potential of breakthroughs of impounded coal refuse slurry into … · 2010-12-23 · potential of...

Potential of Breakthroughs of Impounded Coal Refuse

Slurry into Underground Mines

PETER R. MICHAEL1

U.S. Office of Surface Mining Reclamation and Enforcement, 3 Parkway Center,Pittsburgh, PA 15220

MICHAEL W. RICHMOND

U.S. Office of Surface Mining Reclamation and Enforcement,1027 Virginia Street, East, Charleston, WV 25301

MICHAEL J. SUPERFESKY

U.S. Office of Surface Mining Reclamation and Enforcement, 604 Cheat Road,Suite 150, Morgantown, WV 26508

DONALD E. STUMP, JR.

U.S. Office of Surface Mining Reclamation and Enforcement, 3 Parkway Center,Pittsburgh, PA 15220

LISA K. CHAVEL

U.S. Army Corps of Engineers–Chicago District, 111 N Canal Street, Suite 600,Chicago, IL 60606

Key Terms: Impoundment, Slurry, Refuse, Mining,Coal, Breakthrough

ABSTRACT

On October 11, 2000, an estimated 306 milliongallons of water and fine coal refuse slurry brokethrough a bedrock barrier from an impoundment inMartin County, eastern Kentucky, into an adjacentunderground mine. Approximately 260 million gallonsof the water and coal slurry discharged from twounderground mine portals and affected over 75 miles ofstreams in Kentucky and West Virginia. As a result ofthis and several other breakthroughs over just half adecade, the U.S. Office of Surface Mining Reclamationand Enforcement (OSM) and other institutions under-took investigations to assess the causes of the events,the potential for additional breakthroughs in the future,and available methods for preventing them. In additionto needed improvements in the design, construction, andinspection of the facilities, the studies have addressedissues pertaining to the flow characteristics of refuseslurry, not only in impoundments still receiving pumpedslurry, but also in ‘‘idle’’ and reclaimed facilities.

Related questions concern: (1) the effects on break-through potential of the impoundment abandonmentprocess and construction of slurry cells on top of cappedstructures; and (2) appropriate measures and availablemethods that may be used to ensure that undergroundmines adjacent to or underlying impoundments areknown and accurately located. Current information onthe engineering properties of coal refuse in existingfacilities provides no assurance against fine refuseflowability during any stage in the impoundmentconstruction and reclamation process or after reclama-tion has been completed. Due to this uncertainty,thorough site investigations and conservative measuresin design, construction, reclamation, and quality controlare of paramount importance.

INTRODUCTION

A concern shared by many engineers, geologists,and mine inspectors familiar with coal refuse slurryimpoundments is related to the common occurrenceof underground mine workings adjacent to or beneaththe impoundments: the potential for slurry break-throughs into mine works and subsequent breakoutsinto surface waterways. Events of this nature canendanger underground mine workers and down-stream inhabitants, and negatively impact local

1Corresponding author: Phone: 412-937-2867; fax: 412-937-3012;email: [email protected].

Environmental & Engineering Geoscience, Vol. XVI, No. 3, August 2010, pp. 299–314 299

groundwater resources and stream and river ecosys-tems. This concern is particularly applicable toimpoundments within the steep-slope topography ofCentral Appalachia where the structures rest oninclined slopes within narrow hollows and conse-quently are in contact with numerous coal beds.

On October 11, 2000, a combination of coal refuseslurry and water from the Big Branch impoundmentin Martin County, Kentucky, broke through into anunderground mine and subsequently discharged intothe receiving streams (Figure 1). An estimated 306million gallons of water and coal refuse slurry drainedfrom the impoundment into the adjacent under-ground mine. Approximately 260 million gallonssubsequently discharged from the underground mineat two portals.

This was the second breakthrough event at thisimpoundment, the first having occurred in May 1994.The breakthrough in 2000 differed from the 1994breakthrough in that it resulted in severe streamdegradation and property damage. Fortunately, nopersonal injuries were reported as a result of the 2000breakthrough. However, the water-slurry mixtureaffected over 75 miles of stream in Kentucky andWest Virginia. At some locations, the water-slurrymixture spilled over the banks and deposited slimeonto adjacent property. Six public water intakes wereadversely affected, and alternative water supplies hadto be arranged. It was reported that the cost to cleanup the waterways and affected lands exceeded 56million dollars.

Fortunately, the 2000 event remains the lastbreakthrough to have occurred to date. Otherdocumented breakthroughs, in addition to the 1994event at Big Branch, include three breakthroughs inVirginia in 1996. Owing to the short time period overwhich these events took place and the severity ofeffects from the one in 2000, several investigationswere undertaken with the ultimate goal of preventingfuture impoundment breakthroughs. A prominentstudy among those investigations was ‘‘Coal WasteImpoundments’’ by the National Research Council(NRC, 2002), which examined current engineeringpractices and standards applied to the refuse im-poundments, explored ways to improve undergroundmine location relative to the impoundments, andevaluated alternative technologies that could reducethe amount of coal refuse generated and allowproductive use of the material.

Studies that specifically focused on the Big Branchimpoundment were conducted by the U.S. Depart-ment of Labor, Mine Safety and Health Adminis-tration (U.S. MSHA, 2001) and the U.S. Depart-ment of the Interior, Office of Surface MiningReclamation and Enforcement (U.S. OSM, 2002).

Both studies evaluated on-site conditions and prob-lems with construction and regulation enforcementpractices that led to the failure. MSHA administersthe provisions of the Federal Mine Safety andHealth Act of 1977 (FMSHA) to enforce compliancewith mandatory miner safety and health standards.OSM was established by The Surface MiningControl and Reclamation Act of 1977 (SMCRA)to oversee the enforcement of surface coal miningand reclamation regulations from the standpoint ofpublic safety and environmental protection. Thefindings of the studies are largely in agreement.However, the studies’ conclusion that the mechanismof the 2000 breakthrough involved piping throughweathered rock, colluvium, and loose artificialbarrier material from the underground mine intothe impounded slurry and overlying layer of clearwater was challenged. Thacker (2002) and Hagertyand Curini (2004) used finite-element seepage anal-ysis in an attempt to disprove the piping theory andproposed, by elimination, that the thin and weakbedrock mine barrier succumbed to hydrostaticpressure.

Additional studies in response to the breakthroughconcern were carried out by OSM staff at the requestof management and resulted in internal positionpapers. Michael et al. (2005) conducted a survey ofcurrent knowledge pertaining to the flow propertiesof impounded refuse (henceforth referred to as the‘‘2005 study’’). Under the assumption that at leastsome of the impounded slurry may be in a flowablestate, Michael and Chavel (2008) (henceforth referredto as the ‘‘2008 study’’) evaluated the potential forbreakthroughs resulting from additional surchargesimposed on mine outcrop barriers during part of theimpoundment-reclamation process, i.e., the place-ment of cap material over the slurry-containing‘‘basin’’ of the impoundment.

This paper presents a brief overview of theconstruction and reclamation of coal refuse slurryimpoundments and elaborates the authors’ concernspertaining to the potential for future breakthroughsof fine coal refuse slurry into adjacent or subjacentunderground coal mines.

COAL REFUSE IMPOUNDMENTS

‘‘Coal refuse’’ is waste that results from thepreparation of mined coal for energy production.Some of the material is placed in dry landfills, butmost is disposed in slurry impoundments. In the lattercase, refuse is first separated into relatively coarse andfine fractions. The coarser material (grain sizesranging from 0.1 to 70 mm) is kept relatively dryand is used to construct the embankment of the

Michael, Richmond, Superfesky, Stump, and Chavel

300 Environmental & Engineering Geoscience, Vol. XVI, No. 3, August 2010, pp. 299–314

impoundment. The fine fraction of the refuse (from0.001 or less to 20 mm) is mixed with water andtransported in pipes as slurry to the impoundment‘‘basin.’’

The proportion between the dry coarse refuse andfine refuse slurry constituting an impoundmentdepends on the relative amounts of the materialsproduced at the contributing preparation plant(s) and

Figure 1. Post-breakthrough overview of Big Branch slurry impoundment (A) and ground-level view of one of two portal breakoutpoints (B).

Impounded Slurry Breakthroughs into Underground Mines


commonly governs the way in which the impound-ment is constructed. Figure 2 is a schematic diagramof the ‘‘downstream,’’ ‘‘upstream,’’ and ‘‘centerline’’construction methods. Many impoundments amountto a hybrid among these three construction methods.It is important to note that there generally is nopreconceived final design for these structures. Moreoften than not, coal refuse slurry impoundments areperiodically redesigned, i.e., enlarged to accommo-date additional refuse as it is generated at the

preparation plant(s). In the narrow hollows ofsteep-slope Appalachia, this enlargement of impound-ments requires the structures to increase in sizevertically and the impounded slurry to deepen, thusincrementally adding pressure on mine outcropbarriers.

MSHA and OSM regulatory requirements for theengineering design, construction, maintenance, in-spection, and elimination of slurry impoundmentsare provided in Title 30 of the Code of Federal

Figure 2. Schematic cross sections of downstream (A), centerline (B), and upstream (C) slurry impoundment construction methods.(Diagrams are from D’Appolonia, 2009.)



Regulations (30 CFR), Section 77.216 and Section780.25, respectively (U.S. Office of the FederalRegister, 2007). Detailed guidance is available inMSHA’s Engineering and Design Manual—CoalRefuse Facilities (D’Appolonia Engineering, 2009).The latter document includes information and guid-ance pertinent to on-site reconnaissance of founda-tion conditions—including the identification of un-derground mines—and evaluation of and preventionagainst mine subsidence and breakthrough potential(see Chapter 8). Preventative techniques are designedto ensure that adequate horizontal barriers (i.e.,outcrop barriers) and vertical barriers (barriers abovemines that lie below the bottom of the impoundment)exist or that barriers are improved when necessary (byback-stowing underground mines or covering theoutcrops of mined coal beds with a seepage barrier).

The status of a coal refuse slurry impoundmentmay be active (i.e., still receiving coal refuse slurryfrom an active coal-cleaning operation), inactive, orreclaimed (Figure 3). Those structures that areinactive may be under a current permit or ‘‘or-phaned.’’ Orphan impoundments are those for whichthere is no responsible mine operator accountable forthe structure and where the impoundment has notbeen properly reclaimed under FMSHA and SMCRAstandards. Generally, inactive impoundments con-structed prior to the passage of the SMCRA are notreclaimed unless they are re-permitted or they presentan imminent safety hazard to the public or harm tothe environment. Impoundments on bond-forfeituresites may not be reclaimed if there is insufficient bondor insurance money to cover the cost of the work.Most impoundments that are reclaimed are cappedwith coarse refuse or surface mine spoil and re-vegetated. Some have been converted into ponds orlakes for recreational use under the SMCRA exper-imental practice program. Other impoundments arere-mined. Still others are capped when there areconcerns that a breakthrough may occur, but theymay also be converted to slurry cell structures toallow for additional refuse disposal.

We are unaware of any national or regionaldatabases that have complete and up-to-date infor-mation on the status or condition of all existing finecoal refuse impoundments. As of the 2008 study, theMSHA National Impoundment and Refuse PileInventory included a total of 632 structures that metthe size criteria for MSHA jurisdiction (see 30 CFR77.216 (a) for size criteria). However, information onthe status of these impoundments was not yetcomplete. The 2008 study did obtain the followinginformation directly from three Appalachian stateregulatory authorities: out of a total of 113 fine coalrefuse impoundments in Kentucky, six have been

reclaimed; only a ‘‘handful’’ from a total of 110 WestVirginia impoundments have been capped; and thereare 17 active fine coal refuse impoundments, onepre-law inactive impoundment, and zero reclaimedimpoundments in Virginia.

THE BREAKTHROUGH ISSUE

Principle Unknowns

Fine coal refuse slurry breakthroughs into under-ground mines can take place via ‘‘punch-ins’’ throughweak horizontal mine barriers or through sinkholesor sag subsidence cracks in thin vertical barriers.Concerns relating to potential breakthroughs centeron two unknowns. One of those is site specific innature: the location of underground mines that occurbeneath or adjacent to the footprint of an impound-ment (see Figure 4), and the consequent thickness ofthe horizontal and vertical barriers between the minesand the impoundment. The other unknown is whetherimpounded fine coal refuse slurry remains in a liquidstate or can be changed into such a state throughliquefaction or thixotropic agitation.

Mine Barriers

With respect to mine outcrop barriers, there aremine maps available that can be employed to estimatedistances between the boundary of an impoundmentand the closest reach of the mines. However, there arenumerous mines that are not located or are notaccurately located on mine maps.

Based on past and present underground miningtechnology, coal beds that are approximately 2 ft ormore thick are potentially minable underground. Incases where there are no records of adjacent mining inseams that intersect the impoundment, there are othermethods of mine or mine-void detection, includinginterviews with experienced miners and the localpopulation, visual field reconnaissance for undocu-mented mine entries, horizontal drilling, and shallow-subsurface geophysics. However, the employment ofthese methods—within reasonable economic con-straints—cannot guarantee that all mines surround-ing the entire perimeter of an impoundment will beaccounted for. For mines below the impoundment,barriers above known or possible workings can easilybe established through vertical drilling. However,detection of undocumented mines can be even moreproblematic in comparison to adjacent mines sincemine adits may not be proximate to the impoundmentsite, drill holes may penetrate barriers betweenunderground mine panels or mine pillars, and geo-physical methods are more costly and less effective



with depth. It is noteworthy that even where minesmay be too deep for potential formation of sinkholesor wide subsidence tension cracks, ground movementresulting from the failure of mine pillars couldsimultaneously destabilize shallower mine barriersand mobilize impounded slurry. Consequently, accu-

rate mining information for relatively deep coal bedsbelow an impoundment is also important.

It is also important to note that even when coalmines have been identified and accurately locatedrelative to an impoundment, the competence of themine barriers can be over-estimated. This is what in

Figure 3. Comparison between an active (A) and reclaimed impoundment (B).



fact happened at the Big Branch impoundment (U.S.OSM, 2002).

Slurry Flowability

Uncertainties pertaining to the presence of under-ground mines, conditions of mine barriers, andstability of mine pillars would not be as significantas they are if we were confident that deposited finerefuse slurry consolidated, through dewatering anddensification, into a statically and dynamically stablematerial in a predictable time frame. However, theresults of the 2005 study provided no such assurance.

The 2005 investigation was primarily concernedwith slurry in active and uncapped inactive impound-ments. The subsequent 2008 study concluded that anunderground mine breakthrough during impound-ment capping is a theoretical but remote possibility.The authors of that assessment also expressed thestrong opinion that reclaiming (which includescapping) a fine coal refuse impoundment generallydecreases the risk of breakthrough. The processeliminates clear water above the slurry (Figure 5),dewaters the fine refuse to some extent, and decreasessurface water and groundwater infiltration into thereservoir. Consequently, hydrostatic pressure on mine

Figure 4. Schematic depiction of a slurry impoundment basin and adjacent and subjacent coal mine workings. (Diagram from NRC, 2002.)

Figure 5. Top of dewatered slurry being capped with mine spoil (from Shinavski, 2006).



barriers and the flowability of the material arereduced.

Some impoundments are capped specifically be-cause of breakthrough concerns but are also convert-ed to slurry cell structures for continued refusedisposal (Figure 6). Slurry cells are small impoundingstructures holding fine refuse separated by dikes ofcompacted coarse refuse. They are constructed inlayers, the total depth of which can equal or exceedthat of the original impoundment (Figure 7). Theiruse limits the volume and flowability of slurry thatwould be released (relative to an active impoundmentof equal size) should a breakthrough into anunderground mine occur. However, they do notnecessarily diminish breakthrough potential fromthe original impoundment. Surcharge from thestacked slurry cells can still increase hydrostaticpressure in the fine refuse slurry below the impound-ment cap.

Findings of the 2005 Study

The purpose of the 2005 study was to reviewcurrent knowledge of—or applicable to—the poten-tial flow characteristics of impounded coal refuse.

The review explored two inter-related issues: (1)Given the occurrence of a breakthrough event thatwould result in a potential flow conduit between anunderground mine and an impoundment, should coalrefuse be expected to flow into the mine (i.e., is itflowable)? (2) If the refuse would flow, what would bethe nature (e.g., velocity and extent) of that flow? Thispaper summarizes the findings pertaining to only thefirst (and more important) question.

Sources employed in the review included: (1)interviews with other OSM staff working withimpoundments, (2) interviews with geotechnicalexperts in coal refuse and metal tailings impoundmentconstruction from academia, other federal agencies,and the industry, and (3) geotechnical articlesobtained through literature searches and throughdirect contacts with the authors. The final reportincluded modifications in response to solicited com-ments on the first draft. Most of the comments wereprovided by geotechnical engineers and engineeringgeologists outside of OSM with expertise in minewaste impoundments or similar structures.

It was recognized that the response of the slurry toa barrier failure should vary site specifically accordingto the strength characteristics of the refuse, depth of

Figure 6. Aerial view of coal refuse slurry cells.



the impoundment (i.e., how much stress might bearon the material if a failure occurred), and nature of anopening into an underground mine (e.g., its positionin the impoundment, and its size, length, andinclination). The search did not encounter any studythat had scrutinized the interaction of all three ofthose variables, either empirically or through model-ing. However, there existed a considerable amount ofwork relating to the static, load-bearing strengths ofcoal refuse, and metal and oil sands tailings. Thereport distinguished several general properties of thismaterial that are assessed in the literature. Mostattention was applied to consolidation strength, i.e.,the development of shear strength in the fine refuseduring the consolidation process. Another propertyof interest, thixotropic strength, was found indocuments pertaining to tailings deposits. The reportincluded consideration of the potential effect ofthixotropy, assuming it was potentially applicable tofine coal refuse. Additional aspects thought to havebearing on fine refuse stability are the design andconstruction of the impoundments, and potentialeffects of chemical additives to the slurry.

Consolidation Strength

One of the properties that impacts the stability ofan impoundment is consolidation. Consolidation of asoil is defined as a void-ratio reduction that takesplace as a function of time and pressure. Consolida-

tion is a gradual process that involves slow drainage,compression (density increase, volume reduction,reduction in void space between particles), and stresstransfer or gradual pressure adjustment. The devel-opment of engineering strength in the soil duringconsolidation results from the buildup of effectivestress among the soil particles. Consolidation isaffected in part by particle size distribution anddensity or void ratio.

The rate and degree of consolidation depend on theinduced load on the soil and the soil’s coefficient ofconsolidation (Cv). Factors influencing Cv includethe soil’s compressibility and its permeability (orhydraulic conductivity). Compressibility is a ratio ofthe strain of the material (specifically the rate ofvolume reduction) to induced stress, and it dependson particle shape, size distribution, and initial voidratio. With higher compressibility, the Cv is lower,and more strain is necessary for the buildup ofeffective stress in the soil. Permeability is the facilitywith which water is able to travel through the pores ofthe soil, and it is a function of the size, number, andinterconnection of voids between the soil particles.The higher is the permeability, the higher is the Cv.Figure 8 shows a relationship between compressibilityand void ratio for copper, gold, composite tailings, aswell as ‘‘coal wash tailings’’ (assumed to the sameas—or similar to—coal refuse slurry). Notable factorsare the relatively high void ratios of the coal washtailings when highly compressible. The graph suggests

Figure 7. Cross section depicting zone of slurry cell construction above capped impoundment and coal seams subjacent to, and belowthe structure.



that unconsolidated fine coal refuse is capable ofrelatively high moisture content.

The stability of impounded fine refuse and tailingsdepends in large part on the development ofconsolidation strength in the material. Insufficientshear strength under static or dynamic load will resultin deformation of the refuse. A major concern is thatinsufficient drainage during the consolidation processmay result in liquefaction, i.e., excessive porepressures under load and consequent material flow.

The following comments from literature pertainingto coal refuse consolidation strength were noted:

Permeability

Consolidation in impounded coal refuse and tailingsis more influenced by permeability than compressibil-ity. The permeability of refuse is low, and the length ofthe drainage path is long, so that consolidation of thematerial can take a long time. One should expect highexcess pore pressure to exist in fine refuse for yearsafter the initial placement, which will significantlyreduce the shear strength of the material. Even afterthe material is fully consolidated under the imposedload, the void ratio of fine refuse is still high (Zeng etal., 1998a, 1998b; Sweigard et al., 1997; and Suthakerand Scott, 1994, 1996, 1997).

Slow Rate of Consolidation

Suthaker and Scott (1994) and Suthaker et al.(1997) conducted large-scale testing of oil sand finetails, using 2-m and a 10-m stand pipes. Whereassedimentation occurred from the bottom up and wasrapid (2.5 days), self-weight consolidation occurredfrom the top down and was long-term (300 days).Tang et al. (1997) examined mature fine tailings with

scanning electron microscopy and found that after acertain void ratio was reached (ratio of 6), the rate ofconsolidation slowed considerably. Suthaker et al.(1997) also found that a decrease in void ratioresulted in a decrease of hydraulic conductivity,resulting in slower consolidation.

Low Shear Strength of Fine Refuse and Tailings inExisting Impoundments

Concern over consolidation strength in impound-ments has been expressed by Busch et al. (1975), Zenget al. (1998a, 1998b), Sweigard et al. (1997), andSuthaker and Scott (1994, 1996, 1997). Relatedobservations include: high moisture contents (somethat are above the liquid limit); increases in moisturecontent, void ratio, compressibility, and pore pressurewith depth in the impoundment; and generally lowshear strength even after years of consolidation.Suthaker and Scott (1994) and Suthaker et al.(1997), in their large-scale consolidation test, foundthat there was no effective stress buildup after14.4 years in the oil-sand tailings except in the bottompart of the stand pipe.

Uncertainty whether the Consolidation ProcessResults in the Development of Shear Strength in theImpounded Refuse

The foregoing points assume that shear strength inrefuse will increase as a result of the consolidationprocess. That is not the case if consolidation occurswithout sufficient dewatering of the material. Swei-gard et al. (1997) attempted to address the long-termeffect of consolidation of refuse at or above its liquidlimit on its shear strength. Their results wereinconclusive.

Susceptibility of Refuse and Tailings to Liquefaction

As previously stated, an important issue pertainingto development of consolidation strength in im-pounded fine refuse and tailings is the avoidance ofliquefaction. Most soils continue to behave in a solidstate after they fail in shear, i.e., shear strength is notcompletely lost, and the amount of strain that occursdepends on the duration of sufficient (‘‘residual’’ or‘‘ultimate’’) shear stress. The exception is whenliquefaction takes place. Liquefaction is a process bywhich the soil structure collapses under shock orother type of loading and is associated with a sudden,temporary increase in pore-water pressure. Thematerial then temporarily transforms into a liquid.Liquefaction can occur in response to dynamic forces

Figure 8. Compressibility of mine tailings as a function of voidratio (from Qiu and Sego, 1998).



such as earthquakes and mine blasting. Staticliquefaction can result from sudden shear stressesinduced by mine-barrier breakthroughs, mine subsi-dence, or other kinds of single-cycle events. Theliquefaction potential of unconsolidated sediments isrelated to the materials’ void ratio and mean effectivestress.

According to Terzaghi et al. (1996), soils mostsusceptible to liquefaction: (1) have clean sands andsilty sands with minimal clay content, (2) are looseenough to be contractive, and (3) are of sufficientlylow permeability to experience no significant drainageduring static or dynamic loading. Figure 9 comparesgrain-size distribution boundaries associated in theliterature with liquefiable natural soils and liquefiable‘‘tailings slimes’’ with the general grain-size distribu-tion boundaries of fine coal refuse reported byD’Appolonia (2009). It is noted that the liquefiablesoils boundaries may need to be adjusted to accountfor gravelly sands also prone to disturbance undercyclic loading (e.g., Evans and Harder, 1993). Forpurposes of this discussion, it is especially noteworthythat the coal refuse boundaries do not match theliquefiable silts and sands of the tailings and soils,respectively. This suggests that fine refuse with lowresistance to liquefaction may have its own uniquepair of boundaries.

Fourie (2004) emphasized the important role ofliquefaction, referencing his studies of a tailings-damfailure in South Africa (Fourie and Papageorgiou,2001; Fourie et al., 2001). He pointed out that thesusceptibility of a material to liquefaction is governedby the relative values of the material in situ density (orvoid ratio) and effective stress.

Liquefaction potential relates to the conditionsunder which deposition of tailings occurred and theinter-grain ‘‘fabric’’ that thus developed. For exam-ple, subaqueous deposition will result in a much lowerdensity and effective stress than subaerial deposition,even when the deposit is fully consolidated. Suchmaterial then behaves in a contractive fashion whenloaded undrained, i.e., the solid content of thematerial contracts and induces excess pore-waterpressures. Recognition that fine coal refuse consoli-dated in upstream impoundments is ‘‘usually highlycontractive’’ is documented by Genes et al. (2000).Volpe (2004) cited conditions in fine refuse conduciveto contractive behavior in reference to the tragicBuffalo Creek failure in 1972, i.e., very low densityand a high void ratio.

Moisture Content

The work of Huang et al. (1987) emphasizes howthe flowability and flow behavior of coal refuse slurrycan vary. They point out that unless the refuse isrelatively dry, the undrained shear strength of evenpartially saturated fine coal refuse is very sensitive tomoisture contents. A change in moisture content ofonly one percent may cause a large change in theundrained strength.

Thixotropic Strength

Usui (1998) and Suthaker et al. (1997) identifiedcoal-water slurry and oil-sand tailings, respectively, asthixotropic in nature. Since coal refuse slurry mayhave some engineering properties similar to those

Figure 9. Gradation curves defining boundaries for liquefiable soils (Tsuchida, 1970), liquefiable tailings (Ishihara, 1985), and fine coalrefuse (modified from D’Appolonia, 2009). (Liquefiable soils and tailings boundaries were first combined in Terzaghi et al., 1996.)



mixtures, the study in this report considered thixot-ropy as a potential factor affecting the shear strengthof impounded refuse.

The term thixotropy was originally introduced todescribe the well-known phenomenon of isothermal,reversible gel-sol (solid-liquid) transformation in col-loidal suspensions due to mechanical agitation. Thix-otropy is the property of some substances to behavelike a fluid when worked or agitated and settle to asemisolid state when at rest. Thixotropy is alsoclassified as a rheological process. From this perspec-tive, it is described as a continuous decrease inapparent viscosity with time under shear, and subse-quent recovery of viscosity after cessation of flow.From a geotechnical point of view, thixotropy is aprocess of softening caused by remolding, followed bya time-dependent return to the original harder state ata constant water content and constant porosity (incontrast to consolidation). For remolded clays, themechanism of the softening is thought to includedestruction of orderly arrangement of water moleculesand ions in the adsorbed layers of the soil, damage tothe structure acquired during sedimentation andconsolidation, and realignment of clay plates (Terzaghiet al., 1996). Thixotropic strength gain is measured as aratio of the strength after an elapsed time to thestrength immediately after remolding (or compaction)and is called thixotropic strength ratio. Generally, theratio is determined in terms of undrained shearstrength, not effective (i.e., drained) shear strength.

Information about thixotropy as it may pertain tothe stability of coal refuse impoundments wasacquired from the work of Suthaker et al. (1997) onoil-sand tailings. Several of his comments andfindings are as follows:

Factors Affecting Thixotropy

Several factors such as the mineralogy of the clay,water content, and rate of loading directly affectthixotropy of fine tailings. Some clays exhibit highthixotropy naturally. Kaolinite and illite of oil sandsare not thixotropic. However, because of the additionof a dispersing agent, organic matter in the form ofbitumen, and organic acids in the pore water, finetailings are thixotropic in nature.

Duration of Thixotropic Strength Gain

Using cavity-expansion, vane-shear, and viscome-ter testing equipment, Suthaker et al. (1997) conduct-ed three tests at different water contents up to450 days. Thixotropic strength increased quadratical-ly and was still increasing at 450 days.

Relationship between Thixotropy and Consolidation

Long-term strength development in slurries can beeither thixotropic or have a combination of thixotro-pic and consolidation strengths. At low water content(,100 percent), there was no consolidation strengthdevelopment, as the water content remained un-changed. At higher water contents, water contentchanges were prominent, indicating the existence ofconsolidation effects.

Effect of Self-Weight Consolidation on Thixotropy

Shearing strains resulting from consolidation im-pede the physicochemical bonding that producesthixotropic strength. The higher is the rate ofconsolidation, that is, the shearing rate, the smalleris the thixotropic strength gain.

Effect of Moisture Content on Thixotropy

The thixotropy of fine tailings is highly dependenton water content. Substantial thixotropic strengthincreases were seen for fine tailings with watercontents less than 150 percent (Figure 10). The loweris the water content, the higher is the thixotropicstrength.

Design, Construction, and Performanceof Impoundments

It is apparent from the points listed previously thatone very important consideration in the design andconstruction of an impoundment should be theeffective drainage of the impounded fine refuse ortailings. The following papers focus on the danger ofembankment failure, but the negative impact of

Figure 10. Thixotropic strength with time for a number of watercontents (Suthaker et al., 1997).



inadequate drainage is also pertinent to breakthroughpotential:

Comparison of Failed and Stable Impoundments

Mittal and Morgenstern (1977) compared thetailings dams of five large mines in Canada and listedthe characteristics of failed and stable impoundments.Failed impoundments had high phreatic surfaces, inaddition to overly steep downstream slopes and weakfoundation soils. The qualities of stable impoundingstructures included pervious foundations, in additionto relatively course impounded fines and slowimpoundment construction rates.

Consolidation Rate and Rate ofImpoundment Construction

Huang et al. (1987) and Zeng et al. (1998a) bothrecommended a rate of upstream impoundmentconstruction slow enough to provide sufficient timefor consolidation strength development in the finerefuse. For instance, Zeng et al. (1998a and b) con-ducted centrifuge experiments on three models toassess the response of coal refuse impoundments toseismic loading: The models corresponded to thedownstream method, upstream method, and center-line method with induced consolidation. The down-stream model resulted in a small amount of defor-mation but remained stable. The upstream modelresulted in catastrophic failure. The centerline modelhad more deformation than the downstream but wasstill stable after the simulated earthquake. Zeng et al.concluded that it is imperative for consolidation of in-place coal refuse to be monitored in the field duringupstream impoundment construction.

Case Histories of Impoundment Failures

Most documented impoundment failure eventsentail failures of the dams or embankments of theimpoundments. Quite frequently, these follow stormevents that suddenly and drastically increase the loadof the impounded material on the dam and, followingthe breach, lubricate the flow of the fine material. Insuch cases, the potential flow properties, or rheology,of the slurry may become considerably different fromwhat they were prior to the storm event. Such casesprovide little information about the potential rheol-ogy—and even flowability—of impounded refuse thatmay enter into an underground mine. This isparticularly significant with respect to capped struc-tures overlain by layers of slurry cells, since there is noclear water above the impounded slurry. Two possibleexceptions follow.

The Martin County Coal Company, Big BranchSurface Impoundment Breakthrough in MartinCounty, KY

Several studies of this breakthrough event concludedthat the breakthrough resulted from water seepingthrough a thin barrier to an underground coal mineand weakening of the barrier to where it could nolonger withstand the stress exerted by the coal slurry(U.S. MSHA, 2001; NRC, 2002). Under this scenario,it may be an open question as to if the slurry itselfflowed into the mine, or was piped into it by clearwater. Short an answer, we know nothing of theflowability of the slurry itself prior to the break-through. However, two documents question thesignificance of the piping effect (Thacker, 2002;Hagerty and Curini, 2004). Using finite-elementseepage analyses, they demonstrated that the pipingprocess would have drained the clear water and slurrypore water enough to dissipate the excess pore pressurein the fine refuse well before reaching the elevation ofthe clear water. The reduction of pore pressure shouldhave forestalled flow into the mine. Their alternativeconclusion is that the hydrostatic pressure from theimpounded slurry and rainwater surcharge was suffi-cient to push through the barrier (i.e., without theassistance of seepage). If that is correct, the hydrody-namic pressures at the time of the breakthrough, size ofthe conduit to the mine, and initial slurry rheologywere enough for the flow to begin.

Failure of the Los Frailes Tailings Dam

Another example of tailings flow without theassistance of clear-water piping is available from theweb site: http://www.antenna.nl/wise/uranium/mdaflf.html. The failure of the Los Frailes tailings impound-ment in Aznalcollar, Spain, in 1998, involved a breachof the dam. However, the mechanism of the breach wasa bearing failure in the dam foundation, composed ofimpervious Tertiary marl, in response to extra stressbuildup from water above the fine tailings during aheavy rain. The tailings flow is depicted as initiallyfollowing the slide of the foundation material fromunderneath the impoundment.

Effect of Additives on Impounded Refuseand Tailings

In our review, it became apparent that at least someof the existing impounded coal refuse may have beentreated with additives for various purposes. At thistime, we do not know how frequently such treatmenttakes place or how much of it is strictly experimentalversus standard practice. Indications that additives



are applied to coal refuse slurry or tailings in somecases come from Puri et al. (1990), Tang et al. (1997),Suthaker et al. (1997), Liu and McKenna (1999), andHeywood and Alderman (2003). Potential objectivesof the additives include accelerating consolidationand increasing shear strength of impounded refuse,and reduction of slurry viscosity for pipeline trans-port to the impoundment. Michalek (2005) has statedthat, in the experience of U.S. MSHA, miningcompanies generally use additives to accelerate slurryparticle sedimentation in order to decant clear waterback into the mine operation. We believe that anaccurate analysis of the strength and stability of finecoal refuse will have to account for the extent andvariety of additive use, regardless of its purpose, andidentify its effect(s) on the material.

Assessment of Literature Published after the2005 Study

From our review of literature published from 2005to the present, it is apparent that there remains keeninterest in the flow-related engineering properties offine coal refuse and other types of tailings deposits.Ongoing work includes the development of field andlaboratory methods to determine the liquefactionpotential of the materials. For example, Chang andHeymann (2005) tested the relationships betweenshear wave velocity and void ratios and effectivestresses in gold tailings. They confirmed that therelationships for the silty material differed fromconventional correlations associated with sands.Fourie and Tshabalala (2005) proposed use of the‘‘collapse surface’’ determined from the locus of peakstress values from undrained compression tests onisotropically consolidated tailings specimens to pre-dict the onset of liquefaction. Kalinski and Phillips(2008) employed several field and laboratory meth-ods, including standard penetration, seismic conepenetration, surface wave, and vane shear testing,with the objective of developing a universal approachto predicting the seismic behavior of coal refuse slurryimpoundments. Recent work also includes attemptsto model tailings dewatering (de Oliveira-Filho andvan Zyl, 2006) and the effect of capillarity on thestability of nickel tailings dams (Zandarin et al.,2009). The latter modeling study is particularlysignificant with respect to the subject of this paper,since it not only: (1) confirms that low tailingspermeability can maintain an elevated phreaticsurface during and after the impoundment construc-tion, but (2) it also suggests that capillarity in thetailings causes a rapid rise in the phreatic surfaceduring rain storms. This seems to support theprospect of coal refuse slurry remaining saturated

for indefinite time periods even in capped impound-ments if they cover points of groundwater discharge.

Recent literature also presents potential solutionsto slow consolidation in tailings deposits. Wong et al.(2008) found that ‘‘nonsegregating tailings,’’ i.e.,homogenized fine and coarse oil sands tailings,exhibit enhanced performance in consolidation andstrength, and reduction in water retention in com-parison to fine tailings. Wickland and Wilson (2005)described mixtures of gold tailings and ‘‘waste rock’’(analogous to surface coal mine spoil) as havinglower total settlements and faster consolidation timesthan tailings alone. They suggested that combiningtailings with waste rock helps to solve ‘‘…the twobiggest problems associated with conventional meth-ods of mine waste disposal….’’ The low permeabilityof the fine tailings control acid mine drainage fromthe waste rock, and the high shear strength providedby the waste rock prevents catastrophic liquefactionof impounded tailings. Finally, Noakes (2005)reported that as early as 1974, professor David Bogeradded polymers to tailings slurry to keep it flowingthrough pipelines without adding water, thus allow-ing the dry stacking of the tailings and limiting landuse to half that required by conventional impound-ments. Researchers are attempting to develop thistechnology into an economically viable tool fortailings disposal.

A final note pertains to improvements in minebarrier stability analysis. Rohlf and Sweigard (2009)have developed a two-dimensional analysis thatconsiders inclined slices through the barrier aspotential critical failure surfaces. Interestingly, anapplication of their analysis to an example barrierresults in a small increase in safety factor with slurrydepth (from 12.2 to 48.8 m). This implies that barrierstability (assuming piping is not a factor) shouldprogressively become less of a concern as theelevation of an impoundment—or stack of slurrycells above a capped impoundment—increases. This isgood news if true. However, the catastrophic effectsof a slurry breakthrough, if a barrier fails, should stillincrease with load. Further, the utility of thisanalytical approach, like all others, depends onaccurate information about the location of the minerelative to the edge of the impoundment.

CONCLUSIONS AND RECOMMENDATIONS

Based on our review of published information onrefuse and tailings shear strength, we are notconfident that all or even the majority of existingimpoundments (yet under construction or reclaimed)would avoid flows of fine refuse through break-throughs into underground mines. There is significant



uncertainty regarding the effectiveness of strengthdevelopment through consolidation in the fine refuse.Other reasons for our concern include the influence ofpore-water pressure in the refuse and potentiality ofliquefaction—and the sense that at least someimpoundments are not constructed to effectivelydrain water from the material. Further, whereas inone sense, the effect of thixotropy may temporarilysupplement what consolidation strength may occurunder conditions of high moisture content, itsreversibility may only induce a ‘‘false sense ofsecurity.’’ That is, wet, thixotropic refuse may appearto be stable under static conditions before changing toa liquid state when agitated.

The peer review of the 2005 report included severalrecommendations for further assessment of fine refuseslurry flowability, including: an in-depth review of therheology of other materials (e.g., mud, ceramics,refractory clays, and pharmaceuticals); laboratoryand in situ testing of slurry consolidation, shearstrength, liquefaction potential, and rheology; andmodeling of fine coal refuse response to break-throughs. We feel that the suggested studies wouldbe instrumental in providing a better understandingof the magnitude of the problem and of factorsaffecting slurry flowability. However, those studieswould not address breakthrough concerns pertinentto specific impoundment sites. There are optionalpreventative site-specific construction practices thatshould be evaluated. For instance where, after carefulsite investigation, there remains uncertainty whetherthick coal beds intersecting the impoundment foot-print have been underground mined within a ‘‘safetyzone,’’ the coal seams can be surface mined somedistance into that zone; and designed artificialbarriers can be constructed on the benches againstthe high walls. Also, where there are plans to increasethe size of active impoundments (beyond originaldesigns) or construct slurry cells on top of cappedimpoundments, the impounded slurry can be sampledand tested to ensure that the material’s water contentis not above its liquid limit.

Finally, research and development of methods tobetter detect and position inadequately documentedunderground mines and to prevent coal refuse slurryflowability are strongly encouraged.

ACKNOWLEDGMENTS

The authors wish to thank Roger Calhoun, LoisUranowski, David Lane, and this journal’s peerreviewers for their constructive comments on thedraft manuscript. The assistance of Tiara Neal andTom Mastrorocco with the illustrations is also deeplyappreciated. Finally, special thanks are due to Brian

Greene for inviting us to make this contribution tothe Dams Special Edition.

REFERENCES

BUSCH, R. A.; BACKER, R. R.; ATKINS, L. A.; AND KEALY, C. D.,1975, Physical Property Data on fine Coal Refuse: U.S.Department of Interior Bureau of Mines Report of Investi-gation 8062.

CHANG, H. P. N. AND HEYMANN, G., 2005, Shear wave velocity ofgold tailings: Journal of the South African Institution of CivilEngineering, Vol. 47, No. 2, pp. 15–19.

D’APPOLONIA ENGINEERING, INC., 2009, Engineering and DesignManual: Coal Refuse Disposal Facilities: prepared for theU.S. Mine Safety and Health Administration, Pittsburgh,PA, 826 p., plus appendices.

DE OLIVEIRA-FILHO, W. L. AND VAN ZYL, D., 2006, Modelingdischarge of interstitial water from tailings followingdeposition. Part 1: Phenomenology and model description:Solos e Rochas, Vol. 29, No. 2, pp. 199–209.

EVANS, M. D. AND HARDER, L. F., 1993, Evaluating liquefactionpotential of gravelly soil in dams. In Geotechnical Practice inDam Rehabilitation: American Society of Civil EngineersSpecial Publication 35. pp. 467–481.

FOURIE, A. B., 2004, Comments to Peter Michael of the U.S. Officeof Surface Mining re: Request for Assistance—Review of DraftReport: University of Witwatersrand Department of CivilEngineering, Witwatersrand, South Africa, 2 p.

FOURIE, A. B.; BLIGHT, G. E.; AND PAPAGEORGIOU, G., 2001, Staticliquefaction as a possible explanation for the Merriespruitdam failure: Canadian Geotechnical Journal, Vol. 38,pp. 707–719.

FOURIE, A. B. AND PAPAGEORGIOU, G., 2001, Defining anappropriate steady state line for Merriespruit gold tailings:Canadian Geotechnical Journal, Vol. 38, pp. 695–707.

FOURIE, A. B. AND TSHABALALA, L., 2005, Initiation of staticliquefaction and the role of K0 consolidation: CanadianGeotechnical Journal, Vol. 42, pp. 892–906.

GENES, B.; KELLER, T.; AND LAIRD, J., 2000, Steady stateliquefaction susceptibility of high hazard upstream-construct-ed coal refuse disposal facilities. In Proceedings, TailingsDams 2000: Association of State Dam Safety Officials andU.S. Society of Dams, Las Vegas, NV, pp. 47–58.

HAGERTY, D. J. AND CURINI, A., 2004, Impoundment failureseepage analyses: Environmental and Engineering Geoscience,Vol. 10, No. 1, pp. 57–68.

HEYWOOD, N. I. AND ALDERMAN, N., 2003, Developments in slurrypipeline technologies: CEP Magazine: April 2003.

HUANG, YANG H.; LI JUNLI; AND WEERATUNGA GAMINI, 1987,Strength and Consolidation Characteristics of Fine CoalRefuse: University of Kentucky Contract Report J5140126prepared for U.S. Department of Interior Office of SurfaceMining Reclamation and Enforcement, Lexington, KY,115 p.

ISHIHARA, K., 1985, Stability of natural deposits during earth-quakes. In Proceedings of the 11th International Conferenceon Soil Mechanics and Foundation Engineering. AA. Bolhema,Vol. 1, pp. 321–326.

KALINSKI, M. E. AND PHILLIPS, J. L., 2008, Development ofmethods to predict the dynamic behavior of fine coal refuse:Preliminary results from two sites in Appalachia. In Zeng, X.;Manzari, M. T.; and Hitunen, D. R. (Editors), Proceedings ofthe Conference of Geotechnical Earthquake Engineering andSoil Dynamics IV: GSP-181, pp. 1–10, American Society ofCivil Engineers, Reston, VA, 10 p.



LIU, B. Y. AND MCKENNA, G., 1999, Application of wickdrains incomposite tailings. In Proceedings of the 52nd Canadian

Geotechnical Conference: Canadian Geotechnical Society,Regina, SK (Canada), pp. 487–494.

MICHAEL, P. R. AND CHAVEL, L., 2008, Environmental Risks

Associated with Coal Refuse Impoundment Reclamation: An

Assessment of the Possibility of an Underground Mine

Breakthrough Occurring as a Result of the Impoundment

Reclamation Process: U.S. Office of Surface Mining Appa-lachian Region Management Council Open-File Report,Pittsburgh, PA, 14 p.

MICHAEL, P.; MURGUIA, R.; AND KOSAREO, L., 2005, The

Flowability of Impounded Coal Refuse: A Review of Recent

Work and Current Ideas in the Engineering Profession: U.S.Office of Surface Mining Appalachian Region ManagementCouncil Open-File Report, Pittsburgh, PA, 25 p.

MICHALEK, S. J., 2005, Comments to Peter Michael of the U.S.

Office of Surface Mining re: Comments on Office of Surface

Mining Report—‘‘Flow Characteristics of Impounded Coal

Refuse Material’’: U.S. Mine Safety and Health Administra-tion, Pittsburgh Safety and Health Technology Center, MineWaste and Geotechnical Engineering Division, Bruceton, PA,3 p.

MITTAL, H. K. AND MORGENSTERN, N., 1977, Design andperformance of tailings dams. In Landva, A. and Knowles,G. D. (Editors), Proceedings of the Conference on Geotech-

nical Practice for Disposal of Solid Waste Materials:American Society of Civil Engineers, Ann Arbor, MI,pp. 475–492.

NATIONAL RESEARCH COUNCIL (NRC), 2002, Coal Waste Impound-

ments: Risks, Responses, and Alternatives: National AcademyPress, Washington, D.C., 185 p., plus appendices.

NOAKES, F., 2005, An end to tailings dams is in sight wastedisposal: Australian Mining, Vol. 97, No. 10, p. 18.

PURI, V. K.; DAS, B. M.; AND DEVKOTA, B. C., 1990, Geotechnicalproperties of coal mine slurry waste. In Chugh, Y. P.; Davin,D. C.; and Dietz, K. R. (Editors), Proceedings First

Midwestern Region Reclamation Conference: Southern IllinoisUniversity, Carbondale, IL, pp. 11.1–11.10.

QIU YUNXIN AND SEGO, D. C., 1998, Engineering properties ofmine tailings. In Proceedings of the 51st Canadian Geotech-

nical Conference: Canadian Geotechnical Society, Vancou-ver, BC (Canada), pp. 149–154.

ROHLF, R. A. AND SWEIGARD, R. J., 2009, Stability analysis ofbarriers protecting underground mine entries in coal slurryimpoundments: Mining Engineering, Vol. 60, pp. 37–42.

SHINAVSKI, L., 2006, Overview of impoundment abandonment. InPower Point Presentation at the U.S. Mine Safety and Health

Administration 2006 Annual Dam Safety Seminar: NationalMine Health and Safety Academy, Beaver, WV, 48 slides.

SUTHAKER, N. N. AND SCOTT, J. D., 1994, Large scale consolidationtesting of oil sand fine tails. In Proceedings of the

International; Congress of Environmental Geotechnics: Geo-

technical and Related Aspects of Waste Management Associ-

ated with Municipal, Mine, Industrial, and Nuclear Wastes:Canadian Geotechnical Society, Edmonton, AB (Canada),pp. 149–154.

SUTHAKER, N. N. AND SCOTT, J. D., 1996, Measurement ofhydraulic conductivity in oil sand tailings slurries: Canadian

Geotechnical Journal, Vol. 33, No. 1, February 1996,pp. 642–653.

SUTHAKER, N. N. AND SCOTT, J. D., 1997, Thixotropic strengthmeasurement of oil sand fine tailings: Canadian Geotechnical

Journal, Vol. 34, pp. 974–984.

SUTHAKER, N. N.; SCOTT, J. D.; AND MILLER, W. G., 1997, The firstfifteen years of a large scale consolidation test. In Bauer, G.E. (Editor), Proceedings of the 50th Canadian GeotechnicalConference: Canadian Geotechnical Society, Ottawa, On-tario, pp. 476–483.

SWEIGARD, R. J.; HOPKINS, T. C.; JUSTICE, C. T.; BECKMAN, T.;THOMPSON, E. D.; AND ROHLF, R. A., 1997, Impact ofconsolidation on the shear strength of coal refuse impound-ments: Transactions of the Society for Mining, Metallurgy,and Exploration, Vol. 302, pp. 141–144.

TANG, J.; BIGGAR, K. W.; SCOTT, J. D.; AND SEGO, D. C., 1997,Examination of mature fine tailings using a scanning electronmicroscope. In Bauer, G. E. (Editor), Proceedings of the 50thCanadian Geotechnical Conference: Canadian GeotechnicalSociety, Ottawa, Ontario, pp. 746–753.

TERZAGHI, K.; PECK, R.; AND MESRI, G., 1996, Soil Mechanics inEngineering Practice, 3rd ed.: John Wiley & Sons, Inc., NewYork, NY, pp. 193–198.

THACKER, B. K., 2002, Big Branch slurry impoundment break-through: Why it happened and lessons learned. In Proceed-ings, Tailings Dams 2002: Association of State Dam SafetyOfficials and U.S. Society of Dams, Las Vegas, NV,pp. 47–58.

TSUCHIDA, H., 1970, Prediction and countermeans against theliquefaction in sand deposits. In Abstract of the Seminar in thePort and Harbor Research Institute. Yokohama, Japan,pp. 3.1–3.33.

U.S. DEPARTMENT OF INTERIOR, OFFICE OF SURFACE MINING

RECLAMATION AND ENFORCEMENT (U.S. OSM), 2002, Reporton October 2000 Breakthrough at the Big Branch SlurryImpoundment: Office of Surface Mining, Lexington, KY,80 p., plus appendices.

U.S. DEPARTMENT OF LABOR, MINE SAFETY AND HEALTH ADMINIS-

TRATION (U.S. MSHA), 2001, Report of Investigation:Noninjury Impoundment Failure/Mine Inundation Accident,October 11, 2000: Mine Safety and Health Administration,Arlington, VA, 71 p., plus appendices.

U.S. OFFICE OF THE FEDERAL REGISTER, NATIONAL ARCHIVES AND

RECORDS ADMINISTRATION, 2007, Title 30 of the Code ofFederal Regulations: U.S. Government Printing Office,Washington, D.C.

USUI, H., 1998, Prediction of slurry viscosity by a thixotropymodel for dispersion systems. In Wypych, P. W. (Editor),Proceedings from the 6th International Conference on BulkMaterials Storage, Handling, and Transportation: Institutionof Engineers, Wollongong, NSW, Australia, pp. 257–263.

VOLPE, R., 2004, Comments to Peter Michael of the U.S. Office ofSurface Mining re: Draft Refuse Flowability Report, SantaClara Valley Water District, Santa Clara, CA, 2 p.

WICKLAND, B. AND WILSON, G. W., 2005, Research of co-disposalof tailings and waste rock: Geotechnical News, Vol. 23, No. 3,pp. 35–38.

WONG, R. C. K.; MILLS, B. N.; AND LIU, Y. B., 2008, Mechanisticmodel for one-dimensional consolidation behavior of non-segregating oil sands tailings: Journal of Geotechnical andGeoenvironmental Engineering, Vol. 134, No. 2, pp. 195–202.

ZANDARIN, M. T.; OLDECOP, L. A.; RODREQUEZ, R.; AND ZABALA,F., 2009, The role of capillary water in the stability of tailingdams: Engineering Geology, Vol. 105, pp. 108–118.

ZENG, XIANGWU; WU, JIAER; AND ROHLF, R., 1998a, Modeling theseismic response of coal-waste tailings dams: GeotechnicalNews: June 1998, pp. 29–32.

ZENG, XIANGWU; WU, JIAER; AND ROHLF, R., 1998b, Seismicstability of coal-waste tailings dams: Geotechnical EarthquakeEngineering and Soil Dynamics III: pp. 951–961.



potential of breakthroughs of impounded coal refuse slurry into … · 2010-12-23 · potential of...

Documents