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Uniaxial compression test of frozen tailings
Cold Regions Science and Technology 129 (2016) 60–68
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Uniaxial compression test of frozen tailings Yonghao Yang a,b, Zuoan Wei a,b,⁎, Guangzhi Yin a,b, J.G. Wang c, Wensong Wang d, Yulong Chen a,b a
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400030, China State and Local Joint Engineering Laboratory of Methane Drainage in Complex Coal Gas Seam, Chongqing University, Chongqing 400030, China c School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, China d College of Resource and Environmental Science, Chongqing University, Chongqing 400030, China b
a r t i c l e
i n f o
Article history: Received 10 December 2015 Received in revised form 15 May 2016 Accepted 17 June 2016 Available online 23 June 2016 Keywords: Frozen tailings Uniaxial compression Stress-strain relationship Compressive strength Deformation modulus
a b s t r a c t The mechanical properties (uniaxial compressive strength and deformation modulus) of frozen tailings are key parameters for the safety assessment of a tailing dam. However, the experimental data on such mechanical properties are limited. In this study, more than sixty tailings samples with four tailings (medium sand, fine sand, silty sand, and silt) were frozen at the temperature of −16 °C, and then tested under uniaxial compression in order to investigate their mechanical properties. The effects of four parameters (average particle size, dry density, water content and strain rate) on the uniaxial compressive strength (UCS) and the deformation modulus of frozen tailings were investigated. The test results show that three failure patterns of samples were observed: inclined plane shear failure, lateral tensile failure, and composite failure involving both. Their uniaxial compressive strength is related logarithmically to average particle size, exponentially to dry density, linearly to moisture content, and parabolically to strain rate. On the other hand, their deformation modulus is related logarithmically to average particle size, parabolically to moisture content, exponentially to both dry density and strain rate. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tailings, the byproducts of mineral processing, are special solid wastes. Large-scale mining and mineral processing in China and even in the world inevitably generate huge amounts of tailings. These tailings are to be economically disposed in an environmentally acceptable manner. The tailings impoundments, as one of practical solutions, can dispose of huge masses of tailings and other mineral wastes (Wei et al., 2009). Generally, a slurry waste (tailings after ore extraction), which is composed of residual ore, water, sand, silt and fine clay particles, is hydraulically transported and stored in these surface tailings impoundments, where an increasable dam is constructed to form a storage reservoir. The statistics shows that more than 12,000 tailings storage facilities (TSFs) had been constructed in China up to 2009 (Wei et al., 2013). The geographical locations of these tailings storage facilities are summarized in Table 1. Tailings dam, as the main form of TSF, are considered as the largest man-made structures in the world (Shamsai et al., 2007). However, a number of particular characteristics make tailings dams more vulnerable to failure than water storage dams (Rico et al., 2008). In this sense, tailings dams are classified as one of the high hazard risks in China's industry. Table 2 summarizes the main disastrous failures of tailings dams in China since 1962. Two to five major tailings ⁎ Corresponding author at: State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400030, China. E-mail address: [email protected] (Z. Wei).
http://dx.doi.org/10.1016/j.coldregions.2016.06.007 0165-232X/© 2016 Elsevier B.V. All rights reserved.
impoundment failures per year occurred during the last thirty years. This failure frequency is ten times higher than that of water retention dams (Dixon-Hardy and Engels, 2007). Therefore, the safety of tailings impoundments is of the highest concern for local residents and governments. Frozen tailings may have special physical and mechanical properties (Zhao et al., 2014). Based on their locations (see Table 1), most of the tailings impoundments in China may seasonally freeze in winter. Fig. 1 shows a typical example of frozen tailings impoundments covered by snow. Most of frozen tailings impoundments in winter are at 0 °C to −16 °C. When the air temperature in these cold regions is below 0 °C, the pore water in subsurface soil layers may be frozen to a certain depth to form frozen ground. This frozen ground becomes deeper over a longer freezing period and at lower air temperature. Either freezing and thawing or alternative wetting and drying of a hill slope causes upward expansion of the ground surface perpendicular to the slope face, resulting in a net down-slope movement of the soil (Pipkin and Trent, 1997; Liu et al., 2008). Fig. 2 presents a typical cross section of a tailings dam under freezing conditions in winter. Hard frozen layers were formed from the surface of the impoundments. These frozen layers have great impacts on the stability of tailings impoundments, which affects the seepage of a tailings dam, reduce its stability, and results in large deformation and even the failure of the tailings dam. Therefore, in order to improve the safe management of tailings impoundments, it is necessary to analyze the stability of tailings dams under these conditions. While using the finite element method (FEM) to analyze the
Y. Yang et al. / Cold Regions Science and Technology 129 (2016) 60–68 Table 1 Distribution of tailings impoundments in mainland China (up to 2012). No.
Province
Total tailings impoundments
Active
Inactive
Closed
Constructing
1 2 3 4 5 6 7 8 9 10 11a 12 13a 14 15 16 17 18a 19 20 21 22a 23 24 25
Jiangsu Beijing Qinghai Heilongjiang Zhejiang Xinjiang Sichuan Jilin Gansu Guizhou Guangdong Hubei Fujian Shanxi Anhui Jiangxi Shandong Guangxi Hunan Henan Inner Mongolia Yunnan Liaoning Shanxi Hebei
19 37 57 62 78 103 137 167 183 219 226 236 247 269 341 380 494 504 651 681 685 692 1475 1735 2888
10 9 10 45 44 47 87 112 107 103 63 54 136 125 263 234 252 184 356 275 449 457 1091 602 1773
0 0 44 3 8 0 6 2 24 36 101 149 0 25 6 45 25 136 109 281 118 54 90 247 343
5 28 1 0 25 11 7 22 8 50 47 3 36 49 27 26 136 116 160 5 14 104 120 330 614
1 0 2 14 1 45 37 31 44 30 15 30 75 70 45 75 81 68 26 120 104 77 174 556 158
a These tailings impoundments are located in non-cold regions. The rest may be seasonally frozen in winter.
stability of a tailings dam, the mechanical properties of frozen tailings, such as compressive strength and deformation modulus, are necessary. The strength and deformation properties of frozen soils have been investigated. For example, Wijeweera and Joshi (1990) studied the effects of dry unit weight, water content, temperature, and soil type on the peak compressive strength of six fine-grained frozen soils by using uniaxial compression tests. They obtained an empirical formula with which to predict the peak compressive strength of fine-grained frozen soils at a particular temperature. Christ and Kim (2009) studied frozen silt by using uniaxial shear tests. They found that the compressive and tensile strengths of frozen silts increased with the reduction of temperature and the increase of moisture content. Therefore, the freezing process can largely modify the mechanical properties of frozen soils. The mechanical properties of frozen soils are important to engineering design and construction in cold regions. China has implemented several huge engineering projects such as the Qinghai-Tibet Railway and the Golmud-Lhasa oil-pipelines, in past decades. These projects were constructed in permanent frozen soil regions of the QinghaiTibet Plateau. This sped up the investigations of the mechanical properties of frozen soils (Lai et al., 2013). For example, Li et al. (1995)
Table 2 Main failures of tailings dams in China. Name of dam
Tailings types
Construction method
Year of failure
Persons killed
Huogudu, Yunnan Tin Group Co., Yunnan province Niujiaolong, Shizhuyuan Non-ferrous Metals Co., Hunan province Longjiaoshan, Daye Iron Ore mine, Hubei province Dachang, Nandan Tin mine, Guangxi province Zhenan Gold mine, Shanxi province Xiangfen tailings pond, Shanxi province Xinyi Zijin Tin mine, Guangdong province
Tin
Upstream
1962
171
Copper
Upstream
1985
49
Iron
Upstream
1994
31
Tin
Upstream
2000
28
Gold Iron
Upstream Upstream
2006 2008
17 277
Tin
Upstream
2010
28
61
conducted uniaxial compression tests in order to investigate the relationship of uniaxial compressive strength with the strain rate of frozen soils and frozen temperature. They found that the uniaxial compressive strength of frozen soils increases linearly with the decrease of frozen temperatures and follows a power function with strain rate. Yang et al. (2010) carried out a series of triaxial compression tests on artificial frozen sand at a temperature of −6 °C. In their experiments, the water content varied from 10% to 20% and the constant confining pressure varied from 0.0 to 14.0 MPa. They found that the nonlinear Mohr-Coulomb criterion can describe the strength of frozen sand with high confining pressure well. Further, the stress-strain relationship of this frozen sand can be expressed by a hyperbolic function. Liu et al. (2007) studied the load versus displacement curves of frozen clays and the failure patterns of the samples using uniaxial compression tests. The test samples of frozen clay were directly taken from the site at a temperature of −15 °C. Chen et al. (2012) investigated the impact of soil moisture on the uniaxial compressive strength and the failure strain under freezing conditions. They observed that the uniaxial compressive strength of frozen salty silt increases with the increase of moisture content when the moisture content is low. Cui et al. (2014) studied the mechanical properties of a silty clay under freezing-thawing conditions. The dynamic constitutive behaviors of frozen soils were experimentally investigated under impact loading (Xie et al., 2014; Ning et al., 2014). It was found that the dynamic stress - strain responses of the frozen soil exhibits a positive strain rate sensitivity and negative temperature dependence. However, no study has been done on the mechanical properties of frozen tailings so far. The tailings, as solid residues, are different from natural soils in terms of formation, particle geometry, and particle size distribution (Yin et al., 2012). In general, the mechanical properties of frozen soils depend on both external conditions, such as temperature, loading type (static or cyclic), as well as strain rate, and the internal parameters of soils, such as soil composition, microstructure, and moisture content. Soil type, microstructure, mineral composition, amount of ice, and dry density are identified as the most important internal parameters. They may have significant impacts on the mechanical behaviors of frozen soils. Han (2011) conducted uniaxial compression tests in order to investigate the strength and deformation behaviors of frozen cemented tailings for the backfill of underground mining. In his study, artificial tailings, distilled water, and dry snow were used to mix a uniform water-snow-solid particle mixture. The cylindrical testing specimen was frozen at − 6 °C. So far, numerous experiments have been conducted in order to measure the strength and stress to strain relationship of tailings and frozen natural soils, but few publications about the mechanical properties of frozen tailings are available. The effects of strain rate on the mechanical characteristics of rocks have been widely investigated (Leandro et al., 2014). Bragg and Andersland (1981) studied the effects of strain rate, temperature, and sample size on the compression and tensile properties of frozen sand. It was found that the compressive peak strength and the initial tangent modulus of frozen sand increase with decreasing temperatures and increasing strain rates. Li et al. (1995) carried out uniaxial compression tests for frozen silt under different temperatures and strain rates. The test results showed that the compressive strength of frozen silt was sensitive to strain rate. The sensitivity can be divided into three phases. When the strain rate is less than 10− 4 s− 1, it is generally sensitive. When the strain rate is larger than 10−3 s−1, it is the most sensitive. However, few publications on this sensitivity study on frozen tailings are currently available. In this study, uniaxial compression tests were carried out in order to thoroughly investigate the mechanical properties of frozen tailings. The tests measured the mechanical behaviors of more than sixty frozen tailings samples at a temperature of −16 °C. The samples were made from four tailings. The uniaxial compressive strength and the deformation modulus were quantitatively analyzed. These tests investigated the effects of four influential factors on the mechanical properties of frozen
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Fig. 1. A tailings dam covered with snow and ice in winter in Shanxi province.
tailings. It is expected that the test results may be helpful not only to the understanding of the basic mechanical behaviors of frozen tailings but also to the enhancement of the safety management of tailings impoundments in cold regions.
were poured into cylindrical molds for sample casting using a layerby-layer compaction method. Each sample was divided into five layers. In each layer, the dry density was controlled by weighing the test samples. The cast samples were wrapped with preservative films, as shown in Fig. 4, and put into a refrigerating box for 24-h freezing at −16 °C.
2. Uniaxial compression tests of frozen tailings 2.3. Test schemes 2.1. Tailings gradations The test tailings were sampled from the processing mill of Lala Copper Mine (Sichuan province, China). The Lala Copper Mine is the biggest open-pit copper mine in southwest China. According to the national technical Code for Geotechnical Engineering of Tailings Embankment (GB50547-2010), the tailings are divided into three classes and seven sub-classes based on their grain size gradations. They are tailings sand (the sub-classes are gravelly sand, coarse sand, medium sand, fine sand, and silty sand), tailings silt (including silts), and tailings clay (the sub-classes are silty clay and clays). In order to investigate the effects of gradations on the mechanical properties of tailings, the original tailings (#5) sampled from this mine site were manually sieved to form four groups of tailings samples based on their grain size gradations. The four groups are medium sand (#4), fine sand (#3), silty sand (#2), and silty clay (#1). Their main physical properties are summarized in Table 3, and their particle size distributions are presented in Fig. 3. Further, Table 4 presents their mineralogical compositions obtained through the x ray fluorescence (XRF) analysis. 2.2. Preparation of test samples The test samples are cylinder-shaped. Each sample is 50 mm in diameter and 100 mm in height. The preparation obeys the following procedure. First, the tailings samples were mixed to predetermined moisture contents. The well-mixed tailings were then sealed in plastic bags. After 24 h and further mixing in the seal bags, the mixed soils
Uniaxial compression tests were performed to investigate the mechanical properties of frozen tailings. The key influential factors that affect the compressive strength and deformation modulus of frozen tailings were explored. Four influential parameters were preliminarily selected as average particle size, dry density, moisture content, and strain rate. Table 5 presents the test schemes and the physical properties of the tailings samples. These tailings samples were taken from the original tailings (#5) as well as another four tailings (#1, #2, #3, and #4). The tests were divided into four groups. Group 1 was used to investigate the impact of average particle size. The initial size ranges from 31.97 μm to 310.50 μm. Group 2 was used to study the impact of dry density. The density varies from 1.48 g/cm3 to 1.71 g/cm3. Group 3 was used to explore the impact of moisture content, which varies from 5% to 20%. Group 4 was used to study the effect of strain rate. In these tests, the strain rate varied from 0.05/min to 0.2/min. 2.4. Test procedure The uniaxial compression tests for frozen tailings were carried out using the Japan Shimadzu AG-I25KN electronic precision materials testing machine (see Fig. 5) in the State Key Laboratory of Coal Mine Disaster Dynamics and Control in Chongqing University. After putting the test samples in place and setting loading parameters, the whole compression procedure was automatically controlled by computer to realize real-time and automatic acquisition of test data. Due to the temperature difference, a rubber insulation tube that was 15 mm thick was used to
Fig. 2. A typical cross section of a tailings dam under freezing conditions in winter.
Y. Yang et al. / Cold Regions Science and Technology 129 (2016) 60–68 Table 3 Physical index of test tailings.
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Table 4 Main mineralogical compositions of tailings from Lala copper mine.
Sample no.
Tailings class
Plastic limit Liquid limit Plastic (%) (%) index
d50(μm) Cu
Cc
Mineralogical compositions (analyzed by XRF)
Results (mg per kg)
#1 #2 #3 #4
Silty clay Silty sand Fine sand Medium sand Silty sand
14.5 13.3 13.1 12.1
25.8 24.1 23.0 21.4
11.3 10.8 9.9 9.3
31.97 83.58 168.92 310.50
7.35 3.52 3.65 3.38
1.29 1.26 1.14 1.40
12.7
20.8
8.1
102.25
8.99 1.22
Oxygen Calcium Magnesium Silicon Iron Aluminum Kalium Manganese Natrium Titanium Copper
458,960 261,363 160,181 115,240 9735 8754 3522 2451 2313 905 614
#5
Note: d50 refers to the average particle size.
hold the frozen sample for heat insulation. An insulating plastic sheet (see details in Fig. 5) was placed on the contact surface between the end of sample and the press of test machine. This prevented the contact surface at the end of the test samples from melting during the test. The tests were conducted in winter. The temperature in the testing room was always kept below 0 °C during the test procedure in order to prevent the samples from melting. In addition, the test time was as short as possible. The whole compression procedure for each test was completed within 1 to 3 min. Each test was repeated at least three times (up to six times for some samples) in order to ensure the repeatability of the results. On the other hand, the surface temperatures on the sample end and middle were measured during the test process (Fig. 6a). The change of temperature with time was summarized in Fig. 6b. The measurement results showed that the temperature increment was between +0.4 °C and +0.6 °C. The temperature of the sample was still close to −16 °C.
3. Experimental results and analysis
3.1.2. Pattern 2: lateral tensile failure This failure pattern is shown in Fig. 7(b). Under the axial compressive stress, lateral tensile stress is produced within the sample. When the tensile stress exceeds the ultimate tensile strength of the material, a crack initiates along the axial direction throughout the sample and causes splitting failure. This failure pattern easily occurs for the tailings with relatively smaller average particle sizes, or those with less moisture content or at lower strain rates. 3.1.3. Pattern 3: composite failure This failure pattern is shown in Fig. 7(c). In this pattern, partial ballooning appears in the middle of the test sample, and obliquely crossing cracks are generated, causing sample failure. For some samples, stripshaped fracture failure occurred on the lateral side and shear failure happened in the remaining parts. When the dry density of tailings is lower, or the strain rate is greater, this failure mode easily occurs.
3.1. Failure pattern The failure pattern of frozen tailings samples was observed under uniaxial compression. The following three patterns were observed.
3.1.1. Pattern 1: inclined plane shear failure This failure pattern is shown in Fig. 7(a). The fracture occurs in a part of the sample at first, and then expands constantly to form the crack via interpenetration and finally becomes an inclined shear plane throughout the whole sample. In some cases, two mutually crossing shear planes were formed. When the average particle size of tailings is greater, or the moisture content of test samples is higher, this failure pattern will happen more easily.
Fig. 3. Particle size distribution of tailings.
3.2. Typical stress-strain curve In the uniaxial compression tests, typical stress-strain (σ-ε) curves of frozen tailings at − 16 °C are shown in Fig. 8 for the samples with two dry densities. Strain hardening and strain softening were observed during the compression test process. The stress-strain relation can be divided into four deformation stages. 3.2.1. Stage 1: initial compaction stage (OA segment in Fig. 8) This stage occurs at the initial deformation and lasts for a very short time. This stress-strain curve soon bends upwards from the point A with the increase of vertical stress. This stage may be due to initial partial contact and partially melt induced by the temperature difference. This can be ruled out through the extension of the next stage.
Fig. 4. Samples of frozen tailings.
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Table 5 Test schemes. Test group
Average particle size (groups of tailings samples) d50 (μm)
Dry density ρd (g cm−3)
Moisture content ω (%)
Strain rate v (10−1 min−1)
Group 1
1.58
15
2
Group 2
31.97(#1),83.58(#2), 168.92(#3), 310.50(#4) 102.25(#5)
15
2
Group 3 Group 4
102.25(#5) 102.25(#5)
1.48, 1.58, 1.64, 1.71 1.58 1.58
5, 10, 15, 20 15
2 0.5, 1, 1.5, 2
3.2.2. Stage 2: linear strain hardening stage (AB segment in Fig. 8) The stress-strain curve is an approximately straight line. The stressstrain relationship of frozen tailings is of an initial linear elastic property.
3.2.3. Stage 3: nonlinear strain hardening stage (BC segment in Fig. 8) The axial stress increases continuously with the increase of axial strain, but the stress-strain curve bends downward. The slope of the stress-strain curve gradually decreases. That is, its deformation modulus decays gradually. Finally, the stress reaches its peak compressive strength, σc, and the deformation modulus approaches zero. The strains at point C are between 6.2% and 8.6%, which are proportional to the dry density and the average particle size and the moisture content of sample. That is, the greater the dry density and the average particle size and the moisture content of sample, the larger the strain at point C is. During this deformation stage, slight slanting and longitudinal microcracks were initially observed on the surfaces of samples. The number of these micro-cracks becomes greater with the increase of vertical or axial stress.
3.2.4. Stage 4: nonlinear strain softening stage (the segment beyond the peak or the point C) After the axial stress reaches the peak compressive strength, the sample undergoes the strain softening stage, wherein the stress is gradually decreased with the increase of strain and the slope of the stressstrain curve becomes negative. In this stage, the micro-cracks on the sample surface were observed to penetrate into the internal zone and amalgamated to form larger macro-cracks. Partial fractures occur in the sample along with slippage and, thus, finally cause sample failure. The final strains are between 16% and 23%. The final failure times are
Fig. 6. The results and schematic diagram for measuring the sample temperature.
between 1.2 min and 4.6 min. The loading strain rate has the biggest impact on the failure time of sample. 3.3. Uniaxial compressive strength and deformation modulus The effects of average particle size, dry density, moisture content, and strain rate on uniaxial compressive strength and deformation modulus of the frozen tailings were analyzed. 3.3.1. Effect of average particle size The physical and mechanical properties of a soil are strongly influenced by its particle size distribution. Fig. 9 presents the effects of average particle size on the uniaxial compressive strength and deformation modulus of frozen tailings. The tests were conducted under a constant strain rate of 0.2/min. This figure clearly indicates that both uniaxial compressive strength and deformation modulus of the frozen tailings are logarithmically related to the average particle size of tailings. They can be described by the following two regression expressions:
Fig. 5. Uniaxial compression test setup for frozen tailings samples.
σ c ¼ 1:2581 ln ðdave Þ þ 0:5971
ð1Þ
Y. Yang et al. / Cold Regions Science and Technology 129 (2016) 60–68
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Fig. 7. Three failure patterns of frozen tailings samples.
with a correlation coefficient of R2 = 0.9708. E ¼ 147:88 ln ðdave Þ−258:79
For uniaxial compressive strength ð2Þ
with a correlation coefficient of R2 = 0.9429. Where σc (MPa) is the uniaxial compressive strength. dave (μm) is the average diameter of tailings particles, and E (MPa) refers to the deformation modulus at the linear stage (the AB segment of the σ-ε curve in Fig. 8). 3.3.2. Effect of dry density The effect of dry density of tailings was investigated under a constant strain rate of 0.2/min. Fig. 10(a) presents the change of uniaxial compressive strength with dry density, and Fig. 10(b) presents the change of deformation modulus. Obviously, both the uniaxial compressive strength and the deformation modulus of frozen tailings gradually increase with the increase of dry density. These changes were fitted by following expressions:
σ c ¼ 0:3333e1:8485γ
ð3Þ
with a correlation coefficient of R2 = 0.9635. For deformation modulus E ¼ 54:461γ4:0741
ð4Þ
with a correlation coefficient of R2 = 0.9496. Where γ is the dry density of tailings (g/cm3). Eq. (3) shows that the uniaxial compressive strength is exponentially related to dry density, but the power function of Eq. (4) is applicable to the relationship between deformation modulus and dry density. This is because a relatively higher dry density of tailings means lower porosity. The particles of tailings have closer contact each other, the cohesive force between ices within the frozen tailings pores increases, and thus the bearing capability of the frozen tailings can be greater.
Fig. 8. Typical stress-strain curves of frozen tailings in uniaxial compression tests.
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Fig. 9. Effect of average particle size on both uniaxial compressive strength and deformation modulus of frozen tailings.
Fig. 10. Impact of dry density on both uniaxial compressive strength and deformation modulus of frozen tailings.
3.3.3. Impact of moisture content The moisture content is a key parameter of the physical and mechanical properties of frozen tailings. Tailings are man-made soils that are usually three-phase composites of solid mineral particles, pore water, and pore air. When the soil temperature drops below 0 °C, water within soil pores starts to form ice particles. This ice cementation on particles increases the strength of the soil. In order to investigate the impact of moisture content, this study specified the moisture content as 5%, 10%, 15%, and 20% during sample preparations. The impact of moisture content on the uniaxial compressive strength is presented in Fig. 11(a) and Fig. 11(b), on the deformation modulus. It was observed that the uniaxial compressive strength of frozen tailings is linearly related to moisture content. The reason for this property of frozen tailings is that the higher moisture content has more ice content within frozen tailings pores. The ice bonds the particles together and significantly increases the strength of the tailings. The deformation modulus is parabolically related to the moisture content. Curve fitting obtained the following expressions (where moisture content is in %). For uniaxial compressive strength, σ c ¼ 0:5442w−1:694
ð5Þ
with a correlation coefficient of R2 = 0.9971. For the deformation modulus, E ¼ 0:6295w2 þ 43:061w−153:93
ð6Þ
with a correlation coefficient of R2 = 0.9962. 3.3.4. Effect of strain rate Strain rate may influence the mechanical properties of frozen tailings. The loading rate-dependence of compressive strength and deformation modulus of frozen tailings was investigated under four strain rates of 0.05, 0.1, 0.15, and 0.2/min. Fig. 12(a) presents the effect of
Fig. 11. Change of uniaxial compressive strength and deformation modulus of frozen tailings with moisture content.
Y. Yang et al. / Cold Regions Science and Technology 129 (2016) 60–68
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strain rate on the uniaxial compressive strength, and Fig. 12(b) shows the effect of strain rate on the deformation modulus. Fig. 12(a) shows that the uniaxial compressive strength of frozen tailings is parabolically related to strain rate. With the increase of strain rate, the uniaxial compressive strength of frozen tailings increases linearly at the beginning and gradually becomes stationary at high strain rates. The following expressions are fitted (where the strain rate, v, is in 10−2/min). σ c ¼ −0:015ν2 þ 0:5183ν þ 1:9962
ð7Þ
with a correlation coefficient of R2 = 0.9971. The deformation modulus of frozen tailings is exponentially related to strain rate, as shown in Fig. 12(b) and fitted by the following expression: E ¼ 223:42ν0:1367
ð8Þ
with a correlation coefficient of R2 = 0.9982. The existence of ice particles has an impact on this property of frozen tailings. With the decrease of strain rate, the ice in frozen tailings, especially the ice at the mineral grain contact points, exhibits slow plastic flow and creep properties. The capacity of frozen tailings restricting external force increases with the increase of strain rate, and the failure patterns of frozen tailings samples change from fragile failure to plastic failure. 3.4. Relation between the failure strain and four parameters
Fig. 12. Impact of strain rate on both uniaxial compressive strength and deformation modulus of frozen tailings.
The failure strain of the frozen tailings is corresponding to the strain at point C (see Fig. 8). The value of the failure strain is between 2.7% and 8.6%. The relation between the failure strain and four parameters were
Fig. 13. The relation between the failure strain and four parameters of frozen tailings.
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analyzed. Fig. 13 presents the effects of four parameters on the failure strain. The results show that the failure strain is related parabolically to the average particle size (see Fig. 13(a)), the dry density (see Fig. 13(b)) and the strain rate (see Fig. 13(d)), logarithmically to the moisture content (see Fig. 13(c)). That is, the greater the dry density and the average particle size and the moisture content of sample, the larger the failure strain at point C is. 4. Conclusions A series of uniaxial compressive tests were conducted on frozen tailings at a temperature of −16 °C. The stress-strain curves of the tailings samples were analyzed, and their failure patterns were observed. Particularly, the effects of particle size, dry density, moisture content, and strain rate on the uniaxial compressive strength and deformation modulus of frozen tailings were analyzed. Based on these preliminary test results on the mechanical properties and the observations of failure patterns, the following conclusions can be drawn. (1) Three failure patterns of the tailings samples were observed in the uniaxial compression tests. They are inclined plane shear failure, lateral tensile failure, and composite failure. Further, the average particle size, dry density, and strain rate significantly influence the failure patterns. (2) The stress-strain curves of frozen tailings under uniaxial compression have the following typical deformation stages: linear strain hardening, nonlinear strain hardening, and nonlinear strain softening. During the deformation stage of nonlinear strain hardening, slight slanting and longitudinal micro-cracks were initially observed on the surfaces of samples. The number of these micro-cracks becomes greater with the increase of vertical or axial stress. In the softening stage, the micro-cracks on the sample surface were observed to penetrate into the internal zone and amalgamated to form larger macro-cracks. Partial fractures occur in the sample along with slippage and finally cause the sample failure. (3) Both uniaxial compressive strength and deformation modulus of frozen tailings are closely related to average particle size, dry density, moisture content, and strain rate. It was found that the uniaxial compressive strength of frozen tailings is related logarithmically to average particle size, exponentially to dry density, linearly to moisture content and parabolically to strain rate. The deformation modulus of frozen tailings is related logarithmically to average particle size, exponentially to dry density, parabolically to moisture content, and exponentially to strain rate. Acknowledgements The authors are very grateful to Dr. Ryan Madeley and anonymous reviewers for their valuable comments and suggestions. This research
was funded by the National Natural Science Foundation of China (No. 11372363) and “Gan-po Talent 555” Project of Jiangxi Province. References Bragg, R.A., Andersland, O.B., 1981. Strain rate, temperature, and sample size effects on compression and tensile properties of frozen sand. Eng. Geol. 18 (1–4), 35–46. Chen, J., Li, D., Bing, H., 2012. An experimental study of influence of water content on uniaxial compression strength of frozen salty silt. J. Glaciol. Geocryol. 34 (2), 441–446 (in Chinese). Christ, M., Kim, Y.-C., 2009. Experimental study on the physical- mechanical properties of frozen silt. KSCE J. Civ. Eng. 13 (5), 317–324. Cui, Z., He, P., Yang, W., 2014. Mechanical properties of a silty clay subjected to freezing– thawing. Cold Reg. Sci. Technol. 98, 26–34. Dixon-Hardy, D.W., Engels, J.M., 2007. Guidelines and recommendations for the safe operation of tailings management facilities. Environ. Eng. Sci. 24 (5), 625–637. Han, F.S., 2011. Geotechnical Behavior of Frozen Mine Backfill. University of Ottawa, Canada, pp. 44–77. Lai, Y., Xu, X., Dong, Y., Li, S., 2013. Present situation and prospect of mechanical research on frozen soils in China. Cold Reg. Sci. Technol. 87, 6–18. Leandro, R.A., Áurea, P., Claudio, O., Jiménez, R., 2014. Rock Engineering and Rock Mechanics: Structures in and on Rock Masses. CRC Press, pp. 119–124. Li, H., Yang, H., Chang, C., 1995. The strain rate sensitivity analysis of compression strength of frozen soil. J. Glaciol. Geocryol. 17 (1), 40–48 (in Chinese). Liu, S., Chen, Z., Zhang, Z., 2008. Harm of frozen earth to tailings reservoirs in high- cold are in winter and its prevention and control measures. Eng. Constr. 40 (1), 22–26 (in Chinese). Liu, Z., Zhang, X., Li, H., 2007. Experimental study of uniaxial compression of clay frozen in situ. Rock Soil Mech. 28 (12), 2657–2660 (in Chinese). Ning, J., He, Y., Zhu, Z., 2014. Dynamic constitutive modeling of frozen soil under impact loading. Chin. Sci. Bull. 59 (26), 3255–3259. Pipkin, B.W., Trent, D.D., 1997. Geology and the Environmental (2nd Ver.). Wadsworth Publishing Company, pp. 182–213. Rico, M., Benito, G., Diez-Herrero, A., 2008. Floods from tailings dam failures. J. Hazard. Mater. 154, 79–87. Shamsai, A., Pak, A., Bateni, S.M., Ayatollahi, S.A.H., 2007. Geotechnical characteristics of copper mine tailings: a case study. Geotech. Geol. Eng. 25 (5), 591–602. Wei, Z., Yin, G., Li, G., Wang, J.G., Wan, L., Shen, L., 2009. Reinforced terraced fields method for fine tailings disposal. Miner. Eng. 22, 1053–1059. Wei, Z., Yin, G., Wang, J.G., Wan, L., Li, G., 2013. Design, construction and management of tailings storage facilities for surface disposal in China: case studies of failures. Waste Manag. Res. 31 (1), 106–112. Wijeweera, H., Joshi, R.C., 1990. Compressive strength behavior of fine-grained frozen soils. Can. Geotech. J. 27, 472–483. Xie, Q., Zhu, Z., Kang, G., 2014. Dynamic stress–strain behavior of frozen soil: experiments and modeling. Cold Reg. Sci. Technol. 106-107, 153–160. Yang, Y., Lai, Y., Chang, X., 2010. Laboratory and theoretical investigations on the deformation and strength behaviors of artificial frozen soil. Cold Reg. Sci. Technol. 64, 39–45. Yin, G., Zhang, Q., Wei, Z., 2012. Experimental study of migration characteristics of pore water and its effect on meso-structure of tailings. Chin. J. Rock Mech. Eng. 31 (1), 71–79 (in Chinese). Zhao, L., Yang, P., Wang, J.G., Zhang, L., 2014. Cyclic direct shear behaviors of frozen soilstructure interface under constant normal stiffness condition. Cold Reg. Sci. Technol. 102, 52–62.
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Cold Regions Science and Technology journal homepage: www.elsevier.com/locate/coldregions
Uniaxial compression test of frozen tailings Yonghao Yang a,b, Zuoan Wei a,b,⁎, Guangzhi Yin a,b, J.G. Wang c, Wensong Wang d, Yulong Chen a,b a
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400030, China State and Local Joint Engineering Laboratory of Methane Drainage in Complex Coal Gas Seam, Chongqing University, Chongqing 400030, China c School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, China d College of Resource and Environmental Science, Chongqing University, Chongqing 400030, China b
a r t i c l e
i n f o
Article history: Received 10 December 2015 Received in revised form 15 May 2016 Accepted 17 June 2016 Available online 23 June 2016 Keywords: Frozen tailings Uniaxial compression Stress-strain relationship Compressive strength Deformation modulus
a b s t r a c t The mechanical properties (uniaxial compressive strength and deformation modulus) of frozen tailings are key parameters for the safety assessment of a tailing dam. However, the experimental data on such mechanical properties are limited. In this study, more than sixty tailings samples with four tailings (medium sand, fine sand, silty sand, and silt) were frozen at the temperature of −16 °C, and then tested under uniaxial compression in order to investigate their mechanical properties. The effects of four parameters (average particle size, dry density, water content and strain rate) on the uniaxial compressive strength (UCS) and the deformation modulus of frozen tailings were investigated. The test results show that three failure patterns of samples were observed: inclined plane shear failure, lateral tensile failure, and composite failure involving both. Their uniaxial compressive strength is related logarithmically to average particle size, exponentially to dry density, linearly to moisture content, and parabolically to strain rate. On the other hand, their deformation modulus is related logarithmically to average particle size, parabolically to moisture content, exponentially to both dry density and strain rate. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tailings, the byproducts of mineral processing, are special solid wastes. Large-scale mining and mineral processing in China and even in the world inevitably generate huge amounts of tailings. These tailings are to be economically disposed in an environmentally acceptable manner. The tailings impoundments, as one of practical solutions, can dispose of huge masses of tailings and other mineral wastes (Wei et al., 2009). Generally, a slurry waste (tailings after ore extraction), which is composed of residual ore, water, sand, silt and fine clay particles, is hydraulically transported and stored in these surface tailings impoundments, where an increasable dam is constructed to form a storage reservoir. The statistics shows that more than 12,000 tailings storage facilities (TSFs) had been constructed in China up to 2009 (Wei et al., 2013). The geographical locations of these tailings storage facilities are summarized in Table 1. Tailings dam, as the main form of TSF, are considered as the largest man-made structures in the world (Shamsai et al., 2007). However, a number of particular characteristics make tailings dams more vulnerable to failure than water storage dams (Rico et al., 2008). In this sense, tailings dams are classified as one of the high hazard risks in China's industry. Table 2 summarizes the main disastrous failures of tailings dams in China since 1962. Two to five major tailings ⁎ Corresponding author at: State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400030, China. E-mail address: [email protected] (Z. Wei).
http://dx.doi.org/10.1016/j.coldregions.2016.06.007 0165-232X/© 2016 Elsevier B.V. All rights reserved.
impoundment failures per year occurred during the last thirty years. This failure frequency is ten times higher than that of water retention dams (Dixon-Hardy and Engels, 2007). Therefore, the safety of tailings impoundments is of the highest concern for local residents and governments. Frozen tailings may have special physical and mechanical properties (Zhao et al., 2014). Based on their locations (see Table 1), most of the tailings impoundments in China may seasonally freeze in winter. Fig. 1 shows a typical example of frozen tailings impoundments covered by snow. Most of frozen tailings impoundments in winter are at 0 °C to −16 °C. When the air temperature in these cold regions is below 0 °C, the pore water in subsurface soil layers may be frozen to a certain depth to form frozen ground. This frozen ground becomes deeper over a longer freezing period and at lower air temperature. Either freezing and thawing or alternative wetting and drying of a hill slope causes upward expansion of the ground surface perpendicular to the slope face, resulting in a net down-slope movement of the soil (Pipkin and Trent, 1997; Liu et al., 2008). Fig. 2 presents a typical cross section of a tailings dam under freezing conditions in winter. Hard frozen layers were formed from the surface of the impoundments. These frozen layers have great impacts on the stability of tailings impoundments, which affects the seepage of a tailings dam, reduce its stability, and results in large deformation and even the failure of the tailings dam. Therefore, in order to improve the safe management of tailings impoundments, it is necessary to analyze the stability of tailings dams under these conditions. While using the finite element method (FEM) to analyze the
Y. Yang et al. / Cold Regions Science and Technology 129 (2016) 60–68 Table 1 Distribution of tailings impoundments in mainland China (up to 2012). No.
Province
Total tailings impoundments
Active
Inactive
Closed
Constructing
1 2 3 4 5 6 7 8 9 10 11a 12 13a 14 15 16 17 18a 19 20 21 22a 23 24 25
Jiangsu Beijing Qinghai Heilongjiang Zhejiang Xinjiang Sichuan Jilin Gansu Guizhou Guangdong Hubei Fujian Shanxi Anhui Jiangxi Shandong Guangxi Hunan Henan Inner Mongolia Yunnan Liaoning Shanxi Hebei
19 37 57 62 78 103 137 167 183 219 226 236 247 269 341 380 494 504 651 681 685 692 1475 1735 2888
10 9 10 45 44 47 87 112 107 103 63 54 136 125 263 234 252 184 356 275 449 457 1091 602 1773
0 0 44 3 8 0 6 2 24 36 101 149 0 25 6 45 25 136 109 281 118 54 90 247 343
5 28 1 0 25 11 7 22 8 50 47 3 36 49 27 26 136 116 160 5 14 104 120 330 614
1 0 2 14 1 45 37 31 44 30 15 30 75 70 45 75 81 68 26 120 104 77 174 556 158
a These tailings impoundments are located in non-cold regions. The rest may be seasonally frozen in winter.
stability of a tailings dam, the mechanical properties of frozen tailings, such as compressive strength and deformation modulus, are necessary. The strength and deformation properties of frozen soils have been investigated. For example, Wijeweera and Joshi (1990) studied the effects of dry unit weight, water content, temperature, and soil type on the peak compressive strength of six fine-grained frozen soils by using uniaxial compression tests. They obtained an empirical formula with which to predict the peak compressive strength of fine-grained frozen soils at a particular temperature. Christ and Kim (2009) studied frozen silt by using uniaxial shear tests. They found that the compressive and tensile strengths of frozen silts increased with the reduction of temperature and the increase of moisture content. Therefore, the freezing process can largely modify the mechanical properties of frozen soils. The mechanical properties of frozen soils are important to engineering design and construction in cold regions. China has implemented several huge engineering projects such as the Qinghai-Tibet Railway and the Golmud-Lhasa oil-pipelines, in past decades. These projects were constructed in permanent frozen soil regions of the QinghaiTibet Plateau. This sped up the investigations of the mechanical properties of frozen soils (Lai et al., 2013). For example, Li et al. (1995)
Table 2 Main failures of tailings dams in China. Name of dam
Tailings types
Construction method
Year of failure
Persons killed
Huogudu, Yunnan Tin Group Co., Yunnan province Niujiaolong, Shizhuyuan Non-ferrous Metals Co., Hunan province Longjiaoshan, Daye Iron Ore mine, Hubei province Dachang, Nandan Tin mine, Guangxi province Zhenan Gold mine, Shanxi province Xiangfen tailings pond, Shanxi province Xinyi Zijin Tin mine, Guangdong province
Tin
Upstream
1962
171
Copper
Upstream
1985
49
Iron
Upstream
1994
31
Tin
Upstream
2000
28
Gold Iron
Upstream Upstream
2006 2008
17 277
Tin
Upstream
2010
28
61
conducted uniaxial compression tests in order to investigate the relationship of uniaxial compressive strength with the strain rate of frozen soils and frozen temperature. They found that the uniaxial compressive strength of frozen soils increases linearly with the decrease of frozen temperatures and follows a power function with strain rate. Yang et al. (2010) carried out a series of triaxial compression tests on artificial frozen sand at a temperature of −6 °C. In their experiments, the water content varied from 10% to 20% and the constant confining pressure varied from 0.0 to 14.0 MPa. They found that the nonlinear Mohr-Coulomb criterion can describe the strength of frozen sand with high confining pressure well. Further, the stress-strain relationship of this frozen sand can be expressed by a hyperbolic function. Liu et al. (2007) studied the load versus displacement curves of frozen clays and the failure patterns of the samples using uniaxial compression tests. The test samples of frozen clay were directly taken from the site at a temperature of −15 °C. Chen et al. (2012) investigated the impact of soil moisture on the uniaxial compressive strength and the failure strain under freezing conditions. They observed that the uniaxial compressive strength of frozen salty silt increases with the increase of moisture content when the moisture content is low. Cui et al. (2014) studied the mechanical properties of a silty clay under freezing-thawing conditions. The dynamic constitutive behaviors of frozen soils were experimentally investigated under impact loading (Xie et al., 2014; Ning et al., 2014). It was found that the dynamic stress - strain responses of the frozen soil exhibits a positive strain rate sensitivity and negative temperature dependence. However, no study has been done on the mechanical properties of frozen tailings so far. The tailings, as solid residues, are different from natural soils in terms of formation, particle geometry, and particle size distribution (Yin et al., 2012). In general, the mechanical properties of frozen soils depend on both external conditions, such as temperature, loading type (static or cyclic), as well as strain rate, and the internal parameters of soils, such as soil composition, microstructure, and moisture content. Soil type, microstructure, mineral composition, amount of ice, and dry density are identified as the most important internal parameters. They may have significant impacts on the mechanical behaviors of frozen soils. Han (2011) conducted uniaxial compression tests in order to investigate the strength and deformation behaviors of frozen cemented tailings for the backfill of underground mining. In his study, artificial tailings, distilled water, and dry snow were used to mix a uniform water-snow-solid particle mixture. The cylindrical testing specimen was frozen at − 6 °C. So far, numerous experiments have been conducted in order to measure the strength and stress to strain relationship of tailings and frozen natural soils, but few publications about the mechanical properties of frozen tailings are available. The effects of strain rate on the mechanical characteristics of rocks have been widely investigated (Leandro et al., 2014). Bragg and Andersland (1981) studied the effects of strain rate, temperature, and sample size on the compression and tensile properties of frozen sand. It was found that the compressive peak strength and the initial tangent modulus of frozen sand increase with decreasing temperatures and increasing strain rates. Li et al. (1995) carried out uniaxial compression tests for frozen silt under different temperatures and strain rates. The test results showed that the compressive strength of frozen silt was sensitive to strain rate. The sensitivity can be divided into three phases. When the strain rate is less than 10− 4 s− 1, it is generally sensitive. When the strain rate is larger than 10−3 s−1, it is the most sensitive. However, few publications on this sensitivity study on frozen tailings are currently available. In this study, uniaxial compression tests were carried out in order to thoroughly investigate the mechanical properties of frozen tailings. The tests measured the mechanical behaviors of more than sixty frozen tailings samples at a temperature of −16 °C. The samples were made from four tailings. The uniaxial compressive strength and the deformation modulus were quantitatively analyzed. These tests investigated the effects of four influential factors on the mechanical properties of frozen
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Y. Yang et al. / Cold Regions Science and Technology 129 (2016) 60–68
Fig. 1. A tailings dam covered with snow and ice in winter in Shanxi province.
tailings. It is expected that the test results may be helpful not only to the understanding of the basic mechanical behaviors of frozen tailings but also to the enhancement of the safety management of tailings impoundments in cold regions.
were poured into cylindrical molds for sample casting using a layerby-layer compaction method. Each sample was divided into five layers. In each layer, the dry density was controlled by weighing the test samples. The cast samples were wrapped with preservative films, as shown in Fig. 4, and put into a refrigerating box for 24-h freezing at −16 °C.
2. Uniaxial compression tests of frozen tailings 2.3. Test schemes 2.1. Tailings gradations The test tailings were sampled from the processing mill of Lala Copper Mine (Sichuan province, China). The Lala Copper Mine is the biggest open-pit copper mine in southwest China. According to the national technical Code for Geotechnical Engineering of Tailings Embankment (GB50547-2010), the tailings are divided into three classes and seven sub-classes based on their grain size gradations. They are tailings sand (the sub-classes are gravelly sand, coarse sand, medium sand, fine sand, and silty sand), tailings silt (including silts), and tailings clay (the sub-classes are silty clay and clays). In order to investigate the effects of gradations on the mechanical properties of tailings, the original tailings (#5) sampled from this mine site were manually sieved to form four groups of tailings samples based on their grain size gradations. The four groups are medium sand (#4), fine sand (#3), silty sand (#2), and silty clay (#1). Their main physical properties are summarized in Table 3, and their particle size distributions are presented in Fig. 3. Further, Table 4 presents their mineralogical compositions obtained through the x ray fluorescence (XRF) analysis. 2.2. Preparation of test samples The test samples are cylinder-shaped. Each sample is 50 mm in diameter and 100 mm in height. The preparation obeys the following procedure. First, the tailings samples were mixed to predetermined moisture contents. The well-mixed tailings were then sealed in plastic bags. After 24 h and further mixing in the seal bags, the mixed soils
Uniaxial compression tests were performed to investigate the mechanical properties of frozen tailings. The key influential factors that affect the compressive strength and deformation modulus of frozen tailings were explored. Four influential parameters were preliminarily selected as average particle size, dry density, moisture content, and strain rate. Table 5 presents the test schemes and the physical properties of the tailings samples. These tailings samples were taken from the original tailings (#5) as well as another four tailings (#1, #2, #3, and #4). The tests were divided into four groups. Group 1 was used to investigate the impact of average particle size. The initial size ranges from 31.97 μm to 310.50 μm. Group 2 was used to study the impact of dry density. The density varies from 1.48 g/cm3 to 1.71 g/cm3. Group 3 was used to explore the impact of moisture content, which varies from 5% to 20%. Group 4 was used to study the effect of strain rate. In these tests, the strain rate varied from 0.05/min to 0.2/min. 2.4. Test procedure The uniaxial compression tests for frozen tailings were carried out using the Japan Shimadzu AG-I25KN electronic precision materials testing machine (see Fig. 5) in the State Key Laboratory of Coal Mine Disaster Dynamics and Control in Chongqing University. After putting the test samples in place and setting loading parameters, the whole compression procedure was automatically controlled by computer to realize real-time and automatic acquisition of test data. Due to the temperature difference, a rubber insulation tube that was 15 mm thick was used to
Fig. 2. A typical cross section of a tailings dam under freezing conditions in winter.
Y. Yang et al. / Cold Regions Science and Technology 129 (2016) 60–68 Table 3 Physical index of test tailings.
63
Table 4 Main mineralogical compositions of tailings from Lala copper mine.
Sample no.
Tailings class
Plastic limit Liquid limit Plastic (%) (%) index
d50(μm) Cu
Cc
Mineralogical compositions (analyzed by XRF)
Results (mg per kg)
#1 #2 #3 #4
Silty clay Silty sand Fine sand Medium sand Silty sand
14.5 13.3 13.1 12.1
25.8 24.1 23.0 21.4
11.3 10.8 9.9 9.3
31.97 83.58 168.92 310.50
7.35 3.52 3.65 3.38
1.29 1.26 1.14 1.40
12.7
20.8
8.1
102.25
8.99 1.22
Oxygen Calcium Magnesium Silicon Iron Aluminum Kalium Manganese Natrium Titanium Copper
458,960 261,363 160,181 115,240 9735 8754 3522 2451 2313 905 614
#5
Note: d50 refers to the average particle size.
hold the frozen sample for heat insulation. An insulating plastic sheet (see details in Fig. 5) was placed on the contact surface between the end of sample and the press of test machine. This prevented the contact surface at the end of the test samples from melting during the test. The tests were conducted in winter. The temperature in the testing room was always kept below 0 °C during the test procedure in order to prevent the samples from melting. In addition, the test time was as short as possible. The whole compression procedure for each test was completed within 1 to 3 min. Each test was repeated at least three times (up to six times for some samples) in order to ensure the repeatability of the results. On the other hand, the surface temperatures on the sample end and middle were measured during the test process (Fig. 6a). The change of temperature with time was summarized in Fig. 6b. The measurement results showed that the temperature increment was between +0.4 °C and +0.6 °C. The temperature of the sample was still close to −16 °C.
3. Experimental results and analysis
3.1.2. Pattern 2: lateral tensile failure This failure pattern is shown in Fig. 7(b). Under the axial compressive stress, lateral tensile stress is produced within the sample. When the tensile stress exceeds the ultimate tensile strength of the material, a crack initiates along the axial direction throughout the sample and causes splitting failure. This failure pattern easily occurs for the tailings with relatively smaller average particle sizes, or those with less moisture content or at lower strain rates. 3.1.3. Pattern 3: composite failure This failure pattern is shown in Fig. 7(c). In this pattern, partial ballooning appears in the middle of the test sample, and obliquely crossing cracks are generated, causing sample failure. For some samples, stripshaped fracture failure occurred on the lateral side and shear failure happened in the remaining parts. When the dry density of tailings is lower, or the strain rate is greater, this failure mode easily occurs.
3.1. Failure pattern The failure pattern of frozen tailings samples was observed under uniaxial compression. The following three patterns were observed.
3.1.1. Pattern 1: inclined plane shear failure This failure pattern is shown in Fig. 7(a). The fracture occurs in a part of the sample at first, and then expands constantly to form the crack via interpenetration and finally becomes an inclined shear plane throughout the whole sample. In some cases, two mutually crossing shear planes were formed. When the average particle size of tailings is greater, or the moisture content of test samples is higher, this failure pattern will happen more easily.
Fig. 3. Particle size distribution of tailings.
3.2. Typical stress-strain curve In the uniaxial compression tests, typical stress-strain (σ-ε) curves of frozen tailings at − 16 °C are shown in Fig. 8 for the samples with two dry densities. Strain hardening and strain softening were observed during the compression test process. The stress-strain relation can be divided into four deformation stages. 3.2.1. Stage 1: initial compaction stage (OA segment in Fig. 8) This stage occurs at the initial deformation and lasts for a very short time. This stress-strain curve soon bends upwards from the point A with the increase of vertical stress. This stage may be due to initial partial contact and partially melt induced by the temperature difference. This can be ruled out through the extension of the next stage.
Fig. 4. Samples of frozen tailings.
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Table 5 Test schemes. Test group
Average particle size (groups of tailings samples) d50 (μm)
Dry density ρd (g cm−3)
Moisture content ω (%)
Strain rate v (10−1 min−1)
Group 1
1.58
15
2
Group 2
31.97(#1),83.58(#2), 168.92(#3), 310.50(#4) 102.25(#5)
15
2
Group 3 Group 4
102.25(#5) 102.25(#5)
1.48, 1.58, 1.64, 1.71 1.58 1.58
5, 10, 15, 20 15
2 0.5, 1, 1.5, 2
3.2.2. Stage 2: linear strain hardening stage (AB segment in Fig. 8) The stress-strain curve is an approximately straight line. The stressstrain relationship of frozen tailings is of an initial linear elastic property.
3.2.3. Stage 3: nonlinear strain hardening stage (BC segment in Fig. 8) The axial stress increases continuously with the increase of axial strain, but the stress-strain curve bends downward. The slope of the stress-strain curve gradually decreases. That is, its deformation modulus decays gradually. Finally, the stress reaches its peak compressive strength, σc, and the deformation modulus approaches zero. The strains at point C are between 6.2% and 8.6%, which are proportional to the dry density and the average particle size and the moisture content of sample. That is, the greater the dry density and the average particle size and the moisture content of sample, the larger the strain at point C is. During this deformation stage, slight slanting and longitudinal microcracks were initially observed on the surfaces of samples. The number of these micro-cracks becomes greater with the increase of vertical or axial stress.
3.2.4. Stage 4: nonlinear strain softening stage (the segment beyond the peak or the point C) After the axial stress reaches the peak compressive strength, the sample undergoes the strain softening stage, wherein the stress is gradually decreased with the increase of strain and the slope of the stressstrain curve becomes negative. In this stage, the micro-cracks on the sample surface were observed to penetrate into the internal zone and amalgamated to form larger macro-cracks. Partial fractures occur in the sample along with slippage and, thus, finally cause sample failure. The final strains are between 16% and 23%. The final failure times are
Fig. 6. The results and schematic diagram for measuring the sample temperature.
between 1.2 min and 4.6 min. The loading strain rate has the biggest impact on the failure time of sample. 3.3. Uniaxial compressive strength and deformation modulus The effects of average particle size, dry density, moisture content, and strain rate on uniaxial compressive strength and deformation modulus of the frozen tailings were analyzed. 3.3.1. Effect of average particle size The physical and mechanical properties of a soil are strongly influenced by its particle size distribution. Fig. 9 presents the effects of average particle size on the uniaxial compressive strength and deformation modulus of frozen tailings. The tests were conducted under a constant strain rate of 0.2/min. This figure clearly indicates that both uniaxial compressive strength and deformation modulus of the frozen tailings are logarithmically related to the average particle size of tailings. They can be described by the following two regression expressions:
Fig. 5. Uniaxial compression test setup for frozen tailings samples.
σ c ¼ 1:2581 ln ðdave Þ þ 0:5971
ð1Þ
Y. Yang et al. / Cold Regions Science and Technology 129 (2016) 60–68
65
Fig. 7. Three failure patterns of frozen tailings samples.
with a correlation coefficient of R2 = 0.9708. E ¼ 147:88 ln ðdave Þ−258:79
For uniaxial compressive strength ð2Þ
with a correlation coefficient of R2 = 0.9429. Where σc (MPa) is the uniaxial compressive strength. dave (μm) is the average diameter of tailings particles, and E (MPa) refers to the deformation modulus at the linear stage (the AB segment of the σ-ε curve in Fig. 8). 3.3.2. Effect of dry density The effect of dry density of tailings was investigated under a constant strain rate of 0.2/min. Fig. 10(a) presents the change of uniaxial compressive strength with dry density, and Fig. 10(b) presents the change of deformation modulus. Obviously, both the uniaxial compressive strength and the deformation modulus of frozen tailings gradually increase with the increase of dry density. These changes were fitted by following expressions:
σ c ¼ 0:3333e1:8485γ
ð3Þ
with a correlation coefficient of R2 = 0.9635. For deformation modulus E ¼ 54:461γ4:0741
ð4Þ
with a correlation coefficient of R2 = 0.9496. Where γ is the dry density of tailings (g/cm3). Eq. (3) shows that the uniaxial compressive strength is exponentially related to dry density, but the power function of Eq. (4) is applicable to the relationship between deformation modulus and dry density. This is because a relatively higher dry density of tailings means lower porosity. The particles of tailings have closer contact each other, the cohesive force between ices within the frozen tailings pores increases, and thus the bearing capability of the frozen tailings can be greater.
Fig. 8. Typical stress-strain curves of frozen tailings in uniaxial compression tests.
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Fig. 9. Effect of average particle size on both uniaxial compressive strength and deformation modulus of frozen tailings.
Fig. 10. Impact of dry density on both uniaxial compressive strength and deformation modulus of frozen tailings.
3.3.3. Impact of moisture content The moisture content is a key parameter of the physical and mechanical properties of frozen tailings. Tailings are man-made soils that are usually three-phase composites of solid mineral particles, pore water, and pore air. When the soil temperature drops below 0 °C, water within soil pores starts to form ice particles. This ice cementation on particles increases the strength of the soil. In order to investigate the impact of moisture content, this study specified the moisture content as 5%, 10%, 15%, and 20% during sample preparations. The impact of moisture content on the uniaxial compressive strength is presented in Fig. 11(a) and Fig. 11(b), on the deformation modulus. It was observed that the uniaxial compressive strength of frozen tailings is linearly related to moisture content. The reason for this property of frozen tailings is that the higher moisture content has more ice content within frozen tailings pores. The ice bonds the particles together and significantly increases the strength of the tailings. The deformation modulus is parabolically related to the moisture content. Curve fitting obtained the following expressions (where moisture content is in %). For uniaxial compressive strength, σ c ¼ 0:5442w−1:694
ð5Þ
with a correlation coefficient of R2 = 0.9971. For the deformation modulus, E ¼ 0:6295w2 þ 43:061w−153:93
ð6Þ
with a correlation coefficient of R2 = 0.9962. 3.3.4. Effect of strain rate Strain rate may influence the mechanical properties of frozen tailings. The loading rate-dependence of compressive strength and deformation modulus of frozen tailings was investigated under four strain rates of 0.05, 0.1, 0.15, and 0.2/min. Fig. 12(a) presents the effect of
Fig. 11. Change of uniaxial compressive strength and deformation modulus of frozen tailings with moisture content.
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strain rate on the uniaxial compressive strength, and Fig. 12(b) shows the effect of strain rate on the deformation modulus. Fig. 12(a) shows that the uniaxial compressive strength of frozen tailings is parabolically related to strain rate. With the increase of strain rate, the uniaxial compressive strength of frozen tailings increases linearly at the beginning and gradually becomes stationary at high strain rates. The following expressions are fitted (where the strain rate, v, is in 10−2/min). σ c ¼ −0:015ν2 þ 0:5183ν þ 1:9962
ð7Þ
with a correlation coefficient of R2 = 0.9971. The deformation modulus of frozen tailings is exponentially related to strain rate, as shown in Fig. 12(b) and fitted by the following expression: E ¼ 223:42ν0:1367
ð8Þ
with a correlation coefficient of R2 = 0.9982. The existence of ice particles has an impact on this property of frozen tailings. With the decrease of strain rate, the ice in frozen tailings, especially the ice at the mineral grain contact points, exhibits slow plastic flow and creep properties. The capacity of frozen tailings restricting external force increases with the increase of strain rate, and the failure patterns of frozen tailings samples change from fragile failure to plastic failure. 3.4. Relation between the failure strain and four parameters
Fig. 12. Impact of strain rate on both uniaxial compressive strength and deformation modulus of frozen tailings.
The failure strain of the frozen tailings is corresponding to the strain at point C (see Fig. 8). The value of the failure strain is between 2.7% and 8.6%. The relation between the failure strain and four parameters were
Fig. 13. The relation between the failure strain and four parameters of frozen tailings.
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analyzed. Fig. 13 presents the effects of four parameters on the failure strain. The results show that the failure strain is related parabolically to the average particle size (see Fig. 13(a)), the dry density (see Fig. 13(b)) and the strain rate (see Fig. 13(d)), logarithmically to the moisture content (see Fig. 13(c)). That is, the greater the dry density and the average particle size and the moisture content of sample, the larger the failure strain at point C is. 4. Conclusions A series of uniaxial compressive tests were conducted on frozen tailings at a temperature of −16 °C. The stress-strain curves of the tailings samples were analyzed, and their failure patterns were observed. Particularly, the effects of particle size, dry density, moisture content, and strain rate on the uniaxial compressive strength and deformation modulus of frozen tailings were analyzed. Based on these preliminary test results on the mechanical properties and the observations of failure patterns, the following conclusions can be drawn. (1) Three failure patterns of the tailings samples were observed in the uniaxial compression tests. They are inclined plane shear failure, lateral tensile failure, and composite failure. Further, the average particle size, dry density, and strain rate significantly influence the failure patterns. (2) The stress-strain curves of frozen tailings under uniaxial compression have the following typical deformation stages: linear strain hardening, nonlinear strain hardening, and nonlinear strain softening. During the deformation stage of nonlinear strain hardening, slight slanting and longitudinal micro-cracks were initially observed on the surfaces of samples. The number of these micro-cracks becomes greater with the increase of vertical or axial stress. In the softening stage, the micro-cracks on the sample surface were observed to penetrate into the internal zone and amalgamated to form larger macro-cracks. Partial fractures occur in the sample along with slippage and finally cause the sample failure. (3) Both uniaxial compressive strength and deformation modulus of frozen tailings are closely related to average particle size, dry density, moisture content, and strain rate. It was found that the uniaxial compressive strength of frozen tailings is related logarithmically to average particle size, exponentially to dry density, linearly to moisture content and parabolically to strain rate. The deformation modulus of frozen tailings is related logarithmically to average particle size, exponentially to dry density, parabolically to moisture content, and exponentially to strain rate. Acknowledgements The authors are very grateful to Dr. Ryan Madeley and anonymous reviewers for their valuable comments and suggestions. This research
was funded by the National Natural Science Foundation of China (No. 11372363) and “Gan-po Talent 555” Project of Jiangxi Province. References Bragg, R.A., Andersland, O.B., 1981. Strain rate, temperature, and sample size effects on compression and tensile properties of frozen sand. Eng. Geol. 18 (1–4), 35–46. Chen, J., Li, D., Bing, H., 2012. An experimental study of influence of water content on uniaxial compression strength of frozen salty silt. J. Glaciol. Geocryol. 34 (2), 441–446 (in Chinese). Christ, M., Kim, Y.-C., 2009. Experimental study on the physical- mechanical properties of frozen silt. KSCE J. Civ. Eng. 13 (5), 317–324. Cui, Z., He, P., Yang, W., 2014. Mechanical properties of a silty clay subjected to freezing– thawing. Cold Reg. Sci. Technol. 98, 26–34. Dixon-Hardy, D.W., Engels, J.M., 2007. Guidelines and recommendations for the safe operation of tailings management facilities. Environ. Eng. Sci. 24 (5), 625–637. Han, F.S., 2011. Geotechnical Behavior of Frozen Mine Backfill. University of Ottawa, Canada, pp. 44–77. Lai, Y., Xu, X., Dong, Y., Li, S., 2013. Present situation and prospect of mechanical research on frozen soils in China. Cold Reg. Sci. Technol. 87, 6–18. Leandro, R.A., Áurea, P., Claudio, O., Jiménez, R., 2014. Rock Engineering and Rock Mechanics: Structures in and on Rock Masses. CRC Press, pp. 119–124. Li, H., Yang, H., Chang, C., 1995. The strain rate sensitivity analysis of compression strength of frozen soil. J. Glaciol. Geocryol. 17 (1), 40–48 (in Chinese). Liu, S., Chen, Z., Zhang, Z., 2008. Harm of frozen earth to tailings reservoirs in high- cold are in winter and its prevention and control measures. Eng. Constr. 40 (1), 22–26 (in Chinese). Liu, Z., Zhang, X., Li, H., 2007. Experimental study of uniaxial compression of clay frozen in situ. Rock Soil Mech. 28 (12), 2657–2660 (in Chinese). Ning, J., He, Y., Zhu, Z., 2014. Dynamic constitutive modeling of frozen soil under impact loading. Chin. Sci. Bull. 59 (26), 3255–3259. Pipkin, B.W., Trent, D.D., 1997. Geology and the Environmental (2nd Ver.). Wadsworth Publishing Company, pp. 182–213. Rico, M., Benito, G., Diez-Herrero, A., 2008. Floods from tailings dam failures. J. Hazard. Mater. 154, 79–87. Shamsai, A., Pak, A., Bateni, S.M., Ayatollahi, S.A.H., 2007. Geotechnical characteristics of copper mine tailings: a case study. Geotech. Geol. Eng. 25 (5), 591–602. Wei, Z., Yin, G., Li, G., Wang, J.G., Wan, L., Shen, L., 2009. Reinforced terraced fields method for fine tailings disposal. Miner. Eng. 22, 1053–1059. Wei, Z., Yin, G., Wang, J.G., Wan, L., Li, G., 2013. Design, construction and management of tailings storage facilities for surface disposal in China: case studies of failures. Waste Manag. Res. 31 (1), 106–112. Wijeweera, H., Joshi, R.C., 1990. Compressive strength behavior of fine-grained frozen soils. Can. Geotech. J. 27, 472–483. Xie, Q., Zhu, Z., Kang, G., 2014. Dynamic stress–strain behavior of frozen soil: experiments and modeling. Cold Reg. Sci. Technol. 106-107, 153–160. Yang, Y., Lai, Y., Chang, X., 2010. Laboratory and theoretical investigations on the deformation and strength behaviors of artificial frozen soil. Cold Reg. Sci. Technol. 64, 39–45. Yin, G., Zhang, Q., Wei, Z., 2012. Experimental study of migration characteristics of pore water and its effect on meso-structure of tailings. Chin. J. Rock Mech. Eng. 31 (1), 71–79 (in Chinese). Zhao, L., Yang, P., Wang, J.G., Zhang, L., 2014. Cyclic direct shear behaviors of frozen soilstructure interface under constant normal stiffness condition. Cold Reg. Sci. Technol. 102, 52–62.