Overtopping Failure of a Reinforced Tailings Dam: Laboratory Investigation and Forecasting Model of Dam Failure

water

Article Overtopping Failure of a Reinforced Tailings Dam: Laboratory Investigation and Forecasting Model of Dam Failure

Xiaofei Jing 1, Yulong Chen 2,*, David J. Williams 3, Marcelo L. Serna 3 and Hengwei Zheng 4 1 School of Safety Engineering, Chongqing University of Science and Technology, Chongqing 401331, China; [email protected] 2 Department of Hydraulic Engineering, Tsinghua University, Beijing 100084, China 3 School of Civil Engineering, The University of Queensland, Brisbane, QLD 4072, Australia; [email protected] (D.J.W.); [email protected] (M.L.S.) 4 School of Mathematics and Physics, Chongqing University of Science and Technology, Chongqing 401331, China; [email protected] * Correspondence: [email protected]

Received: 29 December 2018; Accepted: 8 February 2019; Published: 12 February 2019

Abstract: Overtopping failure of reinforced tailings dam may cause significant damage to the environment and even loss of life. In order to investigate the feature of overtopping of the reinforced tailings dam, which has rarely appeared in the literature, the displacement, the phreatic level and the internal stress of dam during overtopping were measured by a series of physical model tests. This study conclusively showed that, as the number of reinforcement layers increased, the anti-erosion capacity of tailings dam was notably improved. It could be supported by the change of the dimension of dam breach, the reduction of stress loss rate, and the rise of phreatic level from the tests. Based on the erosion principle, a mathematical model was proposed to predict the width of the tailings dam breach, considering the number of reinforcement layers. This research provided a framework for the exploration of the overtopping erosion of reinforced tailings dam, and all presented expressions could be applied to predict the development of breach during overtopping.

Keywords: tailings dam; overtopping test; reinforcement; dam breach; erosion evolves

1. Introduction Tailings, which are generally stored in a slurry form and then pumped into a sedimentation pond, are waste products of the ore-dressing process [1]. Currently, tighter legislation and regulations on tailings disposal are forcing mine operators to play a more prominent role in controlling the disposal of vast quantities of highly visible wastes [2]. Over the last few decades, there were several failures of tailings impoundments due to various reasons, including liquefaction of the retained tailings, overtopping erosion of the dam, or an excessive water seepage that would produce breach [3–7]. However, ﬂood overtopping was recognized as the major cause of tailings dam failure [8]. Overtopping had been of particular interest in the last few decades due to the protection from potential environmental damages and the loss of property [9–13]. However, actual measurements of overtopping erosion is a costly and time-consuming process that is usually limited to small experimental tests [14–18]. The main influence of overtopping on dam is the erosion by water, so the characteristic of soil erosion is very important to study and many people focused on the soil erosion in recent years. A great many of the theories and methods of erosion have been studied and lots of achievements and experience were obtained in many fields. In the coast erosion, Mulder et al. [19], Van Rijn et al. [20] and Giardino et al. [21] studied the characteristics of coastal erosion and proposed some effective

Water 2019, 11, 315; doi:10.3390/w11020315 www.mdpi.com/journal/water Water 2019, 11, 315 2 of 16 methods to control it. In the research on erosion of earth dams, the characteristics of dam erosion by water was researched by Carlsten et al. [22], Ran et al. [23] and Wu et al. [24]. Callaghan et al. [25], Serpa et al. [26] and Stefanidis et al. [27] studied the effect of climate change on soil erosion in a mountainous Mediterranean catchment to assess soil erosion changes. Simonneaux et al. [28] and Teng et al. [29] also studied the climate change effects on soil erosion in a semi-arid mountainous watershed and the assessment of soil erosion by water on the Tibetan Plateau based on the Revised Universal Soil Equation (RUSLE) and the Coupled Model Intercomparison Project (CMIP5) climate models, respectively. Meanwhile, Wang et al. [30] studied the response of the meltwater erosion to runoff energy consumption on loessal slopes, and Sujatha et al. [31] used the RUSLE to predict the erosion risk of a small mountainous watershed in Kodaikanal, South India. Deng et al. [32] presented a coupled model for simulating bed deformation and bank erosion that focuses on the erosion of the bank with a composite structure in the Lower Jingjiang Reach. Choi et al. [33] proposed a combined erosion framework; the framework adopts a dam-breach erosion model that can capture the progressive nature of fluvial erosion by considering the particle size distribution of the soil being eroded. The findings from the field investigation were then used to back-analyze fluvial erosion along the riverbank. All the aforementioned research presented a great many theories and methods of erosion that can provide a good reference for the study of overtopping failure of the tailings dam. In terms of overtopping failure research of the tailings dam, to analyze the tailings dam failure evolution pattern, Zhang et al. [34] studied a tailings dam overtopping evolution model based on a physical model experiment method according to the model similarity theory and dam breach erosion model. Sun et al. [8] found out that the displacement of dam was dependent on the degree of dam saturation during overtopping erosion. Sliding displacement was increased as the saturation line rose. Liu et al. [35] proposed a physical-based model to analyze the flood process, integrating the theory of non-equilibrium sediment transposition and a conceptual model of the process of lateral erosion. However, the above research results made a great contribution to study tailings dam overtopping evolution, but the dam was built without reinforcement. The geosynthetics have been applied extensively for strengthening and/or increasing the height of tailings dam built from low-shear strength fine-grain tailings for decades [36–38]. Yin et al. [39] studied the characteristics of interaction between geosynthetics (geobelt and geo-grid) and fine copper ore tailings through laboratory pull-out tests, and the results revealed that the interaction was influenced by tailings particle size, density, moisture content, and vertical load. James et al. [40] found out that reinforcement could improve the seismic stability of tailings impoundments, which could also help dissipate porewater pressures after shaking. Liu et al. [41] focused the variation in properties of composite materials after immersion to evaluate the shear behaviors of polymer–sand composite material after immersion with direct shear tests. The shear behaviors were improved with an increment in the curing time, polymer content and sand dry density while there is a decrease in the shear behaviors with increasing immersion time. Consoli et al. [42] studied the mechanical properties of gold tailings with the insertion of fibers. The research advanced in understanding the parameters that control durability and strength of fiber-reinforced compacted gold tailings. Although overtopping and reinforcement of tailings dam have been studied separately over the past few years, rather less attention had been paid to the effect of reinforcement on the overtopping process. Since geosynthetics have been extensively used in tailings dam projects, the feature of overtopping erosion considering the influence of reinforcement, which has rarely appeared in the literature, is worth investigating. It will help to provide theoretical framework for the protection of dam failure. This study aimed to analyze the feature of overtopping erosion of the tailings dam in terms of the number of reinforcement layers. It is achieved by the use a self-designed Tailings Dam Overtopping Failure Model Test System, which consisted of a glass flume, an observation system for phreatic line, and a rainfall system. In addition, a mathematical model for overtopping failure prediction considering the reinforcement layers was proposed based on the river dynamics theory and the erosion principle. This model could be used for predicting the width of breach caused by erosion and the discharge of water through the breach. It will provide data for the risk assessment of reinforced tailings dam overtopping. Water 2019, 11, 315 3 of 16

2. Experimental

2.1. Experimental Facilities Water 2019, 11, 315 3 of 16 The experimental facilities consist of a glass flume, an observation system for phreatic line, and artificial rainfall simulator (Figure1). Experiment tracer points and equipment layout are shown the reinforcement layers was proposed based on the river dynamics theory and the erosion principle. in Figure2. This model could be used for predicting the width of breach caused by erosion and the discharge of The glass flume had an internal dimension of 200 cm (length) by 60 cm (width) by 50 cm (height). water through the breach. It will provide data for the risk assessment of reinforced tailings dam The inner wall of flume was lubricated by the Vaseline in order to reduce the friction. The process of overtopping. flooding and the change of the breach could be monitored using a high resolution camera (Camera model: Sony/HDR-CX680 (SONY, Beijing, China), Camera resolution: 1920×1080/50p). There were 2. Experimental three U-type tubes that were evenly spaced at 30 cm monitoring the phreatic line (JR-01, JR-02, JR-03), 2.1. withExperimental a diameter Facilities of 20 mm and height of 50 cm (Figure2). Each U-type tube was divided into two segments: the one in the dam was made of perforated Polyvinyl Chlorid (PVC) and the other that was outsideThe experimental the dam was facilities made of consist transparent of a glass acrylic. flume, Geo-grid an observation was used system as the reinforcing for phreatic materials line, and of artificialthe model rainfall test simulator [43], and the(Figure basic 1). properties Experiment were tracer shown points in Table and1. equipment layout are shown in Figure 2.

Figure 1. Tailings dam overtopping failure model test system. Figure 1. Tailings dam overtopping failure model test system. WaterWater 20192019, 11, 11, 315, 315 44 ofof 16 16

FigureFigure 2. 2. MonitoringMonitoring distribution distribution of of tailings tailings dam dam failure failure experiment experiment.. ( (aa)) Side Side view; view; ( (bb)) Top Top view.

The glass flume had an internalTable dimension 1. Basic of property 200 cm of (length) geo-grid. by 60 cm (width) by 50 cm (height). The inner wall of flume was lubricated by the Vaseline in order to reduce the friction. The process of Grid Size (mm) Tensile Strength (kN·m−1) flooding Typeand the change of the breach could be monitored using a highElongation resolution at Breakcamera (%) (Camera model: Sony/HDRVertical-CX680 (SONY Horizontal, Beijing Longitudinal, China), Camera Transverse resolution: 1920×1080/50p). There were three U-typeJT1101 tubes that 25 were evenly 25 spaced at 30 6.5 cm monitoring 6.5 the phreatic line 12.7(JR-01, JR-02, JR-03), with a diameter of 20 mm and height of 50 cm (Figure 2). Each U-type tube was divided into two segments: the one in the dam was made of perforated Polyvinyl Chlorid (PVC) and the other that was In order to measure the deformation of the dam, tracer points were located at the crest, the slope outside the dam was made of transparent acrylic. Geo-grid was used as the reinforcing materials of the and the toe of the model tailings dam (Figure2). Two soil pressure sensors (BX-202, BX-203, Dandong model test [43], and the basic properties were shown in Table 1. Electronic Instrument Factory, Dandong, China) were installed in the middle of the dam slope at a depth of 20 cm to measure the stress of theTable tailings 1. Bas inic property the dam, of and geo two-grid other. soil pressure sensors (BX-201, BX-204) were installed 20 cm horizontally away from BX-202 and BX-203. BX-201 and BX-203 were Grid Size (mm) Tensile Strength (kN·m−1) placedType horizontally while BX-202 and BX-204 were placed vertically, in orderElongation to measure at Break the vertical (%) and horizontal stresses,Vertical respectively Horizontal (Figure 2Longitudinal). The soil pressure Transverse sensor had a dimension of Φ12 × 4.2 mm, and theJT measuring1101 25 range is from25 0 to 50 kPa. The6.5 model of dynamic6.5 strain tester is12.7 TST3828EW (Jiangsu Test Electronic Equipment Manufacturing Co., Ltd., Jingjiang, China), the maximum sampling frequency In order to measure the deformation of the dam, tracer points were located at the crest, the slope is 1 kHz, and the A/D (Analog to Digital) conversion resolution of dynamic strain tester is 24 bits. and the toe of the model tailings dam (Figure 2). Two soil pressure sensors (BX-202, BX-203, Dandong All of these equipment were calibrated before their placement in the tailings dam. The model dam was Electronic Instrument Factory, Dandong, China) were installed in the middle of the dam slope at a a 1.4 m × 0.6 m × 0.4 m upstream dam with an upstream slope ratio of 1:6 and a downstream slope depth of 20 cm to measure the stress of the tailings in the dam, and two other soil pressure sensors ratio of 1:2. (BX-201, BX-204) were installed 20 cm horizontally away from BX-202 and BX-203. BX-201 and BX-203 A series of dam overtopping tests were carried out with different reinforcement layers were placed horizontally while BX-202 and BX-204 were placed vertically, in order to measure the (reinforcement layers number N = 0, 1, 2, 3, 4; see Figure3). vertical and horizontal stresses, respectively (Figure 2). The soil pressure sensor had a dimension of Φ12×4.2 mm, and the measuring range is from 0 to 50 kPa. The model of dynamic strain tester is TST3828EW (Jiangsu Test Electronic Equipment Manufacturing Co., Ltd., Jingjiang, China), the maximum sampling frequency is 1 kHz, and the A/D (Analog to Digital) conversion resolution of dynamic strain tester is 24 bits. All of these equipment were calibrated before their placement in the tailings dam. The model dam was a 1.4 m × 0.6 m × 0.4 m upstream dam with an upstream slope ratio of 1:6 and a downstream slope ratio of 1:2. A series of dam overtopping tests were carried out with different reinforcement layers (reinforcement layers number N = 0, 1, 2, 3, 4; see Figure 3). Water 2019, 11, 315 5 of 16 Water 2019, 11, 315 5 of 16

(a) N = 0 (b) N = 1 1: 6 1: 2 1: 6 1: 2

0 cm 0 Tailings 40 cm40 Water 20194 , 11, 315 Tailings dam 20 cm 5 of 16 dam

(a) N = 0 (b) N = 1 1: 6 1: 2 (c) N = 2 1: 6 1: 2 (d) N = 3 1: 6 1: 2 1: 6 1: 2

m 10 cm

0 cm 0 13 cm Tailings 40 cm40 4 TailingsTailings dam Tailings 1020 cm cm 40 cm 40 40 c 40 13 cm dam dam dam 10 cm

m 10 cm 8 13cm cm Tailings 8 cm Tailings 10 cm 40 cm 40 40 c 40 Tailings 40 cm dam 138 cm cm dam 10 cm dam 8 cm

Figure 3. TheThe schematic diagram of geo geo-grid-grid laying. (a) N = 0; ( b) N = 1; 1; ( (cc)) NN == 2; 2; ( (dd)) NN == 3; 3; (e (e)) NN == 4. 4. (e) N = 4 1: 6 2.2. Experimental Materials1: 2 and Procedures 2.2. Experimental Materials8 cm and Procedures 8 cm Tailings

The40 cm copper tailings8 utilized cm in the all tests were extracted from a tailings disposal site located in The copperdam tailings utilized in the all tests were extracted from a tailings disposal site located in Yunnan Province, China, being8 cm classified as medium sand approximately (code for design of tailings Yunnan Province, China, being classified as medium sand approximately (code for design of tailings facilities. The particle mass percent with grain size greater than 0.25 mm is more than 50%; the tailings facilitiesFigure. The 3. particle The schematic mass percent diagram with of geo grain-grid sizelaying. greater (a) N =than 0; (b 0.25) N = mm1; (c) is N more= 2; (d than) N = 3;50% (e); Nthe = 4. tailings are classified as medium sand.) The copper tailings particle size distribution was shown in Figure4, are classified as medium sand.) The copper tailings particle size distribution was shown in Figure 4, and the characterization test by Standard for soil test method and results were presented in Table2. 2.2.and Experimentalthe characterization Materials test and by Procedures Standard for soil test method and results were presented in Table 2. According to the similarity theory, the tailings dam model should be built first, and all the sensors andThe geo-grid copper should tailings be utilized installed in completely. the all tests Then, were through extracted the from rainfall a tailings system todisposal pump watersite located into the in 100 Yunnanflood control Province, area China, until the being beach classified length reachedas medium 10 cm sand (as wellapproximately as local beach (code length for 40design m , the of rainfalltailings facilitiesintensity. The was particle 80 mm/day, mass percent which waswith as grain well size as the greater local ofthan Yunnan 0.25 mm Province, is more China. than 50% While; the the tailings water arehad classified saturated as themedium beach80 sand. and the) The dam, copper the current tailings phreatic particle level size was distribution measured was until shown the phreatic in Figure level 4, andwas the stabilized. characterization Finally, ittest rained by Standard again until for the soil tailings test method dam model and results flood overtoppingwere presented failure in Table happened. 2. 60 100 40 80 Percent Percent Finer (%) 20 60 0 400.001 0.01 0.1 1 10 Particle Size (mm)

Percent Percent Finer (%) 20 Figure 4. Grain size distribution curve of the copper tailings sample. 0 Table 2. Properties of the copper tailings sample. 0.001 0.01 0.1 1 10 IndexParticle Size (mm) Values Specific gravity 2.10 Figure 4. Grain size distribution curve of the copper tailings sample. Figure 4. Grain sizeMoisture distribution content curve (% of) the copper tailings12.5 sample. Porosity 0.024 TableModulus 2. Propert of compressionies of the copper (MPa tailings) sample.17.6 −1 Coefficient of compressibilityIndex (MPa ) V0.058alues −6 −1 PermeabilitySpecific coefficient gravity (×10 m·s ) 2.5.8610 Moisture content (%) 12.5 Porosity 0.024 Modulus of compression (MPa) 17.6 Coefficient of compressibility (MPa−1) 0.058 Permeability coefficient (×10−6 m·s−1) 5.86 Water 2019, 11, 315 6 of 16

Water 2019, 11, 315 Table 2. Properties of the copper tailings sample. 6 of 16

Index Values CohesionSpecific gravity (kPa) 7.52 2.10 InternalMoisture friction content angle (%) (Φ/°) 31.2 12.5 Porosity 0.024 According to the similarity Modulustheory, the of compressiontailings dam (MPa) model should 17.6 be built first, and all the sensors and geo-grid should be installedCoefficient complete of compressibilityly. Then, through (MPa the−1) rainfall0.058 system to pump water into the −6 −1 flood control area until the beachPermeability length coefficientreached 1 (0× c10m (asm well·s ) as local5.86 beach length 40 m , the rainfall Cohesion (kPa) 7.52 intensity was 80 mm/day, which was as well as the local of Yunnan Province, China. While the water Internal friction angle (Φ/◦) 31.2 had saturated the beach and the dam, the current phreatic level was measured until the phreatic level was stabilized. Finally, it rained again until the tailings dam model flood overtopping failure happenedThe. dam displacements, internal stress of the dam, phreatic level and erosion of the dam were recordedThe dam by thedisplacements, sensors introduced internal above. stress of the dam, phreatic level and erosion of the dam were recordedAll by the the detail sensors procedures introduced of tailings above. dam construction were listed as below: 1. AllThe the tailings detail procedures with 10% water of tailings content dam were construction prepared according were listed to theas below: test requirements (as shown 1. Thein tailings Figure5 a).with 10% water content were prepared according to the test requirements (as shown 2. inLayer Figure compaction 5a). method for construction tailings dam, each layer was 8 cm, the compactness of 2. Layereach compaction layer is 85% method (as shown for inconstruction Figure5b). tailings dam, each layer was 8 cm, the compactness of 3. eachLay layer down is the85% reinforcement (as shown in andFigure sensors 5b). (as shown in Figure5c). 3.4. LayThe down displacement the reinforcement monitoring and mark sensors were (as placed shown on in the Figure side of5c). the dam every 10 cm according to 4. Thethe displacement drawn grid lines monitor (as showning mark in Figurewere placed5d). on the side of the dam every 10 cm according to 5. theInstall drawn the grid camera, lines and(as shown calibrate in Figure these equipment 5d). after their placement in the tailings dam. 5. Install the camera, and calibrate these equipment after their placement in the tailings dam.

(a) (b) (c) (d)

FigureFigure 5. 5. TheThe procedures procedures of tailings of tailings dam construction. dam construction. (a) Tailings (a) sample;Tailings (b )sample tailings; dam(b) construction; tailings dam construction(c) tailings; dam (c) tailings construction; dam construction (d) displacement; (d) displacement monitor. monitor.

3.3. Results Results and and Discussion Discussion

3.1.3.1. Reinforcement Reinforcement Density Density on on Dam Dam Displacement Displacement ToTo analy analyzeze the the displacement displacement of ofthe the tailings tailings dam dam during during overtoppin overtopping,g, the the displacement displacement of the of thekey tracerkey tracer points points W1, W1, W8, W8, W10 W10 were were recorded recorded throughout throughout the the test test.. F Foror the the number of of different different reinforcementreinforcement layers layers (N (N == 0, 0, 1, 1, 2, 2, 3, 3, 4), 4), the final ﬁnal vertical andand horizontalhorizontal displacementdisplacement werewereplotted plotted in in FigFigureure 66. . Water 2019, 11, 315 7 of 16 Water 2019, 11, 315 7 of 16

2.1 -2.6

displacement(cm) Vertical (a) 2.0 -2.8

1.9 -3.0 1.8

-3.2 1.7 W1-Horizontal W1-Vertical

Horizontal displacement (cm) 1.6 -3.4 0 1 2 3 4 Reinforcement layers -1.6

(cm) displacement Vertical 3.2 (b) -1.7

3.0 -1.8

-1.9 2.8 -2.0 2.6 W8-Horizontal -2.1 W8-Vertical

Horizontal displacementHorizontal (cm) 2.4 -2.2 0 1 2 3 4 Reinforcement layers 2.6 -1.6 (c) displacement (cm) Vertical 2.4 -2.0 2.2

2.0 -2.4

1.8 W10-Horizontal -2.8 1.6 W10-Vertical

Horizontal displacement (cm) Horizontal displacement 1.4 -3.2 0 1 2 3 4 Reinforcement layers FigureFigure 6. TheThe ﬁnalfinal vertical vertical and and horizontal horizontal displacement displacement of tracer of pointstracer duringpoints overtopping during overtopping for different for differentreinforcement reinforcement layers. (a ) layers Displacement. (a) Displacement of point W1; of (b ) point Displacement W1; (b) of Displacement point W8; (c) Displacement of point W8; of (c) Displacementpoint W10. of point W10.

3.2.It Reinforcement was shown Layersthat, as and the Flooding number Time of reinforcemen on the Phreatict layer Levels increased, the final displacement of W1, W8, andAs theW10 rainfall reduced continued, significantly. the height The ofmain the phreaticreason was level that in the reinforcement observation tubes layers was could recorded combine at tailingsa time intervaltogether of and 4 min. enhance In order the to analyze strength the of inﬂuence tailings damof reinforcement. With the increase layers on of the the height number of the of reinforcementphreatic level in(geo the-grid) observation layers, tubes, the distance the data between of JR-01, geo JR-02-grid and layers JR-03 decrease were collected.d and the The interaction phreatic betweenlevel monitor two adjacent tube JR-02 layers was increase located neard considerably the crest of, thewhich dam, effectively which was restrain the mosted the representative deformation to of tailingreﬂects thedam. tailings dam phreatic level changes during overtopping. The change of phreatic level from JR-02 was shown in Figure7. The phreatic level rose as rainfall 3.2.continued, Reinforcement but the Layers rate ofand rise Flooding was slowing Time on down the Phreati withc the Level increased number of reinforcement layers. Water 2019, 11, 315 8 of 16

As the rainfall continued, the height of the phreatic level in the observation tubes was recorded at a time interval of 4 min. In order to analyze the influence of reinforcement layers on the height of the phreatic level in the observation tubes, the data of JR-01, JR-02 and JR-03 were collected. The phreatic level monitor tube JR-02 was located near the crest of the dam, which was the most representative to reflectWater 2019 the, 11tailings, 315 dam phreatic level changes during overtopping. 8 of 16 The change of phreatic level from JR-02 was shown in Figure 7. The phreatic level rose as rainfall continued, but the rate of rise was slowing down with the increased number of reinforcement layers. ComparingComparing to to the the dam dam without without reinforcement, reinforcement, the the peak of thethe phreaticphreatic level level of of the the dam dam with with four four reinforcementreinforcement layers layers delayed delayed about about 36 36 min. The The main reason waswas thatthat the the ﬁne fine tailings’ tailings’ particles particles attachattacheded to to the the geotextile geotextile and and form formeded a awater water impermeable impermeable layer layer in in the the dam, dam, which which impeded impeded the the rise rise of phreaticof phreatic level level..

50 Reach the peak level Overtopping happened 86 min 122 min 139 min 40 Test finished 150 min 30

20 NN=0 = 0 NN=1 = 1 NN=2 = 2 10 NN=3 = 3 N=4 Phreatic level (cm)Phreatic level N = 4

0 60 80 100 120 140 160 Time (min) Figure 7.7. Phreatic levellevel of JR-02JR-02 during test.test.

TailingsTailings dam dam overtopping overtopping occurred occurred at at 139 139 min, min, and and the the phreatic phreatic level level decline declinedd in in the the following following 11 min11 minuntil until the thewater water flowed flowed out outcompletely. completely. Figure Figure 8 display8 displayeded the thefinal final phreatic phreatic level level of tailings of tailings dam withdam different with different reinforcement reinforcement layers layerswhen overtopping when overtopping was ended. was ended.It indicated It indicated that the that final the phreatic final levelphreatic was level higher was ashigher the as number the number of reinforcement of reinforcement layer layer increased. increased. The The main main reason waswas that that ddeformationeformation during during overtopping overtopping would would reduce reduce the height of of tailings tailings dam dam while while the the reinforcement reinforcement layerslayers effectively effectively restrained restrained the the deformation. deformation. Hence, Hence, the morethe more the reinforcements the reinforcements there were, there the were, higher the Water 2019, 11, 315 9 of 16 higherthe dam the was, dam which was, which resulted resulted in the in ascent the ascen of thet of final the phreatic final phreatic level. level.

25 JR-01 JR-02 20 JR-03

10 Phreaticlevel (cm)

5 0 1 2 3 4 5 Reinforcement layers Figure 8. Final phreatic level at the end of time.

3.3. Stress Change during Overtopping The internal stress of dam during overtopping was obtained by the sensors throughout the tests. When the phreatic level reached the peak, the internal stress of dam was measured as peak stress (SP). When the test finished, the internal stress of dam was measured as final stress (SF), and the loss rate of stress (SL) was calculated by Equation (1):

SL  (SF  SP)/ SP100% . (1)

As shown in Figure 9, with the increase of the reinforcement layers, the SL of tailings dam gradually decreased. From the reading of sensor BX-201, the SP and SF (Figure 9a) without reinforcement were 0.73 kPa cm and 0.23 kPa, and the SL was 68.9%; the SP and SF with four reinforcement layers were 0.73 kPa cm and 0.57 kPa, and the SL was only 21.3%. Meanwhile, the data from sensor BX-202 suggested that the SP and SF (Figure 9b) without reinforcement were 0.43 kPa cm and 0.11 kPa, and the SL was 72.1%; the SP and SF with four reinforcement layers were 0.44 kPa cm and 0.37 kPa and the SL was only 16.8%.

1.0 0.80 0.73 0.74 0.74 0.73 0.8 0.58 0.57 0.6 0.43 0.53

0.4 0.23 0.2 0.0 0 1 2 3 4 -0.2

Stress (kPa) 22.6% 21.3% -0.4 33.7% -0.6 42.1%

-0.8 68.9% Reinforcement layers （(N)N） Peak stress （(SP)SP） (a) Final stress （(SF)SF） Loss rates of stress（ (SL)SL）

Water 2019, 11, 315 9 of 16

25 JR-01 JR-02 20 JR-03

10 Phreaticlevel (cm)

5 0 1 2 3 4 5 Water 2019, 11, 315 Reinforcement layers 9 of 16 Figure 8. Final phreatic level at the end of time. 3.3. Stress Change during Overtopping 3.3. Stress Change during Overtopping The internal stress of dam during overtopping was obtained by the sensors throughout the tests. WhenThe the internal phreatic stress level of reached dam during the peak, overtopping the internal was stress obtained of dam by was the measured sensors throughout as peak stress the( SPtest).s. WWhenhen the the phreatic test ﬁnished, level thereached internal the stress peak, of the dam internal was measured stress of dam as ﬁnal was stress measured (SF), and as peak the loss stress rate (SP of ). Whenstress the (SL )test was finis calculatedhed, the byinternal Equation stress (1): of dam was measured as final stress (SF), and the loss rate of stress (SL) was calculated by Equation (1): SL = (SF − SP)/SP × 100%. (1) SL  (SF  SP)/ SP100% . (1)

AsAs shown in in FigureFigure9, with9, with the increasethe increase of the of reinforcement the reinforcement layers, the layersSL of, the tailings SL of dam tailings gradually dam grdecreased.adually decreas From theed. readingFrom ofthe sensor reading BX-201, of sensor the SP and BX-201,SF (Figure the SP9a) and without SF reinforcement(Figure 9a) without were reinforcement0.73 kPa cm and were 0.23 0.7 kPa,3 kPa and cm the andSL was 0.23 68.9%; kPa, theandSP theand SLSF waswith 68 four.9%; reinforcement the SP and SF layers with were four reinforcement0.73 kPa cm and layers 0.57 kPa,were and 0.73 the kPaSL cmwas and only 0.5 21.3%.7 kPa Meanwhile,, and the SL the was data only from 21.3% sensor. Meanwhile, BX-202 suggested the data fromthat sensor the SP BXand-202SF suggested(Figure9b) that without the SP reinforcement and SF (Figure were 9b) 0.43without kPa reinforcement cm and 0.11 kPa, were and 0.43 the kPaSL cm andwas 0. 72.1%;11 kPa the, andSP theand SLSF waswith 72.1 four%; reinforcementthe SP and SF layerswith four were reinforcement 0.44 kPa cm and layers 0.37 were kPa and0.44 thekPaSL cmwas and 0.only37 kPa 16.8%. and the SL was only 16.8%.

1.0 0.80 0.73 0.74 0.74 0.73 0.8 0.58 0.57 0.6 0.43 0.53

0.4 0.23 0.2 0.0 0 1 2 3 4 -0.2

Stress (kPa) 22.6% 21.3% -0.4 33.7% -0.6 42.1%

-0.8 68.9% Reinforcement layers （(N)N） Peak stress （(SP)SP） Water 2019, 11, 315 (a) Final stress （(SF)SF） 10 of 16 Loss rates of stress（ (SL)SL）

0.6 0.43 0.43 0.45 0.43 0.44 0.37 0.4 0.22 0.27 0.33 0.2 0.11

0.0 0 1 2 3 4 -0.2 16.8% 19.7% -0.4 Stress (kPa) 38.7% -0.6 46.5%

-0.8 72.1% Reinforcement layers（ (N)N） (b) Peak stress （(SP)SP） Final stress （(SF)SF） Loss rates of stress （(SL)SL）

FigureFigure 9. 9. TheThe characteristics characteristics of of dam dam internal stress, ( (a)) stressstress of of sensor sensor BX-201; BX-201; ( b (b)) stress stress of of sensor sensor BXBX-202,-202, SL =( (SFSF−SPSP))//SPSP×100100%.% .

In view of the mechanics analysis, the flood overtopping carried away a large number of tailings sands during the overtopping process, which led to a significant reduction in the internal stress. However, the erosion of adjacent tailings was impeded by reinforcement layers to a certain degree. As the number of the reinforcement layers increased, the erosion of tailings was gradually reduced.

3.4. Analysis on Evolution of the Overtopping Failure Process The failure process of the tailings dam varies significantly with different numbers of reinforcement layers, which directly affected the shape of the breach and the discharge of tailings. The final breach shape of tailings dam for different reinforcement layers was shown in Figure 10.

Reinforcement

(a) Before test (b) N = 0 (c) N = 1

Water 2019, 11, 315 10 of 16

0.6 0.43 0.43 0.45 0.43 0.44 0.37 0.4 0.22 0.27 0.33 0.2 0.11

0.0 0 1 2 3 4 -0.2 16.8% 19.7% -0.4 Stress (kPa) 38.7% -0.6 46.5%

-0.8 72.1% Reinforcement layers（ (N)N） (b) Peak stress （(SP)SP） Final stress （(SF)SF） Loss rates of stress （(SL)SL）

Figure 9. The characteristics of dam internal stress, (a) stress of sensor BX-201; (b) stress of sensor BX-202, SL  (SF  SP) / SP100% .

Water 2019, 11, 315 10 of 16 In view of the mechanics analysis, the flood overtopping carried away a large number of tailings sands during the overtopping process, which led to a significant reduction in the internal stress. However,In view the oferosion the mechanics of adjacent analysis, tailings the was ﬂood impeded overtopping by reinforcement carried away layers a large to a number certain ofdegree. tailings As thesands number during of the the reinforcement overtopping layer process,s increased, which led the toerosion a signiﬁcant of tailing reductions was gradually in the internalreduced stress.. However, the erosion of adjacent tailings was impeded by reinforcement layers to a certain degree. 3.4.As Analysis the number on Evolution of the reinforcement of the Overtopping layers Failure increased, Process the erosion of tailings was gradually reduced.

3.4.T Analysishe failure on Evolution process of of the the Overtopping tailings Failure dam Process varies significantly with different numbers of reinforcement layers, which directly affected the shape of the breach and the discharge of tailings. The final breachThe failure shape process of tailings of the dam tailings for different dam varies reinforcement signiﬁcantly layers with different was shown numbers in Fig ofure reinforcement 10. layers, which directly affected the shape of the breach and the discharge of tailings. The ﬁnal breach shape of tailings dam for different reinforcement layers was shown in Figure 10. Reinforcement

Water 2019, 11, 315 (a) Before test (b) N = 0 (c) N = 1 11 of 16

Reinforcement Reinforcement Reinforcement

(d) N = 2 (e) N = 3 (f) N = 4

FigureFigure 10. 10. TheThe breach breach shape shape forfor different different reinforcement reinforcement layers layers.. (a) Before (a) Before test; test;(b) N ( b=) 0;N (c=) N 0; = (c 1;) N (d=) N 1; = 2;( d(e))N N == 2;3; ((ef) N = 4. 3; (f) N = 4.

AsAs shown shown in in Fig Figureure 10 10, ,it it was was found found that that more reinforcement layerslayers willwill leadlead toto a a relatively relatively less less damagedamage to to the the tailings tailings dam dam during during overtopping. For For the unreinforced tailings tailings dam dam ((NN == 0),0),a a large large amountamount of of tailings tailings on on the the top top were were washed washed away away,, formingforming aa deepdeep breach.breach. ThisThis erosionerosion process process would would causecause the the shore shore of of the the breach breach to to become instable instable,, which could result inin collapsecollapse ofof bothboth sidessides of of the the breachbreach.. Since Since the the debris debris of ofthe the dam dam accumulated accumulated at atthe the toe toe and and blocked blocked the the erosion, erosion, the the bottom bottom of ofthe damthe damwas not was greatly not greatly affected. affected. The breach The breach was waseventually eventually shaped shaped as an as hourglass an hourglass (see (see Figure Figure 10b 10). Fb).or theFor case the case with with four four layers layers of reinforcement of reinforcement (N (=N 4=), 4), both both sides sides of of the the dam dam breach breach did notnot showshow an an obviousobvious trench trench collapse collapse.. The The main main reason reason was was that that reinforcement hadhad aa lockinglocking effecteffect onon the the tailings tailings particle,particle, so so the the erosion erosion of of adjacent adjacent tailings tailings was was impeded impeded by by reinforcement. reinforcement. The The large large-scale-scale collapse collapse did not occur at the main body of the dam, finally showing a clear ladder-shaped slope, as shown in Figure 10f. Figure 11 shows that plots of width and depth of the breach versus time for different reinforcement layers. It was found that the maximum width and depth of the breach for the unreinforced tailings dam was about 43 cm and 22 cm, respectively. At the end of the overtopping test, the maximum width and depth had decreased to 8 cm and 14 cm for two layers of reinforcement, and 7 cm and 18 cm for four layers of reinforcement. In addition, it was noticed that, when the reinforcement was more than two layers, the further improvement was not significant. This result showed that the optimum number was two layers of reinforcement.

50 NN=0 = 0 NN=1 = 1 (a) 40 NN=2 = 2 NN=3 = 3 N=4 30 N = 4

10 The breach width (cm) 0 0 100 200 300 400 Time (s) Water 2019, 11, 315 11 of 16 Reinforcement Reinforcement Reinforcement

(d) N = 2 (e) N = 3 (f) N = 4

Figure 10. The breach shape for different reinforcement layers. (a) Before test; (b) N = 0; (c) N = 1; (d) N = 2; (e) N = 3; (f) N = 4.

As shown in Figure 10, it was found that more reinforcement layers will lead to a relatively less damage to the tailings dam during overtopping. For the unreinforced tailings dam (N = 0), a large amount of tailings on the top were washed away, forming a deep breach. This erosion process would cause the shore of the breach to become instable, which could result in collapse of both sides of the breach. Since the debris of the dam accumulated at the toe and blocked the erosion, the bottom of the dam was not greatly affected. The breach was eventually shaped as an hourglass (see Figure 10b). For the case with four layers of reinforcement (N = 4), both sides of the dam breach did not show an Water 2019, 11, 315 11 of 16 obvious trench collapse. The main reason was that reinforcement had a locking effect on the tailings particle, so the erosion of adjacent tailings was impeded by reinforcement. The large-scale collapse did notdid occur not occur at the at main the mainbody bodyof the of dam the, dam,finally ﬁnally showing showing a clear a ladder clear ladder-shaped-shaped slope, slope, as shown as shown in Figure in 10Figuref. 10f. FigFigureure 1111 showsshows that that plots plots of width of width and depth and of depth the breach of the versus breach time for versus different time reinforcement for different reinforcementlayers. It was foundlayers. thatIt was the maximumfound that width the and maximum depth ofwidth the breach and depth for the of unreinforced the breach tailings for the unreinforceddam was about tailings 43 cm dam and was 22 cm, about respectively. 43 cm and At 22 the cm end, respectively of the overtopping. At the end test, ofthe themaximum overtopping width test, theand maximum depth had width decreased and depth to 8 cmhad and decreased 14 cm for to two8 cm layers and 14 of cm reinforcement, for two layers and of reinforcement 7 cm and 18 cm, and for 7 cmfour and layers 18 cm of for reinforcement. four layers of In reinforcement. addition, it was In noticedaddition that,, it was when noticed the reinforcement that, when the was reinforcement more than wtwoas more layers, than the two further layers improvement, the further was improvement not signiﬁcant. was This not result significant. showed This that result the optimum showed number that the optimumwas two number layers of w reinforcement.as two layers of reinforcement.

50 NN=0 = 0 NN=1 = 1 (a) 40 NN=2 = 2 NN=3 = 3 N=4 30 N = 4

10 The breach width (cm) 0 Water 2019, 11, 315 0 100 200 300 400 12 of 16 Time (s) 25 NN=0 = 0 NN=1 = 1 (b) 20 NN=2 = 2 NN=3 = 3 N=4 N = 4 15

5 The breach depth (cm) 0 0 100 200 300 400 Time (s) FigureFigure 11 11.. TheThe width/depth width/depth changes changes graph graph of breach of breach,, (a) (bareach) breach width; width; (b) ( b)reach breach depth. depth.

4. Prediction4. Prediction Model Model of Overtopping of Overtopping Failure Failure A predicA predictiontion model model was was established established to to quantify quantify the the process process of ofthe the erosion erosion of of the the breach breach and and validatedvalidated with with the the experimental experimental data data obtained. obtained. Based onon thethe results results of of the the tailings tailings dam dam overtopping overtopping tests, testthes, the changing changing process process of the of breach the breach width width on the on top the of topthe tailings of the tailin damgs should dam satisfy should the satisfy following the followingEquation Equation [44]: [44]: 2 2 Ct ν − νk ∆b =2 2 ∆t, (2) Ct vρs v k νk b   t， (2) sv k where ρs was the density of the tailings (kg/m3), Ct was the erosion coefficient of tailings (kg/m3), b was the increment of breach width in t (m), v was the velocity of water in breach (m/s), and vk was the velocity of impact resistance (m/s), which was related to the properties of the dam material. The hydraulic characteristics of overtopping flow were similar to the flow of broad crested weir [44]. The flow discharge could be obtained by Equation (3):

1.5 2.5 Qout  ksm (c1bh0  c2mh0 ) . (3)

Among them, c1 = 1.7, c2 = −1.3 [45], the slope parameter was m = 0.36 according to the test model, ksm was the coefficient of submergence, ksm = 0.95, b was the breach width (m), h0 was the breach depth (m), and in this paper, h0 = z. The relationship about time, breach depth and the number of reinforcement layers (N) was shown in Table 3, which could be calculated by the following Equation (4), the coefficients were obtained by best-fitting using Matrix Laboratory (Matlab, The MathWorks, Natick, MA, USA) based on the test results:

2 2 z  5.287  0.102t  2.983N  0.009tN  0.0001t  0.395N . (4)

Ct was the erosion coefficient of reinforced tailings dam. From the test results, it was found that the Ct was changing with the number of reinforcement layers. The relationship between Ct and the number of reinforcement layers (N) was shown in Table 4, which could be represented by Equation (5):

3 1 Ct f( N )  C t0  ( N  9) (5) 2 , where the N was the number of reinforcement layers, and Ct0 was the erosion coefficient of tailings dam without reinforcement. Water 2019, 11, 315 12 of 16

3 3 where ρs was the density of the tailings (kg/m ), Ct was the erosion coefficient of tailings (kg/m ), ∆b was the increment of breach width in ∆t(m), v was the velocity of water in breach (m/s), and νk was the velocity of impact resistance (m/s), which was related to the properties of the dam material. The hydraulic characteristics of overtopping flow were similar to the flow of broad crested weir [44]. The flow discharge could be obtained by Equation (3):

1.5 2.5 Qout = ksm(c1bh0 + c2mh0 ). (3)

Among them, c1= 1.7, c2= −1.3 [45], the slope parameter was m = 0.36 according to the test model, ksm was the coefficient of submergence, ksm = 0.95, b was the breach width (m), h0 was the breach depth (m), and in this paper, h0 = z. The relationship about time, breach depth and the number of reinforcement layers (N) was shown in Table3, which could be calculated by the following Equation (4), the coefficients were obtained by best-fitting using Matrix Laboratory (Matlab, The MathWorks, Natick, MA, USA) based on the test results: z = 5.287 + 0.102t − 2.983N − 0.009tN − 0.0001t2 + 0.395N2. (4)

Table 3. Relationship about breach depth, reinforcement layers (N) and time.

Index Values Time (s) 0 20 40 60 80 100 120 140 160 180 200 220 240 280 300 320 N = 0 0 10 10 12 12.5 14 16 18 20 20 22 22 22 22 22 22 N = 1 0 6 8 11 11 13 13 15 15 17 17 20 20 20 20 20 Breach N = 2 0 4 6 7.2 8 8 8 8 10 12 14 14 14 14 14 14 Depth (cm) N = 3 0 3 3 3 4 4 6 7 8 8 9 10 10 10 10 10 N = 4 0 2 2 2 4 4 5 7 7 8 8 10 10 10 10 10

Ct was the erosion coefﬁcient of reinforced tailings dam. From the test results, it was found that the Ct was changing with the number of reinforcement layers. The relationship between Ct and the number of reinforcement layers (N) was shown in Table4, which could be represented by Equation (5):

3 −1 C = f (N) = C × ( N + 9) , (5) t t0 2 where the N was the number of reinforcement layers, and Ct0 was the erosion coefﬁcient of tailings dam without reinforcement.

Table 4. Relationship between Ct and reinforcement.

Reinforcement Layers (N) 0 1 2 3 4 −4 3 Ct (×10 kg/cm ) 38.62 13.51 9.92 6.10 5.59

Substituting Equation (5) into Equation (2), an equation of the width of the breach after introducing reinforcement layers number (N) can be obtained:

C ν2 − ν2 ∆b = t0 × k ∆t. (6) 3 ν ρs( 2 N + 9) k

According to the initial conditions above, the width of the dam breach with four reinforcement layers could be predicted by Equation (6), showing a high goodness of ﬁt. The calculated breach width values were compared to the measured ones, as shown in Figure 12. Water 2019, 11, 315 13 of 16

Substituting Equation (5) into Equation (2), an equation of the width of the breach after introducing reinforcement layers number (N) can be obtained: C v2 v 2 b t0  k  t 3 vk (6) s (N  9) 2 .

Table 3. Relationship about breach depth, reinforcement layers (N) and time.

Index Values Time (s) 0 20 40 60 80 100 120 140 160 180 200 220 240 280 300 320 N = 0 0 10 10 12 12.5 14 16 18 20 20 22 22 22 22 22 22 N = 1 0 6 8 11 11 13 13 15 15 17 17 20 20 20 20 20 Breach Depth N = 2 0 4 6 7.2 8 8 8 8 10 12 14 14 14 14 14 14 (cm) N = 3 0 3 3 3 4 4 6 7 8 8 9 10 10 10 10 10 N = 4 0 2 2 2 4 4 5 7 7 8 8 10 10 10 10 10

Table 4. Relationship between Ct and reinforcement.

Reinforcement Layers (N) 0 1 2 3 4 Ct (×10−4 kg/cm3) 38.62 13.51 9.92 6.10 5.59

According to the initial conditions above, the width of the dam breach with four reinforcement layersWater could2019, 11 be, 315 predicted by Equation (6), showing a high goodness of fit. The calculated breach 13width of 16 values were compared to the measured ones, as shown in Figure 12.

8 Measured value Calculated value

2 The breach width (cm)

0 0 50 100 150 200 250 Time (s) FigureFigure 12 12.. CComparisonomparison of of the the calculated calculated and and measured measured breach breach width width for for four four reinforcement reinforcement layers layers..

SubstitutingSubstituting Equation Equation (4) (4)into into Equation Equation (3),(3), the theflow ﬂow through through the thebreach breach of reinforc of reinforcementement dam dam was calculatedwas calculated by Equation by Equation (3), as shown (3), as in shown Figure in 13 Figure. It was 13 found. It was that found the reinforcement that the reinforcement could effectively could Water 2019, 11, 315 14 of 16 reduceeffectively the discharge reduce the of the discharge overtopping of the. overtopping.

10 N = 0 /s) N=0 3 NN=2 = 2 cm 8 3 NN=4 = 4 6

0 50 100 150 200 250 Flow (×10 through breach Time (s) Figure 1 13.3. Calculated of flow ﬂow through breach with different reinforcement layers.layers.

5.5. Conclusi Conclusionsons OvertoppingOvertopping was was recognized recognized as as the the major major cause of tailings damdam failure.failure. InIn orderorder to to investigate investigate the the featurefeature of of overtopping overtopping of of the the reinforced reinforced tailings tailings dam, a series ofof physicalphysical modelmodel teststests werewere carried carried out. out. TheThe findings findings of of this this research research provide providedd insights for thethe overtoppingovertopping erosionerosion ofof thethe tailings tailings dam dam with with differentdifferent number numberss of of reinforcement reinforcement.. A Allll presented presented expressions expressions were were applicable applicable to tothe the prediction prediction of ofthe developmentthe development of breach of breach in terms in terms of of time. time. From From the the study study developed developed in in this paper, paper, the the following following conclconclusionsusions could could be be dr drawn:awn: 1.1. TheThe reinforcement reinforcement layers layers significantly significantly affected affected the phreatic level.level. AsAs thethe numbernumber ofof reinforcement reinforcement layerlayerss increased, increased, the the rate rate of of th thee rise rise of of the the phreatic phreatic level was slowingslowing down,down, butbut thethe final final phreatic phreatic levellevel became became higher. higher. 2.2. TheThe number number of of reinforcement reinforcement layers layers had had an an impact impact on the final final shape shape of of dam dam breach breach after after overtoppingovertopping erosion. erosion. The The final final breach breach was was shaped shaped as a ladder withwith fourfour reinforcementreinforcement layers layers and and anan hourglass hourglass without without reinforcement. reinforcement. 3. The reinforcement layers could improve the anti-erosion capability of tailings dam. With the increase of reinforcement layers, the size of breach and the loss rate of stress were reduced significantly. 4. Based on the erosion principle, a mathematical model including the number of reinforcement layers was proposed to predict the width and flow of the tailings dam breach.

Funding: The research reported in this paper has been funded by the National Natural Science Foundation of China (No. 51404049), the Natural Science Foundation project of Chongqing Science and Technology Commission (No. cstc2016jcyjA0319, No. cstc2018jcyjAX0231, No. cstc2015jcyjA50010), the China Scholarship Council (No. 201608505157), the China Postdoctoral Science Foundation (No. 2017M620048, No. 2018T110103), and grant chongqing-0004-2017AQ, Chongqing-0009-2018AQ.

Acknowledgments: The authors would like to thank the reviewers for their constructive comments that improved the paper.

Conflicts of Interest: The authors declare no conflict of interest.

References

3. The reinforcement layers could improve the anti-erosion capability of tailings dam. With the increase of reinforcement layers, the size of breach and the loss rate of stress were reduced signiﬁcantly. 4. Based on the erosion principle, a mathematical model including the number of reinforcement layers was proposed to predict the width and ﬂow of the tailings dam breach.

Author Contributions: Conceptualization, X.J.; Methodology, X.J.; Validation, Y.C.; Supervision, D.J.W. and M.L.S.; Writing—Original Draft Preparation, X.J. and Y.C.; Writing—Review and Editing, X.J., Y.C. and H.Z. Funding: The research reported in this paper has been funded by the National Natural Science Foundation of China (No. 51404049), the Natural Science Foundation project of Chongqing Science and Technology Commission (No. cstc2016jcyjA0319, No. cstc2018jcyjAX0231, No. cstc2015jcyjA50010), the China Scholarship Council (No. 201608505157), the China Postdoctoral Science Foundation (No. 2017M620048, No. 2018T110103), and grant chongqing-0004-2017AQ, Chongqing-0009-2018AQ. Acknowledgments: The authors would like to thank the reviewers for their constructive comments that improved the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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