minerals Article Static Liquefaction Causes the Flow Failure of a Tailings Dam: A Case Study of El Descargador, Cartagena–La Unión Mining Region, SE Spain (October 1963) Roberto Rodríguez-Pacheco 1, * , Ana Vanesa Caparrós 2 , Alberto Alcolea 2 , Pedro Martínez-Pagán 3 , Marcos A. Martínez-Segura 3 , Cristóbal García-García 3 , Ángel Faz 4 , Isaac Corral 5 , Carles Roque 6 and Mario Zarroca 5 1 Instituto Geológico y Minero de España, CSIC, Ríos Rosas, 23, 28003 Madrid, Spain 2 Servicio de Apoyo a la Investigación Tecnológica (SAIT), Universidad Politécnica de Cartagena, Edificio I+D+i, 30202 Cartagena, Spain 3 Department of Mining and Civil Engineering, Universidad Politécnica de Cartagena, 30203 Cartagena, Spain 4 Sustainable Use, Management and Reclamation of Soil and Water Research Group, Escuela Técnica Superior de Ingeniería Agronómica, Universidad Politécnica de Cartagena, Paseo Alfonso XIII, 52, 30203 Cartagena, Spain 5 Departamento de Geología, Universitat Autònoma de Barcelona, Campus UAB, s/n, Bellaterra, 08193 Barcelona, Spain 6 Àrea de Geodinàmica Externa i Geomorfologia, Universitat de Girona, Campus Montilivi, 17003 Girona, Spain * Correspondence: roberto.rodriguez@igme.es; Tel.: +34-917287235 Abstract: The aim of this work is to investigate the causes of the El Descargador tailings dam failure, Citation: Rodríguez-Pacheco, R.; at the mine district Cartagena–La Unión (SE Spain), in October 1963. Dam stability back analyses Caparrós, A.V.; Alcolea, A.; have been carried out by applying a geotechnical and geophysical approach. The failure occurred Martínez-Pagán, P.; Martínez-Segura, in the form of several landslides in five different points along the dam structure. The rise in the M.A.; García-García, C.; Faz, Á.; pore pressure and the steeped slopes of the tails, scaling up to 40◦ in some sectors, were the main Corral, I.; Roque, C.; Zarroca, M. Static Liquefaction Causes the Flow causal preparatory factors. Here we propose that static liquefaction is the most plausible cause Failure of a Tailings Dam: A Case of the tailings dam flow failure. The presence of sand dikes and sand volcanoes with atypical Study of El Descargador, stratigraphic architecture, both in the lagoon and at the surfaces exposed in the landslide areas, Cartagena–La Unión Mining Region, as well as the evidence of conspicuous sand fraction on the surface support the occurrence of the SE Spain (October 1963). Minerals liquefaction processes. Major landslides were located near the drainage pipe and the flow directions 2022, 12, 1488. https://doi.org/ were controlled by its position. Our results reveal that the liquefaction processes were triggered and 10.3390/min12121488 aggravated by the poor drainage capability of the tailings dam structure. Academic Editor: Abbas Taheri Keywords: dam collapse; flow failure; mine tailings; electrical resistivity imaging Received: 20 September 2022 Accepted: 19 November 2022 Published: 23 November 2022 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in El Descargador tailings dam failed catastrophically at 7:45 on Monday, 14 October published maps and institutional affil- 1963, causing an environmental disaster in the Cartagena–La Unión mining district, in the iations. province of Murcia, Spain (Figures 1 and 2). The Cartagena–La Unión mining district is part of the easternmost foothills of the Betic mountain ranges, and it is well known as a highly productive area of metals such as lead, zinc, iron, manganese and silver since ancient times [1]. The tailings dam was part of a large underground mining complex that had Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. been in operation for more than nine years in La Unión municipality. The El Descargador This article is an open access article tailings dam stored tailings from the Belleza mining group. This group is integrated by distributed under the terms and the following Fortuna, Felicidad, Maestra, Belleza, Resucitada, Pronta, Virgendel Carmen, conditions of the Creative Commons Convención and Ferrocarril mining concessions. The activity of the Belleza group included Attribution (CC BY) license (https:// the exploitation of pyrite (12%), sphalerite (10%) and galena (7%). The flotation process creativecommons.org/licenses/by/ of the mine had a capacity of 300 tons of mineral per day. The 75% of the ore entering the 4.0/). flotation process was discharged into the dam as tailings. Considering the volume of daily Minerals 2022, 12, 1488. https://doi.org/10.3390/min12121488 https://www.mdpi.com/journal/minerals Minerals 2022, 12, 1488 2 of 20 production, half a million tons were stored at the time of the dam failure. The confining embankment was conceived in the original design as an upstream construction using the tailings themselves. The method of tailings discharge was hydraulic backfill. The tailings dam construction began in November 1953 and it was 42 m height at the time of the failure. The flow failure developed in five different landslide points (labeled LL; Figure 2). The height of the main landslide (LL3, Figure 3) was 27 m. 66,000 m3 of tailings and high acidic waters were released into the environment affecting: (1) an area of 4 km2 ; (2) different infrastructures (Cartagena–Cabo de Palos highway, the Cartagena–Los Blancos Minerals 2022, 12, x railroad); (3) the watercourse of Beal wadis stream; and (4) the El Mar Menor lagoon, 2 of 22an area of great ecological and patrimonial value in the Mediterranean Sea. Catastrophic mine tailings dam failures are increasing globally [2–12]. However, while the study of its ecologi- cal by theandfollowing socioeconomic Fortuna,consequences has been Felicidad, Maestra, widely Belleza, addressedPronta, Resucitada, in the literature Virgendel[11–17] Car- studies, attempts to address the hydro-geomechanical factors that caused men, Convención and Ferrocarril mining concessions. The activity of the Belleza the failures group are scarce included [18–20]. There is still the exploitation more (12%), of pyrite uncertainty regarding sphalerite historical (10%) and galenafailures (7%). Theof dams that flotation happened process of many decades the mine had aago, as is the capacity casetons of 300 of El of Descargador. ThisThe mineral per day. is an important 75% of the oregap of knowledge, since more than 80 uncontrolled old dams have been inventoried entering the flotation process was discharged into the dam as tailings. Considering the in this mining region [20]. The study of the causes that triggered and controlled the volume of daily production, half a million tons were stored at the time of the dam failure.failure and collapse of ancient The confining tailings dams embankment such as ElinDescargador was conceived the original can provide design as an valuable upstreaminformation construc- to tion using the tailings themselves. The method of tailings discharge wasand be considered in the design and management of such installations its associated hydraulic back- hazards, fill. which are instrumental for the success of any mining operation. Figure 1. (a) Location Location ofof studied studied area area on on the theSpanish Spanishseismic seismicbasebasemap; map;source: source:NCSR-2 NCSR-2(2004) (2004) atat Region of Murcia, (b) El Descargador tailings dam and its area of influence, (c) Beal Region of Murcia, (b) El Descargador tailings dam and its area of influence, (c) Beal wadis and wadis and the oxidation the products oxidation of sulphides products can becan of sulphides seenbeinseen whiteincolor white in color the area in covered the areaby tailings,by(d)tailings, covered Car- tagena–Cabo de Palos highway and the Cartagena–Los Blancos railroad affected by flow failure and (d) Cartagena–Cabo de Palos highway and the Cartagena–Los Blancos railroad affected by flow (e) abandoned metallurgical plant. The discontinuous arrow indicates the direction of the runoff. failure and orthorectified Base maps (e) abandonedaerial metallurgical plant.source photographs; The discontinuous Spanish NationalarrowGeographic indicates the direction Institute of the (IGN). runoff. SchemesBase maps follow theorthorectified aerial photographs; source Spanish National Geographic Institute same formatting. (IGN). Schemes follow the same formatting. Minerals 2022,12, Minerals2022, 12,1488 x 3 of2220 3 of Figure Figure 2.2. (a) Layout of (a) Layout of El El Descargador Descargadortailings tailingsdam damdepicting depictingthe thelocalization localizationofofdifferent differentlandslides landslides (LL), (LL), lagoons lagoons (Lg), (Lg), and and fragments fragments ofof breakwater breakwater (SB) (SB) and and some someareareaffected affectedfor forflow flowfailure. failure.The The discontinuous discontinuous arrow indicates indicates the the direction directionofofthe therunoff, runoff,(b–d) (b–d)Obliquus Obliquusaerial aerialviews viewsofofthe thedam dam from different from different perspectives; source source Spanish SpanishNational NationalGeographic GeographicInstitute-IGN Institute-IGNand andGoogle GoogleEarth© . Earth©. Schemes follow Schemes follow the the same same formatting. formatting. The tailings Here we aimdam construction to stress began in the hypothesis November that 1953 and it was the El Descargador 42 mdam tailings height at the failed due time to of the static failure. The liquefaction. Toflow meetfailure developed the goal, in five characterization a geotechnical different landslide points of the (labeled different ele- LL; Figure ments of the2). The height tailings dam andof the main landslide a back-analysis of the(LL3, Figure failure 3) was 27 m. by geophysical and66,000 m3 of geotechnical tailings and modeling high have acidic been waters carried out.were The released failure ofinto the environment a sloping ground dueaffecting: to static (1) an area liquefaction of 4 km 2; (2) different infrastructures (Cartagena–Cabo de Palos occurs when the shear stress applied by a monotonic triggering load exceeds the undrainedhighway, the Cartagena– Los Blancos yield (peak) shearrailroad); (3) the strength of watercourse the saturatedofliquefiable Beal wadiscohesionless stream; andsoil. (4) the El Mar Menor Triggering mecha- lagoon, nisms foranthis area of great type ecological of tailings dam and patrimonial failure in contractivevalueandin the Mediterranean saturated granular Sea. Cata- materials strophic are usually: mine (1)tailings dam failures slope instability; (2) are increasing a rise globally [2–12]. in the hydraulic However, head; (3) while toe erosion duetheto study of piping its ecological (seepage, anderosion, internal socioeconomic consequences runoff erosion); has been widely (4) overtopping; addressed (5) rapid in theof application literature surface [11–17] loading duestudies, attemptsoftonew to the discharge address materials;the and hydro-geomechanical (6) important storage factors volume thatof causedonthe water failures the surface are orscarce [18–20]. as a result Theremechanisms of other is still more uncertainty regarding to [1,20–22]. According historical existing failures and reviews of dams that happened databases on tailingsmany decades static dam failures, ago, as is the case isofnot liquefaction El Descargador. This considered relevant is one as an important gap of knowledge, of the fundamental mechanisms sinceofmore failure than 80 uncontrolled [1,12,22,23]. old damsanhave Unfortunately, been important inventoried number in thisdam of tailings mining region failures due[20]. The liquefaction to static study of thehave causes beenthat triggered and misinterpreted ascon- other trolledmodes, failure the failure and and onlycollapse few caseofstudies ancienthavetailings beendams suchinterpreted correctly as El Descargador can pro- due to static lique- vide valuable faction [20,24].information to be considered Static liquefaction phenomena in theand design and management the resulting land slidingof such instal- and tailings lations and its liquefaction haveassociated hazards,related been commonly which to arethe instrumental more hazardous for the success historic of anydam tailings mining flow operation. failures. Noticeable incidents of destructive mudflows caused by tailings dam breaching are: (1) the Befokeng platinum ring-dike tailings dam failed as a result of overtopping after a large storm [17,25]; (2) Merriespruit, Harmony Mine, South Africa ring-dike gold tailings dam failed in 1994 in many similar aspects to Bafokeng failure. The failure occurred after a significant rainfall event [26]; (3) active iron pond tailings impoundment at the Sullivan Mine, Canada, failed in August of 1991 due to static liquefaction [27]. (4) The two tailings dams in a fluorite mine located near Stava (northern Italy) suddenly failed and released approximately 240,000 m3 of liquefied tailings. The failure occurred on 19 July 1985. This Minerals 2022, 12, 1488 4 of 20 was perhaps the most tragic tailings dam failure in history. The liquefied mass moved up to a speed of 60 km/h obliterating everything in its path for a length of ~4 km. The failure was initiated not because of increasing shear stresses and undrained loading but because of the decreasing effective stress due to the rising phreatic surface under drained conditions [28]; and (5) tailings dam Los Frailes mine, Spain, 1998, which is possibly the most reported tailings dam failure. A shallow foundation failure led to the release of more than 3 million m3 of processed water and tailings from one of the two adjacent ponds within an overall Minerals 2022, 12, x impoundment [18]. According to different studies, these five cases are clear 4 of 22 examples of static liquefaction [1,18,20,21]. Figure Figure3.3.(a) (a)ERI ERIprofiles layout profiles on aon layout hill a shaded elevation hill shaded modelsmodels elevation of El Descargador tailings dam; of El Descargador tailings dam; (a) actual state; (b) tentative reconstruction of the relief before the 1963′s failure; 0 and (a) actual state; (b) tentative reconstruction of the relief before the 1963 s failure; and (c) Representa- (c) Representative tive topographic profiles of the main landslide before (green) and actual surface of failure dashed topographic profiles of the main landslide before (green) and actual surface of failure dashed lines. lines. The white dashed arrow indicates the stream runoff direction. The white dashed arrow indicates the stream runoff direction. Here we aim to stress the hypothesis that the El Descargador tailings dam failed due The importance of modeling the mechanical behavior of earth dams is the knowledge to static liquefaction. To meet the goal, a geotechnical characterization of the different el- ements ofgeotechnical on their the tailings dam features. Geotechnical and a back-analysis of characterization of largeand the failure by geophysical earth structures is a geotech- challenging nical modeling issue. haveNumerical been carriedsolutions requireofselecting out. The failure a slopingmanyground parameters, due to staticand sometimes- lique- geological faction models occurs when theare shear oversimplified stress applied to achieve simplertriggering by a monotonic numerical loadmodels. exceedsDirectthe obser- vations undrained byyield means(peak)of boreholes shear strength andoflaboratory the saturated tests offer valuable liquefiable information, cohesionless soil. Trig- but they have several gering mechanismseconomical and of for this type logistical tailings damconstraints failure inand sometimes contractive they failgran- and saturated to be enough ular materials areof representatives usually: (1) slope instability; the characteristics of such(2)large a risestructures. in the hydraulic In thishead; (3) toe regard, moreero-extensive sion due to piping geophysical (seepage, surveying internal erosion, techniques, althoughrunoff erosion); they (4) overtopping; are penalized (5) rapid by a lower resolution, application may be useful of surface loading more to construct due torepresentative the discharge ofgeological new materials; and at models (6)field-scale. important Electri- storage volumeimaging cal resistivity of water on the has (ERI) surface or as a result experienced of other major mechanismsin improvements [1,20–22]. Ac- recent decades [27]. cording to existing reviews and databases on tailings dam failures, static liquefaction is Among other applications, ERI has been increasingly used for geotechnical characterization not considered relevant as one of the fundamental mechanisms of failure [1,12,22,23]. Un- of tailings dams and their emplacements [28–34]. Bulk resistivity of materials is mainly fortunately, an important number of tailings dam failures due to static liquefaction have controlled been by porosity, misinterpreted fluid as other content, failure modes, hydrochemistry and only few case and mineralogy. studies have been Therefore, correctly ERI has interpreted due to static liquefaction [20,24]. Static liquefaction phenomena and the result- and the strong capabilities to characterize the stratigraphy of the tailings dams, resistivity groundwater ing land slidingflow and conditions [11,30]. have tailings liquefaction A tailings dam should been commonly be considered related to the more as haz-a complex multilayer aquifer system, in which there are different hydraulic ardous historic tailings dam flow failures. Noticeable incidents of destructive mudflows relationships between the waters caused by stored in the tailings damlagoons breaching and the(1)groundwater are: the Befokeng that flowsring-dike platinum throughtailings sedimentary dam layers failed as a result of tailings of overtopping [11,28–36]. In addition, afterstudies a large ofstorm ERT[17,25]; work on (2) mine Merriespruit, tailingsHarmony dams that assess Mine, South Africa ring-dike gold tailings dam failed in 1994 in many similar aspects to Bafokeng failure. The failure occurred after a significant rainfall event [26]; (3) active iron pond tailings impoundment at the Sullivan Mine, Canada, failed in August of 1991 due to static liquefaction [27]. (4) The two tailings dams in a fluorite mine located near Stava Minerals 2022, 12, 1488 5 of 20 environmental risk and groundwater flow are relatively abundant [11,28–36]. However, combined ERT and geotechnical studies are more limited [18]. The main limitation of ERT studies is that they do not determine the key parameters for the stability of tailings dams: gravimetric moisture, volumetric water content, degree of saturation, density, etc. Hence, ERI data combined with geotechnical and hydrogeological information may be useful for modeling the flow conditions that could have triggered and controlled the assumed static liquefaction phenomenon. 2. Materials and Methods 2.1. Characterization of the Tailings Dam Geotechnical mapping of the tailings dam was performed by fieldwork and inspection of the aerial photographs and satellite images and topographical data (resolution 1 m × 1 m; source: Instituto Geográfico Nacional) [36–38]. The topographical model was constructed by Global Mapper software (V.18.1; Figure 3a). A tentative reconstruction of the previous topography of the tailings dam failure was carried out by AutoCAD (Autodesk) software, V 10 (Figure 3b). Such pre-failure topographical reconstruction was used as a base model for the numerical stability back-analysis (Figure 3c). A sampling campaign was performed in May 2015 in two different zones: trial-pit profile and sampling of the sand dikes (Figure 4). The sampling aimed at comparing both unaltered and liquefacted materials. Up to nine representative tailings beds were sampled Minerals 2022, 12, x on the left side of a trial pit (1.7 m high and 1 m wide), which was dug in6the northern of 22 sector of the East wall of the base of the tailings dam (Figure 2). Around 2 kg samples were collected, P1 being the lowest and P9 the shallowest sampling points. Once collected, they were preserved in sealed plastic bags to avoid quick oxidation. An additional 13 samples were extracted from several subvertical sand dikes crosscutting the fault plane in the north were wall extracted of the damfrom several (Figure subvertical 4), W1 sand dikes being the bottom crosscutting and W13 being thethe topfault plane sample in the north (Figure wall 4b). of the dam (Figure 4), W1 being the bottom and W13 being the top sample (Figure 4b). Figure 4. Figure 4. (a) (a)El ElDescargador Descargador tailings dam, tailings (b) trial dam, pit excavated (b) trial in the in pit excavated East wall, the East(c)wall, sampling points points (c) sampling of sand dikes exposed on the surface of the landslide four (LL4), (d) detail sedimentary layer and of sand dikes exposed on the surface of the landslide four (LL4), (d) detail sedimentary layer and sampling points by different layer in vertical profile. The white color indicates the precipitation of sampling points the different typesby ofdifferent sulphateslayer in verticaltoprofile. that contribute The white the cementation of color indicates the tailings in thethe precipitation dam. The of the different types discontinuous arrowof sulphates indicates that contribute the direction to therunoff. of the stream cementation of the tailings in the dam. The discontinuous arrow indicates the direction of the stream runoff. 2.2. Particle Size Distribution The particle size disftribution (PSD) was determined by a Mastersizer 2000LF (Mal- vern Instruments Ltd. [39]), a coarse grain-size analyzer with a coupled dispersant unit (Hydro 2000 G). The analyzer covers a measurement range from 0.02 to 2000 µ m with two light sources: the red light (633 nm) comes from a He-Ne laser and the blue light (466 nm) comes from a solid-state light source. Laser diffraction (LD) technique bases on the mod- Minerals 2022, 12, 1488 6 of 20 2.2. Particle Size Distribution The particle size disftribution (PSD) was determined by a Mastersizer 2000LF (Malvern Instruments Ltd. [39]), a coarse grain-size analyzer with a coupled dispersant unit (Hydro 2000 G). The analyzer covers a measurement range from 0.02 to 2000 µm with two light sources: the red light (633 nm) comes from a He-Ne laser and the blue light (466 nm) comes from a solid-state light source. Laser diffraction (LD) technique bases on the modeled PSD calculation from an angular pattern of equivalent scattered light intensities, which provides a volume-based size distribution of a group of spherical particles. In this case, the LD system analyzed the dispersed dry soil fraction in water, performing the measurements off-line. A medium-to-low particulate concentration is required to have enough signal-to- noise ratios to avoid multiple scattering. Obscuration—the percentage of incident light removed by particles—indicates the suitable concentration limits. An ideal obscuration range for this type of suspensions is 5%–15%. 2.3. Electrical Resistivity Imaging (ERI) ERI is a non-invasive geophysical method devoted to capture the resistivity distri- bution in the subsoil by multielectrode schemes [25,40,41]. The moisture content and the hydrochemistry of the pore fluids are probably the dominant factors controlling the bulk resistivity of sediments in many cases [28–36]. Mild changes in such parameters cause rough variations in resistivity, which can reach up to several magnitude orders. Since tailings typically exhibit significant changes in moisture, hydrochemistry and textural features, ERI is postulated as a useful technique for tailings dam exploration [11,29–35]. The ERI survey was conducted in April 2004. A Syscal R1 Plus (IRIS Instruments) resistivity meter, equipped with 75 electrodes, spaced every 5 m was used to gather two parallel ERI profiles, oriented N-S (Figure 3). Several reverse measuring cycles and long current injection periods were set to deal with self-potential cancellation. The hybrid Wenner–Schlumberger array was selected as the collecting array since it provides enhanced data coverage compared to Wenner, while it maintains an analogous high signal-to-noise ratio [42–52]. The apparent resistivity records (pseudosection) (Figure 5a) were inverted to construct “real” resistivity images (best fitted models) by EarthImager2D software (Advanced Geosciences, Inc.). The numerical solutions were solved by the least-square smoothness-damping constrained method (Occam’s inversion) [52–55]. A smoothed in- version was selected due to its stability and consistency, and a homogeneous half-space with a resistivity equal to the average of the recorded resistivity datasets was used as the starting model [56]. Several quality control criteria were considered prior to the geolog- ical interpretation of the resistivity images. On the one hand, potential noisy data were excluded from the inversion based on the root mean square (RMS) of the residuals from a preliminary inversion, considering a cutoff threshold of ±15% RMS (Figure 5b). Overall, just some noticeable data points exhibited significant RMS in the central part of the images. More rigorous cutoff criteria were not imposed, since some features are interpreted as inversion artifacts that could offer relevant information on the presence of fractures [28,57–59]. The final model residuals do not exceed an RMS of 5%. On the other hand, the DOI- index was computed with Res2DInv software (Geotomo) to identify the depth at which the resistivity images were poorly constrained by the data [60]. Two constant resistivity- starting models of 0.1 and 10 times the average of the measured apparent resistivity were used for the calculation. The average of apparent resistivities was 17 Ωm (ERI1) and 16 Ωm (ERI2) in this case. Several authors advise a DOI cutoff value of 0.1 to 0.2 or, alternatively, considering the limit according to the maximum gradient [60,61]. Based on both criteria, it is possible to identify poorly constrained sectors, as those located in the northern sector of ERI1, at position x = 250 m (Figure 5c), which should be interpreted with extreme caution. starting model [56]. Several quality control criteria were considered prior to the geological interpretation of the resistivity images. On the one hand, potential noisy data were ex- cluded from the inversion based on the root mean square (RMS) of the residuals from a preliminary inversion, considering a cutoff threshold of ±15% RMS (Figure 5b). Overall, Minerals 2022, 12, 1488 just some noticeable data points exhibited significant RMS in the central part of the 7im-of 20 ages. Qualityassessment Figure5.5.Quality Figure assessment ofof the the ERI1 ERI1 profile, profile, (a)(a)recorded recorded pseudosection, pseudosection, (b)(b) Data Data misfit misfit pseu- pseu- dosectionrepresenting dosection representingthe therelative relativedata dataerror errorbetween betweenthe thecalculated calculatedand andthe themeasured measuredapparent apparent resistivity resistivityrecords records(model (modelresiduals), residuals), (c) (c) Depth of investigation investigationindex index(DOI) (DOI)computed computedasasthethe differ- difference between subsequent inversions setting the starting models to 1.7 and 170 Ωm, respectively. ence between subsequent inversions setting the starting models to 1.7 and 170 Ωm, respectively. 2.4.More Numerical Slopecutoff rigorous Stability Modeling criteria were not imposed, since some features are interpreted as inversion artifactsof The modeling that thecould offer stability relevant factors wasinformation performed by on means the presence of fractures of a coupled hydro- mechanical [28,57–59]. Thefine element final modelformulation, residuals do considering not exceed andifferent RMS ofscenarios 5%. On the and boundary other con- hand, the ditions (Tables DOI-index S2 and S3).with was computed TheRes2DInv computation was made software by theto (Geotomo) software identifySlide (Rock at the depth Sci- ence).the which Slide is a 2D images resistivity slope stability analysis were poorly softwareby constrained that theuses datalimit [60].equilibrium Two constant methods resis- for stability computation. tivity-starting models of 0.1 andThis10includes times thethe following average of thecalculation methods:resistivity measured apparent simplified Bishop method (1955); simplified Janbu method (1954), Janbu corrected method (1968) and GLE/Morgenstern–Price method (1965). These methods are based on the Mohr–Coulomb failure criterion that considers a linear relationship between normal and shear stresses at failure planes (Equation (1)), which has proven to be a robust hypothesis to model earth slope failures. Geotechnical parameters for the computation were set according to the results obtained (Table S2) and values proposed by [12,62,63]. τ = (σ − u) × tang ϕ + c + s (1) where τ is the shear stress, σ is the normal stress on the sliding plane, u is the pore water pressure, ϕ is the internal friction angle, c is the cohesion, and s is suction. The last two variables in Equation (1) are the shear stress interparticle components that are characteristics of the mine tailings material. The value of cohesion in the mine tailings studied is zero. If they are saturated, the suction is zero. In our case, 13 boundary conditions have been considered for the calculation of the safety factors (Table S3). 3. Results 3.1. Geotechnical and Geophysical Features of the Tailings Dam In general, the tailings dam is an irregular pyramidal structure, with platforms and lagoons distributed at different elevations. The local relief is slightly more than 40 m (Figures 2 and 3). The dam settles on two different geological materials, with contrasted geomechanical features. While the northern sector overlies quaternary alluvial sediments Minerals 2022, 12, 1488 8 of 20 (sandstone, limestone, and conglomerate), whose thickness may exceed 20 m, the southern sector already settles on the local Betics complex bedrock are formed by micaschists, white quartzites, marbles and “green” rocks [10,64,65]) (Figure S1). Such bedrock is shown in resistivity images as medium-resistivity materials (ρ > 50 Ωm; Figure 6). A different pattern is observed from north to south on both ERI profiles. While the resistivity is similar in the southern sector (ρ > 50 Ωm), it is more heterogeneous and lower toward the north, where the metamorphic bedrock is overlain by quaternary alluviums. On the other hand, Minerals 2022, 12, x noticeable inversion artifacts are shown beneath the midpoint of the profiles. It should be 9 of 22 noted that these features are shown in an area well-constrained by the data, according to the computed DOI-indexes (Figure 5b). Figure 6. (a) Modeled geological profile S–N oriented along the El Descargador tailings dam Figure 6. (a) Modeled geological profile S–N oriented along the El Descargador tailings dam em- emplacement. placement. (b) Inverted (b) Inverted resistivity resistivity imagesimages and tentative and tentative of geological of geological interpretation interpretation of ERI1 of ERI1 and and ERI2 collected by Wenner–Schlumberger array on the dam structure (see emplacements ERI2 collected by Wenner–Schlumberger array on the dam structure (see emplacements ERI1 ERI1 and ERI2 and ERI2 in3a). in Figure Figure 3a). Therefore, we interpret the inversion artifacts as model misfits caused by rough Therefore,variation resistivity we interpret the inversion gradients, artifacts assolved which are awkwardly modelby misfits caused by rough re- the smoothed-constrained sistivity variation gradients, which are awkwardly solved by the smoothed-constrained inversion method. Such rough gradients have been frequently related to fault areas, in inversion method. different Such roughcontexts geomorphological gradients have been [28,58,59]. frequently The direct related observation to fault of the areas, fault was notin different geomorphological possible since the area is contexts covered by[28,58,59]. Theand the tailings direct observation the alluvial of the fault by fans sedimented wasthenot possible since the two ravines area is covered bordering bydam. the tailings the tailings However, and thethe alluvial break in the fans slopesedimented that occurs inbythe the area—even two ravines a possible bordering thedegraded fault However, tailings dam. facet is observed in the the break inNW–sector—as well asin the slope that occurs the the accommodation area—even of the a possible alluvial fault degraded fans themselves are considered facet is observed as the possible geomorphic in the NW–sector—as well as the accommodation of the alluvial fans themselves are considered as the possible geomorphic expression of the fault imaged by ERI, and which would form part of the East-trending fault system already mapped (e.g., Roma fault) (Figure 6a). The mine wastes are shown as low-resistivity materials (ρ < 20 Ωm). The internal Minerals 2022, 12, 1488 9 of 20 expression of the fault imaged by ERI, and which would form part of the East-trending fault system already mapped (e.g., Roma fault) (Figure 6a). The mine wastes are shown as low-resistivity materials (ρ < 20 Ωm). The internal structure of the sedimentary layer of tailings could be observed on the walls of the test pit. The pit exposed an interbedded sequence of sands and a very fine-grained dark gray material (Figure 4a). The tailings layers thickness ranged from a few mimileters to several centimeters. Coarse- to medium-grained dark gray materials are coated by red-to- white hydrated sulphates, iron hydroxisulphates, and iron oxyhydroxides precipitation compounds. All sediments were completely or almost completely saturated in water regardless of their texture. It should be noted that the apparent cohesion of materials disappears when they become dry. The resistivity is even lower (ρ < 5 Ωm) beneath the lagoons, up to at least 10 m-depth (Figure 6) and within the slid mass, at the foot of the northern sector of the tailings dam. These low-resistivity zones are interpreted as the geophysical signature of the materials saturated in saline waters related to acid mine drainage processes, which are characterized by extremely low resistivity. The ERI profiles image the preferential groundwater flow paths as anomalous low-resistivity areas. Groundwater is recharged preferentially by the lagoons (e.g., Lg2, Lg3 and Lg4), and the preferential flow pathways are controlled by the local hydraulic gradients, induced by the geometry of the basin and the dam itself, stratification and desiccation cracks. Noticeable is that the landslide areas (e.g., LL3 and LL4) also show low-resistivity features, even decades after 1963 (Figure 6). This is because the tailings saturate quickly but dry very slowly due to their low permeability and suction [18]. The very low-resistivity values in the vertical profile below the lagoon are consistent with other work [11,18,29–35]. 3.2. Landslides Geometry The reconstructed topographic model prior to the dam failure restores the localization of the pre-existent five decantation ponds (Figures 2 and 3b). The flow failure directly affected four of these lagoons (Lg1, Lg3, Lg4 and Lg5), which stored a large amount of water at the time of collapse. The lagoons had a little freeboard not exceeding 2 m (Figure 3). The flow failure of the tailings dam took place in five different points (Figures 2 and 3). Landslide LL1 (Figure 2) caused a breach in the wall of the impoundment 10 m wide and 6 m high, through which about 2000 m3 of tailings slurry were released to the stream. LL1 affected a small section of lagoon 1 (Lg1, Figure 2). The water runoff eroded the slope and part of the foot of the dam, where it is still possible observing the contact between tailings and underlain quaternary and metamorphic materials. The landslide LL2 (Figures 2 and 3) breached the impoundment wall along 30 m wide and 15 m high, and 5.000 m3 of slurries reached the stream. Here, whitish precipitation compounds produced by the oxidation of the sulphides, mainly magnesium sulphates, still coat some sectors of the exposed slope surfaces. Landslide LL3 (Figure 2, Figure 3, Figure 9 and Figure 10) was the biggest and the only one where a drainage pipe was installed. The movement of the sliding mass from LL3 came to obstruct temporarily the stream channel. LL3 caused a breach 60 m wide and 27 m height, and ca 50,000 m3 tailings flowed up to temporarily dam the stream channel and buried the drainage pipe. The inspection in 2015 revealed that a drainage pipe was filled with tailings, before the sliding, suggesting that the obstruction and failure of the drainage system contributed to the dam slope destabilization. Some noticeable stratigraphic unconformities characterize LL3 deformation upstream. In LL3, some up to 30◦ dip-tilted tailings beds are juxtaposed to subhorizontal other ones. It should be noted that settling slopes for tailings dams are majorly in the order of 1 to 2◦ . In the north, LL4 breached the impoundment along 35 m wide (Figure 2, Figure 3 and Figure 11) and released ca. 6000 m3 of tailings. The lack of topographic impediments favored the flowing and sedimentation of mining slurries along a surface of 0.2 km2 . Landslide LL5, in the west, depicted a similar behavior compared to LL4, with a breach of 40 m wide and the mobilization of ca. 3000 m3 of tailings along 0.3 km2 . In general, the tailings dam is an irregular pyramidal structure, with platforms and lagoons distributed at different elevations. Minerals 2022, 12, 1488 10 of 20 3.3. Sand Dikes The subhorizontal interbedded sequences of tailings are crosscut by sand dikes, which are typical water-escape structures. Clastic dikes depart from the dam’s foot, with a sheet thickness of 5–10 mm that expanded up to 10–20 cm during the upwelling. Inter- connected sand dikes forming irregular polygonal shapes (quadrilaterals, pentagons, or hexagons), as well as sand volcanoes, are observable on the exposed surfaces along the fault planes (Figures 7–11). The textural characteristics of the tailings present in the sand Minerals 2022, 12, x dikes are very similar to the sands exposed in the trial pit. There are no sand dams 11 of and 22 sand volcanoes in Lg2; it is because the liquefied tailings were released by Lg4, placed at the downstream, while the presence of sand volcanoes has been related to tailings dam failures[13,18,21,66–68], failures [13,18,21,66–68],totoour ourknowledge. knowledge.The Themanifestation manifestationofofsandsand dikes dikes ofof this this mag- mag- nitude has not been previously reported in the literature [1,2,20,21]. The nitude has not been previously reported in the literature [1,2,20,21]. The reconstructed reconstructed topographicmodel topographic modelprior priortotothe thedam damfailure failurerestores restoresthe thelocalization localizationofofthe thepre-existent pre-existentfive five decantation ponds (Figures 2 and decantation ponds (Figures 2 and 3b). 3b). Figure Figure7.7.(a)(a)Massive Massivesandy sandylayer layerininthe thelagoon lagoonone one(Lg1). (Lg1).(b) (b)Detail Detailofofthe theatypical atypicalstratification stratificationofof sand sand volcanoes compared with stratigraphic signature of mining tailings. (c) Sand dikesininlagoon volcanoes compared with stratigraphic signature of mining tailings. (c) Sand dikes lagoon one (Lg1). (d) Sand dikes in landslide one (LL1). (e) Detail of the sand dike emplacement, which cut one (Lg1). (d) Sand dikes in landslide one (LL1). (e) Detail of the sand dike emplacement, which cut crosses the subhorizontal tailings bedding. crosses the subhorizontal tailings bedding. Minerals 2022, 12, 1488 Minerals 2022, 12, x 12 of 22 11 of 20 Figure Figure8. (a) 8. Obliquus view of (a) Obliquus landslide view one (LL1) one of landslide and two (LL2); (LL1) andthetwo discontinuous (LL2); thearrow indicates arrow indicates discontinuous the direction of the stream runoff; source Spanish National Geographic Institute-IGN and Google the direction Earth© of the landslide one. stream Detailed runoff; views source landslide Spanish one (LL1) National (b) and landside 2Geographic Institute-IGN (LL2) (c). (d) Sand dikes and Google Minerals 2022, 12, x Earth©onlandslide exposed the surfaceone. Detailed of landslide twoviews (LL2).landslide one (LL1) (b) and landside 2 (LL2) (c). (d) Sand dikes 13 of 22 exposed on the surface of landslide two (LL2). Figure 9. (a) Obliquus view of landslide three (LL3); source: Spanish National Geographic Institute- Figure 9. (a) Obliquus view of landslide three (LL3); source: Spanish National Geographic Institute- IGN and Google Earth© . Note that the slid mass reached the opposite bank of the stream up to 20 IGN m andfrom height Google Earth©. the channel NoteThe bottom. that the slid dammed obstruction mass reached the hindering the stream, opposite the bank of thetostream up to 20 m mudflow expand height downstream. (b) Remains from the channel of the The bottom. existing drainage pipe obstruction at the time dammed of stream, the the failure. (c) Close- the mudflow to hindering up views of the broken pipeline that is filled of tailings. (d) Pipeline degradation and collapse due expand downstream. (b) Remains of the existing drainage pipe at the time of the failure. (c) Close-up to the inside tailings oxidation. (e) Drainage pipeline decoupling. views of the broken pipeline that is filled of tailings. (d) Pipeline degradation and collapse due to the inside tailings oxidation. (e) Drainage pipeline decoupling. Figure 9. (a) Obliquus view of landslide three (LL3); source: Spanish National Geographic Institute- IGN and Google Earth© . Note that the slid mass reached the opposite bank of the stream up to 20 m height from the channel bottom. The obstruction dammed the stream, hindering the mudflow to expand downstream. (b) Remains of the existing drainage pipe at the time of the failure. (c) Close- Minerals 2022, 12, 1488 12 of 20 up views of the broken pipeline that is filled of tailings. (d) Pipeline degradation and collapse due to the inside tailings oxidation. (e) Drainage pipeline decoupling. Minerals 2022, 12, x 14 of 22 Figure 10. (a) View of landslide three (LL3) from lagoon one (Lg1). (b) View of the dam slide Figure 10. (a) View of landslide three (LL3) from lagoon one (Lg1). (b) View of the dam slide deposit deposit remnants remnants on the stream on the opposite opposite stream bank, bank, which which caused thecaused streamthe stream obstruction. obstruction. (c) View of(c) theView tiltedof the tilted tailings bedding in the right margin of the landslide LL3, which is in contrast tailings bedding in the right margin of the landslide LL3, which is in contrast with the subhorizontal with the subhorizontal bedding bedding of mining of mining tailings. (d) Viewtailings. of the (d) Viewhorizontal almost of the almost horizontal bedding beddingofcharacteristic characteristic the tailings of the tailings deposition, deposition, in the in the left margin left margin of the of the landslide. Thelandslide. The discontinuous discontinuous arrow indicatesarrow the indicates direction the of the runoff. direction of the runoff. Figure11. Figure (a)View 11.(a) Viewofofthe the breach breach in the wall of of the the impoundment impoundmentininlandslide landslidefour four(LL4). (LL4).(b) (b)Close-up Close- up viewview of of a sand a sand dikecross dike crosscutting cuttingthe thesubhorizontal subhorizontaltailings tailings bedding. bedding. Plan view view of ofthe thepolygonal polygonal arrangement arrangementofofsandsanddikes dikesononthe theLL4 LL4surface surface(c) (c)and andlagoon lagoonfour four(Lg4) (Lg4)(d). (d).(e) (e)View Viewofofaasand sanddike dike exposed exposedon onthe thebreach breachplane planeofoflandslide landslidefive five(LL5). (LL5).The Thediscontinuous discontinuousarrow arrowindicates indicatesthe thedirection direction ofofthe therunoff. runoff. 4. Discussion The El Descargador tailings dam failure in 1963 caused significant damages that have lasted until nowadays. More than 65,000 m3 were released by landslides that covered an area of ca. 4 km2 in the periurban area of La Unión town, affecting different infrastructures such as the Cartagena–Cabo de Palos highway and the Cartagena–Los Blancos railroad), and the Beal wadis stream. A large amount of released acidic waters polluted the El Mar Minerals 2022, 12, 1488 13 of 20 4. Discussion The El Descargador tailings dam failure in 1963 caused significant damages that have lasted until nowadays. More than 65,000 m3 were released by landslides that covered an area of ca. 4 km2 in the periurban area of La Unión town, affecting different infrastructures such as the Cartagena–Cabo de Palos highway and the Cartagena–Los Blancos railroad), and the Beal wadis stream. A large amount of released acidic waters polluted the El Mar Menor coastal lagoon, which currently shows concentrations of metals and other mining pollutants well above the quality standards [63,69,70]. The impacts of the dam flow failure could have been of greater magnitude if LL3 had not dammed the stream flow in the early stages of the tailing slurries releasing. 4.1. Failure of the Tailings Dam: Conceptual Model Several causal factors should be considered in the analysis of the tailings dam failure. According to [71], distinct groups of factors may concatenate in the development of land- slides. The mechanical contact between the pre-existent terrain and the previous dip slope around 10◦ toward the plain is considered as predisposing factors in El Descargador site. Several dynamic preparatory factors concurred in El Descargador, which shifted the dam from stable to marginally stable state. The geometric arrangement of the materials that reached very steep slopes of up to 40◦ , higher than the internal fiction angle of the sandy tailings (32◦ ), was one of the most influential [70]. However, even more influential were the rapid raising of the tailings dam (4.2 m/years) and its poor drainage conditions, which was not able to relieve the pore-fluid overpressures. Pore-fluid overpressurization minimizes the shear strength of earths and, therefore, compromises the resisting efforts within the dam [17,70]. Noticeable is that El Descargador lacked any water drainage system, except for the drainage from the lagoon Lg3 by siphon method. The samples taken from the trial pit were already saturated (Table S2), indicating that mine tailings remain fully saturated and for a long time after their deposition. The El Descar- gador area is a semiarid climate and rainwater recharge is just seasonal (300 mm/year), with a precipitation deficit of 900 mm/year. However, the low resistivity of tailings recorded by ERI revealed that tailings keep on saturated down to more than 10 m-depth beneath the lagoons, and that the two active decantation ponds continue recharging the tailings deposit at present by their rainwater storage (Figure 6). According to these results, an important volume of tailings could have remained saturated almost 45 years after the flow failure. This also points to the low drainage capacity of these materials, although being deposited in a semiarid climate. The endorheic nature of the lagoons and the almost non-existence of the drainage systems increase infiltration to the detriment of surface runoff and evacuation of water outside the dam. Consequently, any contribution of water by direct infiltration from the surface, or from the alluvial systems underlain the dam, may favor overpressures due to the drainage deficiencies of the tailings dam. It should be noted that ERI has been very useful to image the areas of preferential groundwater recharging and groundwater flow paths. Such areas were characterized by their anomalous low resistivity (ρ < 5 Ωm), which is consistent with tailings saturated in briny acidic mining lixiviates. This information is very difficult to obtain through direct observations or other geophysical techniques; hence, this work illustrates that ERI has strong capabilities to support tailings dam investigations. ERI also enabled imaging the tailings slid mass, which depicts analogous low-resistivity geophysical signature. However, this rainwater stored in the lagoons would have contributed to the ground- water recharge and the rise in pore pressure and flowing, which favored the liquefaction processes. The groundwater recharge increases the saturation of the tailings and the granu- lar material forming the foundation. The saturation of the porous material and the rapid increase of load by tailings discharge sustained over time (300 ton/day) in a small area (23,500 m2 ), and the rapid raising of the tailings dam (more than 4.2 m/years), caused the rise in the pore pressure and the static liquefaction of the sandy tailings (Figure 12 stage 2). The liquefied materials ascended vertically crosscutting strata of different textural and Minerals 2022, 12, 1488 14 of 20 sedimentary features and filled the decantation lagoons (Figure 12 stage 3). This implied the collapse of the structure and the overtopping of the three lagoons (1, 3 and 4) affected by landslides. The expulsion of the pore water and sediment from the beds caused an excessive water table rising in the lagoons. Consequently, the embankment overtopping Minerals 2022, 12, x 16 of 22 and the toe of the dam become unstable by the erosion, altering the equilibrium of forces and eventually releasing the stress by the mass sliding (Figure 12, stage 4). Figure Figure 12. (a)12. (a) Conceptual Conceptual model model of the of the “El “El Descargador” Descargador” tailings tailings dam flowdam flowThe failure. failure. modelThepro-model posesproposes a historyaof static liquefaction history favored favored of static liquefaction by the gain sizegain by the of tailings size of at base levels tailings at baseand the and levels rise the in rise the hydraulic head byhead in the hydraulic the effective groundwater by the effective recharge groundwater from the recharge fromlagoons (stage(stage the lagoons 2). The2).upwelling The upwelling of sands and water of sands caused and water the clogging caused of theoflagoons the clogging and the the lagoons andfreeboards overtopping the freeboards (stage(stage overtopping 3). The3). The flowing of tailings and water on the slopes together with the internal erosion and the flowing of tailings and water on the slopes together with the internal erosion and the pore-fluid over pore-fluid over pressuring of the tailings mass triggered the land sliding (stage 4). 1. Basement; 2. Sandy tail- pressuring of the tailings mass triggered the land sliding (stage 4). 1. Basement; 2. Sandy tailings ings layers; 3. Silty tailings layers; 4. Water and 5. Sand dikes and volcanoes. (b) Textural features layers; 3. Silty tailings layers; 4. Water and 5. Sand dikes and volcanoes. (b) Textural features of mine of mine tailings: samples P1 to P9, collected from the trial-pit profile and W1 to W11, collected from tailings: the sand dike samples P1 toto from bottom P9,top collected from theoftrial-pit on the surface profile landslide fourand(seeW1 to W11, Figure 4 andcollected from the sand Table S2). dike from bottom to top on the surface of landslide four (see Figure 4 and Table S2). Sediment extrusion structures, such as the sand dikes identified both in the five land- Sediment extrusion structures, such as the sand dikes identified both in the five land- slides and the sand dikes and sand volcanoes in Lg1 (Figures 7, 8 and 11), together with slides and the sand dikes and sand volcanoes in Lg1 (Figures 7, 8 and 11), together with the the conspicuous sand fraction on the surface (Figures 7a,b) do not present typical stratifi- conspicuous sand fraction on the surface (Figure 7a,b) do not present typical stratification cation of tailings, and support the occurrence of liquefaction processes. SEM observations of tailings, and support the occurrence of liquefaction processes. SEM observations of of the mine wastes from El Descargador exhibited quite different shapes, depending on the mine wastes from El Descargador exhibited quite different shapes, depending on the the degree of weathering and liquefaction. Differences are in the irregularity (major for P9 degree of weathering and liquefaction. Differences are in the irregularity (major for P9 than than W11) and surface roughness (major for W11 than P9) of samples P9 and W11 [17,71]. W11) and surface roughness (major for W11 than P9) of samples P9 and W11 [17,71]. P9 P9 was wasextracted extractedfromfromthethetest testpit pitand andisisa afine-grained fine-grainedmaterial material(Figure (Figure13a) 13a)with witha astruc- structure ture unaltered since its deposition (see Figure 9b; more information unaltered since its deposition (see Figure 9b; more information in [52]). W11 in [52]). W11 is coarse- is coarse- grained material grained (Figure material 13b) 13b) (Figure fromfromthe sand the sanddike dike cutting the surface cutting of theoflandslide the surface four four the landslide (LL4)(LL4) see Figure 9a in [71]. Therefore, W11 is a weathered post-flotation waste with see Figure 9a in [71]. Therefore, W11 is a weathered post-flotation waste with a shear a shear history of liquefaction history because of liquefaction of a previous because of a previous severe mudslide. severe mudslide. Minerals Minerals 12, x 12, 1488 2022,2022, 17 of 22 15 of 20 FigureFigure 13. The13. Theshows graph graphallshows possibleallcircular possible circular failure failure surfaces with surfaces withofa less a safety factor safety thanfactor one of less than one in in orange; orange; look look atscale at the vertical the vertical scaleand on the right onthe themost rightcritical and the most of all criticalinofgreen. is marked all is Nu- marked in green. merical modeling modeling Numerical of the stability of thefactors based stability on the factors reconstruction based of the topography on the reconstruction of theconditions topography conditions beforebefore the failure (back-analysis), the failure considering (back-analysis), static scenarios, considering static with the hydraulic scenarios, with the head set at the hydraulic head set at the surface. (a) Janbu Method, (b) simplified Bishop Method, (c) GLE/Morgenstern–Price Method and surface. (a) Janbu Method, (b) simplified Bishop Method, (c) GLE/Morgenstern–Price Method and (d) Janbu corrected Method. (d) Janbu corrected Method. 4.2. Numerical Modeling 4.2. Numerical Modeling The slope stability analysis using the reconstructed topography before the failure The slope stability analysis using the reconstructed topography before the failure shows that the dam broke under static and pseudostatic for 13 boundary conditions (Table shows that the dam broke under static and pseudostatic for 13 boundary conditions S3). The layers and dam with different thicknesses were considered. The calculation ac- (Table S3). The layers and dam with different thicknesses were considered. The calcu- celeration used was 0.18 PGA. The article does not show the images of the results of the lation acceleration pseudo-static analysis, butused it doeswasshow0.18thePGA. The results in article Table S3 does andnot show Figure S2. the Theimages geotech-of the results of the pseudo-static analysis, but it does show the results nical parameters as specific weight, cohesion, frictional angle of mining tailings were in Table S3 and setFigure S2. The geotechnical parameters as specific weight, cohesion, frictional according to Table S2 and [71]. Figure 13 shows the results of the static stability calculation angle of mining tailings were set according to Table S2 and [71]. Figure 13 shows the for the LL4 slip. Four materials have been considered: silty tailings with the internal frac- results of the static stability calculation tion angle of 27 °C,for the LL4 sandy slip. tailings Four with thematerials have been internal fraction angleconsidered: of 29 °C (see silty Tabletailings S2), with the internal dam with fractionfraction the internal angle of 27 ◦of angle C,37sandy °C and tailings with the foundation withinternal the internalfraction angle of 29 ◦ C fraction angle(see of 41Table °C. The S2),cohesion dam with is zero. theThe water fraction internal head wasangle considered of 37 at◦ Cthe and tailings dam foundation with the surface, reproducing internal fraction theangle hydraulic ◦ of 41condition C. The of liquefaction. cohesion is zero.ThisThe is considered water head a mini- was considered mumat since thethe hydraulic tailings damhead couldreproducing surface, have reachedthe even an elevation hydraulic higher than condition that of of liquefaction. This is the lagoons. consideredThe achieved a minimum numerical since simulations the hydraulic are consistent head could with havethe reached hypothesis eventhatan elevation the tailings highermassthansliding occurred that of due to aThe the lagoons. combination achievedanumerical dysfunction of the drainage simulations are sys- consistent with tem and static liquefaction. This concatenation of factors caused the the hypothesis that the tailings mass sliding occurred due to a combination a dysfunctionascent of liquefied material to the lagoon and the overflow. The debris caused the erosion of the slope and of the drainage system and static liquefaction. This concatenation of factors caused the its foot which altered the stress equilibrium, leading to the tailings dam failure. The ge- ascent of liquefied material to the lagoon and the overflow. The debris caused the erosion ometry of failure is, according to the model, a subcircular type landslide with stability of the slope and its foot which altered the stress equilibrium, leading to the tailings dam factors ranging from 0.773 to 0.880 (less than one is considered as unstable condition; Fig- failure. ure 13). The geometry This geometry of failure is consistent with is, according a failure within tothe themass model, a subcircular of tailings, which can type landslide be considered as a reasonably homogeneous material for modeling purposes. Part of the as unstable with stability factors ranging from 0.773 to 0.880 (less than one is considered condition; liquefied tailingsFigure 13). This can be seen on thegeometry left side ofisthe consistent with a failure stream (Figures 9 and 10). within the mass of tailings, The released tailings caused the obstruction of the valley and the invasion of the national highway. In purposes. which can be considered as a reasonably homogeneous material for modeling Part ofthe this regard, thedamming liquefied oftailings the stream canvalley be seenhadon the leftimpact a positive side offrom the stream a hazard’s (Figures per- 9 and 10). Thesince spective released tailingsthe it minimized caused abilitythe of obstruction the mine wastes of the valley to flow andand the downstream spread invasion of the national of thehighway. basin. In this regard, the damming of the stream valley had a positive impact from a This work perspective hazard’s illustrates why static since liquefaction the it minimized should not of ability be the underestimated mine wastesastoa flow trig- and spread geringdownstream factor of tailingsof thedams’ basin.failures, with respect to seismicity or extreme weather This work illustrates why static liquefaction should not be underestimated as a trigger- ing factor of tailings dams’ failures, with respect to seismicity or extreme weather events that have been more frequently invoked. It also shows that a multidisciplinary investigation approach, by integrating geotechnical, hydrogeological and geophysical characterization Minerals 2022, 12, 1488 16 of 20 22, 12, x 18 of 22 and modeling, is required to not misinterpret the main causes of a dam failure. The new events that have been more frequently invoked. It also shows that a multidisciplinary in- acquired knowledge on the El Descargador tailings dam failure could be replicated in other vestigation approach, by integrating geotechnical, hydrogeological and geophysical char- regions acterization and modeling, worldwide, is required to notwhich may contribute misinterpret to a better the main causes design of a dam fail- and management of these critical mining operations and their side impacts. ure. The new acquired knowledge on the El Descargador tailings dam failure could be replicated in other regions worldwide, which may contribute to a better design and man- 5. Conclusions agement of these critical mining operations and their side impacts. 1. The work is a unique case study. Its contribution lies in the magnitude and nature of 5. Conclusions the static liquefaction. This phenomenon has not been reported in this way before. 1. The work is a unique caseIn no caseItsincontribution study. the literature orthe lies in in the databases magnitude has a tailings and nature of dam been reported to the static liquefaction. This developphenomenon has notflow simultaneous beenfailure reported atin thispoints five way before. In [1,2,5–11,18]. no case in the literature 2. or in The the databases has a tailingscharacterization geophysical–geotechnical dam been reported and tomodeling de- of the El Descargador velop simultaneous flow failuredam tailings at five points [1,2,5–11,18]. supported that the static liquefaction was the most plausible cause of its 2. The geophysical–geotechnical characterization flow failure in 1963. Thisandoccurred modeling in of the the El formDescargador of five different landslides along the tailings dam supported that the static liquefaction was the most plausible 3 cause 2of its dam structure, which mobilized 66,000 m on 4 km of the La Unión periurban area. flow failure in 1963. This occurred in the form of five different landslides along the 3. The rapid growing of the2tailings dam, exceeding 4.2 m/year, the steeped slopes of dam structure, which mobilized 66,000 m3 on 4 km of the La Unión periurban area. 3. The rapid growing of the tails, scaling the tailings up to 40◦ ,4.2 dam, exceeding them/year, saturation of tailings the steeped dueofto recharge from the lagoons, slopes the failure of the drainage system and the rise the tails, scaling up to 40°, the saturation of tailings due to recharge from the lagoons, in the hydraulic head were the main the failure of the drainage system and the rise in the hydraulic head were the mainand 7–14). causal preparatory factors for flow failure (Figures 2–4 causal preparatory Thefor 4. factors dysfunction flow failureof the drainage (Figures system and the static liquefaction caused the dam’s 2–4, 7–14). 4. The dysfunction of thelagoons drainageovertopping system and the and the liquefaction static erosion of the slope caused theand the foot of the structure, which dam’s lagoons overtopping and the erosion of the slope and the foot of the structure, triggered mechanism the flow failure the El Descargador tailings dam. which triggered mechanism5. the flow failuremodeling Geophysical the El Descargador tailings of the tailings dam dam. revealed that the materials remain saturated 5. Geophysical modelingbeneath of the tailings the lagoons to a 10 m depth or more,remain dam revealed that the materials satu- after the dam failure. This even decades rated beneath the lagoonsobservation highlights the relevant role of the lagoonsfail- to a 10 m depth or more, even decades after the dam in the groundwater recharge in ure. This observation highlights the relevant role of the lagoons in the groundwater tailings dam (Figure 14). recharge in tailings dam (Figure 14). Figure 14. (a) Lagoon Lg1 and Lg3 after a rainstorm and (b) detail an Lg1 (October 2015). The spatio-temporal evolution of drying and wetting of the lagoons can be seen in the Google Earth clock (Since June 2002). Minerals 2022, 12, 1488 17 of 20 Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/min12121488/s1, Figure S1. The image shows the contact between the different geological materials and tailings. The discontinuous arrow indicates the direction of the runoff. Figure S2. Factors of safety for the 13 boundary conditions analyzed, (a) in static conditions and (b) in pseudo-static conditions. Table S1. Local magnitude (Richter scale) of the seismic events during 1967 over 500 km ratio from El Descargador tailings dam; source: Spanish National Geographic Institute (IGN); Table S2. Particle size distribution (PSD) of the tailings’ samples collected in the trial pit (P1–P9) and the sand dikes (W1–W13). D10, D30, D50, D60, D90 are the particle size at the 10, 30, 50, 60, 90% point of the cumulative undersize PSD, Cu is the coefficient of uniformity, Cc is the coefficient of curvature, Sr is degree of saturation, e is void ratio, w is gravimetric water content, n is porosity; Table S3. Safety factors for different calculation hypotheses. Fs is phreatic surface, BS is Bishop simplify method, MP is Morgenstem–Price Method, JS is Janbu simplify method, JC is Janbu simplify method. Sp is that the height of the water table is at the surface of the dam and Md is at the middle and D means that the dam of tailings dam is dry. * In scenario 13, the lagoons are considered full of water. Author Contributions: Conceptualization, R.R.-P. and M.Z.; methodology, R.R.-P., A.V.C., M.Z., A.A., R.R.-P., I.C. and C.R.; software, R.R.-P. and M.Z.; validation, R.R.-P. and M.Z.; formal analysis, R.R.-P., I.C., C.R. and M.Z.; investigation, R.R.-P., A.V.C., A.A., P.M.-P., M.A.M.-S., C.G.-G., Á.F., I.C., C.R. and M.Z.; resources, R.R.-P., A.A. and Á.F.; data curation, R.R.-P., A.V.C., A.A., M.Z., I.C. and C.R.; writing—original draft preparation, R.R.-P. and M.Z.; writing—review and editing, R.R.-P., I.C., C.R. and M.Z.; visualization, R.R.-P. and M.Z.; supervision, R.R.-P. and M.Z.; project administration, R.R.-P. and C.G.-G.; funding acquisition, R.R.-P., A.A., C.G.-G. and Á.F. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Data Availability Statement: Not applicable. Acknowledgments: This research was supported by the Spanish Ministry of Industry, Commerce and Tourism and from a management entrustment with the Spanish Ministry of Agriculture, Food and Environment. Authors thanks the anonymous reviewers for their work. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. References 1. Oldecop, L.; Rodríguez, R. Estabilidad y seguridad de depósitos de residuos mineros. In Los Residuos Minero Metalúrgicos en el Medio Ambiente; Rodríguez, R., García-Cortés, A., Eds.; Instituto Geológico y Minero de España (IGME): Madrid, Spain, 2006; pp. 197–243. 2. Rico, M.; Benito, G.; Salgueiro, A.R.; Díez-Herrero, A.; Pereira, H.G. Reported tailings dam failures: A review of the European incidents in the worldwide context. J. Hazard. Mater. 2008, 152, 846–852. 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