Environ Earth Sci (2016)75:284 DOI 10.1007/s12665-015-5193-x

ORIGINAL ARTICLE

A laboratory study of the effect of confining pressure on permeable property in soil-rock mixture Y. Wang1 • X. Li1 • B. Zheng1 • S. D. Li1 • Y. T. Duan1

Received: 20 May 2015 / Accepted: 23 October 2015 Ó Springer-Verlag Berlin Heidelberg 2016

Abstract This paper aims at investigating the effect of confining pressure on fluid–solid properties in soil and rock mixture (SRM) specimens with different rock block percentage. Self-developed servo-permeable testing system was used to carry out the flow-stress coupling testing. Fifty cylindrical SRM specimens (50 mm diameter and 100 mm height) with staggered rock block proportions (20, 30, 40, 50, 60 and 70 % by mass) were produced by compaction test with different count hammers to roughly insure the same void ratio of soil matrix. From the test results, the effect of loading and unloading confining pressure on permeability coefficient is discussed. Permeability coefficient of SRM presents decreasing trend with the increase of confining pressure. At confining pressure descent stage, permeability coefficient begins to increase from the minimum, but it is smaller than the confining pressure ascent stage, the decreasing rate of permeability coefficient is more rapid than increasing rate. In addition, the relationship between permeability coefficient and confining pressure follows quadratic polynomial function, and the

& X. Li [email protected] Y. Wang [email protected] B. Zheng [email protected] S. D. Li [email protected] Y. T. Duan [email protected] 1

Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

correlation coefficients are greater than 0.98 for all studied SRM specimens. Test results also show that at different hydraulic gradients, permeability coefficient increases with time. The results show that interaction between soil matrix and rock blocks is the main reason leading to the variation of permeability coefficient. Keywords Soil and rock mixture (SRM)  Confining pressure  Permeability coefficient  Rock block percentage

Introduction Study on the flow characteristics of SRM is the theoretical basis for various engineering geological problems concerning to the geomaterial of loose deposit. With the largescale construction of large hydropower stations all over the world, especially in the southwest of China, the SRM is usually the main geomaterials for slopes, landslides, subgrades, and dam basements in the reservoir area; in addition, rainfall is a key factor inducing typical geological disaster, such as landslides and debris flow, etc. (Xu and Wang 2010b; Wang et al. 2014, 2015). So, the study on flow characteristics of SRM is critical to the stability of geological body, which causes great concerns by many scholars and engineers. When fluid flows in SRM, interaction between SRM and fluid appear inevitably. On the one hand, the flowing fluid has obvious influence on the physical and mechanical properties of SRM (soil matrix, rock blocks and rock-soil interface, etc.); also, it has important influence on the stress field distribution in SRM. On the other hand, rock-soil interfaces, pores, cracks as seepage path, which control the flow characteristics of water. The stress variety alters the flow characteristics of ground water; the change of the flow characteristics further

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Fig. 1 Structural of SRM, which is composed of rock blocks, soil matrix, pores and cracks (The picture is taken from the slope located at Jianchangtou tunnel, the Fengning Country)

alters the distribution of stress field in SRM. The coupling interaction between stress field and seepage field restricts the stability of SRM body, the study of the fluid–solid coupling properties is significant to SRM engineering, and the development of novel theory in geotechnical mechanics. Figure 1 shows that the SRM has a special structure which is different from both soil and rock. It has been long concerned that confining pressure has obvious influence on the permeability coefficient of porous medium. Zoback and Byerlee (1976) presented the seepage experimental results of Ottawa sandstone and granite broken particles, and found that permeability coefficient decreased with the increase of confining pressure. Bear et al. (1993) carried out numerical simulation test studied the change of seepage vector field, and found that the pores, cracks in rock were compressed smaller, which not only resulted in the decrease of permeability coefficient of rock, but also the seepage path was changed obviously. Oda et al. (2002) conducted triaxial compression tests of Inada granite and found that permeable of rock decrease with increasing of confining pressure, resulting from the closure of pores and micro-cracks. Li et al. (2005) conducted experiment to study the permeable property of sandstone at stress path of unloading confining pressure, and obtained the relationship between permeability coefficient and confining pressure. Li and Luo (2006), Guo (2009) have conducted experiment for undistributed loess and distributed loess with various confining pressure, and drawn the conclusion that permeability coefficient decreased with the increasing of confining pressure and consolidation pressure. Also, they found that permeability coefficient of loess was smaller when exerting the confining to the target value at one stage than multistage. Ameta ¨ berg-ho¨gsta (2002) and Wayal (2008), Sa¨llfors and O

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reported that permeability coefficient of bentonite decreases with the increase of confining pressure; at a certain confining pressure, permeability coefficient also decreased with the increasing of time. Xu et al. (2007) carried out experiment on soft rock and the results demonstrated that the permeability coefficient decreased with the increasing of confining pressure, and increased when unloading. Fang et al. (2005) studied the change law of permeability coefficient for concrete block with different confining pressure, found that inversely exponential relationship existed between permeability coefficients and confining pressure. After the above literature review, the fluid–solid coupling study of the relationship between permeability coefficient and confining pressure is almost concentrated on soil, rock, and rock-like material (e.g., concrete, mortar, etc.). It is clear that few reports about the effect of confining pressure on permeability coefficient for SRM material have been published. As a special geomaterial, SRM formed in the quaternary period, is made up of a mixture of fine soil aggregates, rock blocks, cracks and pores (Medley et al. 1995; Coli et al. 2011; Wang et al. 2014). These individual components of SRM usually have different mechanical and physical properties and different responses under flow-stress condition. Random distributed rock blocks alter the seepage path of fluid, large seepage force drop happens at rock-soil interfaces. The seepage characteristic of SRM is obviously controlled by the inhomogeneous structure. Although the seepage characteristics of SRM have been studied by some scholars (Shelley and Daniel 1993; Indrawan et al. 2006; Shafiee 2008; Xu et al. 2010a; Alonso et al. 2001; Chen et al. 2012), however, the study of permeability coefficient of SRM in laboratory test mostly based on conventional penetration test (e.g., constant head test) (Shafiee 2008;

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Fig. 2 The soil and rock blocks used in the preparation of SRM specimens. a Particle size distribution of soil used in the test; b rock blocks used in SRM specimens, size range between 2 and 5 mm

Chen et al. 2012, 2014), it is difficult to simulate the actual stress state and hydraulic pressure. The experimental results so far have not considered the confining pressure state and hydraulic gradient condition for SRM. In addition, although field experiments can obtain macroscopic permeability coefficient of SRM, it is impossible to study the change of permeability coefficient with different confining pressure and hydraulic gradient (Gao et al. 2009; Chen et al. 2012; Wu et al. 2013). What is more, the laboratory permeable test of conventional penetration test, it cannot ensure the accurate constant rate or pressure increment of injected water. Therefore, the basic objective of the present work is to investigate the relationship between permeability coefficient and confining pressure for SRM with different rock block percentage. Self-specific developed servo pressurized water supply test system which can accurately control the water injection rate and pressure increment, is used to perform the flow-stress test.

Experimental methods The testing material In this paper, SRM specimens were cylinder shaped with diameter 50 mm and height 100 mm. According to the soil specimen preparation standard (GB/T 50123-1999) and the geotechnical test technical manuals of BS1377-1 (1990), diameter of blocks should not more than 10 mm, threshold value for rock block and soil is defined as 2 mm. The soil matrix was obtained from the pit at 10 m deep in the Chinese Academy of Science Institute of Atmospheric Physics. Ten sieving tests were performed to determine the classification of soil, and showed that the studied soil matrix belonged to clay soil, as shown in Fig. 2a. Rock

Table 1 Basic mechanical and physical parameters for the studied soil matrix and rock blocks from geo-technical test Property index

Soil

Rock blocks

Natural weighted density (g/cm3)

1.64

2.52

Dry density (g/cm3)

2.06

2.61

Water ratio (%)

10.2

0.41

Specific gravity (GS)

2.73

Wet compressive strength (MPa)

0.57

50.65

Dry compressive strength (MPa)

2.272

100.741

3.07

Water content corresponding to the ‘‘dry’’ and ‘‘wet’’ for soil specimens were 2 and 10 %, respectively; and for rock specimens were 3 and 85 %, respectively

blocks were aggregate marble gravels with size between 2 and 5 mm (Fig. 2b). Some basic mechanical and physical parameters by geotechnical testing were also shown in Table 1. The soil contained plenty of clay mineral with strong hydrophilic, the liquid limit and plastic limit of the studied hard clay can reach to 63.72 and 36 %, respectively; the plasticity index and liquidity index were about 27.72 and 0.05–0.127, respectively. The above experimental indices indicated that the soil matrix belonged to the typical high plastic and hard plastic clay. To identify the mineral composition and mineral content, we both conducted scanning electron microscope (SEM) and X-ray diffraction (XRD) tests on the soil matrix. According to the result of the SEM tests, as shown in Fig. 3, it can be seen rodlike and irregular quartz grains with grain size about 0.01–0.03 mm and probably surrounded by clay minerals. The XRD tests revealed the mineralogical composition and shown in Table 2. According to Table 2, it is clear that the soil has higher percentage of clay mineral, such as kaolinite, montmorillonite, and illite (Wang et al. 2015).

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Fig. 3 SEM results for soil sample1# and sample2#

Table 2 Mineralogical composition of soil specimen obtained from XRD Mineral

Soil specimen1#

Soil specimen2#

Montmorillonite (%)

61.52

63.28

Kaolinite (%)

26.73

24.66

Illite (%)

6.25

6.58

chlorite (%)

5.5

5.48

70 %. In process of the remolded SRM specimens, extra water with optimal water content of 9.5 % was added into the mixtures, determination of the optimal water ratio was used compaction test for soil matrix. The weight of the hammer is 4.5 kg, diameter is 50 mm, and falling height is 450 mm. The specimen was compacted with three layers, as shown in Fig. 5a. All the tested specimens were cylindershaped and sealed with plastic wrap in Fig. 5b. Experimental system

Preparation of remolded specimen In this paper, remolded soil-rock mixture specimens were prepared for the permeability tests. A total of 50 SRM specimens were produced for the experiment by compaction test (Jan et al. 2013; Wang et al. 2015), the optimal hammer count was determined according to the relationship between the soil density and hammer times. Because the compactness of soil matrix has a great impact on the permeability coefficient of SRM, so during the specimen production, the density of soil matrix was kept the same as much as possible for the specimens with different rock block percentage. As shown in Fig. 4a, the density of soil matrix for SRM specimens with rock block percentage 20–70 % increased with the increase of hammer count. To keep the same soil density (i.e. void ratio) in the SRM specimens, the optimal hammer count for SRM specimens was determined as 2, 3, 3.5, 5,11 and 15 counts (The 0.5 time hammer is achieved by dropping the hammer with half distance at one time), respectively, as shown in Fig. 4b. When the rock block percentage reaches to 70 %, the soil in SRM specimens is difficult to compacted, the rock blocks play the role of skeleton to the large extent. Therefore, considering that the rock blocks in specimens with rock block percentage of 70 % would be crushed with too much hammer counts, so 15 times was determined as the optimal hammer count for specimens with rock block percentage of

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The testing system for the permeability test of SRM includes the rigid loading device, the servo pressurized water-supply system, the specimen chamber system, and the confining pressure system. The overall setup of the system is shown in Fig. 6. In the test, axial stress is applied using the hydraulic jack, and the range of axial load is 0–100 kN. The axial force is measured by the stress sensors during the test. The precision of the load controller is 0.01 kN, and the load controller can record the axial force at every loading stage. Two micrometer gauge with precision of 0.001 mm were installed on the platform, to measure the axial deformation during the test. The servo pressurized water-supply system is the core component of the overall setup. It is composed of speed feedback component, servo and drive motor, full digital servo controller and the computer. The servo pressurized water-supply system is controlled by the Doli servo controller made in German, it is dived by the ball screw stepping servo motor. According to the principle of piston principle, the water can be pressurized and supplied to the specimen. By computer operation, the servo pressurized water supply system can inject water at constant rate or pressure to the specimen chamber. Before the test, the piston is set back firstly, and let the water into the tank, and then the piston is in servo control state, so to control the

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Fig. 4 The methods to keep the same density in SRM specimens (a Relationship between density of soil and hammer count; b determination of the optimal hammer count)

Fig. 5 Preparation of the remolded specimens and specimens used in the flow-stress coupled tests a production of the remolded specimens with Compact Test, the specimen was compacted with three layers; b partial remolded SRM specimens for permeability test)

Fig. 6 Testing system (1 Upper cross beam; 2 Rigid column; 3 Platform; 4 Guide bar; 6 Lower cross beam; 6 SRM specimen; 7 Selfadhesive tape; 8 Permeable cushion; 9 Hose champ; 10 Filter paper; 11 Heat shrink tubing; 12 Seconds counter; 13 Water valve; 14 Force

sensor; 15 Hydraulic jack; 16 Three-way valve; 17 Water tube; 18 Servo-injection water system; 19 Load controller; 20 Measuring cup; 21 Gasbag Hoek Cell-confining pressure device)

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Fig. 7 Photograph of the metal permeability cushions and specimen chamber structure a the lower cushion; b the upper cushion; c structure of specimen chamber)

speed of water-supply and water pressure to the permeability apparatus. The sample chamber system consists of a metal upper cushion, a metal lower cushion, two metal seepage plates, two hose champs and a length of heat shrink tubing accommodating the SRM specimen. The metal upper and lower cushions contain inlet valves, outlet valves, and some grooves. The diameters of the inlet and outlet are 3 mm. To prevent soil particles from clogging the metal cushions, screens with hole-diameter of 0.075 mm are placed on the cushion. The heat shrink tube and metal cushion is connected with self-adhesive type and hose clamps. Figure 6 shows the photograph and schematic representation of the specimen chamber system. The mental permeability cushions are specially designed for the test, dimensions and structure of the cushion, and locations of the inlet and outlet valve are shown in Fig. 7. The confining pressure is applied by a gasbag Hoek cell. The confining pressure is measured by the gas-pressure meter. Measurement range of the gas-pressure meter is 200 kPa with precision of 0.5 kPa. For this kind of extreme heterogeneous geological materials, the stress distribution inside SRM is uneven, the real state is that stress acting on every point of boundary surface are equal (Meng et al. 2004). Therefore, gasbag confining pressure system which can provide flexible boundary loading is used to impose confine pressure, so to realize the equivalent stress condition for SRM specimens.

Start SRM specimen installation

Selecting sevro-controlled mode to inject water

SRM specimens saturate and form steady seepage field Hydraulic pressuredifference control

Confining pressure control Mechanical loading control mode

Closed loop servo-control system

Incremental step loading of axial and lateral stress σ3

Keep the hydraulic gradient constant (e.g., P1=0.03, 0.05, 0.07MPa)

Monitoring variation of hydraulic pressure and flow water volume by computer, collecting experimental data

Obtain the permeability coefficient

Whether to continue the seepage test?

Changing σ3

End

Fig. 8 Technical flowchart for study on the flow-stress coupled tests of SRM specimens

Test procedure To study the effect of confining pressure on permeable characteristics in soil and rock mixture, and to obtain some significant conclusions from the flow-stress coupled test,

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the technical flowchart is used in this paper as shown in Fig. 8. The specimens produced by compaction test are installed by the upper cushion, lower cushion, heating shrink tube, plastic self-adhesive tape and hose clamps.

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Before the stress-seepage coupling test, specimens should be saturated and form steady seepage. During water-injection process, water is supplied by constant flow velocity or constant pressure increment by closed loop servo-control system. Axial stress is applied by the hydraulic jack and confining pressure is applied by the gasbag Hoek cell, as shown in Fig. 6. At a constant confining pressure, we monitored the variation of hydraulic pressure and flow water volume by computer, and record the experimental data. The detailed test procedures are shown in Fig. 8. We calculate the coefficient of permeability, k, as follows: k¼

QL AtðP1  P2 Þ

k¼

QL g  T AtðP1  P2 Þ g20

hydraulic pressure difference of 0.05 MPa. From Fig. 10, the main results are as follows: 1.

ð1Þ

where, k is coefficient of permeability; Q is quantity of water discharged; L is length of specimen; A is cross-sectional area of specimen; t is total time of discharge; P1 and P2 is the hydraulic pressure of the inlet valve and outlet valve, respectively. Furthermore, the permeability coefficient should be corrected to that for 20 °C (68 °F) by multiplying k (Eq. 1) by the ratio of the viscosity of water at test temperature to the viscosity of water at 20 °C (68 °F). The finial expression of permeability coefficient is as below: ð2Þ

2.

3.

where, gT and g20 is the coefficient of water kinematic viscosity at T °C and 20 °C, respectively.

Results and discussions General observation As stated in the test procedures, after steady seepage has formed in SRM specimens, the hydraulic pressure, water outflow volume and time began to be recorded. The relationship of hydraulic pressure, water outflow volume and seepage time is shown in Fig. 9. As shown in Fig. 9, interaction between rock blocks and soil matrix changes strongly with the increase of time. At each stage of confining pressure increment, the curves present fluctuation trend at a certain constant confining pressure. The seepage path changed when changing the confining pressure, which results in the variation of hydraulic pressure and outflow water volume. Relationship between permeability coefficient and confining pressure Figure 10 plots the relationship of permeability coefficient against confining pressure for typical SRM specimens at

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4.

At confining pressure ascending state, the permeability coefficient of SRM decreases gradually. This phenomenon indicates that seepage channel of water is squeezed and clogged by compressive stress, which reduces the permeable ability of fluid flowing in SRM. The reduction rate of permeability coefficient is different for specimens with various rock block percentage. The order is: SRM40 [ SRM20 [ SRM30 [ SRM70 [ SRM60 [ SRM50, this result may be related to the internal structure of SRM specimens. At confining pressure descent state, the permeability coefficient of SRM increases again with the decrease of confining pressure. This phenomenon indicates that the seepage channels which are squeezed compaction under compressive stress re-open again. Porosity of SRM specimens increases again, resulting in the improvement of fluid seepage in SRM. The improvement rate of permeability coefficient is different for specimens with different rock block percentage, the order is: SRM70 [ SRM40 [ SRM50 [ SRM60 [ SRM30 [ SRM20. Comparing of the permeability coefficient reduction rate (K1) in confining pressure ascent state, against increasing rate (K2) in confining pressure descent stage, for the studied specimens, K1 [ K2. This results implies that after the confining pressure ascend stage, internal structure in SRM specimens changes, the change cannot recovery by unloading confining pressure. At the same confining pressure, the permeability coefficient in the confining pressure ascent stage is less than that of decent stage. This result can be better interpreted in Fig. 10. Results indicate that the SRM specimens appear irreversible damage.

In the study, according to the experiment results and refers to the fitting equations of Miller and Low (1963), Jones (1975), Mckee et al. (1988) and Liu and Liu (2001). Linear (y = ax ? b), exponential (y = aebx), power(y = axb) and polynomial curve fitting approximations were executed and the approximation equations that have the highest correlation coefficient was determined for the permeability coefficient- confining pressure equations. The non-linear curve fitting equations for the specimens are listed in Tables 4 and 5. Correlation coefficients were all [0.98, the correlation of the equations are good. Permeability coefficient and confining pressure of SRM shows quadratic polynomial relationship. From the quadratic polynomial fitting equation, the expression of SRM permeability coefficient is defined as below:

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Fig. 9 Plots of hydraulic pressure, water flow volume against time for typical SRM specimens with confining pressure 20 kPa and pressure difference 0.05 MPa (a–f corresponds to SRM 20–5, 30–5, 40–5, 50–5, 60–5 and 70–5, respectively)

KSRM ¼ aP2c þ bPc þ K0

ð3Þ

where, KSRM is the permeability coefficient of SRM; Pc is effective confining pressure; a, b is regression coefficient; Ko is the initial permeability coefficient. Correlation coefficients (r) were greater than 0.98 for SRM20-5, SRM30-5, SRM40-5, SRM50-5, SRM60-5 and SRM70-5, respectively. The fluid–solid equations for the typical specimens at ascent stage and descent stage were listed in Tables 4 and 5, respectively. As shown in Tables 4, 5, the correlation coefficient of all equations is

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highly good, but they do not necessarily indicate the goodness-of-fit of the equations. Thus, validation of the equations was checked by the t- and F-test. The significance of r values can be determined by the t test, which compares the computed t value with the tabulated t value using null hypothesis. The significance of the regressions was determined by analysis of variance (F-test).In these tests, a 95 % of confidence (p B 0.05) was chosen. It is well known that if the computed t and F values are greater than the tabulated t and F values, the null hypothesis is rejected (Wang et al. 2015). In this regard, the computed

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Fig. 9 continued

t and F values are greater than the tabulated t and F values, indicating the validity of these equations. Figure 11 plots the curve fitting results confining pressure ascent stage and descent stage. As stated above, coefficient ‘‘K0’’ in Eq. 3 is the initial permeability coefficient when effective confining pressure is zero. We compare the predict value of ‘‘K0’’ obtaining from Eq. 3 with the measurement value by experiment, the result is shown in Fig. 12. As shown, the predict value was in generally consistent with the measured value.

Comparison analysis of the experimental curve Figure 13 plots the relationship between permeability coefficient and effective confining pressure at the confining pressure ascent stage and descent stage. The curve at descent stage is not consistent with the curve at ascent stage. Permeability coefficient at descent stage is lower than ascent stage, which implies that the SRM specimen encountered irreversible damage during loading of confining pressure. The results indicate that pores and cracks in SRM specimens

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Fig. 10 The relationship between permeability coefficient and confining pressure (a–f corresponds to typical specimen with rock block percentage of 20–70 %)

have plastic deformation characteristics, permeable of SRM cannot completely recover to the loading state at unloading stage. From the point of mesoscopic mechanics, the pores, micro-cracks play critical role in the permeable of SRM. After ascent stages, the specimen experience damage, the flow channel of pores are closed and blocked, even though at unloading stage, the permeable property cannot recover completely, the permeability coefficient was reduced by

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10–30 %. Another important factor that resulting in the reduction of permeability coefficient is the properties changes of rock-soil interfaces. After ascent stage of increasing confining pressure, rock blocks move and rotate so to keep closely contact with soil matrix, pores and cracks closed, the coupling degree between them is very high. When unloading the confining pressure, coupling degree still exists and keeps much degree.

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Table 3 The physical properties of SRM with different rock block percentage Rock percentage (%)

Dry soil (g)

Dry soil and water (g)

Rock block mass (g)

Porosity of soil matrix (%)

Void ratio of soil matrix (%)

20

293.6889

323.0578

73.42222

31.97(0.97)

0.47

30

267.3607

294.0968

76.38879

32.43(1.04)

0.48

40

237.8038

261.5842

79.26792

31.97(1.06)

0.47

50

206.0238

226.6262

82.40952

31.50(1.08)

0.46

60

172.0462

189.2508

86.02308

32.43(1.06)

0.48

70

135.5592

149.1151

90.37282

33.77(1.48)

0.51

The coefficient of variation is indicated inside brackets (%)

Table 4 The fitting result of permeability coefficient and confining pressure (ascent stage) with quadratic polynomial equation for the typical studied SRM specimens

Table 5 The fitting result of permeability coefficient and confining pressure (descent stage) with quadratic polynomial equation for the studied SRM specimens

Specimen no.

Coefficient

Correlation coefficient (r)

a

b

K0

20-5

-0.04707

3.153E-4

2.170

0.9921

20-6

-0.03823

3.832E-4

1.982

0.9922

30-5

-0.03273

1.885E-4

1.719

0.9954

30-7

-0.03682

1.034E-4

1.962

0.9963

40-5

-0.02152

9.262E-4

1.202

0.9909

40-6

-0.02823

8.934E-4

1.023

0.9823

50-5

-0.04428

2.475E-4

2.626

0.9950

50-8

-0.03234

2.034E-4

2.438

0.9833

60-5

-0.03172

9.112E-4

3.089

0.9913

60-6

-0.02873

8.342E-4

3.322

0.9902

70-5 70-7

-0.03622 -0.04220

1.093E-4 2.102E-4

3.782 3.078

0.9909 0.9923

Specimen no.

Coefficient

Correlation coefficient (r)

a

b

K0

20-5

-0.04944

1.974E-4

3.0342

0.9921

20-6

-0.05210

1.643E-4

2.9344

0.9923

30-5

-0.04152

2.334E-4

2.1884

0.9954

30-7

-0.04463

2.544E-4

2.4345

0.9823

40-5

-0.03258

1.560E-4

1.6533

0.9909

40-6

-0.03073

1.985E-4

1.2843

0.9954

50-5

-0.05275

2.396E-4

3.3658

0.9950

50-8

-0.05745

2.645E-4

3.1434

0.9937

60-5

-0.04434

2.060E-4

3.5341

0.9914

60-6

-0.03875

2.458E-4

3.7433

0.9903

70-5 70-7

-0.0334 -0.0483

4.442E-4 4.247E-4

4.0233 3.8934

0.9909 0.9932

Effect of hydraulic gradient on flow-stress property Another factor that affects the fluid–solid coupling property mostly is the hydraulic gradient according to Darcy law. In this work, P1 is the initial hydraulic pressure

applied at the inlet valve. Because the outlet valve is connected to atmosphere, so, P2 = 0. Therefore, P1 is proportional to hydraulic gradient. Because SRM is a kind of special porous medium, when flow reaches to steady seepage in the specimen, the actual hydraulic pressure is

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Fig. 11 Curve fitting results for confining pressure ascent stage and descent stage (a, b is the result for confining pressure ascent stage and descent stage, respectively)

Fig. 12 Comparing of the predict value and measurement value of permeability coefficient

generally lower that P1, as a matter of convenience, this paper, pressure P1 is used to reflect the hydraulic gradient. Figure 14 plots the relationship between permeability coefficient and confining pressure at different hydraulic pressure gradient. The permeability coefficient increases with the increase of hydraulic gradient for SRM specimens with rock block percentage of 20–70 %. In the paper, the hydraulic gradient was below the critical hydraulic gradient, water from the outlet valve was not muddy. However, it can also be seen that change of axial stress in not consistent with permeability coefficient. For specimens with rock block percentage of 20–50 %, axial stress increases with the increase of hydraulic gradient; when rock block percentage rises to 60 and 70 %, axial stress decreases with the increase of hydraulic gradient. This phenomenon may be good interpreted that with lower rock block percentage, the soil particles are easy to compact and block the seepage path, pore pressure in the specimen is high, and the specimen is hard to be compacted. With higher rock block percentage, seepage path around rock-soil interface is easy

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to form and enlarge; soil particles may movement along the seepage path, porosity of specimen further increase, so the ability to resist external force reduces significantly.

Discussion Currently, there are two primary reasons used to interpret why confining pressure has influence on permeability coefficient of soil. One is that with the increasing of confining pressure, force capacity of soil skeleton is greater than water, therefore, the skeleton structure of soil collapses and the porosity decreases; in this case, permeability coefficient of soil decreases greatly (Zoback and Byerlee 1976; Terzaghi et al. 1996). The other reason is that when high confining pressure acts on soil, which makes the soil particles breakage, change of grading of soil particles results in the decrement of permeability coefficient. Actually, grading of soil particles has obvious affect on permeable property. (Kong et al. 2005) reported that when the

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Fig. 13 Comparison of the experimental data at descent stage and ascend stage (a–f corresponds to typical specimen with rock block percentage of 20–70 %)

percentage of fine particles in sand is less than 5 %, there is nearly no change in permeability; when the percentage of fine particles is 5–10 %, permeability coefficient sharply decreases with the increase of fine particle; when the percentage of fine particles is greater than 25 %, permeability coefficient of sand tent to constant value. (Naser 2001) reported that the decrement of permeability coefficient with lower clay particle content is higher than with high content;

when the content was more than 10 %, permeability coefficient almost would not change with increasing of effective confining pressure. Zhang and Wang (2014) conducted experiment to investigate of hydraulic conductivity of sand under high confining pressure. Their results indicated that the volume of the sand specimens decreases during the process of confining pressure increasing and permeation, which lead to the decreasing hydraulic

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Fig. 14 Effect of hydraulic gradient on permeable property of SRM with different rock block percentage (a–f are the results for typical specimen with rock block percentage of 20–70 %)

conductivity. The grain size compositions of the sand specimens before and after the permeability test showed that some sand grains are crushed under high confining pressure, which increased the finer particles and decreased the porosity of the sample, and therefore decrease the hydraulic conductivity. However, for the studied SRM material in this paper, reason for the decrease of

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permeability coefficient is mainly due to the interaction between soil particles and rock blocks, coupling degree improves with the increase of confining pressure. The pores, cracks in soil matrix and the pores at rock-soil interface are compact by the increased stress. Due to the highly elastic mismatch of rock blocks and soil matrix, rock-soil interface is the weakest part. Seepage force

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difference act on rock-soil interface. At unloading stage, proportion of pores would increase, also pores at rock-soil interface increase, resulting in the increase of permeability coefficient again. With the increase of rock block percentage, the permeability coefficient firstly increased and then deceased (Fig. 11). When rock block percentage reaches to 40 %, the permeability coefficient gets to the minimum. In the paper, the test results of permeability coefficient for SRM with different rock block percentage is different from the results of Chen et al. (2014), Liao (2004) and (Xu and Wang 2010a). Chen et al. (2014) used constant head permeameter to obtain the permeability coefficient of SRM. From their research results the permeability coefficient increases with the increment of rock blocks. Liao (2004) and Xu et al. (2010b) used the numerical simulation approach to obtain the permeability of SRM. From their research results, the permeability coefficients decreased with the increase of rock block percentage, e.g., the permeability is negatively correlated with the rock block percentage. However, the result in this paper is consistent with the study by Shafiee (2008). There is a turning point for the variation of permeability coefficient with the increase of rock block percentage. The cause of this phenomenon can be interpreted by the different effects of rock blocks and rock-soil interfaces on permeability coefficient. Firstly, the permeability of rock blocks and soil matrix is obvious discrepant. Because of the relative waterproof of rock blocks, existing of rock blocks decreases the effective flow cross-section, i.e., rock blocks reduce the effective porosity in SRM specimen, so permeability coefficient is supposed to decrease with the increase of rock blocks. Secondly, along the seepage direction, hydraulic pressure sharply decreases in rock blocks, resulting in the appearance of great seepage force at rock-soil interfaces. The rock-soil interfaces are the weakest position of the specimen, together with the great seepage force at rock-soil interface, permeability coefficient at interfaces increases sharply. Although the rock blocks decreases the flow paths in SRM, with the increase of rock-soil interfaces, the overall permeability property is becoming stronger.

Conclusions The present work has encompassed a series of fluid–solid coupling experiment on SRM specimens with different rock percentage (i.e., 20, 30, 40, 50, 60 and 70 %) to investigate the permeable properties of SRM. SRM as a unique geomaterial, which is different from ‘‘soil’’ and ‘‘rock’’, its fluid–solid coupling properties are studied under various confining pressure in the first time. The selfdeveloped servo permeable testing apparatus was used to

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complete the permeability testing. The following conclusions can be drawn based on this experimental study: 1.

2.

3.

The experimental results reveal that permeability coefficient of SRM decreases with the increase of confining pressure. During confining pressure descent stage, permeability coefficient begins to increase from the minimum, but it is smaller than the confining pressure ascent stage. At ascent stage, the increasing rate of permeability coefficient is rapid than decreasing rate in descent state. This implies that the SRM specimen encountered irreversible damage during loading and unloading of confining pressure. The coupling degree greatly improves during ascent stage. For the permeable test, by curve fitting approximation, permeability coefficient and effective confining pressure shows good quadratic polynomial relationship, correlation coefficients are [0.98. During the confining pressure ascent stage and descend stage, the primary reason for variety of permeability coefficient is the interaction between rock blocks and soil matrix. Coupling degree improves with the increase of confining pressure, pores, micro-cracks in soil matrix and rock-soil interface are compact and closed, resulting in the reduction of flow channel in SRM specimen. Unloading confining pressure, it is mainly because of the re-open of rock-soil interfaces that resulting in the increase of permeability coefficient again.

Acknowledgments This work is supported by the National Natural Science Foundation of China (Grants Nos. 41227901, 41330643, 41502294). The authors wish to acknowledge the anonymous reviewers and the editors for their very helpful comments and valuable remarks, which greatly improve the quality of the manuscript. Compliance with ethical standards Conflict of interest

We declare that we have no conflict of interest.

References Alonso EE, Romero E, Hoffmann C (2001) Hydromechanical behaviour of compacted granular expansive mixtures: experimental and constitutive study. Ge´otechnique 61(4):329–344 Ameta NK, Wayal AS (2008) Effect of bentonite on permeability of dune sand. Electron J Geotech Eng 13(A):1–7 Bear J, Tsang CF, Marsily G (1993) Flow and contaminant transport in fractured rocks. Academic Press, San Diego, CA British standard (1990) Methods of test for Soils for civil Engineering purposes-Part 1: General requirements and sample preparation, BS1377-1 Chen ZH, Chen SJ, Chen J, Sheng Q, Yan H, Hu W (2012) In-situ double-ring infiltration test of soil-rock mixture. J Yangtze River Sci Res Insti 29(4):52–56 (in Chinese) Chen XB, Li ZY, Zhang JS (2014) Effect of granite gravel content on improved granular mixtures as railway subgrade fillings. J Cent South Univ 21:3361–3369 (in Chinese)

123

284

Page 16 of 16

Coli N, Berry P, Boldini D (2011) In situ non-conventional shear tests for the mechanical characterisation of a bimrock. Int J Rock Mech Min 48:95–102 Fang YH, Li ZQ, Zhang YT (2005) Permeability of concrete under sustained compressive load. J Chin Ceramic Soc 33(10):1281–1286 (in Chinese) Gao Q, Liu ZH, Xin Li, Li JH (2009) Permeability characteristics of rock and soil aggregate of backfilling open-pit and particle element numerical analysis. Chin J Rock Mech Eng 28(11):2342–2348 (in Chinese) Guo H (2009) Experimental study on traxial seepage of distributed Q3 loess in different regions. Thesis for masters degree, Northwest A & F University (in Chinese) Indrawan IGB, Rahardjo H, Leong EC (2006) Effects of coarsegrained materials on properties of residual soil. J Eng Geol 82(3):154–164 Jones FO (1975) A laboratory study of the effects of confirming pressure on fracture flow and storage capacity in carbonate rocks. J Petrol Tech 9(2):21–27 Kong LW, Li XM, Tian HN (2005) Effect of fines content on permeability coefficient of sand and its correlation with state parameters. J Chin Cere Soc 33(10):1281–1286 Li P, Luo YS (2006) Research on triaxial seepage test of saturated loess. Subgrade Eng 129(6):32–33 (in Chinese) Li XQ, Li WP, Li HL, Sui RH (2005) Experimental study on permeability of sandstone during post—peak unloading of the confining pressure. J Eng Geol 13(4):481–485 (in Chinese) Liao QL (2004) Geological origin and structure model of rock and soil aggregate and study on its mechanical and MH coupled properties. PhD dissertation, Institute of Geology and Geophysics, Chinese Academy of Science, Beijing (in Chinese) Liu JJ, Liu XG (2001) The effect of effective pressure on porosity and permeability of low permeability porous medi. J Geomech 7(1):41–44 (in Chinese) Mckee CR, Bumb AC, Koenig RA (1988) Stress-dependent permeability and porosity of coal and other geologic formations. Spe Reserv Eval Eng 1:81–91 (in Chinese) Medley E, Lindquist ES (1995) The engineering significance of the scale-independence of some Franciscan Melanges in California, USA. In: Daemen JK, Schultz RA (eds) Proceedings of the 35th US rock mechanics symposium. Balkema, Rotterdam, pp 907–914 Meng XY, Li SH, Zhang JF (2004) Study and Manufacture of Flexible Boundary Loading Testing Machine. Chin J Rock Mech Eng 23(10):1760–1764 (in Chinese) Miller RJ, Low PF (1963) Threshold gradient for water flow in clay systems. Proc Soil Sci Soc Am 27(6):605–609

123

Environ Earth Sci (2016)75:284 Ministry of Water Resources of the People’s Republic of China, Standard for soil test method, GB/T 50123-1999 Naser AA (2001) The combined effect of clay and moisture content on the behavior of remolded unsaturated soils. Eng Geol 62(4):319–342 Oda M, Takemura T, Aoki T (2002) Damage growth and permeability change in triaxial compression tests of Inada granite. Mech Mater 34:313–331 Ru¨cknagel Jan, Go¨tze Philipp, Hofmann Bodo (2013) The influence of soil gravel content on compaction behaviour and precompression stress. Geoderma 209–210:226–232 ¨ berg-ho¨gsta AL (2002) Determination of hydraulic Sa¨llfors G, O conductivity of sand-bentonite mixtures engineering purpose. Geotech Geol Eng 20(1):65–80 Shafiee A (2008) Permeability of compacted granule-clay mixtures. Eng Geol 97(7):199–208 Shelley TL, Daniel DE (1993) Effect of gravel on hydraulic conductivity of compacted soil liners. J Geotech Eng ASCE 119(1):54–68 Terzaghi K, Peck RB, Mesri G (1996) Soil mechanics in engineering practice. Wiley, Chichester, UK Wang Y, Li X (2014) Experimental study on cracking damage characteristics of a soil and rock mixture by UPV testing. B Eng Geol Environ. doi:10.1007/s10064-014-0673-x Wang Y, Li X, Wu YF, Lin C, Zhang B (2015) Experimental study on meso-damage cracking characteristics of RSA by CT test. Environ Earth Sci 73(9):5545–5558 Wu JH, Chen JH, Lu CW (2013) Investigation of the Hsien-du-Shan rock avalanche caused by typhoon Morakot in 2009 at Kaohsiung county Taiwan. Int J Rock Mech Min 60:148–159 Xu WJ, Wang YG (2010a) Meso-structural permeability of S-RM based on numerical tests. Chin J Geotech Eng 32(4):543–550 (in Chinese) Xu WJ, Wang YG (2010b) Meso-structural permeability of S-RM based on numerical tests. Chinese J Geotech Eng 32(4):543–550 (in Chinese) Xu DM, Huang RQ, Zhang Q, Deng YE (2007) Experimental Study on Permeability of porous media under high confining Pressure. J Eng Geol 15(6):752–756 (in Chinese) Zhang GL, Wang YJ (2014) Experimental investigation of hydraulic conductivity of sand under high confining pressure. Rock soil Mech 35(10):2748–2786 (in Chinese) Zoback MD, Byerlee JD (1976) Effect of high-pressure deformation on permeability of Ottawa sand. AAPG 60(9):1531–1542