Experimental Study on the Effects of Internal Erosion on the Physical and Mechanical Properties of Tailings Under Unsteady Seepage

: In this paper, the hydraulic sedimentary model was established to investigate the effects of dry beach slope on the sedimentary characteristics of tailings, and the sand column model was built to investigate the effects of seepage erosion on the physical and mechanical properties of sedimentary tailings under unsteady seepage.The results show that the slope of dry beach have a great effect on the sedimentary characteristics of tailings, the average particle size of tailings decreases along the slope of dry beach, and the larger the slope, the more obvious the stratification of the tailings. The migration of fine-grained tailings caused by seepage erosion increases the permeability of the tailings and reduces the shear strength of the tailings. After seepage erosion,the average particle size of 1#tailings sample, 2#tailings sample and 3#tailings sample increased by 6.4%, 12.0% and 2.4% respectively, the hydraulic conductivity of 1# tailings sample, 2# tailings sample and 3# tailings increased by 27.2%,17.9%, and 15.3% respectively after internal erosion, and the shear strength of 1#tailings sample, 2#tailings sample and 3#tailings sample tailings sample decreased by 20.9 %, 15.1% and 12.4% respectively.


Introduction
As the third-largest mining country, China produces about 300 million tons of tailings every year. And most of the tailings were stored in more than 2,000 tailings reservoirs. However, more than 20% of these hydraulic structures are at risk of dam failure, which has been one of the most dangerous sources for the mining enterprises [1][2] . Serious tailings dam-failure accidents have happened all over the world. The United States Committee on Large Dams (USCOLD) evaluated the causes of dam-failure accidents, indicated that heavy rainfall was the critical factor that contributes to 30% of the dam-failure accidents [5][6][7] . when the heavy rainfall infiltrates into the tailings, the saturation line of dam will rise rapidly, the increased hydrostatic and dynamic water pressure will promote the migration of fine-grained tailings in the pore-channel, and further affect the physical and mechanical properties of tailings. Therefore, it is necessary to investigate the erosion mechanism and the effects of seepage erosion on the stability of tailings dams under heavy rainfall.
For the past few years, many researches have been conducted to analyze the influence of seepage erosion on geotechnical structure from theoretical analysis, physical model and numerical simulation. Upstream tailings dam is the most commonly used mode for dam construction of tailings reservoirs in China, its safety has aroused extensive attention due to the disadvantages of poor dam stability, high saturation line and poor seismic performance. Upstream tailings dam is more susceptible to produce seepage erosion under the hydrodynamic and hydrostatic pressure caused by heavy rainfall due to the distinctive deposition characteristics of tailings. Taking an upstream tailings dam in southern China as research object in this paper, the hydraulic sedimentary model was established to analyze the sedimentary characteristics of tailings on the dry beach, the unsteady seepage test was performed to investigate the effect of seepage erosion on the physical and mechanical properties of tailings. The research results play an important role in analyzing and predicting the effect of seepage erosion on the stability of tailings dam under heavy rainfall.

Sedimentary characteristics of tailings under different dry beach slopes
Seepage erosion of tailings dam is influenced by the gradation, spatial distribution and porosity of tailings on the seepage path. The tailings in upstream tailings dam show a distinctive deposition characteristics along the dry beach slope due to hydraulic sedimentation during the discharge process. The fine-grained tailings and coarse-grained tailings locate in the upstream and downstream of seepage field respectively, resulting in fine-grained tailings are easier to migrate to the downstream under the action of seepage force, as shown in Fig.1. Therefore, it is necessary to analyze sedimentary characteristics of the tailings before investigating seepage erosion of upstream tailings dam.  Table 1. The particle characteristics of the raw tailings sample were analyzed by Screening Test, and the particle size smaller than 0.075 mm was determined by Winner2000 Laser Particle Size Analyzer 18 .
The particle size distribution curve was plotted according to test results, as shown in Fig.2.  Table 2.

Experimental model
A physical model of tailings hydraulic sedimentation was established to simulate the tailings discharge process of the upstream tailings dam and to analyze the sedimentary characteristics of tailings on the dry beach. The main body of the model is a length of 500cm plastic groove with trapezoidal cross-section(30cm wide of the topline, 20cm wide of the baseline, and 10cm high) to simulate the dry beach of tailings dam. A height-adjustable cushion set at the upstream of the model to adjust the slope of the groove(1%-2%), which is consistent with the dry beach slope of the tailings dam. A plastics discharge pipe with an inner diameter of 0.3cm is installed at the bottom of the mixing tank for discharging the tailing slurry, as shown in Fig.3.

Fig.3 Schematic diagram of the physical model for the tailings hydraulic sedimentary test
According to the Froude criterion 19 , the model and prototype should have the same Froude number to meet the dynamic similarity conditions when the fluid flow is dominated by inertial force and gravitational force.The Froude number is a ratio of inertial and gravitational forces in fluid flow, Where, r F is the Froude number; g is gravity; v is water velocity; L is the length of fluid flow The flow of the tailings slurry on the dry beach is dominated by inertial and gravitational forces, so the Froude number of the model should be the same as that of the prototype, that is The similarity ratio of the hydraulic parameters between the model and the prototype was derived according to the Froude criterion, as shown in Table 3. The actual flow rate of tailings slurry at the discharge outlet of the tailings dam is measured by a flow meter, is1.72 m/s, as shown in Fig.4.

Fig.4. Flow rate of tailings at the discharge outlet
The hydraulic parameters for the model test were calculated according to the actual flow rate, as shown in Table 4

Experimental procedures
(1) The slope of the trapezoidal groove is maintained at 1 % by adjusting the height of the cushion.
(2) Put the tailings and water in the mixing tank at a mass ratio of 1 : 3. When the tailings slurry is uniformly stirred, drain it into the trapezoidal groove according to the design volume flow of the model until the stack height of upstream tailings exceeds 10cm.
(3) The samples are taken from the tailings at the downstream, midstream and upstream of the trapezoidal groove, which are numbered as 1 # tailing sample, 2 # tailing sample and 3 # tailing sample when the water is completely drained out of the groove,.
(4) Dry the tailings samples with a thermostatic drying chamber (105~110°C) for no less than 10h.
The particle size distribution curves of tailings samples are plotted according to the results of Screening Test and Laser Particle Size Analysis，and the effect of the slope of the Trapezoidal groove on the sedimentary characteristics of tailings are analyzed.
(5) Change the slope of the Trapezoidal groove to 1.5% and 2.0% by adjusting the height of the cushion, and repeat the procedures of step (2) to step (5).

Results and Discussion
The particle size distribution curves of tailings samples at different slopes of the trapezoidal groove were plotted according to test results, as shown in Fig.5.  Fig.5(a). We can see that the average particle size of upstream tailings(3# tailings) is 0.108mm, increased by 11.4% compared to 0.097mm of the raw tailings. And the particle size distribution curves of 2# tailings and 3# tailings have no significant change compared with that of the raw tailings, indicating that 1% slope is not conducive to the sedimentation of downstream tailings due to low horizontal kinetic energy of tailings slurry; when the slope is 1.5%, we can see from Fig.5 When the  value is greater than 1, the horizontal kinetic energy is greater than sedimentary energy of the tailings slurry, and the tailings are hard to deposit; when the  value is less than 1, the tailings are easy to deposit. According to the particle size distribution curves of the hydraulic sedimentary test, the tailings with a diameter smaller than 0.075 mm are hard to deposit, mainly located in the downstream of the model; the tailings with a particle size of 0.075~0.15mm are mid-grained tailings primarily located in the middle of the model, its particle size distribution along the model slope is most influenced by the hydrodynamic value; the tailings with a particle diameter larger than 0.15mm are quickly deposited on the downstream to form the tailings dam.
The average particle size distribution of sedimentary tailings along the slope of dry beach obeys the statistical law. According to the results of hydraulic sedimentary test, the average particle size D 50 as a linear function of the dry beach length L at the dry beach slope of 1.5% can be expressed by the Formula (5).
The relationship of average particle size D50 with the distance of sampling point from upstream L is shown in Fig.6.We can see that the test value of average particle size was consistent with that of on-site measured values, indating that the results of hydraulic sedimentary test can accurately reflect the sedimentary characteristics of dry beach tailings.

The effect of internal erosive on the physical and mechanical
properties of tailings under unsteady seepage

Critical condition of seepage erosion
The hydraulic conductivity K can be derived from Darcy's law due to sedimentary tailings have low hydraulic conductivity, as shown in Formula (6).

V -Volume of fine tailings
The seepage force of fine-grained tailings is F : The initiation of fine-grained tailings should overcome the static friction force between the particles due to the cohesion of saturated tailings is 0: Where, f -the static frictional resistance between the fine-grained tailings (N)  -internal friction angle of underwater fine-grained tailings (°) The critical equilibrium condition for fine-grained tailings initiation is f F  ,so the critical  (3) After the tailings sample is saturated, the outlet flowmeter is adjusted to control the water head gradient of water pressure gauges. When the readings of each water pressure gauges are stable, the flow Q and water head differences of each column are recorded, and the initial hydraulic conductivity of each sand column was calculated.

Experimental model
(4) Unsteady seepage under heavy rainfall was simulated in this test. Keep the water level of the water tank increasing at a rate of 0.1m/h for 24 hours, which is consistent with the rising rate of saturation line of the tailings dam under heavy rainfall in 50-years return period. The flow volume and the water head differences between the water pressure gauges were recorded over time, and the hydraulic conductivity K of each sand column was calculated.

The effect of internal erosion on the hydraulic conductivity of tailings samples
The effect of internal erosion on the hydraulic conductivity of each sand column under unsteady seepage as shown in Fig. 8.

The effect of internal erosion on the shear strength of tailings samples
The tailings samples taken from each sand column in situ for Direct Shear Test 21 after theinternal erosion. The control shear rate was 0.8 mm/min. The shear strength and internal friction angle of each tailings sample before and after internal erosion were shown in Table 5. As can be seen from respectively. It is indicated that seepage erosion changed the structure of tailings samples,and has the greatest influence on the shear strength and internal friction angle of upstream fine-grained tailings.

The effect of internal erosion on the particle size distribution curves of tailings samples
To further analyze the effect of seepage erosion on the physical properties of the tailings samples, the tailings samples after internal erosion were taken for Screening Test. And the comparison of particle-size distribution curves of tailings samples before and after internal erosion is shown in  As can be seen from Fig.9, the average particle size of 1#tailings sample, 2#tailings sample and 3#tailings sample in Column 1, Column 2 and Column 3 increased by 6.4%, 12.0% and 2.4% respectively due to the migration of the fine-grained tailings long the seepage direction from upstream to downstream. There are about 20% tailings with particle size smaller than 0.075mm, and about 10% tailings with the size of 0.075~0.1mm migrated to the downstream in the 1# tailings sample. The change of average particle size is not obvious, however the mass of tailings with the particle size below 0.1 mm increase by about 4% in 3# tailings sample.

Conclusions
Based on the above analysis and discussion, the following conclusions can be drawn: (1) The average particle size D50 of sedimentary tailings decreases along the slope of dry beach.
The slope of beach slope has an important effect on the sedimentary characteristics of tailings, the greater the slope, the more obvious the stratification deposition of tailings.
(2)The average particle size distribution along the groove slope in hydraulic sedimentary test was consistent with that of on-site measured values, indating that the test results can accurately reflect the sedimentary characteristics of dry beach tailings.
(3)The critical hydraulic gradient of 1# tailings sample, 2# tailings sample and 3# tailings sample are 0.32, 0.36, 0.41 respectively. When the hydraulic gradient of various tailings exceeds the critical value, the migration of the fine-grained tailings will result in seepage erosion.
(4)The migration of fine-grained tailings caused by seepage erosion increases the permeability of the tailings and reduces the shear strength of the tailings. After seepage erosion, the hydraulic conductivity of 1# tailings sample, 2# tailings sample and 3# tailings sample increase by 27.2%,17.9%, and 15.3% respectively, and the shear strength of 1#tailings sample, 2#tailings sample and 3#tailings sample tailings sample decreased by 20.9 %, 15.1% and 12.4% respectively.
(5)The average particle size of 1#tailings sample, 2#tailings sample and 3#tailings sample increased by 6.4%, 12.0% and 2.4% respectively due to the migration of the fine-grained tailings along the seepage direction. More than 20% fine-grained tailings in the 1#tailings sample was migrated to the downstream. Figure 1 Sedimentation and seepage diagram of upstream tailings dam Particle size distribution curve of raw tailings sample Flow rate of tailings at the discharge outlet Particle size distribution curves of sedimentary tailings samples at different slopes The relationship between average particle size D 50 and distance from upstream L Figure 7 Schematic diagram of unsteady seepage model Comparison of Particle-size distribution curves of tailings samples before and after seepaging