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Effect of expansive fillings on fracture seepage
M INING SCIENCE AND TECHNOLOGY Mining Science and Technology 19 (2009) 0824–0828
www.elsevier.com/locate/jcumt
Effect of expansive fillings on fracture seepage CHEN Jin-gang, YANG Jun-li, ZHANG Jing-fei Department of Engineering Mechanics, Zhengzhou University, Zhengzhou, Henan 450001, China Abstract: Based on an investigation of the damage process of fractures filled with expansion media, samples of filling media collected on the spot, were obtained. The physical water properties of fracture fillings were tested by a WZ-2 type Dilatometer. Given our test results of the hydrated properties of fracture fillings and the mechanical parameters of altered rock mass collected from the Daye Iron Mine and comparisons between the expansion stress of fracture fillings and mechanical parameters of altered rock mass, the effects of the mechanical response of fillings on fracture seepage are discussed. The results show that the ratio of filling swell pressure to tensile strength of altered rock specimen varied between 11.79 and 36.77 and the ratio of filling swell pressure to shear strength of these rock specimen ranged from 72.11 to 171.18. Therefore, fillings have an important effect on fracture seepage, and the effect of deformation of the fracture caused by swell pressure of the filling mechanics cannot be ignored either. The multiple hydraulic coupling effects of fillings on the impact of fracture permeability are discussed according to theoretical analyses. It is shown that the effect of expansion of fracture fillings greatly affects the deformation of altered rock masses. Both tensile effect and shear effect produced by fracture fillings greatly increase the permeability of fractures. The plastic and liquefaction effects of fracture fillings also improve the permeability of fractures. On the basis of this analysis, a mechanical seepage model of filled fracture is established. Keywords: expansion; filling medium; fracture; seepage
1
Introduction
For a long time, research on fracture seepage has largely concentrated on the characteristics of seepage under stress, while studies on the effect of expansion fillings on seepage are comparatively weak. That is to say, mechanical, plastic and liquefaction effects imposed by expansion fillings on fracture seepage have, up till now, not been given adequate attention by investigators. It is a comparatively general phenomenon that fractures are naturally filled with material. Fillings have an important effect on fracture seepage. In 1989, Tian et al. proved that filled fracture seepage could still be described with cubic rules via a large number of experiments and theoretical analysis, but it needs to be modified now, since 1994 an important coefficient, which only depends on the porosity of the fillings, needs modification[1]. Su et al. conducted model experiments on permeability characteristics of filled fractures and found that the permeability of these fractures not only depends on the filling material, but are also clearly related to the particle granularity of the fillings[2]. They deduced the half-empirical and theoretical formula based on the
Ⱦ.Ɇ.Ɇɢɧɰ half-empirical theory from the analysis of fracture seepage mechanism. Red mud flows in the form of a highly viscous paste through a pipe. The friction between particles and between particles and water, the adhesive water on the surface of the particle and the galvanic-chemical action will all change their rheological properties[3]. In 2002, He et al. studied the physical chemistry and mechanical mechanism of deformation of soft expansion rock, especially for minerals subject to expansion like montmorillonite and also discussed its functional mechanism in terms of mechanical strength characteristics[4]. Expansion and shrinking of a mineral crystal lattice has a comparatively large impact on its volume and the amount of water also has a great effect on its volume[4]. These authors also studied the characteristics of expansion minerals and explained the function mechanism of permeate pressure of expansion and shrinking of a mineral crystal lattice[5]. Montmorillonite has a strong capacity to absorb water. Its volume increases considerably, up to a dozen times, after absorbing water. With this increase in the amount of montmorillonite, the swell pressure increases significantly and the time needed for reach-
Received 23 February 2009; accepted 09 June 2009 Projects 50709030 supported by the National Natural Science Foundation of China, 2008A410002 and 2007570011 by the Natural Science Foundation of Educational Department of Henan Province of China Corresponding author. Tel: +86-13837140487; E-mail address: [email protected]
CHEN Jin-gang et al
Effect of expansive fillings on fracture seepage
ing a maximum increases regularly[5]. The repeated expansion and contraction of fractures are caused by wet and dry filling cycles, where each cycle forms residual deformation. If the deformation continues to accumulate and generates tensile fractures, this process will in the end cause damage to the rock mass because of this gradual accumulation[6]. Fractured zones may be formed if the fillings accumulate and settle at the macro structures. The greater the amount of clay mineral, the more important the effect of sliding and the easier it is to induce slope instability, starting the process of inevitable geological disasters[7]. The model of predicting the strength of filling material was established by applying the theory of artificial neural networks[8]. Taking a broad view of the existing literature, we find that the effect of the fillings on fracture seepage and its mechanism have seldom been studied either at home or abroad. The relevant literature about controlling the effect of expansion fillings on fracture seepage is also largely silent on the issue. It is hard to assess accurately the infiltration characteristics of filled fractures if we know so little about the dynamic permeability of filled fractures, especially under conditions of heavy rainfall and it is therefore difficult to design a drainage program for projects such as mines, water conservation, etc.
2 Characteristics of rocks and appearance of fracture fillings in mining areas 2.1 Characteristics of rocks in the research area The exposed stratum of the Daye Iron Mine is largely of Triassic origin. The main composition of the north and the east sides of the stone pit is diorite, but the south side of the stone pit is mainly made up of marble. Fractures on each side are almost completely filled and the fillings are quartz, garnet, traversellite, etc. To different degrees, changes often take place near ores of various lithological characteristics. The rocks in the slope are eroded from the action of the structure. It is referred to as the change of country rocks near ores and the structural fracture belt. The country rock includes chlorite, kaolinite, montmorillonite, illite, etc., while the fracture belt consists largely of chlorite, montmorillonite and kaolinite. The original mechanical strength varies considerably as a result of changes in rocks. The strength of rocks such as chlorite, kaolinite, and montmorillonite suffers severely from these strong changes and is considerably reduced. 2.2
Characteristics of fracture fillings appearance in the research area
There is an approximately oval cavity in the upper tunnel shoulder, with dimensions 1.2 m×0.8 m and a depth of 1.2 m. Montmorillonite can be found along
825
the entire wall wherever water drips down intermittently. It is because fillings fall from the fractures, due to underground water. Sometimes, two fissure zones, filled with montmorillonite, are exposed where the tunnel collapses. From this observation, we conclude that the fissure zone is from several millimeters to more than 40 cm thick[9].
3
Physical water properties of fillings and mechanical properties of rock samples
Our analysis, based on a number of different tests, indicates that the main mineral composition of the changing rocks is montmorillonite in the open-pit east of the Daye iron ore. Its content ranges from 7.7% to 45%, but generally falls within the 20%–30% range. The lowest swell pressure is 0.125 MPa, while the maximum is 0.684 MPa, with an average of 0.353 MPa. Other parameters of the fillings are shown in Table 1[10]. The water content of most filling samples is greater than 15%, their plastic limit more than 17% and the liquid limit is greater than 33%. The water content is always lower than the plastic limit. Our data indicate that the water-absorbing course of fillings is synchronous with that of swelling and deformation. The state of transfer of the fillings from solid to liquid requires a large amount of water to seep into the fillings and rainfall, especially torrential rains, has become the natural condition for this transfer. Table 1 Physical water properties of fracture fillings Free Minimum Maximum Water Liquid Plastic Sample Plastic swelling swell swell content limit limit number index rate pressure pressure (%) (%) (%) (%) (MPa) (MPa) FDB1 15.67 26.9 17.8 9.1 26.0 0.125 0.163 FDB2
15.86 11.71
34.2
19.9
14.3
13.4
0.200
0.258
0.137
0.192
FDB3
15.56
33.0
17.0
16.0
51.0
0.296
0.329
FDB7
28.74
64.2
30.4
33.8
99.5
0.467
0.582
FDB9
15.56
34.9
20.8
14.1
52.5
0.684
0.684
It has become common knowledge in engineering circles that the tensile strength of a rock mass is far lower than its compression strength. For over 30 years, a large number of tests have been conducted from which we have learnt a great deal about slope rock masses of the open-pit in the east of the Daye Iron Mine which has provided a large amount of information for rock mass fracture parameters in the database. Different parameters of rock samples can be obtained from the relationship between the former test results and the latter empirical tests, shown in Table 2. The fractures of most rock masses are the result from either shearing stress or drawing stress. According to the data of Tables 1 and 2, a curve contrasting the swell pressure of fillings and their tensile and shear strengths can be drawn. An analysis from a
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Mining Science and Technology
comparison of the data shows that the ratio of filling swell pressure to tensile strength of altered rock specimens varies between 11.79 and 36.77, the ratio of filling swell pressure to shear strength of the specimen ranges from 72.11 to 171.18. Therefore, mechanical deformation of the altered rock mass, caused by the effect of swell pressure of the fillings cannot be ignored. We conclude that a mechanical response of the fillings lies at the bottom for the control of fracture seepage characteristics under rainfall conditions. Table 2 Mechanical parameters of altered rock mass Single axle Single axle Single axle Shear strength compression tensile tensile strength strength strength c φ (MPa) (dry) (MPa) (wet) (MPa) (MPa) (°)
Rocks
Marble
70
2.69
1.96
0.25
36
Diopside skarn
94.5
3.18
1.81
0.34
38
Diopside diorite
51.43
2.32
1.14
0.49
33
Weak altered meroxene diorite
59.96
2.49
1.22
0.52
41
[11]
68.31
2.66
1.32
0.53
45
Dolomite marble[11]
63.0
2.55
1.86
0.23
35
Chlorite diorite[11]
57.4
2.44
1.19
0.21
33
Strong altered kaolin[11]
43.4
2.16
1.06
0.16
30
Olafite diorite
4
Effect of physical and mechanical characteristics of fillings on fracture seepage
Fracture deformation primarily contributes to the change of rock structures. The presence of fillings leads to changes in a rock mass, weakens its anti deformation capacity and deforms the entire rock mass. Fracture rock permeability has much to do with the opening, length and connectivity of fractures. At the same time, the deformation of fractures under stress also has an obvious impact. The response of mechanical fillings caused by water promotes the deformation of the fracture, increases its width, shortens the transportation route of water and it is extremely easy for fillings to slip or get lost under the perturbation of stress. Under these conditions, its plastic and liquefaction effects will change rock fracture seepage considerably. 4.1
Effect of expansion of fillings on fracture seepage
Under natural conditions, the original structure of montmorillonite will not be disturbed when fresh rock samples soak in water and swelling is not obvious. The newly opened side of the developed tunnel is exposed to the air flow. During the gradual drying period, the moisture in the fillings will evaporate and the structure of montmorillonite crystals responds to the change in order to keep balance with the outside
Vol.19
No.6
atmospheric humidity. Montmorillonite has an active crystalline grain structure, sensitive to water and is strongly hydrophilic. Besides, crystalline layers shrink after they are dehydrated and swell when absorbing water. So, once the dehydrated montmorillonite meets water, the dehydrated fillings will absorb a great deal of water, leading either to an increase in the depth of adsorbed water between the crystalline layers and the rise of combined water thickness around particles or water drips into the mineral crystals. The wedge pressure of the combined water causes fracture inflation. If the rock has a high content of montmorillonite and the fillings meet water, the water drips into the rock mass rapidly along fractures and runs into the active layers of montmorillonite, causing it to swell. The expansion of the volume can increase considerably up to a dozen times the original volume[5]. Inflation produces great pressure and the pressure further weakens an already weak rock mass. Material damage mostly results from the effect of tensile and shear stress. According to rock tensile failure conditions: σ −σt = 0 (1) where σ is rock stress and σ t tensile (shear) stress. Based on this analysis, it is easy to show that the destruction is largely the result from the effect of tensile and shearing stress. The preceding analysis shows that the swell pressure of fillings has important effects on the deformation of the changing rock specimens, contributing considerably to fracture deformation. We should therefore take this factor into account, in order to analyze the characteristics of the filling fractures more effectively. When fractures in a shearing environment are due to expansion of their own internal media, the swell pressure produced by the fillings can be destroyed by shearing action caused by changing rock cracks. That is to say, in this case, that a single fracture is equivalent to being expanded into several fractures, or that fractures, unconnected with each other, become connected, clearly increasing the permeability of the fractured rock. When fractures in an environment of tensile stress owe their expansion to internal media, the expansion of water-absorbing fillings will generate additional tensile stress in the fractures. Suppose that the swell pressure is σ ′ and because of the effect of its crack expansion wedge, a fracture is relatively pulled and widened, where the stretch displacement is Δμ t = σ ′ /Kn. Considering σ ′ , the rock strain: Δε = σ ′ / E , compresses the rock, where the value of the displacement is: Δμs = − Sσ ′ / E . Therefore, under pressure of the expansion of the fillings, the additional increase of fracture width is: ΔW = (W0 + Δμ t + Δμs ) − W0 = σ ′ / K n + Sσ ′ / E
(2)
CHEN Jin-gang et al
Effect of expansive fillings on fracture seepage
where ΔW is the additional increase of the fracture width, W0 the initial width of the fracture, Kn its normal strength, E the modulus of elasticity and S the mass rate. Research shows that the seepage characteristic of fractures conforms to the famous cube rule, i.e., the fracture permeability is directly proportional with the cube of its width[12]. The seepage of filled fractures can be described with this cubic rule. The formula is:
K = nβ W 3 / L
(3)
where K is the permeability of the fracture, W the fracture width; L the fracture spacing; β a coefficient and n the porosity of the fillings. The increment of fracture permeability, produced by tensile stress, is: ΔK t = nβ ª¬ (W0 + ΔW )3 − W03 º¼ L
(4)
If other factors remain unchanged and if the crack spacing increases, fracture permeability will rise greatly according to this cubic rule. Therefore, a small increase in fracture spacing will have a considerable impact on its permeability. From this exposé, it is clear that the ratio of expansion pressure to tensile strength of changing rock specimens lies between 11.79 and 34.02. It is also clear that the expansion pressure makes the fracture wider. The big slope displacement in the fault (F9) of the Daye Iron Mine is caused by the expansion of the fillings, by rain permeating into the fault. To say the least, even if expansion pressure were not enough to connect the fractures completely, it still has a great impact on its deformation, i.e., a small increase in fracture spacing increases its permeability greatly. The increment of fracture permeability produced by shear stress is: nβ W 3 nβ W 3 − (5) L − ΔL L The permeability of fractures is improved greatly by both tensile and shears stress produced by crevasse fillings. A mechanical model of filled fracture seepage can be constructed from this analysis (Fig. 1). ΔK s =
Tensile stress increases the width of cracks and shear stress increases fracture spacing, both of which improve fracture seepage ability.
4.2 Effect of plastic and liquefaction of fillings on fracture seepage It can be seen from the spot that the seeper in the cave filling areas clearly increases, compared with normal areas. It proves that the loss of the fillings improves the permeability of fractures. The fracture filling is a kind of fluffy medium, characterized by loose structure, high porosity, weak mechanical strength, is prone to plasticity and extremely sensitive to external perturbation. The fillings begin to soften under the action of increasing amounts of water. Over time, the fillings are gradually transformed from a solid to a plastic state. After deformation due to expansion from encountering water, the relatively close, high strength and high dense structures change to loose porous structures, that is the typical of the characteristics of fracture fillings. With further increase in the amount of water, the fillings are transformed from the plastic state to a liquid state. During rainfalls, especially rainstorms, fracture fillings will be eroded by running water. Because these water flows are active and motive forces and generate strong dynamic effects, particles of the fillings will be pushed away and the volume of free space in fracture increases. With further removal of particles, the velocity of the water flow also increases and even thicker particles will be removed and fracture spaces continue to increase gradually. In this way, with the expansion of fracture spaces, filled fractures will be connected and become an open passage. When the filled fractures are fully connected and develop into an open passage, the seepage characteristic indicates no fillings in fractures, i.e., the modified coefficient n (in the cubic rule) does no longer exist in this case. Because of porosity of the fillings n<1, seepage ability of the filled fractures should be less than that of no fillings in fractures. After the fillings are eroded and removed, the permeability of fractures will be higher than that of the original fractures. Therefore, the permeability of fractures will be clearly improved.
5
Fig. 1
Mechanical model of filled fracture
Intuitively, this seepage model shows that two kinds of effects are caused by hydraulic coupling of the filling media, i.e., tensile stress and shear stress.
827
Conclusions
Fracture fillings have a significant impact on mechanical rock properties and permeability. In some sense, the physical and mechanical properties of rock masses depend mainly on that of rock joints, fractures and fillings. The effects of expansive fillings on fracture seepage have been discussed. Given our empirical research and theoretical studies, we come to the following conclusions.
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Mining Science and Technology
1) The ratio of expansion pressure of the fillings to tensile strength of altered rock specimens varies between 11.79 and 36.77 and the ratio of filling swell pressure to shear strength of altered rock specimen ranges from 72.11 to 171.18. Therefore, fillings have an important effect on fracture seepage. Deformation of fractures caused by swell pressure cannot be ignored. When the permeability of filled fractures is studied, this factor must be considered. 2) The rate of increment of fracture permeability caused by tensile stress and shear stress of the filling media are deduced from theoretical analyses. Both tensile and shear stress produced by the expansion of the fillings improve the permeability of filled fractures. It has been shown that the plastic effect and liquefaction effect of fracture fillings also improve the permeability of fractures, given our on the spot observations and theoretical analysis. 3) On the basis of this analysis, a mechanical seepage model of filled fractures has been established.
Acknowledgements The present research work was supported by the National Natural Science Foundation of China (50709030) and the Natural Science Foundation of the Education Department of Henan Province (2008A410002 and 2007570011). The authors gratefully acknowledge the support from the above foundations.
References [1]
Tian K M, Chen M Y, Wang H L. Crevice Water Drift. Beijing: The Publishing House of Learning Centre, 1989.
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No.6
(In Chinese) Su B Y, Zhan M L, Zhan Z T. Experimental research of seepage characteristic for filled fracture. Rock and Soil Mechanics, 1994, 15(4): 46–51. (In Chinese) [3] Wang X, Qu Y Y, Hu W W, Chen J, Zhao X Y, Wu M. Particle characteristics and rheological constitutive relations of high concentration red mud. Journal of China University of Mining & Technology, 2008, 18(2): 266–270. [4] He M C, Jing H H, Sun X M. Engineering Mechanics of Soft Rock. Beijing: Science Press, 2002. (In Chinese) [5] Ju J Y, Shen D X. Development in the Project of Alta-mud and Application. Beijing: The Chinese Publishing House of Building Materials Industry, 2003. (In Chinese) [6] Take W A. Physical Modeling of Seasonal Moisture Cycles and Progressive Failure in Embankments [Ph.D. dissertation]. Cambridge: University of Cambridge, 2003. [7] Xu Z M, Huang R Q, Tang Z G. Clay minerals and failure of slopes. Chinese Journal of Rock Mechanics and Engineering, 2005, 24(5): 729–740. (In Chinese) [8] Chang Q L, Zhou H Q, Hou C J. Using particle swarm optimization algorithm in an artificial neural network to forecast the strength of paste filling material. Journal of China University of Mining & Technology, 2008, 18(4): 551–555. [9] Zhang S X, Xu C Y, Hu X W, Yang M L, Yin J G, Xu X W, Tao W S, Cao T X. The influence of montmorillonite filling in fractures on the stability of a tunnel. Rock and Soil Mechanics, 1996, 17(4): 56–61. (In Chinese) [10] Zhang S X, Xu C Y, Chen J F. Crack Rock Mass Afford to Excavate. Wuhan: The Institute of Technology of Wuhan, 2000. (In Chinese) [11] Xu D J, Chen C X, Xu Y B, Ren W Z, Gu G R. A study of the rock mechanical parameters of slopes in east open-pit slopes of Daye iron mine. Rock and Soil Mechanics, 1999, 20(4): 69–75. (In Chinese) [12] Louis C. A Study of Groundwater Flow in Jointed Rock and Its Influence on the Stability of Rock Masses. London: Imp Coll, 1969. .
[2]
www.elsevier.com/locate/jcumt
Effect of expansive fillings on fracture seepage CHEN Jin-gang, YANG Jun-li, ZHANG Jing-fei Department of Engineering Mechanics, Zhengzhou University, Zhengzhou, Henan 450001, China Abstract: Based on an investigation of the damage process of fractures filled with expansion media, samples of filling media collected on the spot, were obtained. The physical water properties of fracture fillings were tested by a WZ-2 type Dilatometer. Given our test results of the hydrated properties of fracture fillings and the mechanical parameters of altered rock mass collected from the Daye Iron Mine and comparisons between the expansion stress of fracture fillings and mechanical parameters of altered rock mass, the effects of the mechanical response of fillings on fracture seepage are discussed. The results show that the ratio of filling swell pressure to tensile strength of altered rock specimen varied between 11.79 and 36.77 and the ratio of filling swell pressure to shear strength of these rock specimen ranged from 72.11 to 171.18. Therefore, fillings have an important effect on fracture seepage, and the effect of deformation of the fracture caused by swell pressure of the filling mechanics cannot be ignored either. The multiple hydraulic coupling effects of fillings on the impact of fracture permeability are discussed according to theoretical analyses. It is shown that the effect of expansion of fracture fillings greatly affects the deformation of altered rock masses. Both tensile effect and shear effect produced by fracture fillings greatly increase the permeability of fractures. The plastic and liquefaction effects of fracture fillings also improve the permeability of fractures. On the basis of this analysis, a mechanical seepage model of filled fracture is established. Keywords: expansion; filling medium; fracture; seepage
1
Introduction
For a long time, research on fracture seepage has largely concentrated on the characteristics of seepage under stress, while studies on the effect of expansion fillings on seepage are comparatively weak. That is to say, mechanical, plastic and liquefaction effects imposed by expansion fillings on fracture seepage have, up till now, not been given adequate attention by investigators. It is a comparatively general phenomenon that fractures are naturally filled with material. Fillings have an important effect on fracture seepage. In 1989, Tian et al. proved that filled fracture seepage could still be described with cubic rules via a large number of experiments and theoretical analysis, but it needs to be modified now, since 1994 an important coefficient, which only depends on the porosity of the fillings, needs modification[1]. Su et al. conducted model experiments on permeability characteristics of filled fractures and found that the permeability of these fractures not only depends on the filling material, but are also clearly related to the particle granularity of the fillings[2]. They deduced the half-empirical and theoretical formula based on the
Ⱦ.Ɇ.Ɇɢɧɰ half-empirical theory from the analysis of fracture seepage mechanism. Red mud flows in the form of a highly viscous paste through a pipe. The friction between particles and between particles and water, the adhesive water on the surface of the particle and the galvanic-chemical action will all change their rheological properties[3]. In 2002, He et al. studied the physical chemistry and mechanical mechanism of deformation of soft expansion rock, especially for minerals subject to expansion like montmorillonite and also discussed its functional mechanism in terms of mechanical strength characteristics[4]. Expansion and shrinking of a mineral crystal lattice has a comparatively large impact on its volume and the amount of water also has a great effect on its volume[4]. These authors also studied the characteristics of expansion minerals and explained the function mechanism of permeate pressure of expansion and shrinking of a mineral crystal lattice[5]. Montmorillonite has a strong capacity to absorb water. Its volume increases considerably, up to a dozen times, after absorbing water. With this increase in the amount of montmorillonite, the swell pressure increases significantly and the time needed for reach-
Received 23 February 2009; accepted 09 June 2009 Projects 50709030 supported by the National Natural Science Foundation of China, 2008A410002 and 2007570011 by the Natural Science Foundation of Educational Department of Henan Province of China Corresponding author. Tel: +86-13837140487; E-mail address: [email protected]
CHEN Jin-gang et al
Effect of expansive fillings on fracture seepage
ing a maximum increases regularly[5]. The repeated expansion and contraction of fractures are caused by wet and dry filling cycles, where each cycle forms residual deformation. If the deformation continues to accumulate and generates tensile fractures, this process will in the end cause damage to the rock mass because of this gradual accumulation[6]. Fractured zones may be formed if the fillings accumulate and settle at the macro structures. The greater the amount of clay mineral, the more important the effect of sliding and the easier it is to induce slope instability, starting the process of inevitable geological disasters[7]. The model of predicting the strength of filling material was established by applying the theory of artificial neural networks[8]. Taking a broad view of the existing literature, we find that the effect of the fillings on fracture seepage and its mechanism have seldom been studied either at home or abroad. The relevant literature about controlling the effect of expansion fillings on fracture seepage is also largely silent on the issue. It is hard to assess accurately the infiltration characteristics of filled fractures if we know so little about the dynamic permeability of filled fractures, especially under conditions of heavy rainfall and it is therefore difficult to design a drainage program for projects such as mines, water conservation, etc.
2 Characteristics of rocks and appearance of fracture fillings in mining areas 2.1 Characteristics of rocks in the research area The exposed stratum of the Daye Iron Mine is largely of Triassic origin. The main composition of the north and the east sides of the stone pit is diorite, but the south side of the stone pit is mainly made up of marble. Fractures on each side are almost completely filled and the fillings are quartz, garnet, traversellite, etc. To different degrees, changes often take place near ores of various lithological characteristics. The rocks in the slope are eroded from the action of the structure. It is referred to as the change of country rocks near ores and the structural fracture belt. The country rock includes chlorite, kaolinite, montmorillonite, illite, etc., while the fracture belt consists largely of chlorite, montmorillonite and kaolinite. The original mechanical strength varies considerably as a result of changes in rocks. The strength of rocks such as chlorite, kaolinite, and montmorillonite suffers severely from these strong changes and is considerably reduced. 2.2
Characteristics of fracture fillings appearance in the research area
There is an approximately oval cavity in the upper tunnel shoulder, with dimensions 1.2 m×0.8 m and a depth of 1.2 m. Montmorillonite can be found along
825
the entire wall wherever water drips down intermittently. It is because fillings fall from the fractures, due to underground water. Sometimes, two fissure zones, filled with montmorillonite, are exposed where the tunnel collapses. From this observation, we conclude that the fissure zone is from several millimeters to more than 40 cm thick[9].
3
Physical water properties of fillings and mechanical properties of rock samples
Our analysis, based on a number of different tests, indicates that the main mineral composition of the changing rocks is montmorillonite in the open-pit east of the Daye iron ore. Its content ranges from 7.7% to 45%, but generally falls within the 20%–30% range. The lowest swell pressure is 0.125 MPa, while the maximum is 0.684 MPa, with an average of 0.353 MPa. Other parameters of the fillings are shown in Table 1[10]. The water content of most filling samples is greater than 15%, their plastic limit more than 17% and the liquid limit is greater than 33%. The water content is always lower than the plastic limit. Our data indicate that the water-absorbing course of fillings is synchronous with that of swelling and deformation. The state of transfer of the fillings from solid to liquid requires a large amount of water to seep into the fillings and rainfall, especially torrential rains, has become the natural condition for this transfer. Table 1 Physical water properties of fracture fillings Free Minimum Maximum Water Liquid Plastic Sample Plastic swelling swell swell content limit limit number index rate pressure pressure (%) (%) (%) (%) (MPa) (MPa) FDB1 15.67 26.9 17.8 9.1 26.0 0.125 0.163 FDB2
15.86 11.71
34.2
19.9
14.3
13.4
0.200
0.258
0.137
0.192
FDB3
15.56
33.0
17.0
16.0
51.0
0.296
0.329
FDB7
28.74
64.2
30.4
33.8
99.5
0.467
0.582
FDB9
15.56
34.9
20.8
14.1
52.5
0.684
0.684
It has become common knowledge in engineering circles that the tensile strength of a rock mass is far lower than its compression strength. For over 30 years, a large number of tests have been conducted from which we have learnt a great deal about slope rock masses of the open-pit in the east of the Daye Iron Mine which has provided a large amount of information for rock mass fracture parameters in the database. Different parameters of rock samples can be obtained from the relationship between the former test results and the latter empirical tests, shown in Table 2. The fractures of most rock masses are the result from either shearing stress or drawing stress. According to the data of Tables 1 and 2, a curve contrasting the swell pressure of fillings and their tensile and shear strengths can be drawn. An analysis from a
826
Mining Science and Technology
comparison of the data shows that the ratio of filling swell pressure to tensile strength of altered rock specimens varies between 11.79 and 36.77, the ratio of filling swell pressure to shear strength of the specimen ranges from 72.11 to 171.18. Therefore, mechanical deformation of the altered rock mass, caused by the effect of swell pressure of the fillings cannot be ignored. We conclude that a mechanical response of the fillings lies at the bottom for the control of fracture seepage characteristics under rainfall conditions. Table 2 Mechanical parameters of altered rock mass Single axle Single axle Single axle Shear strength compression tensile tensile strength strength strength c φ (MPa) (dry) (MPa) (wet) (MPa) (MPa) (°)
Rocks
Marble
70
2.69
1.96
0.25
36
Diopside skarn
94.5
3.18
1.81
0.34
38
Diopside diorite
51.43
2.32
1.14
0.49
33
Weak altered meroxene diorite
59.96
2.49
1.22
0.52
41
[11]
68.31
2.66
1.32
0.53
45
Dolomite marble[11]
63.0
2.55
1.86
0.23
35
Chlorite diorite[11]
57.4
2.44
1.19
0.21
33
Strong altered kaolin[11]
43.4
2.16
1.06
0.16
30
Olafite diorite
4
Effect of physical and mechanical characteristics of fillings on fracture seepage
Fracture deformation primarily contributes to the change of rock structures. The presence of fillings leads to changes in a rock mass, weakens its anti deformation capacity and deforms the entire rock mass. Fracture rock permeability has much to do with the opening, length and connectivity of fractures. At the same time, the deformation of fractures under stress also has an obvious impact. The response of mechanical fillings caused by water promotes the deformation of the fracture, increases its width, shortens the transportation route of water and it is extremely easy for fillings to slip or get lost under the perturbation of stress. Under these conditions, its plastic and liquefaction effects will change rock fracture seepage considerably. 4.1
Effect of expansion of fillings on fracture seepage
Under natural conditions, the original structure of montmorillonite will not be disturbed when fresh rock samples soak in water and swelling is not obvious. The newly opened side of the developed tunnel is exposed to the air flow. During the gradual drying period, the moisture in the fillings will evaporate and the structure of montmorillonite crystals responds to the change in order to keep balance with the outside
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atmospheric humidity. Montmorillonite has an active crystalline grain structure, sensitive to water and is strongly hydrophilic. Besides, crystalline layers shrink after they are dehydrated and swell when absorbing water. So, once the dehydrated montmorillonite meets water, the dehydrated fillings will absorb a great deal of water, leading either to an increase in the depth of adsorbed water between the crystalline layers and the rise of combined water thickness around particles or water drips into the mineral crystals. The wedge pressure of the combined water causes fracture inflation. If the rock has a high content of montmorillonite and the fillings meet water, the water drips into the rock mass rapidly along fractures and runs into the active layers of montmorillonite, causing it to swell. The expansion of the volume can increase considerably up to a dozen times the original volume[5]. Inflation produces great pressure and the pressure further weakens an already weak rock mass. Material damage mostly results from the effect of tensile and shear stress. According to rock tensile failure conditions: σ −σt = 0 (1) where σ is rock stress and σ t tensile (shear) stress. Based on this analysis, it is easy to show that the destruction is largely the result from the effect of tensile and shearing stress. The preceding analysis shows that the swell pressure of fillings has important effects on the deformation of the changing rock specimens, contributing considerably to fracture deformation. We should therefore take this factor into account, in order to analyze the characteristics of the filling fractures more effectively. When fractures in a shearing environment are due to expansion of their own internal media, the swell pressure produced by the fillings can be destroyed by shearing action caused by changing rock cracks. That is to say, in this case, that a single fracture is equivalent to being expanded into several fractures, or that fractures, unconnected with each other, become connected, clearly increasing the permeability of the fractured rock. When fractures in an environment of tensile stress owe their expansion to internal media, the expansion of water-absorbing fillings will generate additional tensile stress in the fractures. Suppose that the swell pressure is σ ′ and because of the effect of its crack expansion wedge, a fracture is relatively pulled and widened, where the stretch displacement is Δμ t = σ ′ /Kn. Considering σ ′ , the rock strain: Δε = σ ′ / E , compresses the rock, where the value of the displacement is: Δμs = − Sσ ′ / E . Therefore, under pressure of the expansion of the fillings, the additional increase of fracture width is: ΔW = (W0 + Δμ t + Δμs ) − W0 = σ ′ / K n + Sσ ′ / E
(2)
CHEN Jin-gang et al
Effect of expansive fillings on fracture seepage
where ΔW is the additional increase of the fracture width, W0 the initial width of the fracture, Kn its normal strength, E the modulus of elasticity and S the mass rate. Research shows that the seepage characteristic of fractures conforms to the famous cube rule, i.e., the fracture permeability is directly proportional with the cube of its width[12]. The seepage of filled fractures can be described with this cubic rule. The formula is:
K = nβ W 3 / L
(3)
where K is the permeability of the fracture, W the fracture width; L the fracture spacing; β a coefficient and n the porosity of the fillings. The increment of fracture permeability, produced by tensile stress, is: ΔK t = nβ ª¬ (W0 + ΔW )3 − W03 º¼ L
(4)
If other factors remain unchanged and if the crack spacing increases, fracture permeability will rise greatly according to this cubic rule. Therefore, a small increase in fracture spacing will have a considerable impact on its permeability. From this exposé, it is clear that the ratio of expansion pressure to tensile strength of changing rock specimens lies between 11.79 and 34.02. It is also clear that the expansion pressure makes the fracture wider. The big slope displacement in the fault (F9) of the Daye Iron Mine is caused by the expansion of the fillings, by rain permeating into the fault. To say the least, even if expansion pressure were not enough to connect the fractures completely, it still has a great impact on its deformation, i.e., a small increase in fracture spacing increases its permeability greatly. The increment of fracture permeability produced by shear stress is: nβ W 3 nβ W 3 − (5) L − ΔL L The permeability of fractures is improved greatly by both tensile and shears stress produced by crevasse fillings. A mechanical model of filled fracture seepage can be constructed from this analysis (Fig. 1). ΔK s =
Tensile stress increases the width of cracks and shear stress increases fracture spacing, both of which improve fracture seepage ability.
4.2 Effect of plastic and liquefaction of fillings on fracture seepage It can be seen from the spot that the seeper in the cave filling areas clearly increases, compared with normal areas. It proves that the loss of the fillings improves the permeability of fractures. The fracture filling is a kind of fluffy medium, characterized by loose structure, high porosity, weak mechanical strength, is prone to plasticity and extremely sensitive to external perturbation. The fillings begin to soften under the action of increasing amounts of water. Over time, the fillings are gradually transformed from a solid to a plastic state. After deformation due to expansion from encountering water, the relatively close, high strength and high dense structures change to loose porous structures, that is the typical of the characteristics of fracture fillings. With further increase in the amount of water, the fillings are transformed from the plastic state to a liquid state. During rainfalls, especially rainstorms, fracture fillings will be eroded by running water. Because these water flows are active and motive forces and generate strong dynamic effects, particles of the fillings will be pushed away and the volume of free space in fracture increases. With further removal of particles, the velocity of the water flow also increases and even thicker particles will be removed and fracture spaces continue to increase gradually. In this way, with the expansion of fracture spaces, filled fractures will be connected and become an open passage. When the filled fractures are fully connected and develop into an open passage, the seepage characteristic indicates no fillings in fractures, i.e., the modified coefficient n (in the cubic rule) does no longer exist in this case. Because of porosity of the fillings n<1, seepage ability of the filled fractures should be less than that of no fillings in fractures. After the fillings are eroded and removed, the permeability of fractures will be higher than that of the original fractures. Therefore, the permeability of fractures will be clearly improved.
5
Fig. 1
Mechanical model of filled fracture
Intuitively, this seepage model shows that two kinds of effects are caused by hydraulic coupling of the filling media, i.e., tensile stress and shear stress.
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Conclusions
Fracture fillings have a significant impact on mechanical rock properties and permeability. In some sense, the physical and mechanical properties of rock masses depend mainly on that of rock joints, fractures and fillings. The effects of expansive fillings on fracture seepage have been discussed. Given our empirical research and theoretical studies, we come to the following conclusions.
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1) The ratio of expansion pressure of the fillings to tensile strength of altered rock specimens varies between 11.79 and 36.77 and the ratio of filling swell pressure to shear strength of altered rock specimen ranges from 72.11 to 171.18. Therefore, fillings have an important effect on fracture seepage. Deformation of fractures caused by swell pressure cannot be ignored. When the permeability of filled fractures is studied, this factor must be considered. 2) The rate of increment of fracture permeability caused by tensile stress and shear stress of the filling media are deduced from theoretical analyses. Both tensile and shear stress produced by the expansion of the fillings improve the permeability of filled fractures. It has been shown that the plastic effect and liquefaction effect of fracture fillings also improve the permeability of fractures, given our on the spot observations and theoretical analysis. 3) On the basis of this analysis, a mechanical seepage model of filled fractures has been established.
Acknowledgements The present research work was supported by the National Natural Science Foundation of China (50709030) and the Natural Science Foundation of the Education Department of Henan Province (2008A410002 and 2007570011). The authors gratefully acknowledge the support from the above foundations.
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