Geotechnical Properties of Mine Tailings

The produced tailings were transported into a hydrocyclone separation facility, and two kinds of tailings were generated, i.e., coarse tailings from the bottom and fine tailings from the overflow of the hydrocyclone separation facility. The concentrations of the produced coarse iron and fine iron tailings from the Yuhezhai iron mine were about 65–70 and 70 wt%, respectively, and the concentrations of the coarse copper and fine copper tailings from the Bahuerachi copper mine were about 72 and 60 wt%.

A series of conventional laboratory experiments were carried out to investigate the mechanical properties of the tailings, including specific gravity, grain size distribution, Atterberg limits, and hydraulic conductivity. Consolidation tests, triaxial tests, and cyclic triaxial tests were conducted to investigate the compression characteristics and strength of the tailings.

The pycnometer method was chosen to measure the specific gravity. The sieving method and laser particle size analysis were combined for the determination of grain size distribution. The Atterberg limits were measured with the fall cone test. The permeability of the two coarse tailings was measured with a permeameter by applying a constant water head gradient to the bottom and top of the sample, and the permeability of the two fine tailings was studied with a flexible wall permeameter under different consolidation stresses (50, 100, 200, 400, and 600 kPa). The as-received coarse tailings were first saturated under a vacuum and then poured into the sample mold to form the specimen for the permeability test. The fine tailings were first oven dried, then mixed with water at a predetermined water content of 10% and compacted into the sample mold in five layers with a dry density of 1.60 and $1.46\mathrm{g}/{\mathrm{cm}}^3$ for the iron and copper tailings, respectively, and finally saturated under a vacuum. The specimens for the permeability tests were 10 cm in diameter and 10 cm in height. Except for the specimen preparation, tests were completely conformed to ASTM standard D5084-10 (ASTM 2010). The consolidation test was performed in a one-dimensional oedometer 8 cm in diameter according to ASTM standard D2435M-11 (ASTM 2011d). The coarse and fine tailings were naturally deposited in the apparatus and the stepwise vertical stresses were 12.5, 25, 50, 100, 200, 400, 800, and 1,600 kPa.

The unsaturated triaxial testing system (VJTech, Reading, Berkshire, U.K.) was used to conduct the consolidated undrained (CU) and consolidated drained (CD) triaxial compression tests on the tailings according to ASTM standard D4767-11 and D7181-11, respectively (ASTM 2011c, b). The consolidation stresses were 200, 400, and 800 kPa. The specimens for the triaxial tests were 3.91 cm in diameter and 8 cm in height, and the preparation of specimens was the same as that in the permeability tests. The initial void ratios of the samples were 0.94 and 0.93 for the coarse and fine iron tailing samples, and 0.78 and 0.89 for the coarse and fine copper tailing samples, respectively.

A stress-controlled cyclic triaxial system (DDS-70, Beijing New Technology Research Institute, Beijing, China) was used to study the cyclic properties of the tailings. Both the cyclic strength and cyclic modulus tests were carried out according to ASTM standards D5311M-11 and D3999-91 (ASTM 2003, 2013). The size and preparation of the specimens were the same as that in the triaxial tests. The frequency of the applied cyclic load was 1 Hz with sinusoidal wave.

The basic geotechnical properties of the four tailings are summarized in Table 1, and Fig. 1 displays the grain size distributions. The specific gravity of the coarse and fine iron tailings are 3.23 and 3.08, respectively, which are consistent with those of metal tailings but much higher than those of natural soils. The specific gravity of the coarse and fine copper tailings are 2.77 and 2.76, respectively, which are smaller than those of the iron tailings but slightly larger than those of natural soils. Fig. 1 indicates that the mass percentage of grain size less than 0.075 mm is 14.9% for the coarse iron tailings, 78.4% for the fine iron tailings, 14.24% for the coarse copper tailings, and 61.12% for the fine copper tailings. It is worthwhile noting that the grain size distributions of the two coarse tailings are almost the same, and both present an average particle size $D_{50}$ of 0.12 mm. The $D_{50}$ of the fine iron tailings (0.03 mm) is smaller than that of the fine copper tailings (0.06 mm). According to the coefficient of uniformity and coefficient of curvature results, the coarse iron tailings, fine iron tailings, and coarse copper tailings are poorly graded soils, whereas the fine copper tailings present a well-graded grain size distribution. Based on the testing results of Atterberg limits and grain size distribution, the two coarse tailings are classified as silty sand (SM), and the two fine tailings are classified as sandy lean clay (CL) according to the Unified Soil Classification System (ASTM 2011a).

The void ratio ($e$) versus logarithm of the vertical stress ($p$) for the coarse and fine tailings is plotted in Fig. 2. A logarithmic function can be used to fit the compression curves of the coarse and fine tailings (Wong et al. 2008)in which $p_0=1\mathrm{kPa}$; $e_0$ = initial void ratio corresponding to $p_0$; and $C_c$ = compression index. The fitting results of the four tailings are summarized in Table 2. The compression index ($C_c$) is 0.046 and 0.260 for the coarse and fine iron tailings, and 0.025 and 0.085 for the coarse and fine copper tailings. Both the coarse and fine iron tailings display a higher compressibility than the coarse and fine copper tailings. According to the values of $C_c$, the coarse iron tailings are characterized as a moderate compressible soil, and the fine iron tailings are high compressible soil. However, for the copper tailings, the coarse and fine tailings are characterized as low and moderate compressible soil, respectively. The $C_c$ values of the four tailings are in agreement with those reported by Qiu and Sego (2001) and Wong et al. (2008), as displayed in Fig. 2. The copper and gold tailings from Qiu and Sego (2001) and the coarse oil sand tailings from Wong et al. (2008) show a $C_c$ of 0.094, 0.152, and 0.036, respectively, indicating that all of them belong to moderate compressible soil. The values of $C_c$ of the coal wash tailings and the fine oil sand tailings are 0.368 and 0.447, which both belong to high compressible soil. The comparison of $C_c$ for different tailings indicates that the oil sand tailings and coal wash tailings have a higher compressibility than the other metal tailings, and for the same type of tailings, the fine tailings always present higher compressibility than the coarse tailings. For different types of tailings, the compressibility is not only related to the grain size and mine type, but also depends on the grade of mine, mining technology, mill process, deposition method, mineralogical composition, etc.

According to the results of the consolidation tests, the coefficient of compressibility ($m_v$) under different void ratios were calculated, and the relationships between $e$ and $m_v$ were fitted with a logarithmic function as shown in Fig. 3. With a decrease in the vertical stress, the coefficient of compressibility decreases. Compared to the two coarse tailings, the two fine tailings present a larger $m_v$, and moreover, both the coarse and fine iron tailings show a larger $m_v$ than the coarse and fine copper tailings. According to the fitting results in Fig. 3, the relationship between $m_v$ and $e$ can be described with the following equation:in which $m_{v0}({\mathrm{MPa}}^{-1})$ = coefficient of compressibility corresponding to $p_0$ and initial void ratio $e_0$; and $R$ = factor describing the change rate of $m_v$ with the change in void ratio. The fitting of the two fine tailings is better than that of the two coarse tailings because the correlation coefficients are higher than 0.97 for the two fine tailings. The fitting results of the two fine tailings can be expressed as

According to the results of the permeability tests, the average hydraulic conductivities ($k$) of the coarse iron tailings and the coarse copper tailings are $1.0\times {10}^{-4}$ and $2.1\times {10}^{-3}\mathrm{cm}/\mathrm{s}$, respectively. Fig. 4 further shows the relationship between void ratio and hydraulic conductivity of the two fine tailings as well as other kinds of tailings. The hydraulic conductivities of the two fine tailings increase with the increase in void ratio, and the fine copper tailings present a higher hydraulic conductivity than the fine iron tailings. In addition, the hydraulic conductivities of the two tested fine tailings are smaller than those of the copper and gold tailings from Qiu and Sego (2001), and larger than that of the fine oil sand tailings from Wong et al. (2008). Compared to the organic tailings, the metal tailings generally present a larger hydraulic conductivity. The change in hydraulic conductivity with void ratio can be further fitted with a logarithmic function aswhere $k_0$ = initial hydraulic conductivity under initial void ratio ($\mathrm{cm}/\mathrm{s}$). The fitting results of the two fine tailings can be expressed as

The coefficient of consolidation can be derived from Eqs. (2) and (4) aswhere ${\gamma }_w$ = unit weight of water; and $C_{v0}$ = initial coefficient of consolidation, which can be expressed as $k_0/(m_{v0}·{\gamma }_w)$. When $R=M$, the decrease in $m_v$ with decrease in void ratio exactly balances the decrease in $k$, and therefore $C_v$ is constant during the consolidation process. The initial coefficient of consolidation $C_{v0}$ is $3.3\times {10}^{-6}{\mathrm{m}}^2/\mathrm{s}$ and $4.4\times {10}^{-6}{\mathrm{m}}^2/\mathrm{s}$ for the fine iron tailings and fine copper tailings, respectively. When $R>M$, $C_v$ decreases with the decrease in void ratio during consolidation, whereas when $R<M$, $C_v$ increases with the decrease in void ratio. According to the fitting results in Figs. 3 and 4, for the fine iron tailings, the values of $R$ and $M$ are almost the same (0.308 and 0.299); therefore, $C_v$ keeps constant during the test. However, for the fine copper tailings, the value of $R$ is smaller than that of $M$, which means $C_v$ increases during the test.

The consolidation theory by Terzaghi (1943) assumed a constant coefficient of consolidation. Lekha et al. (2003) derived a solution for a one-dimensional consolidation problem considering more realistic assumptions about soil behavior, including the nonlinear variation of compressibility and hydraulic conductivity as described in Eqs. (2) and (4). According to the theory of Lekha et al. (2003), the actual consolidation process takes place faster than expected by Terzaghi’s theory when $R<M$. During and after the construction of the tailing dams, both the coarse and fine tailings gradually consolidate under their own weight, and the consolidation process is important for the safety evaluation and management of the tailing dams. Moreover, because the consolidation of fine tailings is normally slower than the coarse tailings, the analysis of the consolidation of fine tailings is actually even more important for a tailing dam. Based on the above analysis of $m_v$ and $k$ for the two fine tailings, the traditional consolidation theory by Terzaghi (1943) can be used to analyze the consolidation of the fine iron tailings, whereas for the fine copper tailings, the theory proposed by Lekha et al. (2003) is recommended.

Both the CU and CD tests were conducted on the Yuhezhai iron tailings, and CU tests were conducted on the Bahuerachi copper tailings. The friction angle and cohesion of the four tailings were calculated from the results of the triaxial tests as listed in Table 3, where ${\varphi }_{cu}$ and $c_{cu}$ denote the strength indices in terms of total stress calculated from CU tests, ${\varphi }^{\prime }$ and $c^{\prime }$ denote the strength indices in terms of effective stress calculated from CU tests, and ${\varphi }_{cd}$ and $c_{cd}$ denote the strength indices in terms of effective stress calculated from CD tests. For the two iron tailings, the effective friction angle (${\varphi }^{\prime }$) calculated from the CU tests is in accordance with that obtained from the CD tests (${\varphi }_{cd}$), whereas the effective cohesion ($c^{\prime }$) from CU tests is smaller than that from the CD tests ($c_{cd}$). The effective friction angle and cohesion of the coarse tailings are larger than those of the fine tailings, both for the iron and copper tailings. In addition, the comparison also indicates that the strength indices (${\varphi }^{\prime }$ and $c^{\prime }$) of the coarse iron tailings are similar to those of the coarse copper tailings, whereas the strength indices of the fine iron tailings are smaller than those of the fine copper tailings.

As shown in Table 1, the liquid limit of the two fine tailings is the same, and the plastic limit of the fine iron tailings is higher than that of the fine copper tailings; therefore, the fine copper tailings exhibits higher plasticity than the fine iron tailings. According to the Atterberg limits and the natural water content, the liquefaction susceptibility of the two fine tailings can be assessed with three empirical criteria proposed by Wang (1979) and Marcuson et al. (1990) (the Chinese criteria), Seed et al. (2003), and Bray et al. (2004), as shown in Fig. 5. The Chinese criteria suggest that clay soil may be liquefied as a result of cyclic loading if the percent of particles finer than $0.005\mathrm{mm}<15\%$, the liquid limit $<35\%$, and the water content $>0.9\times w_L$. For the fine iron tailings and fine copper tailings, the percent of particles finer than 0.005 mm is about 10.5 and 10.4%, respectively. Therefore, both of the two fine tailings are not safe under cyclic loading [Fig. 5(a)], and further tests are needed such as cyclic triaxial (CTX) or cyclic direct simple shear (CDSS) tests. According to the criteria proposed by Seed et al. (2003) and Bray et al. (2004) [Figs. 5(b and c)], the fine iron tailings have potential to liquefaction, whereas the fine copper tailings need further tests.

The failure criteria adopted in the cyclic strength tests is 5% double amplitude axial strain ($\epsilon$) or 100% pore water pressure ($u$), which means a stage for which the developed pore water pressure during cyclic loading reached the applied consolidation stress. In this study, the 5% double amplitude axial strain is adopted to identify the failure of the tailings specimens. The cyclic strength tests are conducted under the isotropic consolidation condition ($K_c=1.0$). Fig. 6 plots the dimensionless cyclic stress ratio (CSR), defined as the ratio between the applied cyclic shear stress and twice the consolidation stress, against the number of cycles to failure ($N_f$). With an increase in CSR, the number of failure cycles decreases. Similar to the results of copper-gold-zinc tailings and gold tailings reported by Wijewickreme et al. (2005) and James et al. (2011), the CRR of the four tailings are all insensitive to the initial consolidation stress. Furthermore, the relationship between CRR and the number of failure cycles is generally expressed by the following equation (Boulanger and Idriss 2004; James et al. 2011):

The curves in Fig. 6 are fitted with the above function and the results are as following:

As indicated by Eq. (8), the cyclic resistances of the two coarse tailings are larger than those of the two fine tailings, respectively. Furthermore, compared to the iron tailings, the copper tailings possess a higher cyclic resistance.

Some of the previously published data on the cyclic resistance ratio of other kinds of tailings are compared with the results presented in this paper in Fig. 7. The CRRs of the iron and copper tailings in this study are similar to other kinds of tailings such as copper tailings and copper-zinc tailings. As mentioned above, the cyclic resistance of soil can also be studied with the CDSS test, and some of the reported data of CDSS tests are also illustrated in Fig. 7. The comparison between the CTX and CDSS tests indicates that the CRR values obtained from CTR tests are larger than those obtained from CDSS tests.

The CDSS test is more suitable than the CTX test to study the cyclic strength of soil because the CDSS test can simulate the cyclic rotation of principal stress that takes place during earthquake loading. Idriss and Boulanger (2008) recommended an empirical equation for the correction of the CTX results to account for the stress path effectswhere ${\mathrm{CRR}}_{\text{CDSS}}$ = cyclic resistance obtained from CDSS test; ${\mathrm{CRR}}_{\text{CTR}}$ = cyclic resistance obtained from CTR test; and ${(K_0)}_{\mathrm{CDSS}}$ = coefficient of earth pressure at rest in a CDSS device. For normally consolidated soils, the value of ${(K_0)}_{\mathrm{CDSS}}$ can be taken between 0.45 and 0.50, and the empirical equation can be expressed as (Geremew and Yanful 2013)

Pore water pressure increased during cyclic loading, and liquefaction occurred when the pore water pressure reaches the initial consolidation stress, i.e., the effective stress decreased to 0. The development of pore water pressure in the tailings specimens during the cyclic strength tests was monitored, and the results are displayed in Fig. 8. The pore water pressure responding curves of the two coarse tailings are similar to those of tailings reported by James et al. (2011) and sands reported by Seed et al. (1975). In addition, under different consolidation stresses and CSR, the pore water pressure responding curves exhibited similar tendencies; thus, the pore water pressure increased at a relative constant and small rate in the initial stage (except the rapid accumulation at the beginning), and then increased rapidly afterward. The normalized pore water pressure ($u/{\sigma }_3$) increased from 0 to about 0.50–0.60 at normalized number of cycles ($N/N_f$) of 0.6, and then 0.65–0.85 at $N/N_f$ of 0.8, and finally 0.96–1.00 at $N/N_f$ of 1.0. Unlike the coarse tailings, the pore water pressure responding curves for the two fine tailings show a rapid increase stage from the beginning of the cyclic loading. The pore water pressure of the fine tailings developed more quickly than that of the coarse tailings, indicating that liquefaction may occur much easier for fine tailings.

Seed et al. (1975) developed a model to predict the cyclic pore water pressure development for saturated sand at isotropic consolidation condition, in which the relationship between $u/{\sigma }_3$ and $N/N_f$ could be expressed as follows:where $\theta$ = empirical constant determined from laboratory testing.

Zhang et al. (2006) proposed a similar model to predict cyclic pore pressure for mine tailings based on Seed’s model expressed as

To verify the effectiveness of the two models, the pore water pressure responding curves for the four tailings under 200 kPa were chosen to compare with the model predictions as shown in Fig. 8. The model prediction varies dramatically with the change in $\theta$, and therefore different $\theta$ values were examined to identify the optimal model predictions. The development of pore water pressure under 200 kPa during CTX tests was almost the same for the two coarse tailings, and the model prediction from Seed et al. (1975) with $\theta$ of 0.9 matched well with the test results of the two coarse tailings. However, for the two fine tailings, the development of pore water pressure was different, and the model prediction from Zhang et al. (2006) showed better consistency with the test results than that from Seed et al. (1975). To predict the development of pore water pressure in the fine iron tailings and fine copper tailings, the optimal value of $\theta$ in the model of Zhang et al. (2006) should be 2.4 and 1.0, respectively.

The shear modulus ($G$) and damping ratio (${\lambda }_d$) of the coarse and fine tailings were investigated with the cyclic modulus test. The relationship between the normalized shear modulus ($G/{\sigma }_3$) and the shear strain (${\gamma }_d$) is plotted in Fig. 9. The normalized shear modulus decreased with an increase in the axial strain or shear strain because of the increase in pore water pressure during cyclic loading. The shear strength increased with an increase in consolidation pressure, whereas the normalized shear modulus was independent of the consolidation pressure. Compared to the iron tailings, the shear strength of the copper tailings was relatively smaller.

Fig. 10 shows that the damping ratios of the coarse and fine tailings are almost the same, both for the iron and copper tailings. The damping ratio increased with an increase in shear strain during the cyclic modulus test, and the copper tailings exhibited a larger damping ratio than the iron tailings. Comparison with the test results from Geremew and Yanful (2013) indicates that the damping ratios of the four tailings in this study are larger than that of copper-zinc tailings.