Minerals Engineering 24 (2011) 1084–1090

Contents lists available at ScienceDirect

Minerals Engineering

journal homepage: www.elsevier .com/ locate/mineng

Technical Note

A safety assessment of the new Xiangyun phosphogypsum tailings pond

T. Wang a,b,c,⇑, Y. Zhou a, Q. Lv a, Yuanle Zhu a, C. Jiang a

a Wuhan University, Wuhan, Chinab State Key Laboratory of Water Resource and Hydropower Engineering Science, Wuhan, Chinac Key Laboratory of Rock Mechanics in Hydraulic Structural Engineering, Ministry of Education, China

a r t i c l e i n f o

Article history:Received 24 December 2010Accepted 16 May 2011Available online 12 June 2011

Keywords:PhosphogypsumTailings pondSafety assessment

0892-6875/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.mineng.2011.05.013

⇑ Corresponding author at: Wuhan University, W68773941; fax: +86 027 68772310.

E-mail address: tao_wang6@163.com (T. Wang).

a b s t r a c t

Phosphogypsum tailings are piled up to form a phosphogypsum tailings pond. In the design and opera-tion stages of a tailings project, the stability of the tailings pond, the control capacity for flood, and thereliability of the drainage and safety monitoring facilities should be fully evaluated. Key contents ofthe safety assessment are analyzed in view of the new Xiangyun phosphogypsum tailings pond, whichis currently in the design stage. Flood routing calculation results show that the pond cannot meet therequirements of the China National Standards Safety Code in the condition of a 500-year flood even ifthe drainage facilities operate normally. Seepage and static stability of the tailings pond are investigatedthrough numerical and limit equilibrium methods. The results indicate that the sliding stability can meetthe requirements along the starter dike profile. Dynamic calculation results show that the liquefied areais at the top of the dam slope and it cannot influence the dam safety. The aspects of safety monitoringdesign that require attention are also proposed.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, a number of phosphoric acid plants have beenoperated in China (Zhang et al., 2007). During the production pro-cess, the plants produce large amounts of phosphogypsum tailings,which contain many toxic substances, such as sulfates, phosphates,fluorides, and so on (Komnitsas et al., 1998). The pond designshould be stable at all stages of operation, from first commission-ing until closure. Phosphogypsum is usually stacked in one place,so a suitable site for the storage of the phosphogypsum is neces-sity. Construction of a gypsum impoundment to stack the residuesis one of the sound measures in the management of phosphogyp-sum tailings in the production process. The recycled water in thegypsum impoundment is collected through the recycled waterpool, and then sent to the equipment area through a pipeline formultipurpose use. However, it cannot be released because of its po-tential to cause pollution. Gypsum impoundment is a means tomanage the effluent from the chemical production process. Basedon experimental and risk analysis data, a rehabilitation scheme isproposed for the affected areas by tailings, aiming at deactivatingthe pollution sources and rehabilitating the contaminated areaswith remedial actions (Komnitsas et al., 1999). New clean techno-logical developments are especially attractive to governments indeveloping countries since they hold the promise of reducing envi-

ll rights reserved.

uhan, China. Tel.: +86 027

ronmental damage costs while at the same time maintaining thesocial and economic benefits of mining (Warhurst and Bridge,1996). The Mining Association of Canada (MAC), the national orga-nization of the Canadian mining industry, has published a numberof practical waste management guides, and has developed severalenvironmental guidelines for its members (Hilson, 2000).

Research on the safety technology of tailings ponds started late;it was not considered to be worthy of consideration by the Interna-tional Commission of Large Dams (ICOLD) until 1976 (ICME andUNEP, 1998). Before this, some experts and researchers thoughtthat the tailings dam could not be treated as a real dam becauseof its simple design, long time of construction, and inability to storewater. Later, serious disasters caused by accidents related to largetailings ponds gradually have aroused attention and awareness.

A number of characteristics make tailings dams more vulnera-ble to failure than water storage dams (Rico et al., 2008a,b). Con-struction of a tailings dam is more complex than that ofconventional water storage dams (Azizli et al., 1995). Serious tail-ings accidents have occurred from time to time in China (Men andChai, 2009; Li et al., 2006; He et al., 2009). The State Administrationof Work Safety of China has been reporting tailings ponds accidentssince 2001. By the end of 25 November 2007, a total of 43 accidentshad been reported in the country. These include 3 cases in 2001, 2in 2003, 3 in 2004, 9 in 2005, 12 in 2006, and 14 in 2007. The num-ber of accidents shows a rising trend (Men and Chai, 2009). Severalaccidents happened in 2008, and these include the 9.18 accident inXiangfen County in Shanxi Province that killed 262 people andcaused another 1047 to suffer the adverse effects. The 9.18

T. Wang et al. / Minerals Engineering 24 (2011) 1084–1090 1085

accident is the most tragic accident in China so far (SAWSC, 2006).Since 2001, several terrible accidents have happened in China, andcause deaths of 332 people and substantial property losses(Xinhuanet, 2008).

The operation of phosphogypsum tailings is complex. Problemsrelated to it lag, and thereby failing to draw enough attention. Withthe phosphogypsum residue increasing in volume, new problemsmay appear, and should be studied and solved. Experiences showthat prevention, rather than reaction after the fact, should beemphasized (Men and Chai, 2009). Safety assessment of tailingsponds in the design and operation stages is a practical way toanticipate the potential risk. The information on historic tailingsdam failures were compiled with the purpose to establish simplecorrelations between tailings ponds geometric parameters andthe hydraulic characteristics of floods resulting from released tail-ings (Rico et al., 2008a,b). A detailed search and re-evaluation ofthe known historical cases of tailings dam failure in Europe wascarried out (Rico et al., 2008a,b). Rheological properties haveassisted in solving large-scale tailings disposal problems in theAustralian mining and mineral industry (Nguyen and Boger,1998). The laboratory model test was studied for stability analysisof actual tailings dams (Yin et al., 2011). An innovative method ofreinforced terraced fields is presented to satisfy the specificrequirement of fine tailings disposal (Wei et al., 2009). However,there is little or no information available in the literature con-cerned with the synthetic analysis of flood prevention, sliding sta-bility and monitoring. This paper presents a comprehensive safetyassessment for the new Xiangyun phosphogypsum tailings pond,which is currently in the detail design stage. In the course ofassessment, the stability of the tailings pond, the control capacityfor flood, and the reliability of the drainage and safety observationfacilities be fully evaluated in accordance with the China NationalStandards Safety Codes (SAWSC, 2006). Three closely linkedaspects largely determine the safety of tailings operation.

Fig. 1. Detailed topographical map of the new

2. Overview of new Xiangyun phosphogypsum tailings pond

Hubei Xiangyun (Group) Chemical Co., Ltd. has operated for10 years. According to the group’s overall plan, the productioncapacity of the phosphoric acid equipment plant will be expandedto 300,000 tons per year. The new pond needs to be constructed,and the preliminary design of the new pond has to be finishedby May 2010. The safety assessment is focused on the preliminarydesign proposal, and can provide improvement of its design.

2.1. Geological setting and climate

The new Xiangyun phosphogypsum tailings pond (Fig. 1) is lo-cated 30 km north of Wuxue city, which is a medium-sized cityin Hubei Province in Central China. The slope of the pond terrainis 25–45�. The slope area has no history of landslides, avalanches,mud-rock flow, surface subsidence, fall of ground, and other geo-logical disasters. The slopes around the pond are therefore stableunder natural conditions.

The Wuxue region where the tailings pond is located has a typ-ical subtropical monsoon climate with high humidity and four dis-tinct seasons. The largest single-day amount of rainfall is253.2 mm (28 June 1983), and the one-hour maximum amountof rainfall is 41.7 mm (15 May 1983). The average annual amountof rainfall in the region is between 1278.7 and 1442.6 mm.

2.2. Tailings dam and ancillary facilities

The tailings pond is filled upstream style by the flushing meth-od, which has been widely used in China. The starter dike is a rock-filled dam and is 28 m high. The elevation of the dam base is 22 mand the elevation of the dam crest is 50 m above the mean sea le-vel. The perimeter dike is a hydraulic fill dam with phosphogyp-sum, and its average slope ratio is 1/3 (Fig. 2). The cumulated

Xiangyun phosphogypsum tailings pond.

Fig. 2. The arrangement of the drainage and seepage control facilities in the new Xiangyun tailing pond.

Table 1Calculation results of peak discharge and flood discharge.

Designfrequence

Peak discharge Qp ðm3=sÞ Flood discharge

Wpð�104m3Þ

1% 70.90 28.720.2% 93.42 35.06

1086 T. Wang et al. / Minerals Engineering 24 (2011) 1084–1090

elevation of the final design is 125 m, and the storage capacity is24 million m3.

The starter dike will be constructed in a U–valley between theXiangang Hill and the Fox Hill. Due to the favorable geological con-ditions, high mechanical strength, and very shallow cover, the star-ter dike foundation can provide enough bearing capacity. The damabutment is mainly composed of limestone, and the natural vege-tation is abundant.

The relative elevation difference of the dam abutment and theslope crest, the dam crest and the downstream gully are less than10 m. The angle of the slopes of both hillsides is less than 20�. Thefoundation surface of the pond dam is covered by quaternary stra-ta. Big fault structures in the foundation are absent.

3. Key contents of the safety assessment

Analyses of the causes of accidents identify the flood calcula-tion, rock and soil sliding stability as the bases for safety evalua-tion. The use of safety monitoring facilities provides a validationfor the flood calculation.

3.1. Flood prevention calculation

3.1.1. Flood calculation and flood routing methodFlood prevention has two parts: flood calculation and flood

routing. The purpose of the flood calculation is to determine thepeak flow, the flood volume, and the flood hydrograph for use indrainage design and safety evaluation of the constructed tailings.According to the China National Requirements (SAWSC, 2006),and as suggested by related departments, the calculation shouldbe based on the local hydrological atlas or the computing formula,and should be appropriate for a small area. Among the computa-tional methods for the design of flood control systems that havea small drainage, the most widely used are the rational formula,instantaneous unit hydrograph formula, and empirical formula.The first two formulas are the most commonly adopted methodsin calculating the design flood in China. The purpose of the calcu-lation of the flood routing is to determine the flood storage capac-ity and flood flow according to the established drainage system.The flood flow and the flood storage capacity can be determinedby the flood hydrograph from the water balance calculation, con-siderations on the runoff hydrograph, the relation curve of the dis-charge of the drainage construction, and the amount of waterstored in the tailing reservoir.

The water balance equation at any period of time, Dt, is shownby the following equation (Chen, 2007)

12ðQs þ QzÞDt � 1

2ðqs þ qzÞDt ¼ Vz � Vs ð1Þ

where, Qs and Qz are the amounts of water that flow into the tailingspond, qs and qz are the amounts of water that drained out of the tail-ings pond, and Vs and Vz denote the amount of water stored in thetailings pond at the beginning and end of the period, respectively.

Through the calculation of flood prevention, the extent of thewater surface under flood and normal conditions can be deter-mined; this allows the calculation of infiltration flow and slopestability.

3.1.2. Field hydrological conditionsUnder the 1:2000 topographical map of the tailings pond, the

geographic parameters of the tailings field are measured and calcu-lated. The geographic characteristics of the region are as follows:

Catchment area

F = 1.25 km2

Main channel length

L = 1.22 km Average gradient of main channel J = 0.054

Based on its geographic location, the tailings pond is classifiedas a No. 2 hydrology zone in Hubei Province. Thus, the tailingsshould be level 3, and the tailings dam should also be level 3 basedon the China National Codes. Accordingly, the flood control stan-dards for the design frequency is 1% and the check frequency is0.2% (SAWSC, 2006; MCPRC, 1990; DTTF, 1987). As a rule, the max-imum peak discharge of the tailings pond is calculated using thesimplified rational formula as follows (HIWCHEED, 1985)

QP ¼AðSPFÞB

LmJ1=3

� �C � DlF ð2Þ

where QP and SP are the design peak discharge and the storm inten-sity, respectively, with frequency P. F is the catchment area abovethe tailings impoundment, L is the main channel length from thedam site to the watershed, m is the flow concentration coefficient,J is the average gradient of the main channel, l is the average infil-tration rate of the valley during the rainfall duration, and A, B, C, Dare the calculation coefficients of the maximum peak discharge,which can be found in the chart.

In addition, the design flood discharge can be calculated by theformula (HIWCHEED, 1985)

WtP ¼ 1000atHtPF ð3Þ

where WtP and HtP are the flood discharge and rainfall, respectively,for duration t and frequency P; at is the runoff coefficient for therainfall duration t (here, at = 0.9).

Hydrological data are obtained from the latest atlas (HBHWR,2008). The design peak discharge and flood volume under different

T. Wang et al. / Minerals Engineering 24 (2011) 1084–1090 1087

frequencies are obtained after calculation based on the tailingsfilled condition (Table 1).

3.1.3. Calculating flood hydrographThe design flood hydrograph of the tailings ponds or other small

watersheds is often simplified for easy calculation. The generalizedmulti-peak triangular hydrograph is drawn based on calculationfrom certain design hyetographs. It combines the features of therational formula method. This method allows a more realisticdepiction of heavy rains and flooding in the monsoon- or typhoon-prone regions in China (SAWSC, 2006).

As for the new Xiangyun phosphogypsum tailings pond, thegeneralized multi-peak triangular hydrograph method may beused to draw flood hydrographs with different frequencies. Thismethod depends on the result of peaks calculated in the differentfrequency distributions of floods. Figs. 3 and 4 are the flood hydro-graphs with frequencies of 1% and 0.2%, respectively.

3.1.4. Analysis of the discharge processAccording to the topographic map and the elevations of the tail-

ings dam and beach surface, the water surface areas are measuredunder different water levels, and the flood storage capacity is cal-culated based on the following formula (Chen, 2007)

DV ¼ ðS1 þ S2ÞDH=2 ð4Þ

where S1 and S2 represent the surface area of the initial and calcu-lated water levels, respectively.

The cross-sectional area of the drainage chute and culvert is1.00 m2, based on design. From the above analysis, the followingconclusions may be drawn: (1) In the design flood condition withall drainage facilities under normal operation, the flood volumeof the tailings pond is V = 266,404 m3 and the beach water levelis 123.7 m; this occurs in period 21 (each period is 3600 s). (2) Inthe check flood condition, which occurs in period 21,V = 367,427 m3, and the water level is 124.6 m. In terms of the

Fig. 3. Flood hydrograph of design frequency P = 1%.

Fig. 4. Flood hydrograph of check frequency P = 0.2%.

design regulations in China (SAWSC, 2006), the dry beach lengthof this phosphogypsum tailings pond should be at least 70 m, theminimum free height is 0.7 m, and the gradient of the slope isaround 1%. According to the original design, the final height ofthe perimeter dike is 125.0 m, so the highest water level shouldnot exceed 124.3 m. Based on these requirements, the pond is indanger because the free height is 0.3 m below the requirement.Consequently, the design should be revised.

Building a flood-intercepting trench or increasing the cross-sec-tional area of the drainage chute and culvert may be done to ad-dress the problem.

The first method is building a flood-intercepting trench. If this isconstructed around the tailings field and water recycling pool, thenit can intercept the outside flood around the tailings field. The floodmay be discharged directly into the downstream of a nearby ditchthrough the trench, or returned to the water recycling pool for re-use in the dry season. In the design flood condition with all drain-age facilities under normal operation, V = 136,699 m3, and thebeach water level is 121.9 m; in the check flood condition,V = 180,786 m3 and the water level is 122.5 m. The calculationsindicate that the new Xiangyun tailings pond has enough freeheight in the design, and the check frequency of the flood condi-tions is according to the discharge capacity of the drainage system.Therefore, the tailings pond meets the standard requirement.

The second method is increasing the cross-sectional area of thedrainage chute and culverts to 1.8 m2. In the design flood conditionwith all drainage facilities under normal operation, the beachwater level is 123.2 m; in the check flood condition, the water levelis 123.9 m. As before, the tailings pond also meets the standardrequirement.

Both options have their advantages and disadvantages. Due tothe low dip angle of the hillside terrain, this study proposes theconstruction of a flood-intercepting trench around the tailings fieldfor the new Xiangyun tailings pond. An optimal program at the de-sign stage should be selected according to comprehensiveevaluation.

3.2. Calculations of seepage flow and sliding stability

3.2.1. Methods and theory of tailings dam seepage and sliding stabilitycalculation

For simplicity, the steady flow is considered to form under themaximum stage in the process of design. The boundary aroundthe pond appears to be an impervious boundary because of theanti-seepage membrane. The tailings dam can be simplified to ahomogeneous structure, and the seepage field can be calculatedwith the steady seepage formula according to a homogeneousdam cross section. Considering the seepage water along the direc-tion of the main ditches, the seepage section of the calculationshould be selected through the starter dike and along the mainditches.

The seepage and stability calculations can be carried out withFLAC3D, which is used for stress and deformation analyses aroundsurface and underground structures in soil and rock. FLAC3D is anexplicit finite-difference program for engineering mechanics com-putations (Wang and Zhu, 2006; James et al., 2006). Since the timestep is chosen to be small, information cannot be physically trans-mitted from one element to another within that small time step.Each element behaves according to a prescribed linear or nonlinearstress/strain law in response to applied forces or boundaryrestraints.

The sliding stability of the starter dike and perimeter dikesshould be determined by calculations based on the dam material,physical and mechanical properties of the foundation, and the var-ious load combinations. The ordinary (Swedish) method of slices isproposed as a suitable calculation method to obtain the factor of

Table 2Main mechanical properties of materials.

Phosphogypsum Starter dike Foundation

Density (kg/m3) 1150 1700 1650Cohesion (Pa) 6000 20,000 11,000Friction angle (�) 23 28 22Permeability (cm/s) 3e�5 2e�6 1e�6Porosity 0.3 0.4 0.3

1088 T. Wang et al. / Minerals Engineering 24 (2011) 1084–1090

safety (FOS) in the China National Codes. The slope stability analy-sis method based on a rigid limit equilibrium theory has beenwidely used for tailings slope stability analysis. However, thismethod often has difficulties in addressing complex conditions(Duncan, 1996). The numerical method based on the strengthreduction method can handle complex loads and boundaryconditions.

In the method of shear strength reduction technique (SSRT), theFOS of a slope is defined here as the factor by which the originalshear strength parameters (cohesion c0 and friction angle /0) mustbe divided to bring the slope to the point of failure (Griffith andLane, 1999). The factored shear strength parameters c0f and /0f aretherefore given by (Dawson et al., 1999).

c0f ¼ c0=SRF ð5Þ

/0f ¼ arctantan /0

SRF

� �ð6Þ

To find the ‘‘true’’ FOS, the value of the strength reduction factor(SRF) that will just cause the slope to fail must be systematicallysearched. When this value is found, FOS = SRF.

Compared with the roller-compacted sand dam, the buildingmaterial of the tailings pond is relatively loose, and therefore moreprone to earthquake liquefaction. Incidents reported locally andabroad show that the tailings pond is easy to liquefy during anearthquake. Its structure is therefore easily destabilized by earth-quakes. The seismic stability analysis of the tailings pond is mainlydone to analyze the liquefaction resistance.

Existing analytical techniques for the evaluation of pore waterpressure development and liquefaction under seismic loads are lar-gely based on the dynamic behavior of sand; limited research hasbeen conducted on the dynamic behavior of tailings. However,observations indicate that some types of tailings are highly suscep-tible to liquefaction (Ishihara et al., 1980).

The objectives of liquefaction analysis are to determine thepresence of liquefied tailings in the pond area and to outline theextent of the liquefaction zone. Martin, Finn, Seed described themechanism of liquefaction and proposed the calculation model(Itasca, 2006; Wijewickreme et al., 2005).

3.2.2. Calculations of seepage flow and dam stability of the newXiangyun tailings pond

Stability analysis of the tailings pond is mainly focused on thedesigned conditions. The objectives of the analysis are to deter-mine whether the dam meets the safety standard and to providea scientific reference to the reinforcement of the tailings dam.

Seepage calculation is done using FLAC3D, and the coupling ef-fect of strain and seepage are taken into account. Two-dimensionalcalculations are used this time. There are 198 grids in the horizon-tal division, 26 grids in the vertical division, and only one grid inthe third direction (Fig. 5). Calculating parameters from laboratoryexperiments are shown in Table 2. Water head, flow, and imperme-able boundary conditions are used. The seepage calculation can ob-tain the distribution of groundwater seepage line. According tothis, the safety factors of the dam are calculated under various con-

Fig. 5. Calculation mesh an

ditions, such as normal water level, flood level, and specialoperation.

The calculation results show that when the drainage facilitiesare in normal operation, the overflow points are located in thedrainage prism and do not appear in the perimeter dikes and otherparts. Deformation of the operation phase is also analyzed usingthe Mohr–Coulomb model. The full fluid-mechanical coupling inFLAC3D occurs in two directions: pore-pressure changes cause vol-umetric strains to occur, and thus influence the stresses; in turn,the pore pressure is affected by the strain.

After the seepage and deformation calculation, the strengthreduction technique is used to obtain the factor FOS, consideringthe tailings filled condition (Fig. 6). For comparison, the ordinarymethod is also used to calculate the safety factor (Table 3). In theseismic working condition, a quasi-static method is used to obtainthe FOS according China National Codes (SAWSC, 2006; MWRPRC,1988; MWRPRC, 2001), while the liquefaction calculation is carriedout through time history analysis method. Calculation results showthat when the drainage facilities operate properly, the tailings damis safe in a variety of work conditions, and the safety factors meetthe requirements.

The basic intensity of earthquake in this engineering area is lessthan VI. The liquefaction calculation is performed through FLAC3D.The Byrne model (Byrne, 1991; Byrne and Jitno, 1992) is used tosimulate the development of excess pore water pressures withinthe tailings.

For dynamic analysis, the appropriate designed seismic datamust be determined, but this is difficult. Since future earthquakecurves cannot be properly estimated, artificial seismic waves areused to solve this problem (Hu, 2008). According to McGuire’sempirical formula of 90% energy duration, the earthquake lasts22.50 s. According to engineering principles and methods of seis-mology, the seismic acceleration time history is synthesized(Fig. 7).

In general, the criterion of zero value for the effective stress ofphosphogypsum is the basis for determining whether the studiedphosphogypsum is liquefied. However, considering the influenceof the computation accuracy, the effective stress cannot be zero.Thus, the excess pore pressure ratio is commonly used to describethe liquefaction. In FLAC3D, the initial pore water pressure andeffective stress ratio of the pore water pressure ratio (PPR) can beused to determine whether the phosphogypsum is liquefied. Thesignificance of this feature is that when the pore water pressureis less than the initial effective stress, and the PPR is less than100%, the phosphogypsum is not liquefied; when the excess porewater pressure is greater than or equal to the initial effective stress,

d material distribution.

5.0e-4

2.0e-42.5e-43.0e-43.5e-4

Shear Strain RateFOS=1.58

Fig. 6. Calculation results based on strength reduction method at normal condition.

Table 3Safety factors under different states.

Workingconditions

Ordinary method SSRT Required by Chinese codes

Normal 1.388 1.580 1.20Flood 1.301 1.402 1.10Flood and seismic 1.134 1.211 1.05

Fig. 7. Acceleration series created by artificial method.

T. Wang et al. / Minerals Engineering 24 (2011) 1084–1090 1089

and the PPR is greater than 100%, the phosphogypsum is in lique-faction. The PPR expression is (Wijewickreme et al., 2005)

PPR ¼ ur00� 100% ð7Þ

in which u is the pore water pressure; r00 is the initial effectivestress of phosphogypsum, and PPR is a dimensionless scalar.

After static analysis, the dynamic calculation command isexecuted, the mechanical parameters of phosphogypsum are re-designated, the Byrne model parameters are added, nonlineardynamic calculation is conducted, and the PPR is added to theresults in subsequent processing. The calculated dynamic load isapplied to the bottom of the model, and the free field around theboundary is set to reduce wave reflection. Fig. 8 shows the calcu-lated liquefaction area (cyclic PPR greater than 1.0) when the drybeach length is 100 and 70 m. The liquefied area is at the top ofdam slope with a shallow position. It cannot entirely ensure safetyof the dam. The water must be strictly controlled especially duringthe flood season.

In summary, the design of the new Xiangyun tailings pond is inline with relevant national requirements for sliding stability.

Fig. 8. The liquefied area at different dry beach lengths.

3.3. Analysis of the reliability of the safety monitoring facilities

3.3.1. Essential requirements for safety monitoring and managementTo monitor effectively the safety of the dam during operation,

the water level and displacement observation facilities should bedesigned. Operators must strengthen the observation of the seep-age water quality.

To have a timely awareness of the settlement or displacementsituation of the dam, displacement monitoring piles in the outerslope of the starter dike and perimeter dike must be installed. Mea-suring base points are also required on the slope of neighboringhills. Displacements can be observed through the collimation linemethod, and should be observed at least once a month. The fre-quency of observation should be increased when the observed dis-placement values change abnormally.

The water level observations of tailings include flood level andphreatic line observation. Phreatic line observation points shouldbe constructed in succession following the accumulation of tail-ings. The frequency of saturation line observations should be atleast once a month. The frequency of observations should be in-creased during the flood season and during abnormal fluctuationsin water level. At the same time, the water level measurementscale should be set in the tailings so that the observation can becarried out simultaneously with the saturation line.

Through comparison of monitoring results and calculation re-sults, the safety status in construction and operation can be deter-mined. The safety status can be a basis for proposed engineeringimprovements.

3.3.2. The assessment of the safety monitoring in the new Xiangyuntailings pond

In the preliminary design, a displacement observation sectionwith 3–5 observation points is built at every 10 m height differencealong the tailings dam slope surface. A total of seven sections arecreated (Fig. 9). A piezometric line observation profile is set up per-pendicular to the axis of the dam. An observation point is set up atevery 20 m elevation,; this leads to a total of four observationpoints. Piezometric line observation holes should be buried in thedrilling of the dam slope and should be protected from rain. Theobservation well is placed downstream of the pool, and the depthis greater than 5 m. The observation well is mainly used for mon-itoring potential contamination of the groundwater. Settlementobservation points on the top of initial dam are buried as soon asthe dam construction is completed. A pressure-reducing fieldobservation well at the downstream of the pool is buried in theconstruction. Water level monitoring holes are buried after the for-mation of the tailings pond. The safety monitoring facilities abovesatisfy the requirements. If the monitoring facilities are under nor-mal operation, they can aid in the monitoring of safety during con-struction and operation.

The minimum length of dry beach should be more than 70 m inthe early stage of operation, and the minimum length should bemore than 100 m afterward. The water table level must be strictlycontrolled. During the flood season, the water level must be re-duced before the coming of the flood to maintain a sufficientlylow water level for flood storage capacity. The distance betweenthe new Xiangyun tailings pond and the Yangtze River is only

Fig. 9. Layout of tailings pond monitoring.

1090 T. Wang et al. / Minerals Engineering 24 (2011) 1084–1090

about 1 km, and there are several villages in the vicinity, so safetymonitoring is vital.

4. Conclusion

Three important aspects of safety assessment of phosphogypsumtailings pond are interdependence, seepage and sliding stability cal-culations, and monitoring program. The first is the basis for safetyanalysis, and the second is the key to safety assessment. The third in-volves inspection and verification. Together, these factors can bemutually reinforcing, and can lead to a better overall result.

The preliminary design of the new Xiangyun phosphogypsumtailings pond can meet the requirements of the China NationalStandards Safety Code in the condition of a 500–year flood onlyif the design scheme is adjusted according the flood-routing calcu-lation results. From the static and dynamic stability calculation re-sults, the sliding stability can meet the requirements of the ChinaNational Standard Safety Code. The liquefaction area, which is in-duced by an intensity-VI earthquake, is located in the upper por-tion of the dam slope. The liquefaction area is relatively smalland cannot influence the safety of the tailings pond. Safety moni-toring is very important because the tailings pond site is near theYangtze River, along which many residents live. Monitoring thedisplacement, surface, and underground water level should drawenough attention.

Safety evaluation of phosphogypsum tailings is a very complexissue, and further research on methods of tailings safety assess-ment, data mining of the monitoring information, and quick re-sponse to safety judgment should be done. To have acomprehensive evaluation of tailings, water resources and geo-technical and geological survey should be fully utilized. Resultsof these surveys should be used as basis for disaster prevention.

Acknowledgements

This study was funded by the National Natural Science Founda-tion of China (NSFC) under the Contract No. 50879063, ContractNo. 51079111 and Contract No. 90715042.

References

Azizli, K.M., Yau, T.C., Birrel, J., 1995. Design of the Lohan tailings dam, Mamutcopper mining Sdn. Bhd., Malaysia. Mineral Engineering 8 (6), 705–712.

Byrne, P.M., 1991. A cyclic shear-volume coupling and pore-pressure model forsand. In: Proceedings of the Second International Conference on Recent

Advances in Geotechnical Earthquake Engineering and Soil Dynamics, St.Louis, MO, March 1991. ASCE, New York.

Byrne, P.M., Jitno, H., 1992. Earthquake induced displacement of soil-structuressystems. In: Proceedings of the 10th World Conference on EarthquakeEngineering, Madrid, Spain, pp. 1407–1412.

Chen, Q., 2007. Design Manual for Tailings Dam. Metallurgical Industry Press,Beijing.

Dawson, E.M., Roth, W.H., Drecher, A., 1999. Slope stability analysis by strengthreduction. Geotechnique 49 (6), 835–840.

DTTF (Designing Team for Tailings Facilities), 1987. Tailings Facilities DesignReference Material. Metallurgical Industry Press, Beijing.

Duncan, J.M., 1996. State of the Art: limit equilibrium and finite-element analysis ofslopes. Journal of Geotechnical Engineering – ASCE 122 (7), 577–596.

Griffith, D.V., Lane, P.A., 1999. Slope stability analysis by finite elements. Geo-technique 49 (3), 387–403.

HBHWR (Hubei Bureau of Hydrology and Water Resources). Hubei RainstormStatistical Parameter Atlas. Internal document, Wuhan, 2008.

He, Y., Mei, F., Shen, Z., 2009. Analysis of the safety situation and discussion on themanagement measures of the tailing reservoir. Safety and EnvironmentalEngineering 16 (3), 79–82.

Hilson, G., 2000. Barriers to implementing cleaner technologies and cleanerproduction (CP) practices in the mining industry: a case study of theAmericas. Minerals Engineering 13, 699–717.

HIWCHEED (Hubei Institute of Water Conversation & Hydroelectric EngineeringExploration & Design), 1985. Hubei Storm Runoff Check Calculation Chart.Internal Document, Wuhan.

Hu, Y., 2008. Earthquake Engineering, second ed. Earthquake Press, Beijing.ICME and UNEP, 1998. Case studies on tailings management. International Council

on Metals and the Environment (ICME) and United Nations EnvironmentProgramme (UNEP).

Ishihara, K., Troncoso, J., Kawase, Y., 1980. Cyclic strength characteristics of tailingsmaterials. Soils and Foundations 20 (4), 127–142.

Itasca Consulting Group Inc., 2006. FLAC3D – Fast Lagrangian Analysis of Continuain 3 Dimensions. Ver. 3.1 User’s Manual. ICG, Minneapolis.

James, M., Aubertin, M., Wilson, G.W., 2006. Evaluation of the dynamic stability of atailings dam using FLAC. In: Proceedings of the Fourth International FLACSymposium on Numerical Modeling in Geomechanics-2006, Madrid, Spain, May2006. Itasca, Minneapolis, pp. 163–169.

Komnitsas, K., Lazar, I., Petrisor, I.G., 1999. Application of a vegetative cover onphosphogypsum stacks. Minerals Engineering 12, 175–185.

Komnitsas, K., Kontopoulos, A., Lazar, I., Cambridge, M., 1998. Risk assessment andproposed remedial actions in coastal tailings disposal sites in Romania.Minerals Engineering 11, 1179–1190.

Li, Q., Wang, Y., Liao, G., 2006. Study on scientific issue and evaluation method insafety assessment of tailing reservoir. Journal of Safety Science and Technology2 (6), 53–57.

MCPRC (Ministry of Construction of the People’s Republic of China), 1990. ZBJ1-90,Design Code for Tailings Facilities of Mine Concentrator.

Men, Y., Chai, J., 2009. The current safety of tailing reservoir in China and preventivemeasures. Journal of Safety Science and Technology 5 (1), 48–52.

MWRPRC (Ministry of Water Resources of the People’s Republic of China), 1998.SL203-97. Specifications for Seismic Design of Hydraulic Structures. ChinaWaterpower Press, Beijing.

MWRPRC (Ministry of Water Resources of the People’s Republic of China), 2001.SL274-2001. Design Code for Rolled Earth-rock Fill Dams. China WaterpowerPress, Beijing.

Nguyen, Q.D., Boger, D.V., 1998. Application of rheology to solving tailings disposalproblems. International Journal of Mineral Processing 54, 217–233.

Rico, M., Benito, G., Díez-Herrero, A., 2008a. Floods from tailings dam failures.Journal of Hazardous Materials 154, 79–87.

Rico, M., Benito, G., Salgueiro, A.R., Díez-Herrero Pereira, H.G., 2008b. Reportedtailings dam failures, a review of the European incidents in the worldwidecontext. Journal of Hazardous Materials 152, 846–852.

SAWSC (State Administration of Work Safety of China), 2006. Tailings Ponds SafetySupervision and Management Regulations.

Wang, T., Zhu, H., 2006. The application of FLAC and FLAC3D to the support designof underground cavern. In: Proceedings of the Fourth International FLACSymposium on Numerical Modeling in Geomechanics-2006, Madrid, Spain, May2006, Minneapolis, Itasca, pp. 37–40.

Warhurst, A., Bridge, G., 1996. Improving environmental performance throughinnovation: recent trends in the mining industry. Minerals Engineering 9, 907–921.

Wei, Zuoan, Yin, Guangzhi, Li, Guangzhi, Wang, J.G., Wan, Ling, Shen, Louyan, 2009.Reinforced terraced fields method for fine tailings disposal. MineralsEngineering 22, 1053–1059.

Wijewickreme, D., Sriskandakumar, S., Byrne, P.M., 2005. Cyclic loading response ofloose air-pluviated Fraser River sand for validation of numerical modelssimulating centrifuge tests. Canadian Geotechnical Journal 42, 550–561.

Xinhuanet, 2008. Rolling Reports on Tailings Collapse in Xiangfen, Shanxi Province.<http://www.news.xinhuanet.com/newscenter/2008–09/11/content_9926007.htm>.

Yin, G., Li, G., Wei, Z., Wan, L., Shui, G., Jing, X., 2011. Stability analysis of a coppertailings dam via laboratory model tests: a Chinese case study. MineralsEngineering 24, 122–130.

Zhang, C., Yang, C., Yu, K., 2007. Study on physico-mechanical characteristics ofphosphogypsum. Rock and Soil Mechanics 28 (3), 461–466.