A Preliminary Study of the Failure Modes and Process of Landslide Dams Due to Upstream Flow

water

Article A Preliminary Study of the Failure Modes and Process of Landslide Dams Due to Upstream Flow

Xinghua Zhu 1 , Jianbing Peng 1,*, Cheng Jiang 2 and Weilong Guo 3

1 College of Geological Engineering and Surveying of Chang’an University/Key Laboratory of Western China, Mineral Resources and Geological Engineering, Xi’an 710054, China; [email protected] 2 School of Environmental Science and Engineering, Chang’an University, Xi’an 710054, China; [email protected] 3 Shaanxi Institute of Engineering Prospecting Co. Ltd., Xi’an 710068, China; [email protected] * Correspondence: [email protected]; Tel.: +86-158 9139 1126

Received: 5 May 2019; Accepted: 24 May 2019; Published: 28 May 2019

Abstract: In the process of mineral development, large-scale flash floods (or debris flows) can be induced by the failure of landslide dams formed by the disorganized stacking of mine waste. In this study, the modes and processes of mine waste dam failures were explored using 13 experimental tests based on the field investigation of landslide dams in the Xiaoqinling gold mining area in China. Our 13 mine waste dam experiments exhibited three failure modes: (i) Piping, overtopping, and erosion; (ii) overtopping and soil collapse; and (iii) overtopping and erosion. In addition, the failure processes of the landslide dams included impoundment, seepage, overtopping, and soil erosion. Different experimental conditions would inevitably lead to different failure processes and modes, with the failure modes being primarily determined by the seepage characteristics. Overtopping was the triggering condition for dam failure. The landslide dam failure process was determined based on the particle size of the mine waste and the shape of the dam. These findings will provide a scientific reference for the prevention and mitigation of natural hazards in mining areas.

Keywords: mine waste; landslide dam; failure modes; soil erosion; debris ﬂow

1. Introduction The formation and failure of landslide dams include a series of complex processes that occur at the interface between hill slopes and alluvial plain or valley-floor systems. Floods and debris flows caused by the failure of landslide dams constitute a widespread hazard to people and property, in part because of the suddenness and unpredictability of all types of dam failures [1]. Unlike artificial gravity or concrete dams with engineered barriers and filter materials, a landslide dam usually consists of a non-cohesive clast material, which has a broad grain size distribution [2,3] and is in a naturally unstable state [4,5]. Based on historical events, dam failures can generate destructive flash floods and debris flows that cause catastrophic losses [5–8]. The debris flows in the Xiaoqinling gold mining area in Tongguan County, Shaanxi Province (Figure1) are examples of large-scale debris flows that occurred in gullies due to the failure of mine waste landslide dams [9–11]. The unreasonable stacking of mine waste severely blocked the gullies and formed numerous landslide dams with beaded distribution [10]. When these landslide dams failed, large-scale floods (or debris flows) occurred, such as the catastrophic disaster on 11 July 1994, which occurred when heavy rains in the upper reaches of the basin resulted in a large amount of mine waste being delivered by the flood and finally forming a debris flow with a peak flow rate of 260 m3/s. The debris flow resulted in 51 deaths, more than 2000 missing, and economic losses of more than three million dollars [9,12]. We conducted three field investigations in the basin from 2016 to 2018.

Water 2019, 11, 1115; doi:10.3390/w11061115 www.mdpi.com/journal/water Water 2019, 11, 1115 2 of 17

Water 2019, 11, x FOR PEER REVIEW 2 of 18 Our investigations show that the mine waste is still being randomly stacked and blocking the channel, 2018. Our investigations show that the mine waste is still being randomly stacked and blocking the forming many landslide dams that are more than 20 m high. Eight landslide dams with a beaded channel, forming many landslide dams that are more than 20 m high. Eight landslide dams with a distribution were identiﬁed in Daxicha Gully—shown in the left panel of Figure1—which indicated beaded distribution were identified in Daxicha Gully—shown in the left panel of Figure 1—which that the gold miners did not learn from past disasters. The average gradient of the gully is greater than indicated that the gold miners did not learn from past disasters. The average gradient of the gully is 32.5%. Moreover, the average annual precipitation of the basin is 587.4 mm, and the maximum annual greater than 32.5%. Moreover, the average annual precipitation of the basin is 587.4 mm, and the precipitation is 958.6 mm. The maximum daily rainfall was 113.4 mm (23 July 2010), and 76.19% of the maximum annual precipitation is 958.6 mm. The maximum daily rainfall was 113.4 mm (23 July annual rainfall is concentrated in July, August, and September [10]. Therefore, large-scale debris ﬂows 2010), and 76.19% of the annual rainfall is concentrated in July, August, and September [10]. could be triggered by heavy rainfall events in the future. Therefore, large-scale debris flows could be triggered by heavy rainfall events in the future.

Figure 1. Location of the Xiaoqingling gold mining area. Figure 1. Location of the Xiaoqingling gold mining area. A considerable number of important research achievements concerning landslide dams have been madeA in considerable recent years. number Several of multidisciplinary important research studies achievements have investigated, concerning simulated, landslide and dams predicted have beenthe occurrence, made in recent longevity, years. breakdown, Several multidisciplinary and subsequent studies debris flowshave investigated, of landslide damssimulated, [1,5,13 and,14]. predictedDam breaching the occurrence, is very complicated longevity, duebreakdown, to the influence and subsequent of the physical debris flows environment. of landslide Accurate dams [1,5,13,14].knowledge Dam of the breaching dam breach is very process complicated is essential due to to help the designinfluence an earlyof the warning physical system environment. for the Accurateevacuation knowledge of people of and the property. dam breach Many process scholars is essential have carried to help out design model experimentsan early warning on landslide system fordam the failure evacuation [15–18]. of In people addition and to property. experimental Many research, scholars physically-based have carried out mathematical model experiments models have on landslidebeen established dam failure for practical [15–18]. uses In addition [19–23]. Theto experimental application ofresearch, these models physically-based and methods mathematical are useful in modelsdisaster prevention,have been mitigation,established andfor riskpractical assessment uses of[19–23]. floods The caused application by the failure of these of landslide models dams. and methodsHowever, are because useful of in the disaster limitations prevention, for field mitiga monitoring,tion, and it hasrisk been assessment difficult of to floods reproduce caused the by failure the failureprocess of of landslide damsdams. in However, real time. Inbecause addition, of the the variabilitylimitations of for mine field waste’s monitoring, composition it has results been difficultin an unpredictable to reproduce breach the failure mode process and process of landslide response dams when in areal landslide time. In dam addition, fails. the variability of mineIn waste’s this study, composition we used results 13 experimental in an unpredict testsable to reproduce breach mode the failureand process process response of mine when waste a landslidelandslide dam dams fails. under different conditions. Three failure modes were summarized by analyzing the failureIn processthis study, of landslide we used dams.13 experimental In addition, tests we to compared reproduce the the characteristics failure process of the of dammine system waste landslideduring the dams failure under process different and found conditions. that seepage Three wasfailure the modes mainfactor were determiningsummarized theby analyzing failure modes. the failureThis preliminary process of study landslide provides dams. experimental In addition, evidence we compared that can the be characteristics used for planning, of the natural dam hazardssystem duringprevention, the failure and the process mitigation and of found mine wastethat seepag landslidee was dams. the main factor determining the failure modes. This preliminary study provides experimental evidence that can be used for planning, natural2. Experimental hazards prevention, Methods and the mitigation of mine waste landslide dams.

2.2.1. Experimental Experimental Methods Setup Based on the data obtained from the field investigations, an experimental flume was developed to 2.1. Experimental Setup explore the failure modes and process of the mine waste landslide dams in gullies in the Geological HazardBased Simulation on the data Laboratory obtained atfrom Chang’an the field University investigations, [11,24 an]. experiment Figure2a presentsal flume an was overview developed of to explore the failure modes and process of the mine waste landslide dams in gullies in the Geological Hazard Simulation Laboratory at Chang’an University [11,24]. Figure 2a presents an Water 2019, 11, 1115 3 of 17 Water 2019, 11, x FOR PEER REVIEW 3 of 18 theoverview experimental of the experimental setup. The experimental setup. The experi flumemental was composed flume was of composed three sections: of three A storage sections: tank, A astorage flume, tank, and aa tailingsflume, and pond. a tailings The storage pond. tank The was storage 1.3 m tank in length, was 1.3 1.3 m m in in length, width, 1.3 and m 1.8 in mwidth, in height. and Water1.8 m in was height. pumped Water directly was pumped into the directly upper part into of the the upper flume part using of the a water flume pump using installed a water onpump the leftinstalled side ofon the the tank. left side An electromagneticof the tank. An electromagnetic flowmeter and aflowmeter flow balancing and a valveflow balancing were installed valve in were the middleinstalled of in the the pipeline middle to of control the pipeline the flow to rate control of the the water flow with rate anof errorthe water rate ofwith less an than error 5% rate (Figure of 2lessb). Athan triangular 5% (Figure weir 2b). was A also triangular installed weir in the was upper also installed part of the influme the upper and waspart usedof the to flume adjust and flow was stability. used Theto adjust flume flow was stability. 4 m in length, The flume 0.3 m was in width,4 m in andlength, 0.5 0.3 m inm height.in width, The and bottom 0.5 m ofin theheight. flume The consisted bottom of the a rough flume steel consisted plate withof a rough two tempered steel plate glass with sides, two whichtempered made glass it possiblesides, which to observe made theit possible failure to observe the failure process of the dams. The slope of the flume could be adjusted between 6° and process of the dams. The slope of the flume could be adjusted between 6◦ and 35◦. 35°.

Figure 2.2.The The experimental experimental setup. setup. (a) An (a overview) An overview of the experimental of the experimental setup; (b) The setup; electromagnetic (b) The flowmeterelectromagnetic and flow flowmeter balancing and valve; flow (c) Thebalancing data collection valve; ( systemc) The ofdata the experimentalcollection system setup. of the experimental setup. The mine waste material used for our experiments was collected from mine waste dumps in the DaxichaThe Gully.mine waste The grain material composition used for of our the experiment in-situ samples was is collected shown in from Figure mine3. According waste dumps to the in field the specificDaxicha particle Gully. sizeThe distributiongrain composition in Figure of3 the, the in-situ maximum sample particle is shown size was in Figure up to 13. m. According Obviously, to due the tofield the specific limitations particle in the size hydrodynamic distribution conditions in Figure of 3, the the experimental maximum setupparticle in oursize flumewas up tests, to larger1 m. particlesObviously, should due to be the excluded. limitations The criticalin the hydrodynamic starting velocity conditions for cohesionless of the sedimentexperimental is determined setup in our by Zhangflume tests, et al. [25larger]. particles should be excluded. The critical starting velocity for cohesionless 0.14 0.5 h γS γ sediment is determined by Zhang etv =al. ([25].) (17.6 − D) (1) D × γ h γγ− vD=×( )0.14 (17.6S ) 0.5 where v is the flow velocity (m/s), h is the flow depth (m), γs and γ are the bulk density of the sediment(1) and flow (m3/s), respectively, and D is theD particle diameter (m). The velocity of the flow from upstream increaseswhere v is as the the flow flume velocity gradient (m/s), increases, h is the and flow the depth maximum (m), γ startings and γ particle are the sizebulk of density the sediment of the alsosediment increases and flow according (m3/s), to respectively, Equation (1). and Based D is on the our particle previous diameter experiments (m). The [11 velocity,24], the ofD themax flowwas aroundfrom upstream 5 cm when increases the inflow as the discharge flume gradient was 2.5 incr L/seases, (which and is thethe maximummaximum inflowstarting discharge particle size in our of experiments).the sediment also Therefore, increases the according>5 cm particles to Equation were removed (1). Based in on the our material previo usedus experiments in the flume [11,24], test (in Figurethe Dmax3). was The around samples 5 werecm when composed the inflow of less discharge than 10% was fine 2.5 particles L/s (which ( <2 mmis the diameter) maximum and inflow more thandischarge 65% coarsein our particlesexperiments). (>50 mm).Therefore, The particles the >5 cm of particles mine waste were also removed had low in sphericity. the material Because used ofin thesethe flume grading test characteristics, (in Figure 3). theThe mine samples waste were had acomposed high porosity of less and than high 10% permeability. fine particles (<2 mm diameter) and more than 65% coarse particles (>50 mm). The particles of mine waste also had low Water 2019, 11, x FOR PEER REVIEW 4 of 18 sphericity.Water 2019, 11 Because, 1115 of these grading characteristics, the mine waste had a high porosity and 4high of 17 permeability.

100 Grading of sample in situ 90 Grading for the flume test 80

30 Percentagepassing (%)

0 0.01 0.1 1 10 100 1000 Particle size (mm)

Figure 3. Particle size distribution of the modeled mine waste and the in-situ particle size. Figure 3. Particle size distribution of the modeled mine waste and the in-situ particle size. 2.2. Instrumentation and Experimental Testing Procedures 2.2. Instrumentation and Experimental Testing Procedures Channel gradient varied significantly along the length of the gullies. In addition, the precipitation in thisChannel basin variedgradient seasonally, varied significwith heaviestantly along rainfall the occurring length mainlyof the fromgullies. July toIn Septemberaddition, (thethe precipitationmaximum daily in this rainfall basin was varied 113.4 seasonally, mm and occurred with heaviest on 23 July rainfall 2010) occurring [10]. Therefore, mainly 13 from sets of July model to Septemberexperiments (the with maximum different inflowdaily rainfall discharges was and 113.4 rainfall mm intensitiesand occurred were on designed 23 July to2010) reproduce [10]. Therefore, the failure 13processes sets of model of the landslide experiments dams with in the different channel inflow sections. discharges Table1 lists and the rainfall parameters intensities for the were 13 experiments. designed to reproduceSix monitoring the failure points processes were arranged of the landslide inside the dams landslide in the damchannel of each sections. test. EachTablemonitoring 1 lists the parameterspoint contained for the one 13 pore experiments. water pressure sensor, one total stress sensor, and one water content sensor. The positionsSix monitoring and serial points numbers were ofarranged the monitoring inside th pointse landslide are shown dam in of Figure each4 .test. All sensorsEach monitoring have their pointinstructions contained of factory one pore calibrations, water pressure which give sensor, us the one conversion total stress formula sensor, correspondingly. and one water However, content sensor.the general The positions calibration and may serial not numbers be applicable of the moni for ourtoring experiments, points are shown and we in performed Figure 4. All laboratory sensors havecalibration their beforeinstructions using them. of Thefactory soil samplescalibrations, were preparedwhich withgive soilus volumetricthe conversion moisture formula contents correspondingly.of 5%, 7%, 10%, 15%, However, 17%, 27%, the and general 36%, calibration and then the may water not content be applicable sensor was for placedour experiments, in these samples and weto calibrate.performed The laboratory pore water calibration pressure before sensors using were them. calibrated The insoil a watersamples column, were prepared which was with 1.5 msoil in volumetricheight and moisture marked with contents graduations. of 5%, 7%, The 10%, total 15%, stress 17%, sensor 27%, was and loaded 36%, andand unloadedthen the water cyclically content on a sensorset of calibrationwas placed devices in these to samples obtain theto calibrate. relationship The betweenpore water the pressure output electrical sensors were signal calibrated (V) and thein asurface water verticalcolumn, stress which (kPa). was Table1.5 m1 inshows height the and technical marked specifications with graduations. of sensors. The Three total camerasstress sensor were wasused loaded to record and the unloaded erosion andcyclically transport on processesa set of calibration of the mine devices waste from to obtain the front the side, relationship the right betweensides, and the the output top of theelectrical setup. signal The flume (V) and gradient the surface and inflow vertical rates stress were adjusted(kPa). Table before 1 eachshows model the technicaltest according specifications to Table2 .of During sensors. each Three test, camerascameras were were used used to to record record the the experimental erosion and process, transport and processesthe data acquisition of the mine instrumentation waste from the was front turned side, onthe to right record sides, the time-varyingand the top of values the setup. from theThe sensors. flume gradientIn addition, and the inflow depth rates of the were outburst adjusted flow before was recorded each model by a metrictest according ruler placed to Table downstream 2. During from each the test,landslide cameras dam, were and used the flowto record velocities the experimental were determined process, using and the the float data method. acquisition instrumentation was turned on to record the time-varying values from the sensors. In addition, the depth of the outburst flow was recorded byTable a metric 1. The ruler technical placed specifications downstream of sensors. from the landslide dam, and the flow velocities were determined using the float method. Range of Sensor Types Product Model Manufacturers Accuracy Measurement Time Measurement Pore water pressure Model 6007 TSS 0 to 50 kPa 0.5% 10 ms ± Total stress Model 7002 TSS 0 to 50 kPa 2% 10 ms ± Water content EC-5 Decagon 0 to 100% 2% 10 ms ± Water 2019, 11, 1115 5 of 17

Table 2. Summary of the parameters for the 13 experiments.

Number H (cm) α (◦) β (◦) θ (◦) Q0 (L/S) Failure Modes 1 25 32.7 41.6 5 0.3 Stable 2 25 34.1 43.2 6 0.3 Piping, overtopping and erosion 3 25 34.5 42.3 7 0.3 Piping, overtopping and erosion 4 25 32.3 45.1 8 0.3 Piping, overtopping and erosion 5 25 33.8 42.7 9 0.3 Overtopping and soil collapse 6 25 34.3 43.5 5 1.0 Overtopping and erosion 7 25 33.8 42.3 6 1.0 Overtopping and erosion 8 25 32.5 43.6 5 1.5 Overtopping and erosion 9 25 33.1 43.3 6 1.5 Overtopping and erosion 10 25 34.6 44.3 5 2.0 Overtopping and erosion 11 25 33.5 43.8 6 2.0 Overtopping and erosion 12 25 31.8 42.7 5 2.5 Overtopping and erosion 13 25 32.6 43.2 6 2.5 Overtopping and erosion H, α, and β are the height, the angle of the upstream face, and the angle of the downstream face of the landslide Water 2019, 11, x FOR PEER REVIEW 5 of 18 dam, respectively, and θ is slope angle of the ﬂume.

Figure 4. Distribution and serial number of the monitoring points in the landslide dam. Figure 4. Distribution and serial number of the monitoring points in the landslide dam. 3. Experimental Results Table 1. The technical specifications of sensors. Previous studies have shown that three failure mechanisms typically caused the instability and failure of landslideProduct dams: Overtopping, piping, andRange slope failure of [13]. Among them,Measurement overtopping Sensor Types Manufacturers Accuracy is a primary mechanismModel of landslide dam failure whichMeasurement occurrs after the overfilling of theTime dammed reservoirPore water [8,13 ]; piping is a kind of internal erosion caused by the migration of particles to free exits (or Model 6007 TSS 0 to 50 kPa ±0.5% 10 ms intopressure coarse openings) or by the uncontrolled saturation and seepage forces [26]; slope failure can easily occurTotal when stress a landslide Model dam 7002 has steep TSS upstream and downstream 0 to 50 kPa faces, as well ±2% as high pore-water 10 ms [13]. ByWater analyzing content the 13 EC-5experimental Decagontests, we found that 0 theto 100% failure processes ±2% of landslide 10 dams ms were extremely complex. Almost all of the failure processes of the landslide dams in our experiments were the result of multiple failureTable 2. modes. Summary Three of the di parametersfferent failure for the modes 13 experiments. were identified (in Table1) from analyses of the failure processes of the landslide dams in our experiments. Number H (cm) α (°) β (°) θ (°) Q0 (L/S) Failure Modes 3.1. Dam Failure1 Due to25 Piping, 32.7 Overtopping, 41.6 and5 Erosion0.3 Stable 2 25 34.1 43.2 6 0.3 Piping, overtopping and erosion The failures3 in tests 25 2, 3, 34.5 and 4 were 42.3 the 7result of 0.3 piping, Piping, overtopping, overtopping and erosion. and erosion An analysis of test Number4 3 indicates 25 that 32.3 landslide 45.1 dam 8 failure processes0.3 Piping, occurred overtopping with continuous and erosion inflow from upstream, and5 the water 25 level 33.8 gradually 42.7 increased, 9 resulting 0.3 in Overtopping high pore water and soil pressure collapse and seepage (Figure5a).6 The continuous25 34.3 seepage 43.5 through 5 the landslide1.0 damOvertopping formed a completeand erosion seepage path, causing the7 piping eff25ect (Figure33.8 5b).42.3 Subsequently, 6 1.0 a small landslideOvertopping occurred and on erosion the downstream 8 25 32.5 43.6 5 1.5 Overtopping and erosion 9 25 33.1 43.3 6 1.5 Overtopping and erosion 10 25 34.6 44.3 5 2.0 Overtopping and erosion 11 25 33.5 43.8 6 2.0 Overtopping and erosion 12 25 31.8 42.7 5 2.5 Overtopping and erosion 13 25 32.6 43.2 6 2.5 Overtopping and erosion H, α, and β are the height, the angle of the upstream face, and the angle of the downstream face of the landslide dam, respectively, and θ is slope angle of the flume.

3. Experimental Results Previous studies have shown that three failure mechanisms typically caused the instability and failure of landslide dams: Overtopping, piping, and slope failure [13]. Among them, overtopping is a primary mechanism of landslide dam failure which occurrs after the overfilling of the dammed reservoir [8,13]; piping is a kind of internal erosion caused by the migration of particles to free exits (or into coarse openings) or by the uncontrolled saturation and seepage forces [26]; slope failure can easily occur when a landslide dam has steep upstream and downstream faces, as well as high pore-water [13]. By analyzing the 13 experimental tests, we found that the failure processes of Water 2019, 11, x FOR PEER REVIEW 6 of 18

landslide dams were extremely complex. Almost all of the failure processes of the landslide dams in our experiments were the result of multiple failure modes. Three different failure modes were identified (in Table 1) from analyses of the failure processes of the landslide dams in our experiments.

3.1. Dam Failure Due to Piping, Overtopping, and Erosion The failures in tests 2, 3, and 4 were the result of piping, overtopping, and erosion. An analysis of test Number 3 indicates that landslide dam failure processes occurred with continuous inflow Water 2019, 11, 1115 6 of 17 from upstream, and the water level gradually increased, resulting in high pore water pressure and seepage (Figure 5a). The continuous seepage through the landslide dam formed a complete seepage surfacepath, ofcausing the dam, the reducingpiping effect the overall(Figure height 5b). Subsequently, of the dam (Figure a small5c), landslide which was occurred followed on by the dam overtoppingdownstream (Figure surface5 d).of the Then, dam, lateral reducing and headwardthe overall erosionheight of began the dam to occur (Figure throughout 5c), which the was dam (Figurefollowed5e). by Finally, dam theovertopping dam material (Figure was 5d). quickly Then, eroded lateral andand transportedheadward erosion by an outburstbegan to ﬂoodoccur (or debristhroughout ﬂow) (Figure the dam5f). (Figure 5e). Finally, the dam material was quickly eroded and transported by an outburst flood (or debris flow) (Figure 5f).

Figure 5. Failure process of a landslide dam (Q = 0.3 L/s, θ = 7 ). (a) t = 300 s; (b) t = 540 s; (c) t = 544 s; Figure 5. Failure process of a landslide dam (Q00 = 0.3 L/s, θ = 7°).◦ (a) t = 300 s; (b) t = 540 s; (c) t = 544 s; (d()dt) =t =551 551 s; s; ( e(e))t t= = 573573 s; (f) t = 616616 s. s.

DuringDuring the the experiment, experiment, thethe totaltotal stress, pore pore wa waterter pressure, pressure, and and water water content content inside inside the the dam dam werewere recorded recorded by by the the sensors. sensors. Figure Figure6 displays 6 displays the th variatione variation in thein the pore pore water water pressure pressure at the at sixthe sensor six locationssensor locations (i.e., observation (i.e., observation points). points). In Figure In Figure6, the 6, pore the pore water water pressure pressure at sensorsat sensors increases increases in in the sequencethe sequence A-B-D-C-E-F, A-B-D-C-E-F, which which indicates indicates that thethat water the water level level in the in damthe dam slowly slowly increased, increased, and and the the initial water flow and infiltration were mainly horizontal (the pressure increased at sensors A and B first). As the water level gradually increased, the pore water pressure in the upper part of the dam began to increase (the pressure at sensor D increased). Then, the pore water pressure at sensor C began to increase, which indicates that a complete seepage path in the lower layer had formed. Finally, the pore water pressure increased at sensors E and F. In addition, the pore water pressure dramatically changed at about t = 540 s at each of the six sensor locations shown in Figure6, following a pressure change sequence of F-E-D-C-B-A. The variation in the pore water pressure during the experiment indicates that local failure occurred at t = 540 s, which is consistent with the failure process shown in Figure5. Water 2019, 11, x FOR PEER REVIEW 7 of 18

initial water flow and infiltration were mainly horizontal (the pressure increased at sensors A and B first). As the water level gradually increased, the pore water pressure in the upper part of the dam began to increase (the pressure at sensor D increased). Then, the pore water pressure at sensor C began to increase, which indicates that a complete seepage path in the lower layer had formed. Finally, the pore water pressure increased at sensors E and F. In addition, the pore water pressure dramatically changed at about t = 540 s at each of the six sensor locations shown in Figure 6, following a pressure change sequence of F-E-D-C-B-A. The variation in the pore water pressure Waterduring2019 the, 11, experiment 1115 indicates that local failure occurred at t = 540 s, which is consistent with7 of the 17 failure process shown in Figure 5.

FigureFigure 6.6.Variation Variation in in pore pore water water pressure pressure with with time time for for observation observation points points (i.e., (i.e., sensor sensor locations) locations) in the in

landslidethe landslide dam dam (Q0 =(Q0.30 = L0.3/s, L/s,θ = θ7◦ =). 7°).

TheThe transienttransient variations variations in in the the water water content, content, pore pore water water pressure, pressure, and totaland stresstotal stress at point at Bpoint when B Q when0 was Q 0.30 was L/s 0.3 and L/sθ andwas θ 7 ◦wasare 7° shown are shown in Figure in Figure7. The 7. failure The failure process process of the of landslide the landslide dam dam can becan dividedbe divided into into four four stages: stages: In stage In stage I, the I, water the water level level gradually gradually increased increased as the as inflow the inflow discharge discharge filled thefilled dam the reservoir. dam reservoir. However, However, the water the content water co andntent pore and water pore pressure water pressure were unchanged were unchanged because thebecause water the did water not penetrate did not penetrate to observation to observation point B in point this stage.B in this The stage. total The stress total changed stress changed slightly dueslightly to the due slow to the deformation slow deformation of the landslide of the landsl dam.ide In stagedam. II,In waterstage II, penetrated water penetrated observation observation point B, whichpoint B, indicates which indicates that the water that the content water rapidly content increased rapidly increased until the poreuntil space the pore of the space landslide of the damlandslide was saturated.dam was saturated. The pore The water pore pressure water pr andessure total and stress total also stress increased also incr significantly.eased significantly. In stage In III, stage as the III, wateras the levelwater in level the reservoirin the reservoir gradually gradually increased, increa a completesed, a complete seepage seepage path formed, path formed, and the and pore the water pore pressurewater pressure continually continually increased, increased, resulting resulting in a decrease in a decrease in the total in stress.the total In stress. stage IV, In astage small IV, landslide a small occurredlandslide on occurred the downstream on the downstream surface of the surface dam, and of the the overalldam, and height the ofoverall the dam height was of reduced, the dam which was was followed by dam overtopping. Finally, when the dam rapidly failed, several sensors were exposed Waterreduced, 2019, 11 which, x FOR wasPEER followedREVIEW by dam overtopping. Finally, when the dam rapidly failed, several8 of 18 tosensors the upstream were exposed flow, and to the the upstream acquired dataflow, had and no the reference acquired value. data had no reference value.

3.00 100

70 2.00 60

Stress (kPa) Stress 40 water content (%) 1.00 Pore water pressure 30

Total stress 20 Effective stress 10 Ⅰ Ⅱ Ⅲ Water content Ⅳ 0.00 0 0 100 200 300 400 500 600 Time (s) FigureFigure 7. TransientTransient variations in in the the pore water pressure, water content, and total stress at point B in Q thethe experiments experiments ( (Q0 == 0.30.3 L/s, L/s, θθ == 7°).7◦).

This failure mode had a longer impounding time because of the small inflow discharge, which provided sufficient time for seepage. In addition, the loose, porous mine waste allowed for the formation of a complete seepage path, which determined the failure mode and process.

3.2. Dam Failure Due to Overtopping and Soil Collapse Dam failure due to overtopping and soil collapse occurred in experiment Number 5, and the failure processes involved the following. The water level upstream of the dam gradually increased, and a high water head initially formed, which led to seepage through the landslide dam (Figure 8a). However, a complete seepage channel had not formed due to the impediment caused by the fine particles. The complete seepage channel had still not formed by the time the dam reservoir was filled and began to overtop (Figure 8b). The dam became unstable, resulting in sudden soil collapse (Figure 8c). While the entire dam slid downstream (Figure 8d), the mine waste was eroded and rapidly transported by an outburst flood (or debris flow) (Figure 8e) until the end of the test (Figure 8f).

Water 2019, 11, x FOR PEER REVIEW 8 of 18

3.00 100

70 2.00 60

Stress (kPa) Stress 40 water content (%) 1.00 Pore water pressure 30

Total stress 20 Effective stress 10 Ⅰ Ⅱ Ⅲ Water content Ⅳ 0.00 0 0 100 200 300 400 500 600 Time (s)

Water 2019Figure, 11, 1115 7. Transient variations in the pore water pressure, water content, and total stress at point B in 8 of 17 the experiments (Q0 = 0.3 L/s, θ = 7°).

ThisThis failure failure mode mode had had a a longer longer impoundingimpounding time time because because of of the the small small inflow inﬂow discharge, discharge, which which providedprovided su sufficientﬃcient time time for for seepage.seepage. In addition addition,, the the loose, loose, porous porous mine mine waste waste allowed allowed for for the the formationformation of of a a complete complete seepage seepage path, path, whichwhich determined the the failure failure mode mode and and process. process.

3.2.3.2. Dam Dam Failure Failure Due Due to to Overtopping Overtopping andand Soil Collapse DamDam failure failure due due to to overtopping overtopping andand soilsoil collapsecollapse occurred in in experiment experiment Number Number 5, 5,and and the the failurefailure processes processes involved involved the the following.following. The water level level upstream upstream of of the the dam dam gradually gradually increased, increased, andand a higha high water water head head initially initially formed,formed, which led led to to seepage seepage through through the the landslide landslide dam dam (Figure (Figure 8a).8 a). However,However, a a complete complete seepage seepage channelchannel had not formed formed due due to to the the impediment impediment caused caused by bythe the fine fine particles.particles. The The completecomplete seepageseepage channel had had sti stillll not not formed formed by by the the time time the the dam dam reservoir reservoir was was filledfilled and and began began to to overtop overtop (Figure (Figure8 8b).b). TheThe damdam becamebecame unstable, resulting resulting in in sudden sudden soil soil collapse collapse (Figure 8c). While the entire dam slid downstream (Figure 8d), the mine waste was eroded and (Figure8c). While the entire dam slid downstream (Figure8d), the mine waste was eroded and rapidly rapidly transported by an outburst flood (or debris flow) (Figure 8e) until the end of the test (Figure transported by an outburst flood (or debris flow) (Figure8e) until the end of the test (Figure8f). 8f).

Water 2019, 11, x FOR PEER REVIEW 9 of 18

FigureFigure 8. 8.Failure Failure process process of of a a landslide landslide dam dam ( Q(Q00= = 0.3 LL/s,/s, θ = 9°).9◦). (a (a) )t t= =5050 s; s;(b ()b t) =t 175= 175 s; ( s;c) (tc )= t181= 181 s; s; (d)(dt)= t 183= 183 s; s; (e ()e)t t= =187 187 s;s; ((ff)) tt = 199 s. s.

BasedBased on on our our analysis analysis of of thethe variationvariation in pore water water pressure pressure with with time time during during the the experiment, experiment, thethe order order of of the the pore pore water water pressure pressure variationvariation waswas A-D-B-E-F-C. Based Based on on the the experimental experimental process process shownshown in in Figure Figure9, 9, we we conclude conclude that that the the pore pore waterwater pressurepressure at observation observation point point D D increased increased before before thatthat at at point point B B because because the the fine fine particles particles blockedblocked the seepage seepage path. path. Then, Then, the the pore pore water water pressure pressure at at observationobservation point point E substantiallyE substantially increased. increased. The Th poree pore water water pressure pressure at observation at observation point Fpoint increased F beforeincreased the pressure before the at observation pressure at pointobservation C, which point indicates C, which that indicates a complete that seepage a complete channel seepage had not formedchannel when had thenot damformed failed. when the dam failed. TheThe transient transient variations variations in in water water content, content, pore pore water water pressure, pressure, and and total total pressure pressure at pointat point B when B Q0whenwas 0.3Q0 was L/s and0.3 L/sθ was and 9 θ◦ arewas shown 9° are shown in Figure in Figure10. The 10. failure The failure process process of the of landslide the landslide dam dam can be dividedcan be intodivided four into stages. four Instages. stage In I, stage the water I, the levelwater gradually level gradually increased increased as the as inflow the inflow discharge discharge began tobegan fill the to dam fill reservoir.the dam reservoir. However, However, the water the content water and content pore and water pore pressure water remainedpressure remained unchanged unchanged because the water did not penetrate to observation point B in this stage. At the same time, the total stress changed slightly due to the slow deformation of the landslide dam. In stage II, the water penetrated to observation point B, and the pore water pressure and total stress increased. The water content rapidly increased until it was saturated. In stage III, the landslide dam deformed due to overtopping, causing an instantaneous increase in pore water pressure. As the effective stress decreased, the soil collapsed, and the entire dam slid downstream (Figure 8c). In stage IV, the data acquired from the sensors had no reference value because the dam had failed and several of the sensors were exposed to the upstream flow. Water 2019, 11, 1115 9 of 17 because the water did not penetrate to observation point B in this stage. At the same time, the total stress changed slightly due to the slow deformation of the landslide dam. In stage II, the water penetrated to observation point B, and the pore water pressure and total stress increased. The water content rapidly increased until it was saturated. In stage III, the landslide dam deformed due to overtopping, causing an instantaneous increase in pore water pressure. As the effective stress decreased, the soil collapsed, and the entire dam slid downstream (Figure8c). In stage IV, the data acquired from the Water 2019, 11, x FOR PEER REVIEW 10 of 18 sensors had no reference value because the dam had failed and several of the sensors were exposed to Water 2019, 11, x FOR PEER REVIEW 10 of 18 the upstream flow. 2.0 2.0 A B C A B C D E F 1.5 D E F 1.5

1.0 1.0

Pore water pressure (kPa) pressure water Pore 0.5 Pore water pressure (kPa) pressure water Pore 0.5

0.0 0.0 0 50 100 150 200 250 0 50 100Time (s) 150 200 250 Time (s) Figure 9. Variation in water pressure with time at different points in the landslide dam (Q0 = 0.3 L/s, Figure 9. Variation in water pressure with time at diﬀerent points in the landslide dam (Q0 = 0.3 L/s, θFigure = 9°). 9. Variation in water pressure with time at different points in the landslide dam (Q0 = 0.3 L/s, θ = 9 ). θ = 9°).◦ 4.0 80 4.0 Pore water pressure 80 Pore water pressure 3.5 Total stress 70 3.5 EffectiveTotal stress stress 70 3.0 WaterEffective content stress 60 3.0 Water content 60 2.5 50 2.5 50 2.0 40 2.0 40 Stress (kPa) Stress

1.5 30 (%)content Water Stress (kPa) Stress

1.5 30 (%)content Water 1.0 20 1.0 20 0.5 10 0.5 Ⅰ ⅡⅢ Ⅳ 10 Ⅰ ⅡⅢ Ⅳ 0.0 0 0.0 0 50 100 150 200 250 0 0 50 100Time (s) 150 200 250 Time (s) Figure 10.10. Variations inin pore-waterpore-water pressure, pressure, water water content, content, and and total total stress stress with with time time at pointat point B in B the in

experimenttheFigure experiment 10. Variations (Q0 (=Q0.30 = 0.3 Lin/S, L/S,pore-waterθ = θ9 ◦=). 9°). pressure, water content, and total stress with time at point B in the experiment (Q0 = 0.3 L/S, θ = 9°). The characteristics of this failure mode are as follows: The seepage time was longer due to a smallThe inflow characteristics from upstream, of this so failure more mode time arewas as required follows: forThe impounding. seepage time However, was longer during due tothe a seepage,small inflow the completedfrom upstream, seepage so pathmore could time wasnot berequired formed for because impounding. the path However, was blocked during by the migrationseepage, the of fine-grainedcompleted seepage particles. path This could resulted not in be a higherformed pore because water the pressure path was inside blocked the landslide by the dammigration and a ofsharp fine-grained decrease particles.in the effective This resulted stress be infore a higher overtopping pore water occurred. pressure Finally, inside because the landslide of the additionaldam and a shearsharp stressdecrease generated in the effective by the overflow, stress before the overtoppingsoil suddenly occurred. collapsed, Finally, and the because entire of dam the slidadditional downstream. shear stress generated by the overflow, the soil suddenly collapsed, and the entire dam slid downstream. 3.3. Dam Failure Due to Overtopping and Erosion 3.3. Dam Failure Due to Overtopping and Erosion Water 2019, 11, 1115 10 of 17

The characteristics of this failure mode are as follows: The seepage time was longer due to a small inflow from upstream, so more time was required for impounding. However, during the seepage, the completed seepage path could not be formed because the path was blocked by the migration of fine-grained particles. This resulted in a higher pore water pressure inside the landslide dam and a sharp decrease in the effective stress before overtopping occurred. Finally, because of the additional shear stress generated by the overflow, the soil suddenly collapsed, and the entire dam slid downstream.

Water3.3. Dam 2019, Failure11, x FOR Due PEER to REVIEW Overtopping and Erosion 11 of 18 In these experiments, eight tests were conducted (Numbers 6–13 in Table1), and dam failure In these experiments, eight tests were conducted (Numbers 6–13 in Table 1), and dam failure was caused by overtopping and erosion. Taking test Number 9 as an example, the reservoir filled up was caused by overtopping and erosion. Taking test Number 9 as an example, the reservoir filled up in a short time due to a greater amount of inflow from upstream. Then, the dam was overtopped in a short time due to a greater amount of inflow from upstream. Then, the dam was overtopped (Figure 11b). A channel formed and quickly cut down into the dam materials as the overflow (Figure 11b). A channel formed and quickly cut down into the dam materials as the overflow continuously eroded the back surface of the dam (Figure 11c). Afterwards, the dam material on both continuously eroded the back surface of the dam (Figure 11c). Afterwards, the dam material on both sides of the spillway was unstable and experienced continuous failure (Figure 11d). The spillway sides of the spillway was unstable and experienced continuous failure (Figure 11d). The spillway rapidly widened, and the mine wastes was rapidly transported (Figure 11e). Finally, the dam material rapidly widened, and the mine wastes was rapidly transported (Figure 11e). Finally, the dam was completely eroded away (Figure 11f). material was completely eroded away (Figure 11f).

Figure 11. Failure process of a landslide dam (Q0 = 1.5 L/S, θ = 6◦). (a) t = 10 s; (b) t = 40 s; (c) t = 48 s; Figure 11. Failure process of a landslide dam (Q0 = 1.5 L/S, θ = 6°). (a) t = 10 s; (b) t = 40 s; (c) t = 48 s; (d) t = 52 s; (e) t = 60 s; (f) t = 73 s. (d) t = 52 s; (e) t = 60 s; (f) t = 73 s.

Based on our analysis of the variation in pore water pressure with time during the experiment (Figure 12), the pore water pressure increased in the sequence of A-B-D-C-E-F. Combined with the failure process illustrated in Figure 11, the pore water pressure of the lower layer (A and B) initially increased due to seepage during reservoir filling. However, when there was a greater inflow discharge, the water level in the reservoir and the pore water pressure at observation point D increased rapidly. With continuous seepage, the pore water pressure increased at observation point C, and then a complete seepage path formed. Finally, the pore water pressures of the upper layer (E and F) gradually increased due to overtopping. Water 2019, 11, 1115 11 of 17

Based on our analysis of the variation in pore water pressure with time during the experiment (Figure 12), the pore water pressure increased in the sequence of A-B-D-C-E-F. Combined with the failure process illustrated in Figure 11, the pore water pressure of the lower layer (A and B) initially increased due to seepage during reservoir ﬁlling. However, when there was a greater inﬂow discharge, the water level in the reservoir and the pore water pressure at observation point D increased rapidly. With continuous seepage, the pore water pressure increased at observation point C, and then a complete seepage path formed. Finally, the pore water pressures of the upper layer (E and F) gradually increased Water 2019, 11, x FOR PEER REVIEW 12 of 18 due to overtopping.

2 A B C

D E F 1.5

1 Pore water pressure (kPa) pressure water Pore 0.5

0 050100 Time (s)

Figure 12. Variation in pore water pressure with time at different points in a landslide dam (Q0 = 1.5 L/S, Figure 12. Variation in pore water pressure with time at different points in a landslide dam (Q0 = 1.5 θ = 6◦). L/S, θ = 6°). In this failure mode, since the failure process of the landslide dam was extremely short, the data acquiredIn this at observationfailure mode, point since A the constituted failure process the longest of the portion landslide of thedam test. was Therefore, extremely the short, sensor the data fromacquired observation at observation point A point were A chosen constituted to analyze the longest variations portion in the of parameters the test. Therefore, with time. the The sensor variations data infrom water observation content, porepoint water A were pressure, chosen and to totalanalyz stresse variations with time in at the point parameters A are shown with in time. Figure The 13. Thevariations failure in process water content, of the dam pore can water be dividedpressure, into and four total stages. stress with In stage time I, at the point water A are level shown rapidly in increasedFigure 13. asThe the failure inflow process filled the of damthe dam reservoir. can be However, divided into the waterfour stages. content In and stage pore I, the water water pressure level remainedrapidly increased unchanged as the because inflow the filled water the did dam not rese penetratervoir. However, to observation the water point content A in this and stage. pore In water stage II,pressure the water remained penetrated unchanged to observation because point the A,water and did the waternot penetrate content increasedto observation rapidly point until A the in porethis spacestage. wasIn stage saturated. II, the At water the same penetrated time, the to pore obse waterrvation pressure point andA, and total the stress water increased content significantly. increased Inrapidly stage until III, as the the pore upper space layer was of saturated. the mine wasteAt the damsame was time, rapidly the pore eroded, water the pressure total stress and total decreased stress rapidly.increased However, significantly. the poreIn stage water III, pressure as the upper slowly layer decreased of the mi duene towaste the highdam ratewas ofrapidly outflow eroded, over the spillwaytotal stress and decreased the relatively rapidly. high However, water level. the Inpore stage water IV, aspressure the dam slowly failed, decreased several sensors due to were the exposedhigh rate to of the outflow upstream over flow, the spillway so the acquired and the data relatively had no high reference water value. level. In stage IV, as the dam failed, several sensors were exposed to the upstream flow, so the acquired data had no reference value. Water 2019, 11, 1115x FOR PEER REVIEW 1312 of 18 17 Water 2019, 11, x FOR PEER REVIEW 13 of 18 3 100 3 Pore water pressure Total stress 100 Pore water pressure Total stress 90 Effective stress Water content 90 2.5 Effective stress Water content 2.5 80 80 70 2 70 2 60 60 1.5 50 1.5 50

Stress(kPa) 40 Stress(kPa) 40 (%) content Water 1 1 30 (%) content Water 30 20 0.5 20 0.5 10 ⅠⅡ Ⅲ Ⅳ 10 Ⅲ Ⅳ 0 ⅠⅡ 0 0 0 1020304050600 0 102030405060Time (s) Time (s) Figure 13. Transient variations in pore-water pressures, water content, and total stress at point A in Figure 13. Transient variations in pore-water pressures, water content, and total stress at point A in Figureexperiments 13. Transient (Q0 = 1.5 variations L/s, θ = 6°). in pore-water pressures, water content, and total stress at point A in experimentsexperiments ( (QQ00 == 1.51.5 L/s, L/s, θθ = =6°).6◦ ). 4. Discussion Discussion 4. Discussion 4.1. Differences Differences in and Relationship between the Failure Modes 4.1. Differences in and Relationship between the Failure Modes By analyzinganalyzing thethe failure failure modes modes and and processes processes of the of 13 the experimental 13 experimental tests under tests di underfferent different working workingconditions,By analyzing conditions, we concluded the we failure concluded that modes the seepagethat and the processes that seepag occurs eof that beforethe occurs 13 overtoppingexperimental before overtopping is tests extremely under is important.extremelydifferent workingimportant.As the inflow conditions, As discharge the inflowwe continuedconcluded discharge to that fill continued thethe reservoir, seepag toe thefillthat waterthe occurs reservoir, level before gradually the overtopping water increased. level is Ifextremelygradually the grain important.increased.composition IfAs ofthe thethe grain daminflow composition was discharge conducive of the forcontinued seepage,dam wa tos piping conducive fill the might reservoir, for be seepage, formed. the However,piping water mightlevelCosta begradually et formed. al. [13] increased.However,reported that IfCosta the there grain et al. were composition[13] very reported few of casesthat the there ofdam dam werewa failures conducive very causedfew cases for by seepage, of piping. dam pipingfailure In our mightcaused tests, bewe by formed. didpiping. not However,Inobserve our tests, dam Costa we failure etdid al. due not[13] solely observereported to piping.dam that failurethere In test were due Number verysolely few 1, to the cases piping. inflow of damIn discharge test failure Number from caused upstream1, bythe piping. inflow was Indischargeequal our totests, the from we seepage upstream did not discharge, observewas equal so dam theto the damfailure seepage remained due discharge, solely stable. to sopiping. If the the dam damIn testremained remained Number stable. stable, 1, the If itthe inflow would dam dischargeremainedexperience fromstable, overflowing, upstream it would was which experience equal was to a the keyoverflowing, seepage factor and discharge, which triggering wasso the dama damkey failure. remainedfactor In and this stable. triggering stage, If the the damdam remainedfailure.would failIn stable,this due tostage, it soil would collapsethe dam experience (aswould shown failoverflowing, in due test to Number so ilwhich collapse 3) or was soil (as a erosion shownkey factor (asin showntest and Number triggering in tests 3) Number or dam soil failure.erosion6–13). As In(as thethis shown dam stage, materialsin the tests dam Number were would eroded 6–13).fail bydue As the totheoutflow, so damil collapse materials an outburst (as shownwere flood eroded in (or test debris byNumber the flow) outflow, 3) occurred. or soil an erosionoutburstThe failure (as flood processshown (or debrisin and tests the flow) Number relationship occurred. 6–13). between The As failur the the edam process failure materials modesand the were is relationship summarized eroded bybetween in the Figure outflow, the 14 failure. an outburstmodes is flood summarized (or debris in flow)Figure occurred. 14. The failure process and the relationship between the failure modes is summarized in Figure 14.

Figure 14. FailureFailure process process and and the the relationsh relationshipip between the failure modes. Figure 14. Failure process and the relationship between the failure modes. 4.2. Factors Factors Contributing Contributing to to Dam Dam Failure Failure Modes 4.2. Factors Contributing to Dam Failure Modes The failure processes of landslide dams includeinclude impoundment, seepage, overtopping, and soil erosion.The However,failureHowever, processes a a variety variety of of landslide experimentalof experimental dams conditions includ conditionse impoundment, will inevitablywill inevitably leadseepage, to dilead ffovertopping,erent to different failure processesand failure soil erosion.processesand modes. However, and Therefore, modes. a varietyTherefore, the influenceof experimentalthe influence of the relative ofconditions the relative factors will factors on inevitably the on dam the failurelead dam tofailure modes different modes need failure toneed be processestoexplored be explored inand di ffmodes. inerent different stages. Therefore, stages. the influence of the relative factors on the dam failure modes need to be explored in different stages. Water 2019, 11, 1115 13 of 17

Water 2019, 11, x FOR PEER REVIEW 14 of 18 (1) Impoundment Stage (1) Impoundment Stage In this stage, the water level rapidly increased as the inflow discharge began to fill the dam’s In this stage, the water level rapidly increased as the inflow discharge began to fill the dam’s reservoir. At the same time, the dam was infiltrated and deformed (e.g., piping). Figure 15 shows reservoir. At the same time, the dam was infiltrated and deformed (e.g., piping). Figure 15 shows the the physical dimensions of the landslide dam, where Vk is the reservoir storage capacity, hl is the physical dimensions of the landslide dam, where Vk is the reservoir storage capacity, hl is the dam’s dam’s height, θ is the flume gradient, β is the slope angle of the upstream face of the dam, and B is the height, θ is the flume gradient, β is the slope angle of the upstream face of the dam, and B is the dam’s width. dam’s width.

FigureFigure 15. 15. PhysicalPhysical dimensions dimensions of of the the landslide landslide dam. dam.

TheThe reservoir reservoir storage storage can can be be expressed expressed as as follows: follows:

11 2 VhV =×= h（2cot(cotθβθ − cotcot β）) × B (2)(2) k1k 22 × 1 − ×

AccordingAccording toto EquationEquation (1), (1), the the reservoir reservoir storage storageVk will Vk decreasewill decrease as the as flume the gradientflume gradientθ increasesθ increaseswhen all ofwhen the otherall of variables the other arevariables constant. are Basedconsta onnt. theBased principle on the ofprinciple water balance, of water the balance, volume the of volumeseepage of water seepage is: water is: VS ==−Qintin Vk (3) VQtVSinink− (3) wherewhere QQinin isis the the inflow inflow discharge, discharge, ttinin isis the the time time of of water water storage, storage, and and VVss isis the the volume volume of of seepage seepage water.water. A A greater greater upstream upstream flow flow and and a a shorter shorter filling fillingtime time will will result result in in shorter shorter seepage seepage time time before before overtopping.overtopping. Therefore, inin thethe impoundment impoundment stage, stage, thethe process process is mainly is mainly affected affected by two by factors:two factors: Slope Slopegradient gradient and inflow and inflow discharge. discharge. (2) Seepage Stage (2) Seepage Stage In this stage, the fine particles were continuously transported by seepage water [27,28]. Because of theIn randomnessthis stage, the of fine the particles grain composition were continuous and thely distributiontransported ofby dam seepage material, water the [27,28]. seepage Because path ofis the randomness key factor for of controlling the grain composition the results of and the the four distribution different experiments: of dam material, (1) A the complete seepage seepage path is thepath key formed, factorand for thecontrolling inflow discharge the results was of equal the fo tour the different seepage experiments: flow, so the dam (1) A did complete not fail (Numberseepage path1 in Tableformed,1). and (2) Athe complete inflow seepagedischarge path was formed, equal to and the then, seepage piping flow, occurred so the (Numbersdam did not 2–4 fail in (NumberTable1). (3)1 in The Table fine 1). particles (2) A complete were transported seepage pa byth seepageformed, waterand then, and piping blocked occurred the seepage (Numbers path, 2–4increasing in Table the 1). partial (3) The pore fine water particles pressure. were Finally, transported a complete by seepage seepage water path and had blocked not yet formed the seepage when path,overtopping increasing occurred the partial (Number pore 5 inwater Table pressure1). (4) The. Finally, high rate a ofcomplete inflow rate seepage led to path rapid had filling not of theyet formedreservoir. when As theovertopping seepage time occurred was limited (Number in these5 in Tabl tests,e 1). most (4) ofThe the high mine rate waste of inflow were unsaturated rate led to rapidwhen filling overtopping of the reservoir. occurred (NumberAs the seepage 6~13 in time Table was1). limited in these tests, most of the mine waste were Therefore,unsaturated the when factors overtopping influencing occurred landslide (Number behavior 6~13 during in Table this 1). stage are as follows: (1) The seepageTherefore, time, and the (2)factors the grain influencing size distribution landslide andbeha compositionvior during ofthis the stage dam are material, as follows: especially (1) The the seepagecontent andtime, distribution and (2) the grain of the size large distribution particles and and fine composition particles. Theof the large dam particles material, played especially a role the in contentthe skeleton and distribution and support of structures, the large soparticles their content and fine and particles. distribution The large determine particles whether played a completea role in theseepage skeleton path and will support formed. structures, The content so oftheir the cont fineent particles and distribution determines determine whether the whether seepage a complete path will seepagebe blocked. path will formed. The content of the fine particles determines whether the seepage path will be blocked.

(3) Overtopping Stage Water 2019, 11, 1115 14 of 17

(3) Overtopping Stage Water 2019, 11, x FOR PEER REVIEW 15 of 18 When the dam suffered overtopping, a channel was formed and quickly cut down into the dam materialWhen as the dam overflow suffered continuously overtopping, eroded a channel the back was surface formed of theand dam. quickly Afterwards, cut down the into material the dam of materialthe dam onas the both overflow sides of thecontinuously spillway was eroded unstable the back and continuouslysurface of the failed. dam. Afterwards, The spillway the was material rapidly ofwidened the dam by on strong both headward sides of the erosion. spillway Finally, was the un entirestabledam and failedcontinuously and eroded. failed. Quantitative The spillway analysis was rapidlyof this stage widened indicates by thatstrong overland headward flow occurrederosion. onFinally, downstream the entire face whendam thefailed water and overflowed eroded. Quantitativeand some of theanalysis mine of waste this stage started indicates to move that (Figure overland 15). Theflow force occurred analysis on downstream of mine waste face particles when theon thewater downstream overflowed face and is some shown of inthe Figure mine 16waste, in which startedF Dtois move drag (Figure force, F L15).is theThe uplift force force,analysisW0 ofis minegravity, waste and particlesf is frictional on the resistance. downstream Because face the is mineshown waste in Figure was loose 16, in and which accumulated FD is drag naturally, force, FL the is thecohesive uplift force force, was W’ ignoredis gravity, in thisand analysis. f is frictional resistance. Because the mine waste was loose and accumulated naturally, the

Figure 16. ForceForce analysis analysis of of the the mine waste particles in the downstream face.

The gravity W’W0 cancan be be expressed expressed as: π π 33 WDW0'()=−= D (γγγs γ) (4)(4) 66 s − The frictional resistance of the downstream face ff can be expressed as: =+−αθ ϕ fW['cos( )]]tan FL (5) f = [W0 cos(α + θ) FL] tan ϕ (5) − The drag force FD can be expressed: The drag force F can be expressed: D ρ ×ν 2 FCA= (6) DDρ ν2 F = C A ×2 (6) D D 2 The uplift force FL can be expressed as: The uplift force FL can be expressed as: ρ ×ν 2 = FCALL (7) ρ 2ν2 FL = CLA × (7) where D is the particle size, γs is the unit weight of particles,2 γ is the unit weight of water, α is the slope angle of the downstream face of the dam, θ is the flume gradient, φ is the internal friction where D is the particle size, γs is the unit weight of particles, γ is the unit weight of water, α is the slope angle, CD is the drag force coefficient, CL is the uplift force coefficient, v is the flow velocity, A is the angle of the downstream face of the dam, θ is the flume gradient, ϕ is the internal friction angle, CD is maximum cross-sectional area of the particles, and ρ is the water density. In the overtopping stage, the drag force coefficient, CL is the uplift force coefficient, v is the flow velocity, A is the maximum thecross-sectional limit of the area equilibrium of the particles, equation and forρ is the the particles water density. in the In downstream the overtopping face stage, of the the dam limit can of thebe expressedequilibrium as equation follows: for the particles in the downstream face of the dam can be expressed as follows: ρπvv2 π ρ2 CDA2 +−+=−+−33 (γγ )sin()[(αθ Dγγ )cos()αθ CA2 ]tanϕ (8) DSρv π 3 π 3 s Lρv CDA 26+ D (γ γ) sin(α + θ) = [ 6D (γs γ) cos(α + θ) CLA 2] tan ϕ (8) 2 6 S − 6 − − 2 By further simplifying Equation (8), we can obtain the relationship between the maximum particle size that can be moved and the overflow velocity: 3(ρϕvC2 + C tan) D = DL γγ−+−+αθ ϕ αθ (9) 4(s )[cos( ) tan sin( )] Based on our field investigations, the internal friction (φ) of the mine wastes usedin our experiments was 41.6°. The maximum particle size was 5 cm, and the water density was 1 g/cm3. As Water 2019, 11, 1115 15 of 17

By further simplifying Equation (8), we can obtain the relationship between the maximum particle size that can be moved and the overﬂow velocity:

2 3ρv (CD + CL tan ϕ) D = (9) 4(γs γ)[cos(α + θ) tan ϕ sin(α + θ)] − − Based on our field investigations, the internal friction (ϕ) of the mine wastes used in our Water 2019, 11, x FOR PEER REVIEW 16 of 183 experiments was 41.6◦. The maximum particle size was 5 cm, and the water density was 1 g/cm . As in the model test conducted by Chepil [29], the value of CL was set as 0.068, and CL/CD was set in the model test conducted by Chepil [29], the value of CL was set as 0.068, and CL/CD was set as as 0.53. The outflow velocity measured using the float method was from 0.16 m/s to 1.50 m/s in 0.53. The outflow velocity measured using the float method was from 0.16 m/s to 1.50 m/s in the the experimental tests. The initial 40◦ slope of the downstream face gradually decreased during the experimental tests. The initial 40° slope of the downstream face gradually decreased during the experiments due to the headward erosion. Equation (8) can be solved using Matlab. Figure 17 shows experiments due to the headward erosion. Equation (8) can be solved using Matlab. Figure 17 shows the relationship between the maximum particle size that can be moved (D), the flow velocity (v), and the relationship between the maximum particle size that can be moved (D), the flow velocity (v), and the total slope angle of the downstream face (α + θ). The failure process of the landslide dam was the total slope angle of the downstream face (α + θ). The failure process of the landslide dam was determined based on the particle size of the mine waste and the shape of the dam (dam height, slope determined based on the particle size of the mine waste and the shape of the dam (dam height, slope angle of the downstream face, outflow velocity, and discharge). angle of the downstream face, outflow velocity, and discharge).

Figure 17. Relationship between the maximum particle size that can be moved (D), the flow velocity Figure 17. Relationship between the maximum particle size that can be moved (D), the flow velocity (v), and the total slope angle of the downstream face (α + θ). (v), and the total slope angle of the downstream face (α + θ). However, the failure of landslide dams is a complicated process. Because of the limitations of the developedHowever, model, the ourfailure research of landslide is a preliminarily dams is a complicated study of the failureprocess. modes Because of landslideof the limitations dams, and, of thethus, developed it has two model, obvious our shortcomings: research is a preliminarily study of the failure modes of landslide dams, and, thus, it has two obvious shortcomings: (1) Dams formed by the stacking of mine exhibit differences due to the randomness of stacking. (1) DamsThis alsoformed occurred by the in stacking our experiments of mine exhibit because differences it was di ffiduecult to to the artificially randomness control. of stacking. Finally, Thisthe dialsofferences occurred and in randomness our experiments in stacking because led it towas diff difficulterent experimental to artificially results control. between Finally, tests the differencesNumber 2–5 and (Table randomness1), even though in stacking there wasled littleto different di fference experimental in the working results conditions. between tests (2) NumberDuring the2–5 tests,(Table we 1), observedeven though diff erencesthere wa ofs little in the difference dam failure in the modes working and conditions. processes due to (2) Duringchanges the in thetests, inflow we dischargeobserved anddifferences the slope of gradient in the dam of the failure experimental modes flume.and processes However, due it was to changesfound that in the particleinflow discharge composition and and the distributionslope gradient of dam of the material experimental played anflume. important However, role init was found that the particle composition and distribution of dam material played an important role in the failure process, which will need to be investigated further in future research. In addition, the flume was only 0.3 m in width, and the experimental data were inevitably affected by side wall boundary effects. Therefore, in future studies, these research results still need to be verified using a larger experimental model to collect the data.

5. Conclusions Based on field investigations of landslide dams in the Xiaoqinling gold mining area, we determined failure modes and process of landslide dams by 13 flume experiments under different working conditions. The conclusions of this study are as follows: Water 2019, 11, 1115 16 of 17

the failure process, which will need to be investigated further in future research. In addition, the flume was only 0.3 m in width, and the experimental data were inevitably affected by side wall boundary effects. Therefore, in future studies, these research results still need to be verified using a larger experimental model to collect the data.

5. Conclusions Based on field investigations of landslide dams in the Xiaoqinling gold mining area, we determined failure modes and process of landslide dams by 13 flume experiments under different working conditions. The conclusions of this study are as follows:

(1) There were three failure modes of the 13 mine waste dams in our experiments: (i) Piping, overtopping, and erosion; (ii) overtopping and soil collapse; and (iii) overtopping and erosion. (2) The occurrence of the seepage before overtopping was extremely important. If the grain composition of the dam was conductive to seepage, piping occurred. However, if the dam remained stable, it would experience overtopping, which was a key factor triggering the conditions for dam failure. In this stage, the dam failed due to soil collapse or soil erosion. (3) The inflow and slope gradient of the flume had significant impacts on the failure modes and processes of the landslide dams. The reservoir storage was determined using the slope gradient of the upstream face of the dam, the dam height, and the slope gradient of the flume. The seepage time before overtopping occurred was determined using the inflow discharge and the reservoir storage. In addition, the failure process for the landslide dam was determined using the particle size of the mine waste and the shape of the dam (dam height, slope angle of the downstream face, outflow velocity, and discharge), which determine the discharge of the outburst flood (or debris flow).

Author Contributions: Conceptualization, J.P. and X.Z.; methodology, J.P.; software, W.G.; validation, X.Z., W.G. and C.J.; formal analysis, C.J.; investigation, C.J. and W.G.; resources, X.Z.; data curation, X.Z.; writing—original draft preparation, X.Z.; writing—review and editing, X.Z.; visualization, X.Z.; supervision, J.P.; project administration, J.P.; funding acquisition, J.P. and X.Z. Funding: This research was funded by the National Natural Science Foundation of China (Grant No. 41790441, 41877249 and 41402255) and Fundamental Research Foundation of the Central Universities (300102269211). Acknowledgments: We also thank Penghui Ma for his kind assistance with the flume experiments. Conflicts of Interest: The authors declare no conflict of interest.

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