage of mud-water mixture gushing, and the duration time of this phase is T_OA. The experiment also revealed that the greater the augment in mud-water pressure, the shorter the duration time of the incubation period. When the pressure increased from 0.5 MPa to 1.2 MPa, the duration of this stage would decrease from 98s to 45 s. But when the mud-water pressure increases to 1.5 MPa, the incubation stage of mud-water mixture outburst fails to be observed, which showed that the hazard of mud-water inrush with the higher pressure has some characteristics of suddenness and violence. (2) The seepage period of mud-water mixture inrush (AB curve segment): mud-water inrush moves in fracture, and the seepage channels of mud-water forms gradually. The seepage velocity is slow, and the duration time of this phase is T_AB; moreover, with the increase of mud-water pressure, the duration time of this phase shortens. When the pressure is increased from 0.5 MPa to 1.5 MPa, the duration time of this seepage stage would decrease from 23 s to 5 s. (3) The nonstable period of mud-water mixture outburst (BC curve segment): at this phase, mud-water mixture is continuously scouring channels and gushing forward to external. The velocity of mud-water mixture outburst is constant; in addition, with the increase of mud-water pressure, the energy stored in the mud tank and the maximum velocity of mud-water mixture outburst would be increased. The experiment showed that when the pressure is increased from 0.5 MPa to 1.5 MPa, the maximum velocity of mud-water mixture inrush of this stage would increase from 1.155 × 10⁻⁴ m³·s⁻¹ to 3.380 × 10⁻⁴m³·s⁻¹. (4) The stable period of mud-water mixture outburst (CD curve segment): at this stage, the passages of mud-water inrush form, keeping a near constant velocity of mud-water mixture inrush. This stage lasts for a relatively longer time.

4. Conclusions

This paper conducted a series of mud viscosity experiments and chose mud-water with a moisture content of 52% as the tests’ material. The process of mud-water in rock fractures was simulated. The research results can be drawn as follows:(1)The mud viscosity does not depend on the particle size but depends on moisture content. Moreover, the relationship between moisture content and viscosity can be represented by the exponential function.(2)The process of mud-water inrush can be divided into four stages: that is, the preparation stage of mud-water mixture burst, the seepage period of mud-water mixture inrush, the nonstable period of mud-water mixture outburst, and the stable period of mud-water mixture outburst.(3)The duration time of the preparation stage and seepage period is shorter, and the maximum velocity of mud-water mixture inrush is larger with the increase in mud-water pressure.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research is supported by the National Natural Science Foundation of China (nos. 51774131, 51274097, and 51434006), the CRSRI Open Research Program (CKWV2017508/KY), and the Open Projects of State Key Laboratory of Coal Resources and Safe Mining, CUMT (no. SKLCRSM16KF12).

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Copyright

Copyright © 2018 YanLin Zhao and LianYang Zhang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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