Arab J Geosci (2016) 9: 253 DOI 10.1007/s12517-015-2282-9

ORIGINAL PAPER

Experimental research on measurement of permeability coefficient on the fault zone under coal mine in situ Weixin Yi 1 & Enying Wang 1

Received: 20 July 2015 / Accepted: 18 December 2015 / Published online: 24 March 2016 # Saudi Society for Geosciences 2016

Abstract In order to measure permeability coefficient K of fault zone under coal mine quickly, the traditional borehole water pressure test method should be improved. After analyzing the principle of pressure water test in borehole, a new test method suit for borehole water pressure test under coal mine was proposed. In the new method, the time T was measured, and the calculating method was also changed from inverse algorithm to forward algorithm. According to the existing parameters and hydrogeologic conditions of fault zone, an empirical value of permeability coefficient Kc was set and then seven setting values were obtained. P-T figures of seven setting values were drawn by computer, and seven P-T figures were converted to K-T figures; the permeability coefficient could be read directly from the K-T figures. The new method was a fast experimental and calculating method to obtain permeability coefficient by borehole pressure water test on the fault zone in situ. The results were in good agreement with the low permeability coefficient of 7.5 × 10−8 and 8.0 × 10−8 m/s obtained by pressure water test in new borehole. And, the results were also in good agreement with the results of pumping test. This indicated that the pressure water test in the new borehole and the calculating method are accurate.

Keywords Borehole water pressurized test . Fault zone . Permeability coefficient . Experimental principle . Experimental method . Calculating method

* Weixin Yi [email protected]

1

Institute of Resources and Environment, Henan Polytechnic University, Jiaozuo 454003, Henan, China

Introduction A fault zone is the broken zone approximately parallel with the fault plane; it is a result of relative movement of the strata under certain shear stress or tensile stress. With the movement, dislocation, and pressing between the hanging wall and footwall, rock is broken. Comparing with surrounding rock strata, rock of fault zone is relatively unconsolidated, and the fault zone is a weak stress zone (Haakon 2010). In mining process, the fault zone usually is the water inrush channel, and underground water can rush into the mining space through this channel (Hua et al. 2011). When the mining roadway and working face go through the minor fault with good water permeability, it is necessary to carry on grout waterproof treatment on fault zone to prevent water inrush accident which is caused by the connection of fault water with footwall water (Zhang et al. 2011). Before grouting works, the selection of grouting spots, grouting materials, and grouting methods has a great effect on grouting effect; the accurate permeability coefficient K of fault zone is necessary in this process. Furthermore, the permeability coefficient must be measured quickly on the spot to avoid the effect on mining speed. Nowadays, there are some hydrogeological exploration methods and test methods to measure permeability coefficient of rock on ground or in the laboratory. For example is the pumping test which evaluates permeability of various strata in preliminary geological exploration. The Lugeon pressure water test and water injection test are usually used to evaluate permeability of the dam in water system, and permeameter test of rock that measures permeability coefficient of rock samples in room (Hamm et al. 2007). The pumping test is usually used at the stage of coal exploration; it is used to obtain permeability coefficient of strata on ground and is used seldom under coal mine. The permeameter test of rock is done in room; it cannot reflect permeability of strata on the spot. There are a

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few tests to measure permeability coefficient of strata suiting for the application of dozens or hundreds of meters under coal mine in situ, and relative reports are also few. Some research studies just studied the selection of pressure value of high pressure water pressure test in deep parts underground (Huang et al. 2014; Kazmierczak et al. 2007). There are various water pressure tests according to test spot, depth of strata, and level of underground water. And, the water pressure test in borehole is usually used on ground in situ. The borehole is separated into intervals with certain length by plug, and press water into the interval. The permeability of strata can be obtained from the relationship of pressure and flow (Hamm et al. 2007). We believe that borehole water pressure test is an ideal method to measure the permeability coefficient of fault (strata) under coal mine in situ. Because of the small volume of borehole water pressure test apparatus, simple and feasible test method, and the grouting equipment can also be used in water pressure test after simple reform, do not need to make new test apparatus. Fault zone lies in deep underground, pressure is high, distribution of fractures of fault zone is unhomogeneous, permeability coefficient changes greatly in vertical direction or in horizontal direction, so the accuracy of test results of Lugeon borehole water pressure test used in water system is low, and test period is long. Based on the above mentioned, a fast borehole water pressure test method of permeability coefficient that suits for underground can be obtained after analysis of principle of the traditional borehole water pressure test, and improving of test method and calculating method of permeability coefficient. Furthermore, the method does not effect the mining production.

Arab J Geosci (2016) 9: 253 5

3

2

1

1-water sump 2-water pump 6

h1

7

3- flowmeter h2

4- valve

8 9

L/2

5- pressure gauge 6- pipeline 7-borehole

L/2

8-rubber plug 9-closed evaluation area

Fig. 1 Borehole water pressurized test apparatus

Principle of borehole water pressure test and selection of calculating formula Analytic method of water pressure test results Effective injection pressure According to the difference of water pressure test method and test object, there are various calculation formulas and measurement methods, and effective injection pressure is also different. For mechanics properties, wether shear properties or tensile properties, fault zone is affected by ground stress (σ1, σ2, σ3). It is close and no water before exploring of coal seam (as shown in Fig. 2). Before the exploring of coal seam, fault zone shows close and has no water under the effect of mining pressure (σ1, σ2, σ3) (Rao et al. 2014). But, mining stress breaks the fault zone again in the mining process, many fractures generate in the faults zone, and pressure water goes up

Improved borehole water pressure test The borehole water pressure test apparatus (as shown in Fig. 1) is the same with the traditional borehole water pressure test apparatus. The improved borehole water pressure test is as follows: Water in sump is injected into the close borehole space passing through water pump, flowmeter, valve, pressure gauge, and water injection pipeline, raising water injection pressure stage by stage. When water injection pressure runs up to the preset value of the stage, flowmeter keeps a record of the quantity of water injection. When display value of pressure runs up to the maximum, water supply is stopped, valve is closed, the evaluation space is sealed off, and the time Tt when pressure drops from maximum to 90 or 80 % of the maximum (percentage can be changed based on the conditions on the spot) is measured. Namely, water injection pressure was set, the relationship of the decrease of water pressure and time was observed, and permeability coefficient K from the relationship was calculated.

4

σ1

σ1

σ2 σ3

σ3

σ3

σ3 σ1

σ1 shear normal fault

(1)

(2)

tensile normal fault σ1

σ1

(3)

(4)

(1)actually formed fault (2)unformed conjugating fault (3)actually main stress (4)balanced main stress

Fig. 2 States of stress in the formation of faults

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P ¼ P0 þ γ ðh1 −h2 −h3 Þ

ð1Þ

where P is the effective injection pressure, P0 is the water static pressure in borehole (MPa), h1(as shown in Fig. 1) is the elevation difference between pressure gauge and the middle of the close area of test, h2 is the elevation difference of underground water and the middle of the close area of test, h3 is the lost waterhead in borehole (m), and γ is the specific gravity of water (kN/m3). Lost waterhead can be calculated from Eq. 2: h3 ¼ αQ2 L

ð2Þ

Here, Q is the quantity of injection (L/min), L (as shown in Fig. 1) is the length of water injection pipeline from pressure gauge to close area of test (m), and α is the drag coefficient, which can be obtained by conversion of λ frictional resistance coefficient. When diameter and length of pipe are fixed, α is a constant of 7 × 10−5 min2/L2 (according to borehole water pressure test rules of hydraulic and hydroelectricity engineering SL31-2003). It can also be obtained from test. Derivation of calculating formula of permeability coefficient 1. When the mechanics properties of fault are compressive, fault zone is often filled by fault mud. It belongs to the relative even rock with few of network fractures and can be idealized to homogeneous body (as shown in Fig. 3). The seepage movement of underground water basically follows Darcy’s Law:

Vr ¼

Vθ ¼

K ∂P γ ∂r

ð3Þ

K 1 ∂P γ r ∂θ

ð4Þ

Q P=P0

borehole

P=0 (L=5m)

along the fractures. The close borehole area is under the level of underground water and supports the high water pressure and ground stress. Therefore, the effective injection pressure of water pressure test is higher than the traditional borehole water pressure test on ground (Huang et al. 2014; Kazmierczak et al. 2007). In the practical test, the pressure will decrease when injection water goes through water injection pipeline, flowmeter, valve, and pressure gauge. The real effective injection pressure is different at various parts in the same borehole. So, it is necessary to correct the injection pressure at various parts in order to avoid the effect of pipeline structure (Kitagawa et al. 2007; Yamano and Goto 2005; Yokoyama et al. 2014). When the close borehole area is under the level of underground water, the pressure is

r0=L

r0=L 2r0

Fig. 3 Model of homogenous body

Here, Vr is the flow velocity in radiation direction (m/s), Vθ is the flow velocity in circumference direction (m/s), K is the permeability coefficient, γ is the specific gravity of water (kN/m3), and r is the effective effect radius (cm). When the flow in closed part of borehole bottom is constant, water flowing out from borehole bottom is negligible. Then, K dP γ dr dr K Q ¼ −2πL dP r γ Q ¼ −2πrL

ð5Þ

Here, L is the length of close area of borehole (m), P0 is the pressure in borehole (MPa), and r0 is the radius of borehole (cm). Integrating with the above equation, it can be written as Z Z r dr K P ¼ −2πL dP Q γ r0 r P0 r K Qln ¼ −2πrL ðP−P0 Þ ð6Þ r0 γ  r γQln r0 K¼− 2πLðP−P0 Þ When close area of borehole is under the level of underground water, K can be deduced from Eqs. 1 and 2:     r r γQln Qln r0 r0  ¼−   ð7Þ K¼− 2πLγ W h1 −h2 −αQ2 L 2πL h1 −h2 −αQ2 L 2. When mechanics properties of fault are extension, rock of fault zone is relative broken, and the fractures is relative wide and straight. When fractures of rock are idealized to parallel tabular body (as shown in Fig. 4), the movement of underground water basically follows Poiseuille’s law:

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Arab J Geosci (2016) 9: 253 Q

g K¼ 12vL

P=P0 borehole

6Qμln πP0

re r0

re r0 ¼ 2πP0 γQln

ð13Þ

P=0

(L=5m)

Discussion of calculating method of test results

r0=L

r0=L 2r0

Fig. 4 Model of parallel flat fissure

K¼

ga3 12v

ð8Þ

Here, g is the gravitational acceleration (m/s2), a is the width of parallel tabular fractures (cm), and v is the dynamic viscosity coefficient of water (cm/s); if the number the fractures in close area of borehole is m, then K¼

3 1 X m ga j g Xm 3 ¼ a K j¼1 12v j¼1 j L 12vL

ð9Þ

When the injection water in vertical direction borehole seeps into the fractures in parallel direction, the relationship of quantity of injection and pressure follows Poiseuille’s law: P0 ¼

6μQ re ln πa3 r0

ð10Þ

If the number of fractures in close area of borehole is m, and water pressure in borehole holds in P0, then P0 π X m 3 Q¼ a re j¼1 j 6μln r0

ð11Þ

Here, P0 is hydrostatic pressure of water in borehole (MPa), μ is viscosity coefficient of water (gsec/cm2), re is effect radius (cm), and r0 is radius of borehole (cm). Namely, Xm

a3 j¼1 j

6Qμln ¼

It can be seen from the Eqs. 7 and 13 that the relationship of K and Q is not a simple corresponding, the calculation formula changes in different hydrogeological conditions. For the different mechanics properties of faults, the development of rock fractures in fault shear zone is different. When the seepage movement of underground water is turbulent current, which does not follow Darcy’s Law or Poiseuille’s law, the adopted seepage law will change. It is necessary to analyze with threedimension seepage basic differential equations of underground water. Here, more test coefficients are necessary, for example, dynamic viscosity coefficient of water v, specific storativity Ss, and so on. Formulas of calculating K from Q are various. When close area of borehole is above or under the level of underground water, formula of effective injection pressure is different. In brief, the more complex geological conditions, the worse the uniformity of development of rock fractures, and the accuracy of test data is also worse. Therefore, the traditional borehole water pressure test needs to reverse analysis by special analysis software to avoid the effect of various hydrogeological conditions. Analyzing test coefficients, drawing P-Q and K-Q figures, and selecting proper seepage differential equations of underground water conversely, it will take 2 days usually to get the analytical solution. So, it is not suitable for mining of coal mine that takes long time to do a test, it is necessary to improve the method for calculating the test results.

πP0

re r0

ð12Þ

Expressing Eq. 12 with regard for Eq. 9 can be rearranged as

Specific implement method of improved borehole water pressure test under coal mine Improved water pressure test was used in Daping coal mine of Zhengzhou coal group, and Yi’an coal mine of Yima coal group in Henan Province, all of which got the better results. As an example, specific implement method at F29 fault in the middle of Yi’an coal field was explained. F29 fault is a normal strike slip fault with compressive-shear property, the dip angel ranges from 65° to 75°, and fault plane is smooth and straight. The fault shear zone is filled mainly by fault mud. The fault throw is 14–37 m, and width of fault broken zone is 1–5 m (as shown in Fig. 5). In order to prevent water inrush from faults, grouting operation is necessary. Water pressure test apparatus as shown in Fig. 1 can be set in the pipeline of grouting apparatus. Before grouting operation, borehole water pressure test was done two times

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N

70º-80º ( motion angle of stratum) II2 coal 26m 65º-70 º No. 11052 working surface II1 coal 54.5m

O2

O2 65º-70º

H=32 F29

Fig. 5 Profile map of F29 fault attitude on No.11052 working surface passing through Yi’an coal mine

(test method is mentioned above). The length of close area and diameter of borehole were 5.0 and 0.065 m, respectively (Chinese Hydraulic Standard) (according to borehole water pressure test rules of hydraulic and hydroelectricity engineering SL31-2003). And, the widths of fault zone were 2.5 and 4 m, respectively. Injection pressure in close area increased by three steps from 0.4905 MPa (effective pressure is 0 MPa) to 1.4905 MPa, 1.9905 MPa, and 2.4905 MPa (effective pressure is 2 MPa). As the pressure in close area became stable with the increasing of time, quantity of water injection was measured by flowmeter on every step. The test method at the primary stage was nearly the same with Lugeon method. When the pressure in close area became stable and reached the maximum injection pressure (2.4905 MPa, the maximum injection pressure can be determined from the earth pressure of pressured and pressure loss of pipe, the minimum injection pressure is pressure loss of pipe), the valve was turned off, the supply of water was stopped, and the evaluation area was closed. The setting value of injection water pressure in steps and the maximum value of injection water pressure could be adjusted properly with the pressure of pressured aquifer of coal seam floor. Then, the pressure in close area decreased gradually as the water seeped to fault shear zone. The time that pressure decreased 1.8 MPa (90 % of effective pressure) from 2.4905 to 0.6905 MPa was measured. The borehole water pressure test underground was finished.

Improved calculating method of permeability coefficient The traditional borehole water pressure test needs to draw the figures of P-Q and K-Q, and to calculate permeability coefficient after test. It takes long time and cost. In contrast, the improved calculating method uses computer to analyze and get the test results (Yi and Yuan 2011; Liao et al. 2012; Xiao et al. 2005). Calculating method of permeability coefficient of fault zone is explained in detail as follows. According to the results of geological investigations, an empirical value of permeability coefficient Kc was presumed, based on geological conditions of fault zone in drilling, water level reply conditions in drilling process, quantity of water injection in primary, and so on. Around the value, the maximum and minimum values were set in both sides of Kc by divide of 2×101 m/s, and then the maximum and minimum values were divided into equal parts, to get several permeability coefficients Kc. In this test, F29 fault is a normal strike fault, elevation of test position is −200 m, and a mass of fault mud is filled in fault zone. Water level reply time is up to about 15 min in drilling process; the figure of P-Q can be divided into three stages: Test pressure is less than 0.5 MPa, and seepage discharge is less than 1.0 L/min in the first stage. Test pressure is in the range of 0.5–1.5 MPa, and seepage discharge is in the range of 1.0–2.0 L/min in the second stage. Test pressure is in the range of 1.5–2.5 MPa, seepage discharge maintains basically at 1.5 L/min in the third stage. Seepage discharge of fault zone is very small in every stage; it indicates that water permeability of F29 fault is very week. Therefore, empirical permeability coefficient Kc of 5.0 × 10−8 m/s was set, and the maximum and the minimum values of 1.0 × 10−6 and 1.0 × 10−9 m/s were set, respectively. The values from the maximum to the minimum were divided into equal parts by 0.5 m/s. Seven permeability coefficients Kc of 1.0 × 10−9, 5.0 × 10−9, 1.0 × 10−8, 5.0 × 10−8, 1.0 × 10−7, 5.0 × 10−7, and 1.0 × 10−6 m/s were set. The relationship of pressure that of each permeability coefficient in close area and time was analyzed by special hydrogeological software of Aquifer test (it is developed by Waterloo Hydrogeologic Inc. It is an advanced software of pumping test and water pressure test. It mainly solves four cases of pressure water aquifer, unpressurized water aquifer, weak water permeability, and bedrock fractures aquifer. The main functions are document analyzing of pumping test and water pressure test, data processing, parameter determining, and so on). The analysis was based on the unstable seepage basic differential equations and finite element analysis method. In the process of analysis, analysis coefficients of length of close area, drilling radius, and static pressure of water were actual values. The seepage basic differential equations (2D or 3D) were deduced from hydrogeological conditions in situ. The effect radius, coefficient of storage, etc. could also be deduced from hydrogeological conditions in situ. From above

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Arab J Geosci (2016) 9: 253

actual values or deduced values, P-T figures of each permeability coefficient could be obtained after analysis (Fig. 6a–g was drawn from the data of that calculated from the relationship of pressure of each permeability coefficient and passing time in close area).

b 3.0

3.0

2.5

2.5

Pressure (MPa)

Pressure (MPa)

a

2.0 1.5 1.0

2.0 1.5 1.0 0.5

0.5 0.0

0

500

1000 Time (s)

1500

0.0

2000

0

P-T figure at K=1.0× 10-6 (m/s).

c

1.5 1.0 0.5

2.0 1.5 1.0 0.5

0

500

1000

1500

0.0

2000

0

500

1000 Time (s)

Time (s)

1500

2000

P-T figure at K=5.0× 10-8 (m/s).

-7 P-T figure at K=1.0×10 (m/s).

e

f 3 .0

3.0

2 .5

2.5

Pressure (MPa)

Pressure (MPa)

2000

2.5

2.0

2 .0 1 .5 1 .0

2.0 1.5 1.0 0.5

0 .5 0 .0

1500

3.0

Pressure (MPa)

Pressure (MPa)

1000 Time (s)

d

3.0

0.0

500

P-T figure at K=5.0× 10-7 (m/s).

2.5

0

500

1000

1500

0.0

2000

0

Time (s) P-T figure at K=1.0× 10-8 (m/s).

g

500

1000 1500 Time (s) P-T figure at K=5.0× 10-9 (m/s).

2000

h Permeability coefficient (m/s)

3.0 2.5

Pressure (MPa)

Fig. 6 a P-T figure at K = 1.0 × 10−6 (m/s). b P-T figure at K = 5.0 × 10−7 (m/s). c P-T figure at K = 1.0 × 10−7 (m/s). d P-T figure at K = 5.0 × 10−8 (m/s). e P-T figure at K = 1.0 × 10−8(m/ s). f P-T figure at K = 5.0 × 10−9 (m/s). g P-T figure at K = 1.0 × 10−9(m/s). h K-T figure of permeability coefficient and passing time

Figure 6a–g shows that water pressure in close area decreases with increasing of time, but when water pressure decreases to a certain value, the time interval Tc is different with permeability coefficient K. Lower permeability coefficient takes longer time. The time Tc that water pressure decreased

2.0 1.5 1.0 0.5 0.0

0

500

1000 Time (s)

1500

P-T figure at K=1.0× 10-9(m/s).

2000

1.0X10

-5

1.0X10

-6

Δ Δ

1.0X10

-7

1.0X10

-8

Δ Δ Δ Δ

1.0X10 1.0X10

-9

Δ

-10

0

200

400

600

800

1000

Time (s) K-T figure of permeability coefficient and passing time.

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from 2.4905 MPa to 0.6905 MPa can be read from the figures, and then seven groups number (Tc, Kc) could be got. As shown in Fig. 5h, time Tc was X-coordinate, permeability coefficient Kc was Y-coordinate, and K-T figure could be drown from seven groups coordinate by matching the method of least squares in coordinate system. And then, we measured the time Tt that pressure decreased from 2.4905 to 0.6905 MPa in borehole water pressure test in situ; the corresponding permeability coefficient Kt value could be read from K-T figure (Fig. 6h). It was the permeability coefficient of close area. Times Tt measured in two tests were 685 and 700 s; corresponding permeability coefficient Kt was 7.5 × 10−8 and 8.0 × 10−8 m/s.

Results and discussion The measured results of two tests were in good agreement with the results of explore water with mine transient electromagnetic instrument. Two transient electromagnetic tests were done in crossheading no. 1 and no. 2 drilling sites nearby no. 11052 working face of F29 fault zone in Yi’an coal mine (as shown in Fig. 7). The distance from no. 1 and no. 2 drilling sites to F29 fault zone was 50 and 30 m, respectively. The length of transient electromagnetic tests area was 100 m; it covered F29 fault zone. Two test results all showed that the values of whole apparent resistivity were very high (Fig. 8a, b was drawn from the report of advanced detection of transient electromagnetic test in crossheading no. 1 and no. 2 drilling sites at no. 11052 working face in Yi’an coal mine). As shown in Fig. 8a, b, the values of apparent resistivity of advanced detection of transient electromagnetic test were over 1.4 Ω·m, and there were no obvious low resistivity abnormal area. These showed that the fault zone was weak aquifer (division value of low resistivity abnormal zone was below 1.0 Ω·m). Water rich low resistivity abnormal zone was usually a zone with low value of apparent resistivity in middle and high values in two sides; there was no characteristic zone in the figures. The result of explore water with transient

electromagnetic test (7.5 × 10−8 m/s) was in agreement with that of the borehole water pressure test (8.0 × 10−8 m/s). Both of the results all show that F29 fault has weak permeability. On the other hand, no. 3501 borehole explored F29 fault directly in II 1 coal seam during coal mine exploration, and the pumping test was done. The amount of water gush was only 1.1 × 10−6 l/s·m. Although there was a little difference between results of the pumping test and this water pressure test, it was deserved for the different test sites and unhomogeneity of water permeability in fault zone. Furthermore, the amount of unit water gush was only 1.1 × 10−6 l/s·m, it also indicated that F29 fault has weak permeability; it was in good agreement with the results of water pressure test. In another coal mine of Daping coal mine of Zhengmei Group, Henan Province, in order to carry on grouting works at fault zone, new borehole water pressure test was done at F12 fault of tensile normal fault in the middle parts of coal field with fall of 25 m. On contrast, traditional borehole water pressure test was done firstly, and then, new borehole water pressure test was done under coal mine. The results showed that spending times of test course in site were the same basically. But, spending time of subsequent data processing for traditional borehole water pressure test spent about 4 h, while new water pressure test spent only several minutes to get the results. The result of traditional borehole water pressure test was 15 Lu, and new borehole water pressure test was 3.5 × 10−4 (m/s). There exist certain errors by conversion of two test results using empirical formula. According to the analysis, the reason of errors existing has relationship with mechanics properties of faults. F12 fault of Daping coal mine is tensile normal fault. Comparing with compressive-shear normal fault, rock filled in fault zone is broken, water permeability is good, and permeability coefficient is large. These indicate that data processing by Aquifer test is only suitable for stratum or fault with bad water permeability and low permeability coefficient. But, new borehole water pressure test can be done at F12 tensile normal fault smoothly; it shows that this test method is feasible. By only improving the program of data processing, new borehole water pressure test can be used at all faults.

Fig. 7 Profile of 1# and 2# drilling site of 50 and 30 m to F29 fault

N

20m

100m entrance of cut lane

II2 coal

II2 coal 30m

1#drilling 16

20-250

0

2# drilling 85m 180 11m 81m 96m F29 fault

27m

II1 coal

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Arab J Geosci (2016) 9: 253

a 100 2.7

Distance across the drilling direction (m)

Distance across the drilling direction (m)

100

90

80

70

60

50

40

30

20

10

2.6

90

2.5 80

2.4 2.3

70 2.2 2.1

60

2 50

1.9 1.8

40 1.7 1.6

30

1.5 20

1.4

10

0

0 0

10

20

30

40

50

60

70

80

90

100

0

10

Distance along the drilling direction (m)

20

30

40

50

60

70

80

90

100

Distance along the drilling direction (m)

Cross section of transient electromagnetic superior apparent resistivity on 1# drilling site. 100

100

90

90

Distance across the drilling direction (m)

Distance across the drilling direction (m)

b

80

70

60

50

40

30

20

80

70

60

50

40

30

20

10

10

0

0 0

10

20

30

40

50

60

70

80

90

100

Distance along the drilling direction (m)

0

10

20

30

40

50

60

70

80

90

100

Distance along the drilling direction (m)

Cross section of transient electromagnetic superior apparent resistivity on 2# drilling site.

Fig. 8 Cross section of transient electromagnetic superior apparent resistivity on 1# drilling site. b Cross section of transient electromagnetic superior apparent resistivity on 2# drilling site

Conclusions 1. Improved borehole water pressure test and corresponding calculating method could be used to measure permeability coefficient K of fault zone under coal mine quickly. 2. Improved borehole water pressure test not only inherited the advantage of the traditional borehole water pressure test but also innovated it. The test at the primary stage was nearly same with the traditional borehole water pressure test. In order to bring into correspondence with new calculating method, the measurement of time variable was added at the later stage. In theory, stable seepage theory was supplanted by the unstable seepage theory; it fitted well for real hydrogeological conditions on fault zone under coal mine.

3. Computer analytic process of traditional borehole water pressure test was a reverse analytic process; it took longer time. Computer analytic process of improved borehole water pressure test was a forward analytic process; it could be started at the primary stage of borehole water pressure test. The formula and parameters were known or deduced value in the analytic process; analytic time was short. As borehole water pressure test finished, computer analytic process also finished, did not take the time in situ. 4. For the new calculation method, seven permeability coefficients Kc were used to get P-T figure by computer, and P-T figure was converted to K-T figure. Permeability coefficient could be read directly from K-T figure. It was a fast and simple method to get permeability coefficient K.

Arab J Geosci (2016) 9: 253

5. After tests and verification in different coal mines, the permeability coefficient of fault zone obtained from improved borehole water pressure test was in good agreement with the results of physical exploration. 6. Confirmation results indicated that new borehole water pressure test under coal mine was suitable for compressive or compressive-shear normal fault with bad water permeability. It needed to improve the method to suit for tensile fault with good water permeability. For the less test times, it needed to increase the test times to get more test data for using the method in others coal mine. Acknowledgments The authors would like to thank the financial supporting from Important National Science and Technology Specific Projects (2011ZX05060-006), and Scientific and Technical Foundation of China National Coal Association (MTKJ2009-296).

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