Cent. Eur. J. Eng. • 3(2) • 2013 • 316-328 DOI: 10.2478/s13531-012-0056-7

Central European Journal of Engineering

Design of tailing dam using red mud

Research Article

Subrat K. Rout1∗, Tapaswini Sahoo2† , Sarat K. Das3‡

1 Civil Engineering Department, ITER, SOA University, 751030 Bhubaneswar, India 2 Research Scholar, Department of Civil Engineering, National Institute of Technology 769008 Rourkela, India 3 Department of Civil Engineering, National Institute of Technology, 769008 Rourkela, India

Received 21 August 2012; accepted 21 January 2013

Abstract: Red mud, waste industrial product from aluminum industries produced approximately 75 million tonnes every year with less than half of this is used. Storage of this unutilized red mud takes vast tracts of usable land and pollutes, land, air and water. Construction of high embankments, under passes, flyovers, tailing dams uses vast tract of natural resources (top soil) is also matter of concern as its takes thousands of years to form the natural soil. This paper discusses use of red mud for construction of tailing dam based on laboratory findings and finite element analysis. The geotechnical properties such as plasticity, compaction, permeability, shear strength characteristics and dispersion of red mud are presented. Stability and seepage analysis of tailing dams as per finite element analysis using the above geotechnical parameters is presented. Keywords: Red mud • Embankment tailing dam • Stability analysis • Seepage • Finite element method © Versita sp. z o.o.

1. Introduction

is expected to increase to 14.4 million tonnes by 2012 [1]. Out of this less than half of this is used. Storage of this unutilized red mud takes vast tracts of usable land. Highly alkaline red mud, (pH ranges from 10.5 to 13) is typically Red mud is a waste industrial product from aluminum in- deposited as slurries with 15 to 40% solids. Occasional dustries which is produced during the extraction of alu- failure of this red mud tailing pond or dam causes causal- mina from the bauxite ore. Globally there are approxi- ities, flooding the land and polluting the surface water. mately 75 million tonnes of red mud being produced every Ajka tailings pond failed at a corner section on October 4, year and India produces around 4.71 million tonnes, which ∗ 2010, realising 0.6 million cubic meters of sludge, which † ‡ killed ten and injured 120 people [2]. It has been also ob- E-mail: [email protected] served that more number of tailing dams have failed than E-mail: [email protected] E-mail: saratdas@rediffmail.com the water reservoir dams. As the red mud disposed to the 316 S. K. Rout, T. Sahoo, S. K. Das

red mud pond, it is continuously discharged to the dam neers, planners to use red mud as alternate materials for until it is full. Drying and consolidation of the residue construction of tailing dam particularly for difficult soil in does not occur until the dam has been retired. It may borrow area or at least to avoid the environmental degra- cause problem due to instability of dam construction dation of top soil. Few efforts have been made for geotechnical engineering 2. Experimental programme characterization of red mud. Parekh and Goldberger [3] and Li [4] defined red mud as highly alkaline (pH = 11- 2.1. Materials and test programme 13) waste material, whose mineral components can include hematite, goethite, gibbsite, calcite, sodalite and complex silicates. The red mud has more than 50% as clay size par- The red mud used in the experimental work was collected ticles. Vogt [5], observed that the in-situ undrainedφ shear from National Aluminum Company Ltd. NALCO, Daman- strengths are◦ typically◦ very high compared to uncemented jodi Koraput, Odisha, India and a typical discharge point clayey soils and it has very high friction angles ( ) vary- is shown in Figure 1. ingCc from 38 to 42 . Somogyi and Gray [6], Fahey and Newson [7] observed that redk mud has· compression− index − ( ) ranging from 0.27C tov 0.39 similar· to7 silty-clay soils, coefficient of permeability ( )e.g. 2 to 20 10 3 cm/s2 and coeffi- cient of consolidation ( ) as 3 to 50 10 cm /s. Red mud tendsGS to have low plasticity , liquid limit of 45%, and plasticity index of 10% and relatively high specific gravity ( ) of 2.8 to 3.3. There is lack of clay mineralogy and these wastes show many geotechnical properties similar to clayey tailings found in other mineral processing [8]. It was observed that limited studies have been done to find out geotechnical properties of red mud and also little geotechnical information is available particularly for In- dian red mud. For the high embankment, factor of safety Figure 1. Discharge of red mud as slurry into the pond against stability is most important factor which is gener- ally found by slope stability analysis. Most of the stabil- ity analysis of tailing dam is based on limit equilibrium method, where it is difficult to know the stress strain val- The geotechnical properties of red mud like specific grav- ues at the desired points. The slope stability analysis ity, plasticity index, swelling index, linear shrinkage, grain are made without proper consideration of the pore pres- size classification, compaction characteristics, and triaxial sure analysis. Another important aspect of dam design is shear tests were investigated as per relevant Indian stan- hydraulic fracturing [9]. The past analysis of Ajka tail- dard specifications for soil. Limited studies like parti- ing dam failure finding suggests, the seepage through the cle morphology and mineralogy are also made to explain dam is the reason for its failure [2]. Hydraulic fractur- some of the macro properties of red mud. The individ- ing by reservoir water acting on the upstream face of the ual morphology and particle chemistry were studied in dam core causes concentrated leaks of water to enter the a Jeol-840-A model SEM fitted with EDX microanalyzer core. Hydraulic fracturing due to high water pressures and the XRD analyses for the samples were carried out might have caused leakage or failure of many embank- on a Rich-Siefert x-ray diffractometer using copper target ment dams [9, 10] and it takes place in materials with and Ni filter. low permeability such as compact clay core of dams and 3. Results and discussion embankment [11]. In this study as an attempt has been made to charac- terize red mud as an alternate embankment material and accordingly necessary geotechnical laboratory investiga- The basic physical properties of red mud are shown in tions were made. The properties of red mud have been Table 1. It can be seen that the red mud is highly alkaline compared with a local residual soil and an industrial waste with pH value of 11.4 and the specific gravity (3.34) is (fly ash) for comparison. Finite Element Method (FEM) is also very high compared to local residual soil (2.71) and used to study the stability of tailing dam based on above fly ash (2.26). It has low plasticity and low volumetric and geotechnical properties. This study will help the engi- linear shrinkage. As per Indian soil classification system 317 Design of tailing dam using red mud

it can be classified as silty soils of low plasticity as well as clayey soils of low plasticity (ML-CL). Table 1. Geotechnical properties of red mud

Sl. No Properties Red mud 1 PH value 11.4 2 specific Gravity 3.34 Plasticity characteristics 3 Liquid limit (%) 24.75 Plastic limit (%) 17.5 (a) Plasticity index (%) 7.25 4 Volumetric shrinkage (%) 1.6 5 Linear shrinkage (%) 5.26 − 6 USBR ML,CL 4 7 Permeability (m/day) (1-5) x 10

Figure 2 shows the X-ray diffraction pattern of red mud and it can be observed that hematite is the major min- erals with other minerals like goethite, gibbsite, rutile, boehmite, sodalite. The high specific gravity of red mud is due to presence of the above iron rich minerals.

(b)

(c) Figure 2. X-ray diffraction pattern of red mud Figure 3. SEM analysis of (a) red mud, (b) local soil and (c) fly ash

3.1. Grain size classification

Figure 3(a) shows the scanning electron micrographs of red mud and is compared with that of the local soil (Fig- ure 3(b)) and fly ash (Figure 3(c)). The red mud particles The grain size distribution curve of red mud is presented are found to be angular to sub angular shape, whereas in Figure 4. It can be observed that more than 90% of that of local residual shows the irregular plate like struc- particle sizes of red mud are fine grained (< 0.075 mm). tures (Figure 3(b)). In contrast the fly ash particles as The grain size distribution of a local soil and fly ash are shown in Figure 3(c) are of spherical shape with sizes also presented for comparison in Figure 4. It can be ob- varying from 1 µm to 100 µm. The particles shape and served that grain size distribution of red mud and fly ash size has an important role on the physical properties of is comparable and are finer than local soil. However, as material which has been discussed later. shown in Figure 3(c), the fly ash particles are spherical in 318 S. K. Rout, T. Sahoo, S. K. Das

shape but that of red mud are angular. The low plasticity of fly ash is due to spherical particles [12], but though red mud is angular, the low plasticity is due to absence of clay minerals. This low plasticity of red mud may help to 3.2.use it as Compaction a sub-grade material of embankment.

The compaction curve for red mud using light compaction and heavy compaction is shown in Figure 5. Figure 5 also describes the compaction characterisation of the fly ash and the local soil for comparison. It can be seen Figure 4. Grain size distribution curves of red mud with fly ash and that red mud has higher maximum dry density (MDD) in local soil comparison to other materials. This high MDD value may 3.3.be attributed Triaxial to high shear specific strength gravity of red mud.

The stress-strain curves for the as compacted sample of ◦ red mud, local soil and flyφ ash are shown in Figure2 6. The cohesion (c) of red mud is found as 28.8 kN/m and the angle of internal friction ( ) asφ 34.38 . The shear strength value of the red mud is2 found to more as comparedφ to fly ash with c as 18 kN/m and of 28.40. It was also observed that red mud has higher value compared to fly ash and local soil. This may due to the fact that the fly ash particles are spherical and that of local soil is plate like. But the red mud particles are angular to sub granular. The angularity of particle and heavy minerals may be reason for high angle of internal Figure 5. Compaction characterisation of red mud with fly ash and local soil 3.4.friction Dispersiveness of red mud. test

The properties of material such as soils or any kind of waste product like as red mud, slag, crusher dust and coal ash to get dispersed and washed away in water is termed as its dispersiveness. Non-plastic nature of parti- cle and its inadequate inter particle attraction causes to dispersiveness. This is the main property which used to come into consideration during the construction of dykes, embankments and similar kind of water retaining struc- ture [9]. Dispersive clays cannot be identified by the standard laboratory index tests. Different laboratory tests generally performed to identify dispersive clays are crumb test, double hydrometer test, pinhole test, turbidity and SAR (sodium absorption ratio) tests. In present study as the red mud is of low plasticity, dispersive tests were con- Figure 6. Stress- strain curve of red mud, fly ash and soil ducted using the crumb test, double hydrometer test and turbidity test. The crumb test was carried out as it gives a good quick indication of the dispersiveness of the soil. Based on the 319 Design of tailing dam using red mud

(a) (b)

(c) Figure 7. Crumb test showing (a) Prepared samples before test (b) A sample just placed in distilled water (c) Sample in distilled water after seven minutes

tendency for clay particles to go into colloidal suspension In double hydrometer test the dispersion ratio is defined that is observed after 5-10 minutes of immersion, soils as the ratio of the percentage finer than 0.005 mm diame- are classified as nondispersive or dispersive based on the ter measured without any dispersion agents in a hydrom- reaction observed. The specimens of 1.5 cm cube are pre- eter test to that measured with dispersion agents, which is pared as shown in Figure 7(a). Figure 7(b) shows the expressed in percentage. Based on the double hydrometer initial state of the specimen in distilled water. The dis- test [14, 15], the dispersion ratio of red mud is found to persive state of the sample after 7 minutes in distilled be 94%, which is extremely dispersive. Similar conclusion water is shown in Figure 7(c). The red mud is found to was also made for the turbidity test. highly dispersive material as per the crumb test. Based on the above tests, it can be observed that the red mud is highly dispersive and construction of tailing 320 S. K. Rout, T. Sahoo, S. K. Das

Figure 8. Cross section of upstream dam with its materials

Figure 9. Strain diagram dam in upstream method case showing the failure surface

dam/embankment using red mud should be covered. An- dams with different geometry with soil cover is analysed other aspect also is that it should be checked for the hy- for stability and seepage and is presented as follows. draulic fracturing of the dam/embankment, which is basi- cally to check the tension zone or cracked zone and the phreatic line. In the traditional limit equilibrium method it is not possible to find out the stress strain in the body of the embankment. Hence, in the present study tailing

321 Design of tailing dam using red mud

Figure 10. Active pore pressure diagram of upstream method dam

Figure 11. Effective stress diagram of upstream method dam in ZZ direction

322 S. K. Rout, T. Sahoo, S. K. Das

Figure 12. Cross section of centrroid method dam with its materials

4. Finite element analysis

mud as experimentally obtained and used for the present analysis are shown in Table 2. To simulate the field condi- In the present study the finite element analysis of tion, the material parameters of the natural soil and com- the embankment is done using FEM based software- pacted red mud are found by laboratory compaction, but 4.1.PLAXIS Finite [16]. element analysis using PLAXIS for deposited red mud, the material parameters are found by4.2.1. sedimentation Upstream and Tailing drying Dam without compaction.

PLAXIS [16] is a finite element program for geotechni- The height of starter dam and stagings are consid- cal applications in which different soil models are used ered as 10 m each and the side slope considered is to simulate the soil behavior. It is analyzed as an em- 2 horizontal : 1 vertical. The FEM model for the up- bankment using plane strain condition. The material is stream method for stability analysis of the tailing dam modelled as per Mohr-Coulombc model. Thisφ is an elastic as per PLAXIS analysis is shown in Figure 8. The factor perfectly plastic soil model. TheE model uses five soil pa-ν of safety (FOS) is found to be 2.51 and corresponding fail- rameters: Cohesion ( ), Angle of Friction ( ), Dilatancy ure surface is shown in Figure 9. Hence, as FOS is more angle (Ψ), Young’s modulus ( ) and Poisson’s ratio ( ). than 1.5 it may be considered as safe. As red mud has low The Young’s modulus of soil is determined using the cor- permeability and hydraulic fracturing has been observed relation available in the literature based on soil param- as the main reason of failure of red mud tailing dam [18], eters. To compare with the limit equilibrium' method in a study related to hydraulic fracturing is presented here. addition to stress-strain calculation, the factor of safety The phreatic line of the dam as per PLAXFLOW anal- (FOS) of slope is calculated using Phi-c ( -c) reduction ysis and the corresponding pore pressure distribution is method, an option available in PLAXIS. A module called shown in Figure 10. Figure 11 shows the effective stress PLAXFLOW, is available in PLAXIS and has been used diagram as per the stability analysis of tailing dam. It in the presented study for seepage analysis through the can be observed that tensile stress occurs at the surface 4.2.tailing dam.Stability analysis of tailing dam of the dam, and the phreatic line is also in this zone for the starter dyke and in 1st stage construction. This indi- cates hydraulic fracture may develop near above region. So a coarse grained soil layer needs to be provided in 4.2.2. Centriod method tailing dam Unlike water reservoir, the red mud pond is constructed this zone to prevent the hydraulic fracturing. in stages for the optimized cost. Basically there are three methods for the staged construction (i) upstream method, (ii) downstream method and (iii) centroid method based on Similar study as described for the upstream method is sequence of raising the dyke [17]. In the present study re- also done for the centroid method tailing dam (Figure 12). sults pertaining to above three methods are critically dis- The factor of safety (FOS) is found to be 1.67 and corre- cussed as follows. The properties of natural soil and red sponding failure surface is shown in Figure 13. It can be 323 Design of tailing dam using red mud

Figure 13. Strain diagram dam in centroid method case showing the failure surface

Figure 14. Cross section of centroid method dam with phreatic line and pore pressure distribution

324 S. K. Rout, T. Sahoo, S. K. Das

Figure 15. Effective stress diagram of centroid method dam in ZZ direction

Figure 16. Cross section of downstream method dam with its materials

seen that FOS is less compare to upstream method. Sim- zone of starter dyke and 1st stage. So similar to up- ilarly the flow analysis through the tailing dam is shown stream method a coarse grained soil layer near this zone in Figure 14, with the position of the phreatic line and may4.2.3. help Downstream in preventing tailing the hydraulic dam fracturing. distribution of pore water pressure. Figure 15 shows the effective stress diagram and it can be observed that ten- sile stress occurs at the interface of starter dyke and 1st The FOS for the downstream tailing dam (Figure 16) staging of the dam and the phreatic line is close to this is found to be 1.65 and corresponding failure surface is zone. This indicates hydraulic fracture may occur at these shown in Figure 17. Figure 18 shows the PLAXFLOW 325 Design of tailing dam using red mud

Figure 17. Strain diagram dam in downstream method case showing the failure surface

Figure 18. Cross section of downstream method dam with phreatic line and pore pressure distribution

326 S. K. Rout, T. Sahoo, S. K. Das

Figure 19. Effective stress diagram of downstream method dam in ZZ direction

Table 2. Parameters of the Mohr-Coulomb model

Material types Natural soil Compacted red mud Sediment/in-situ red mud 3 Unsaturated unit weight (kN/m ) 15 19.8 14.46 3 Saturated unit weight (kN/m ) 16.28 21 18.37 3 Cohesion (kN/m ) 158.7 28.8 25.3 Angle of internal friction (Degree) 8 34.38 28.6 3 Young’s modulus (kN/m ) 19872− 1771− 1500− Poisson ratio 0.35 0.34 0.32 6 4 4 Permeability (m/day) 1 x 10 4 x 10 4 x 10 5. Conclusion analysis result with the phreatic line and the pore wa- ter pressure distribution. The effective stress diagram is shown in Figure 19 and it can be observed that tensile Based on the experimental investigation and the numerical stress occurs at the 1st and 2nd stage of the dam, which analysis of the tailing dam based following conclusions is below the phreatic line. Hence, this zone is very critical can be made. as far as hydraulic fracture is concerned. Hence, suitable precaution like filter is required at this zone to save the 1. The red mud is fine grained soil of high specific tailing dam. gravity due to presence of iron rich materials. 2. Red mud is having low plasticity due to particle morphology with angular to sub granular shape and absence of clay minerals. 327 Design of tailing dam using red mud

3. The red mud is having high angle of internal friction [7] Fahey M., Newson T. A., and Fujiyasu Y., Engineer- unlike fine grained-soil and low permeability like ing with tailing. Invited Lecture, Proc., 4th Int. Conf. fine grained soil On Environmental Geotechnics. Rio de janeiro, Brazil, 2002, 2, 947-973, Balkema, Lisse 4. Based on the finite element analysis of a particular [8] Vick S. G., Planning, design and analysis of tailing geometry of tailing dam the factor of safety is found dams. Wiley, New York, 1981 to maximum for upstream method (2.57) staged con- [9] Sherard J. L., Hydraulic fracturing in embankment struction followed by centroid method (1.67) and dams. Journal of Geotechnical Engineering, 1986, downstream method (1.65). 112, 10, 905-927 5. Based on the stress analysis and seepage analysis [10] Singh B., and Varshney R. S. Engineering for Em- through the tailing dam, it was observed that hy- bankment Dams. A.A. Balkema/Rotterdam, 1995 draulic fracture may occur in the tailing dam and [11] Kjærnsli B., Valstad T., and Höeg K., Rockfill Dams. combined stress and seepage analysis is required In the series Hydropower Development,. Norwegian to find out the critical points for the design of filter. Institute of Technology, Trondheim, 1992 References [12] Das S.K., and Yudhbir. Geotechnical Characterization of some Indian Fly ashes. Journal of Materials in Civil Engineering, ASCE, 2005 , 17(5), 544- 552 [13] Bhuvaneshwari S., Soundara B., Robinson R.G., Gandhi S.R., Stabilization and microstructural modi- [1] Mukhopadhaya J., Management of Bauxite Residue fication of dispersive clayey soils. 2007. Proc. Indian (Red Mud), Presented to the experts of Asia-Pacific Geotechnical Conference Partnership on Clean Development and Climate held [14] Volk G.M., Method of determination of degree of dis- at Canberra, Australia, January 10-11, 2011 persion of clay fraction of soils. Proc. Soil Science [2] WISE, The Kolontár red mud dam failure. Available Society of America, 1937, 11, 561-565 from http://www.wise-uranium.org/mdafko.html, 2010, [15] ASTM D6572-06 – Standard Test Method for deter- [cited 17 December 2011] mining dispersive characteristics of clayey soils by [3] Parekh B., and Goldberger W., An assessment of Crumb Test technology for possible utilization of Bayer process [16] Brinkgreve R.B.J, Broere W., Waterman D., PLAXIS muds.US EPA, 1976, EPA-600/2-76301 -2D (Version 9.0). Delft University of Technology and [4] Li L. Y., Properties of red mud tailings produced un- PLAXIS b.v., The Netherlands, 2008 der varying process conditions. J. Environ. Eng., 1998, [17] Gandhi S. R., Design and maintenance of ash pond for 124(3), 254-264.ce fly ash disposal. Proc. Indan geotechnical Conference, [5] Vogt M. F., Development studies on dewatering of red 2005 mud. 103rd Annual Meeting of AIME, Dallas, Tex., [18] Zanbak C., Failure mechanism and kinematics 1974, 73-91 of ajka tailings pond incident. http://www.wise- [6] Somogyi F., and Gray D. Engineering properties af- uranium.org/mdafko.html [cited 10 Dec. 2010], 2010 fecting disposal of red mud. Proc., Conf. on Geotech- nical Practice for Disposal of Solid Waste Materials, ACSE, 1977, 1-22

328