Received 05 March 2009; accepted 10 June 2009 Project supported by Qing Lan Project of Jiangsu, China Corresponding author. Tel: +86-516-83591015; E-mail address: dongqh@126.com

Pore pressure fluctuations of overlying aquifer during residual coal mining and water-soil stress coupling analysis

DONG Qing-hong1,2, SUI Wang-hua1,3, ZHANG Xiao-cui1, MAO Zeng-min1

1School of Resources and Geosciences, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China 2School of Geology and Exploration Engineering, Xinjiang University, Urumqi, Xinjiang 830046, China

3State Key Laboratory for Coal Resources and Mining Safety, China University of Mining & Technology, Xuzhou, Jiangsu 221008, China

Abstract: Three test models and a simulation model were constructed based on the prevailing conditions of the Taiping coalmine in order to analyze pore pressure fluctuations of an overlying aquifer during residual coal mining. As well, the relation between pore pressure and soil stress was evaluated. The model tests show the vibrations of pore pressure and soil stress as a result of mining activities. The simulation model tells of the response characteristics of pore pressure after mining and its distribution in the sand aquifer. The comparative analysis reveals that pore pressure and soil stress vibration are activated by unexpected events occurring in mines, such as collapsing roofs. An increased pore pressure zone always lies above the wall in front or behind the working face of a mine. Both pore pressure and vertical stress result in increasing and decreasing processes during movements of the working face of a mine. The vibration of pore pressure always precedes soil stress in the same area and ends with a sharp decline. Changes in pore pressure of sand aquifer are limited to the area of stress changes. Obvious changes are largely located in a very small frame over the mining face. Keywords: pore pressure fluctuations; water-soil stress coupling analysis; residual coal mining

1 Introduction

There are several mining areas in the east and north of China that have increased coal production by im-proving the upper limit level under Quaternary aqui-fers, but there is still a large number of leftover bits and pieces of coal as residual to be developed where the thickness of the overburden rock mass above the coal seams varies largely from 0~60 m in height. Due to the shallow overburden rock strata, the overlying aquifer in the Quaternary above the coal deposits be-comes a serious impediment to mine safety which cannot be determined from traditional experience and regulations. At the same time, aquifers in the uncon-solidated layers of the Quaternary are important water resources for local production and society, requiring that ground water in the Quaternary must be effec-tively protected during mining processes. Given this condition, water resource protection and water proof-ing become two constraints in residual coal mining. It should be pointed out that these are important ele-ments of green techniques in coal mining[1]. As well,

the accumulated experiences over the last 10 years has been summarized about residual coal mining, adjacent to weathered zones and under unconsoli-dated layers[2]. Furthermore, preliminary understand-ing of the factors for the evaluation of residual coal mining and quicksand inrush mechanism during mining, under unconsolidated layers, has been ac-quired[3–5]. Dong and Sui have observed vibrations of pore pressure and soil stress of unconsolidated sand layers before and during water and sand inrush into the workface in their model tests, although the infor-mation of vibrations can be a type of decision-making information to evaluate warnings about water and sand inrush disasters[4,6]. Regulation of water pressure fluctuations and changes in the state of soil stress have not been fully recognized.

This is a percolation process and also a fluid- structure/solid interaction process. Since Darcy estab-lished the linear rule of seepage in 1856, much pro-gress in soil/rock seepage mechanics has been made. The first improvement in understanding the interac-tion between solid deformation and fluid in land sub-

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sidence was provided by Terzaghi, who established the one-dimensional, elastic, saturated porous media consolidation model and the well-known formula for effective stress[7]. So far, the formula is still the basic principle for porous media and fluid interaction. Biot extended Terzlaghi's work to a three-dimensional consolidation problem, which is an isotropic and ani-sotropic elastic consolidation theory of porous media under the condition of small deformation of solids and non-compressible fluids[8–10]. An equivalent con-tinuum model and a fracture network model have, for now, become two basic models for studying the role of soil and water coupling[11–12]. Tests in laboratoria and computer simulations are two of the most impor-tant means to reveal the process of seepage and wa-ter/soil interaction[13].

Based on the geological situation of the Taiping coal mine in Jining, China, three test models and a simulation model are used to reveal the pore pressure fluctuations of overlying aquifers during residual coal mining and water-soil stress coupling processes.

2 Geological condition and model

The Taiping coalmine, which lies in the west of the Yanzhou mining area of Shandong, China, has been mining a residual coal seam for more than ten years. Fig. 1 shows the main geological and hydrogeological structure of the coal deposit. The height of the over-burden rock mass is not more than 60 m and the wa-ter head above the buried erosion surface is more than 15 m. Because of mining adjacent to a weathered zone, the thin, low intensity overburden rock mass is mostly occupied by overburden failure from coal mining. Large displacements and strain will occur during the mining process. This kind of geologi-cal/hydrogeological structure has already led to water inrush or quicksand hazards. It has been pointed out that the pore pressure and stress in the sand aquifer are potential sources of information to assess the situation of roof water above the buried erosion sur-face in the Quaternary system[2].

Fig. 1 Geological model

3 Soil and water pressure fluctuation test

An ideal model for pore pressure and stress tests is presented in Fig. 2. In the model, all formations are horizontal for ease of assembly; the thickness of the overburden rock mass is 20 m and that of the sand aquifer 15 m. The mining thickness of each slice is 2 m. According to our ideal model, a similar experi-mental model was established with a scale of 1:100. Its similarity has been discussed by Sui and Dong in 2008[4]. Fig. 3 is the monitoring system of our model (a~h are the sensor locations and numbers). Part 3 is the sand aquifer including pore pressure sensors a~g and a load sensor h above it. One section of a soft water bag is packaged by unconsolidated sand of a particular particle size to simulate the confined aqui-fer and being loaded by a stability water head as the border condition on each side. The monitoring cable will continue transmitting signals generated by sen-sors to the computer at a frequency of 40 Hz.

Fig. 2 Ideal model for pore pressure and stress test

a b c d e f gh

1

23

4

5a b c d e f g

h

1

23

4

5

Fig. 3 Monitoring system

1. Coal seam No.3; 2. Overburden rock mass; 3. Sand aquifer of Quater-nary; 4. Other unconsolidated layers of Quaternary; 5. Monitoring cable

The model is remolded as a 2D plane strain struc-ture under the vertical load of gravity and horizontal displacement constraints on both sides. Three grain groups, with particle diameters of 0.1~0.5 mm, 0.5~2 mm and 2~5 mm, corresponding to test models W1, W2 and W3, are used to represent different sand aq-uifers. Curves in Fig. 4 are pore pressure (sensor b) and vertical stress (sensor h) fluctuations generated during the mining process (Time) of model W1 and W2 before water or sand rushes into the workface.

Monitoring shows that, if the sensor is located over the back wall of the workface, the pore pressure of the aquifer increased rapidly. When mining to the left of this observation point, the pore pressure would gradually decrease. If the observation point in the front of the workface is far away from the front wall of the mining face, no obvious changes in pore pres-

Weathered zone

Aquifuge of mudstone

Buried erosion surface

Sand aquifer of Quatemary

Coal seam No.3

Sandstone

Failure zone

Aquifuge

Sand aquifenAquifuge

Overburden rock mass

Coal seam No.3

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sure appear. Consequently, while the workface nears to the location below the sensor point, the pore pres-sure increases instantaneously. When mining passes this sensor point, the pore pressure reveals a rapid decline but retains a high water head until the end of the test.

This process tells us that various positions in the upper confined aquifer refer to sensor points at the mining face and that different changes of pore pres-sure will appear. This kind of variation in pore pres-sure originates mostly from mining and the relative position of sensor points relative to the mining face. Stable downward changes in water pressure only oc-cur after closing of the working face.

Simultaneously, the vertical stress in the sand aq-uifer is correspondingly volatile. When the measuring point is located on top of the mining face, the vertical stress increases and when mining passes through the observation point, the vertical stress shows a down-

ward trend after first increasing. The initial breakage, which is caused by first slice mining, always results in greater amplitude fluctuations in vertical stress. The second mining slice caused a small number of increases in pressure but these eventually dropped. However, the third mining slice caused only slight changes or none at all. As a result of mining, there was a great stress release area above the goaf in the profiles, which included the measuring points.

Besides the relationship between pore pres-sure-mining and soil stress-mining, there is a rough correlation between pore pressure and soil stress as a function of time (or mining). But a coupling relation-ship could not be easily derived and stated precisely, because of test errors, interference, the discretionary location of points and the scale of the test model. A simulation model is another way to find some clues about regularity to answer this question.

(a) Model W1 (b) Model W2

Fig. 4 Pore pressure (sensor b) and vertical stress (sensor h) 1. The 1st slice of mining; 2. The 2nd slice of mining; 3. One pass cutting coal mining in sub area

4 Simulation of water stress

According to the geological and hydrogeological conditions of workface 8304 in the Taiping coal mine and to remain consistent with the test models, we de-signed a plane strain model for water-soil coupling simulation, corresponding to the structural test mod-els as shown in Fig. 2. Given that the thickness of the overlying strata above the coal seam is 30 m and the first slice no more than 2 m thick, the water head above the bottom of the Quaternary system is 40 m, imposed by a “pore pressure” load on the sand aqui-fer. Moreover, self-stress is loaded onto the model by gravity. Assuming that the permeability coefficient during the mining process remained constant, the permeability coefficient of the roof and floor forma-tions was set at near zero. The coefficient of the sand aquifer was obtained by tracer experiments between three wells and two underground water points. Pore pressure and self-stress balance were achieved by previous iterations before mining simulation, in which the value of the permeability coefficient be-

came 0.35 m/d. After that, dynamic changes of pore pressure and effective stress were calculated from a number of mining steps. Fig. 5 shows the two step results of pore pressure-location and pore pres-sure-time at each sensor point. The times indicated as 1 and 2 represent the end of mining in areas A and B. It can be seen that pore pressure of the sand aquifer shows a downward trend above the mining area A or B. At the same time, an accumulation of pore pres-sure appeared at the sides of the mined-out area. But

Fig. 5 Relationship between soil and water pressure-time/position

DONG Qing-hong et al Pore pressure fluctuations of overlying aquifer during residual coal mining and … 651

it does not show symmetry at the center of the work-face at time step 1 or 2. The pore pressure changes underwent a number time steps, while the peak of the pore pressure accumulation only existed for a short time.

5 Coupling analysis of soil-water pressure

The diagrams of Fig. 6 show pore pressure, effective stress and vertical stress in the area of the sand aquifer at model time 1+ . The dissipated area of pore pressure, located just above the mining area, can be clearly seen in Fig. 6a, as well as a significant fall in effective stress and vertical stress in the same region of Fig. 6b and 6c. This shows that the total stress has decreased in the area. Additionally, above both sides of the mined-out area, pore pressure and effective/vertical stress increased at the same time. Although characteristics of these results, obtained by

simulation, are mostly controlled by the assumption of the principle of effective stress, a similar trend of soil stress and pore pressure has been observed both in model tests and in our simulations. In summary, numerical simulation explained the phenomena observed in our experiments. Stress and pore pressure in the sand aquifer above the mined-out area were consistent in both research methods. Changes in indicators, which can be tested, just appear in the area above the side of the working face. But it also shows that the results of numerical simulation are not entirely consistent with the characteristics from our experiment. It seems that the time of accumulation and dissipation of pressure/stress is shorter in the simulation model than in the model tests. Another difference is that the zone of horizontal pressure/stress changes was too narrow in the simulation model, but could not be fully observed in model tests because of the sensor layout.

(a) Pore pressure ( =0.00101)

(b) Effective stress ( =0.00101)

(c) Vertical stress ( =0.00101)

Fig. 6 Pore pressure, effective stress and vertical stress in calculation plane

6 Conclusions

1) Cyclical changes of water pressure in sand aquifers over mining areas are basic characteristics, where each cycle of vibration is activated by mining events such as collapsing roofs. Dramatic downward pore pressure occurs instantaneously only after collapsing roofs and overburden deformation. Walls in front or behind a mining face always lie under areas of increased water pressure.

2) Both pore pressure and vertical stresses result in increasing and decreasing processes during the movement of a working face in mines. But vibration of pore pressure always precedes stress in the same area and ends in a sharp decline. Furthermore, the amplitude of pore pressure is significantly higher than that of vertical stress.

3) Changes in pore pressure of sand aquifers during mining are limited to the scope of changes in stress. Obvious changes occur mainly in a very small framework over the mining face.

Our research has tried to discover the relationship between pore pressure and skeleton stress of sand aquifers in residual coal mining under shallow bedrock conditions. A major shortcoming of the study is that it did not consider the conversion of permea- bility. This part of the research is a work in progress.

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