Indian Jo urnal o f Engineering & Material s Sciences Vo l. 8, December 200 I, pp. 31 8-326

Investigation of soil engineering properties for safe design and construction of the iron ore tailing dam

M K Ghose" & P K Senb

"Centre o f Mining Environment, Indi an School o f Mines, Dhanbad 8 16004, Indi a bMECON India Ltd, Ranchi 834002, India

Received 24 luly 2000: accepted II luly 2001

The need fo r the disposal o f iron ore tailings in an environmentall y fri endl y manner is of great concern to the mining autho rity. It is envi saged that by the end of this century, India wo uld be generating 27 Mt of iron ore tailings per year for whi ch sa fe disposal by way of containment in tailing ponds are needed. For the safe design and construction of the tailing dam as well as for the augmentati on o f its exi sting capacity, detailed field and laboratory investi gati ons are described here. To aug ment capacity of the tailing pond, rai sing height o f the existing tailing dam in two stages o f 5 m each is conceived . Samples have been co llected from soil and tailing borrow areas by making pits up to 2 m depth. Bo re holes have been drill ed and seismic studi es are carried out to establish the depth of bed and its thickness. Standard penetration test (SPT) and permeability test have also been conducted . Undi sturbed so il samples along with di sturbed samples have been collected and ex istence o f ground water table is noted. Detai ls of the bore log data and ill situ test results have been d iscussed and laboratory tests are conducted on the collected samples. Bulk geology and fo undation treatment , construction material and design data used. method of stability analysi s and software used for stability analysis are also described.

The rapid industrialisation, particularly in the mining sector, has caused many adverse environmental impacts' . By the end of this century in India, the estimated iron ore production would be 85.0 Mt generating 27 Mt of tailings per year for which safe di sposal by way of containment in tailing ponds are needed to be planned2

. Tailing embankments are susceptible to rapid erosion, down cutting and complete breaching shortly after water over tops the crest]. Conventional design procedures are summarised by US Bureau of Reclamation (1973). Mismanagement of tailings holds the potential for environmental degradation . The surface stability of the impoundment and the water emanating from it have a dominating influence on the environmen{ The stability must be acceptable not only during operation but also after decommissioning and closure. Similarly, seepage from the impoundment must be controlled during operating life and after the closure of the dam5

.6

. The underflow containing thickened tailing slurry having pulp density in the range of 40-50%, produced by typical thickener operation, is conveyed to the tailing pond7

. According to JeypalanB, tailings deposited by thickened discharge method are susceptible to liquefaction flow sliding under moderate to high levels of seismic shaking.

The design and construction of safe earthen dams or rock filled dams for impoundment of tailings calls for proper engineering design9

.1O

. An em1hen dam is

composed of suitable soils obtained from borrow area or selected tailings and compacted by mechanical means. For carrying out detailed des ign, geological and subsurface exploration including finding the engineering propel1ies of fill materials are absolutely essential. The outlet arrangement or decant system must be structurally fool proof since this has been a major source of induced failure as it is difficult to repair once it is functional" . Also. the foundation must be sufficiently strong to SUpp011 the dam without excessive settlement. For computing stability of earthen dam, methods are based on the shearing of the soil and certain assumption with respect to the character of an embankment failure . For the design of the tailing dam in green field site as well as augmentation of existing capacity of tailing pond, detailed soil investigations are required.

Description of the Study Area Barsua Iron Ore Mine meets the iron ore

requirement of Rourkela Steel Plant (RSP) of Steel Authority of India (SAIL). The output of the mine varies from 1.5 Mt/y to 2.1 Mt/y. This mine has a leasehold area of 24.86 sq. km and was set up in 1954. It is located in the Bolani sub divi sion of Orissa at an elevation of 790-910 m in Toda forest. It is 60 km by rail and 100 km by road from Rourkela and is at latitude 21 °50' and longitude 85°8'E. The crushing and ore handling plant went into operation in 1961. A decade later, a beneficiation plant compri sing of

GHOSE & SEN: IRON ORE TAl LING DAM 319

washing and jigging units , was dovetailed into the system. In 1981 , wet re-screening facilities were added to control under-size product (lumps). Initi ally, there was no tailing pond. But at a later stage, in order to mjnimi se the pollution in the Khurhadi river, a temporary tailing pond was constructed followed by the construction of a permanent one on the same si te. Both beneficiable and direct ore occur in the ratio of 60:40 in the deposit and are mixed in that proportion . Mineralogically, the ore is predominantly hematite with common geothite except in latarite in which geothite dominates. The total mine resources at Barsua are 139.17 Mt, and the tailings were estimated as 25 Mt. About 19% of the run-of-mine (ROM) is lost in the form of slime in case of lumps and up to 5% as fines . The tailings form slime with the thickener underflow and waste slime, and are di scharged to the adjoining null ah which ultimately leads to the tailing pond. The null ah is located 1.2 km from the beneficiation plant. The tailing flow diagram for the ore processing plant is shown in Fig. 1. The beneficiation plant was commissioned in the year 1968. The wastes arising are of the order of 500 000 t/y, in the form of jig tailings and slime from the cyclone underflow and thickener underflow. The

eXistIng tailing pond, originally designed to handle four to five years of slime disposal, had reached to its maximum capacity.

Soil Investigation Studies The existing top of the dam (about 1 krn) is 415 .5

m while the tailings are already filled to about 412 m. To meet the future requirements, it is planned to increase the height of existing dam by 5 m from the base by using the tailing materials from the lower reaches of the tailing pond with earth as covering. Borrow areas of tailings and soils were identified and location of foundation pits and bore holes were also selec ted. In all, 33 numbers of disturbed soil samples were collected from both soil and tailing borrow areas. Average depth of these pits was 2 m. Water was observed at the bottom of 6 pits of tailing bon'ow areas. Undi sturbed samples were collected in core cutters (80 mm diameter) from five foundation pits near the upstream and downstream existing toe of the dam. Depth of these pits varied from 0 .5 to 3.5 m.

Three numbers of bore holes (BH1 , BH2 and BH3) were drilled and standard penetration test (SPT) and permeability test were conducted. Undisturbed

LOSSES 20 M3/ h 160 M3/h

680 M 3/ h WATER + JIGGING PLANT

BENEFICIAT ION 780 M3/h

C LARIFIER-

LOSSES 60 M 3/h WATER

+ 62 TONS h (50 M3/ h) SLIME

SPILLv/AY

r

82 TON S (50 M3/ h) SLIME

RETURN BY GRAVITY

600 M3/h

,­,-

640 M3/h

LOSSES

100 M 3/h

160 M 3/ h

/' /' CIRC ULATING WATER PU MP HOUSE E L 1550 M

CLEAN I WATER TAN K 3 PUMPS EACH 320 1-43/h

3 PUMPS EACH 80 M3/ h

(2 WORKING + 1 STAND BY) I <! ..... ..... <! Z

00 u... .... .... 2 0: <! UJ u .... u...<! o~

I I I I I I I I I I I

,- --, TREATME NT I , , PLANT RECYCLE PUMP I

L-!-t _____ .r--L~U~~ ___ J L _...1 60 M3/h

DISCHARGE DURING MONSOON

INTERMED IATE PUMP HOUSE E L 1400 M 3 PU MP EACH 100 M3/h (2 WORKING+l STAND BY)

--IN TAKE PUMP HOUSE EL 1240 M 3 PUMP S fAC H 100 M 3/h (2 WORK ING + 1 STAND BY)

Fig . I-Detailed location map of Barsua iron ore mine site and tai ling dam

320 INDIAN 1 ENG. MATER. SCI., DECEMBER 2001

samples in shelby tubes (38 mm diameter) were also collected. Depth of these bore holes varied from 27 to 33 m. Depth to the water table was observed during boring in BHl and BH2 at 8.5 m and in BH3, it was at 13 .9 m. Fig. 2 shows the position of the soil/t ailing pits, foundation pits and bore holes.

Details of Bore Log Data and Field Tests Bore Hole No.1 was drilled up to 27 m of depth and water table was observed at a depth of 8.5 m. Significant percentage of fines (clay and silt) were generally met to a depth of 16.5 m to 19.5 m and weathered rock from 19.5 to 21 m depth, respectively. Rest of the strata up to the depth of 27 m comprised of soft and hard weathered rock. Undisturbed soil samples were collected in shelby tubes and disturbed samples in core barrel by dry drilling and also while conducting SPT tests. In situ permeability tests were conducted at various depths in this bore hole using pumping in (gravity fed) constant head method. Cased well, open-end test and test sections both were used to determine in situ permeabilities. These tests were conducted as per Bureau of Indian Standard (BIS) code no. IS-5529 (Part 1)14. The value of in situ coefficient of permeability' f(' ranged from 3x 10-6 to

o g

i

o PIT

PIT 8 o

PIT 5 o

30x lO-6 cmls to a depth of 15.5 m. This value of ' K'

was very high between 17 to 21 m depth. SPT test indicated that 'N values were low, (below 7), upto 6 m depth and, thereafter, gradual rise below this depth indicated values of 25 and 47 at depths of 10.5 and 15 .6 m, respectively. 'N values below 15.6 m depth were even higher. In this test, a split spoon sampler resting on bottom of the bore hole was driven by a drive weight of 63 .5 kg.

Bore Hole No.2 was drilled up to a depth of 33 m and water table was observed at a depth of 8.5 m. The strata consists of a large percentage fi nes (clay/silt) up to a depth of 5 m. This was followed by lesser amount of fines with large percentage of sand with few pebbles up to the depth of 19.1 m. Rest of the strata from 19.1 to 33.0 m was soft weathered rock pieces with soil in some places. 'N value by SPT test was 23 at the depth of 2.3 m. It decreased with depth to ' a value of 6 at 9 m and there after increased with depth to a value of 30 at 16.8 m. 'N values below this depth were very high but the 'N value observed at depth of 33 .m again fell to a value of 26. In situ permeability test showed low 'f(' value of 9x lO-Q emlsec (9 ftly) at depth of 3.8 m. Variation of about 500 to 700x lO-6

cmlsec was observed between the depth of 7.5 m to

S-2400

§ i 5 -1100

5-1900

5-2000

GENERAL CROSS SECTION TH RO UGH BORE HOLES

Fig. 2- Localion of bore holes, pil and cross section of the tailing dam showing the exi sting position and proposed position based on the design consideration

GHOSE & SEN : IRON ORE TAILING DAM 321

15.0 m. Very high values of several thousand ftly were observed at depths of 18 and 21 m. The value of 'I(' was 500x 1 0-6 cmls was agai n observed at a depth of 30 m.

Bore Hole No.3 was drilled up to 30 m depth and water table at 13.9 m depth was observed in the hole. The strata up to 15 m comprised of large percentage of fines, followed by weathered rock with soil upto 18.6 m. Strata below thi s was mostly rocky with some soil upto 2l.6 m and hard rock was met when drilled upto the depth 30 m. Two undi sturbed samples in upper 15 m clear soi I strata and di sturbed samples by SPT and dry drilling up to depth of 18 .6 m were collected. Value of in situ permeability ' I(' was 4 to 5 x10-4 cmls in upper 3 to 7.5. ' I(' value variation of 0.10 to 1.20x 10-4 cmls was observed in the depth of 22 to 24 m where as the value increased to about 3x 10-4 cmls. 'N value of 37 was observed at depth of 1.5 m this increased with depth .

Laboratory Tests Grai n size analys is (as per BIS code IS -2720 Part

IV)15 and Atterberg limit tests were conducted on soil , tailing borrow and foundation pit samples including bore hole samples 16. Standard Proctor compaction tests on soils, tailings, borrow pit samples and in-situ density tests on bore holes and foundation pit samples were also carried out. Consolidated undrained (CU) tri axial shear tests were conducted on some selected samples of soil s, tailing borrow, few availab le undisturbed samples of three bore holes. One dimensional consolidation tests were carried out on six borrow area soil and tai lings and two foundation pit samples. Laboratory permeability tests were conducted on three samples each from soil and taili"1gs area. Specific gravity of several samples was also determined. Standard Proctor' s compaction test was conducted as per IS-2720 (Part IV) 15.

Triaxial shear test under consolidated undrained condition (CU) with pore pressure measurement was conducted on 37.5 mm dia 75 mm cylindrical specimens. Borrow area soi l sampl es were moulded at 98% of standard Proctor max imum dry density, while fou ndation pit and bore hole samples were tested at in-situ density . Fully saturated samples from borrow area and foundation were tested. Four soi l specimens were consolidated at 4 different confining pressures for 24 h in tri ax ial cell s. The specimens were sheared at a shearing rate of 0.125 mmlmin . The test was performed as per IS :2720 (Palt XlI)I7. Pore pressure was monitored during the undrained sheari ng.

Results and Discussion Grain size class ification of 17 samples collected

from soil borrow area pits showed that 7 samples were silty and clayey gravel (GM/GC/GM-GC) and 10 samples were silts and clays of intermediate to high plasticity (MlIMl-CIIMHlMG-CH!CH). Classification of 15 samples collected from tailing borrow pits indicated that 4 samples were silty and clayey gravels (GM/GM-GC), 2 samples had SM-SC classifications (silty and clayey sands) classification, 8 samples fell mostly under cl ays of low plasticity and there was 1 sample of MH-CH group (silt and clay of high plasticity) .

Classification of 4 out of 5 samples collected from foundation pits showed that 3 of these fell under CL (clays of low plasticity) and one was of clayey sand (SC) group. Class ification of the 34 samples out of the 41 samples collected from 3 bore hol es in undi sturbed state and di sturbed state by SPT and dry drilling indicated that there were 6 samples of GC (clayey gravels) group, 6 of SM/SC (silty and clayey sands) and 21samples were silt and clay of low to intermediate plasticity (ML, CL, MI and CL in combination). There was only one sample of silt of high plasticity (MH).

In all, 17 soil and 15 tailings borrow area samples were subj ected to Proctor's Compaction test. Standard Proctor's Compaction test was conducted as per IS-2720 (Part VlI)I S. General variation in maximum dry density (MOD) and optimum moi sture content (OMC) for soil borrow area samples were 1.40 to 1.5 1 glcc and 26.00 to 3l.7%, respectively. Variation in general in MOD and OMC for tailing bOlTOW area samples excluding the lower depth of pit number 24, 25 and 32 and also pits 29 and 30 (Fig. 2) were 1.83 to 2.30 glcc and 15 .2 to 23.6%. The values of MOD and OMC for the excl uded pits as mentioned above were close to values reported for soil borrow area samples.

In situ density of undisturbed samples collected in core cutters was determined for 5 samples. Variation in in situ dry density was found to be 1.73 to 2.0 g/mL for 4 samples. General vari ation in in-situ dry density for 10 undi sturbed samples collected in 38 mm tubes was 1.68 to 2. 15 g/cmL.

In all, 8 samples from soil borrow area were tested for triaxial shear (CU) to determjne the strength parameters. General variations in total shear strength parameters i.e. cohesion (C) and angle of shearing resistance (<I» for these samples (mostly silt and clays of intermediate to hi gh plasticity) were 0.21 to 0.27

322 INDIAN J ENG. MATER. SCI.. DECEMBER 2001

kg/cm2 and 8.4 to 2.6° respectively barring an exception. Corresponding variatIOn in effective parameters, i.e. Cohesion C and angle of shearing resistance <1>' were 0.11 to 0.17 kg/cm2 and 25.1 to

28 .5°, respectively. Variation of C and C and <1>' for four samples of CL classification out of 7 samples tested from borrow areas were 0.18 to 0.28 kg/cm2 and 25.8° to 32.9°; 0.12 to 0.17 kg/cm2 and 29.2° to 36.1°, respectively. Excluding the results of samples 94/34B (Pit 32, second lower depth) results of 2 remaining samples of GMlGM-GC group (Pit 24, second lower depth and Pit 10) from tailing borrow area (passing through 4.75 mm sieve) showed variation in C and as 0.16 and 0 .21 kg/cm2, 20.7° and

23.8°, respectively. Corresponding variation barring an exception in C and <1>' were 0.11 and 0.17 kg/cm2, 27.1 ° and 28.5°, respectively. The graphs between c and <1> are given in Figs 3-5.

In all, 10 samples (3 from foundation pits and 7 from bore holes) were tested for triaxial shear. Six of these samples (mostly clays of low plasticity) have C and <1> values with a variation of 0.23 to 0.74 kg/cm2 and 28.5 to 32.8° barring an exception and C and <1> ' values had variation 0 .12 to 0.14 kg/cm2 and 32.9° to 36.0°, respectively. Three samples of CL classification had C and <1>, C and <1>' values around 0.27 kg/cm2 and 23.80, 0.17 kg/cm2 and 30°, respectively. One sample of tailing under SC classification (sample 94/54) had <1>'=39.6° and C=0.7 kg/cm2.

Consolidation test was conducted for 4 soil, 2

tailing borrow area and 2 foundation pit samples.

t c ro L

VI

L ro <lJ ~

lfl

2 .0 __ Mohr envelop Ie considPring the total stress (6 )

____ Mohr envelople considering the effective stress (6 ' )

1·5

1.0

05

0·0 L------,-------r---.----.-0·5 1.0 1.5 2.0

Normal stress ~

Fig. 3--Relation between cohision and ang le of internal friction for so il borrow area samples

Borrow pit samples (passing through 2 mm) were

remoulded at 98% of standard Proctor maximum dry

density. Foundation pit samples were tested at in situ in a consolidation ring of 60 mm diameter and 20 mm

height scatting pressure of 0.25 kg/cm2 was applied to

the soil specimen for 24 h. Loads were doubled after every 24 h, till a normal stress of 8.0 kg/cm2 was

achieved. The compression dial readings at each load level were noted at suitable interval of time. Final dial

readings were noted during specimen unloading which was done in steps of one fourth of the previous

intensity .. The test was performed as per IS :2720 (Part XV)) ?:

20 __ Mohr envelope considering the total stress (6 )

_ __ _ _ Mo hr envelope consider ing the effective stress (6')

i 1·5

-' -'

-,' ./

c ro L

-:n L ro <lJ .c lfl

1·0 ./

/. .6-

05

./ ./

./

./ -'

./

00 L-__ ---,-___ ,--_ _ ---. __ --,

0·5 10 1.5 2.0

Normal stress_

Fig. 4--Relation between cohision and angle of internal friction for tailing borrow area sa mples

i e: 'iii L. -VI

L. (1] Q.I .c Vl

2.0 __ Mohr envelope (on~iderin~ total stres, (6)

____ Mohr envelope con,idering effective sire» (6')

1·5 /.

~

/. .6-

.6-

1.0 ,;0-,,-

7 7

/' ./

0.5 ./ ./

O . O-l----~----,--------r---

0·5 1·0 1.5 2.0

Normal stress _

Fig. 5--Relation between cohision and angle of internal friction for foundation pit and bore hole samples

GHOSE & SEN: IRON ORE TAILING DAM 323

Coefficient of volume compressibility (mv) of 8 cohesive samples were tested for soil , tai lings and foundation pit samples and showed the variation of 0.6 to 4.6xl0-2 kglcm2

. Value of compressive index (Cc) varied from 0.165 to 0.343 and swelling index (Cs) vari ed from 0.014 to 0.066. Coefficient of consolidation (Cv) at various stress level of 0.25 to 8.0 kg/cm2 varied from 1.8 to 20.7x 10-4 cm2/s.

Salient features of Barsua Tailing pond at stage-I & stage-II are given in Table 1. Six samples, 3 each from soil and tailings bOITow pits were tested · for laboratory permeability as per IS :2720 (Part XVII)2o. Readings were taken at suitable interval of time to determine the co-efficient of permeability ' K'. Variation observed in values of ' K' for 3 soil borrow area samples (GM/MlIGM-GC) was 7xlO-6 cm/s to totally impervious . Variation of2.2 to 4.1xlO-6 cm/sec was observed for 3 samples (CUSM-SC) from tailing borrow areas. Average minimum values observed for so il and tailing borrow area samples; foundation and bore hole samples are given in Table 2. Average compression index (Cc) for 8 samples soil, foundation pit cohesive samples was found in the order of 0.040. Laboratory permeability test on three soil borrow area samples were conducted and those were found to be impervious in nature and other three samples tested of tailing borrow area samples were found to be closer to lower boundary of semi-impervious ranging from 2x 10-6 to 4xl 0-6 cm/s.

Foundation The rai sing of the dam on the upstream side would

involve founding of the dam on the deposited slime. To know the extent of height that can be raised, consolidation tests and SPT tests were carried out. The compression index 'C' of the slime foundation and remoulded slime at 98% of Proctor density were found to be more or less same. Thi s showed that the slime had attained a fair degree of consolidation .. It was observed that the coefficient of permeability in the upper strata was low and a 4 m layer of having

Table I - Salient features of Barsua tailing pond at stage-I and stage- II

Characteristics Stage-I Stage-II

Top level of the dam (m) 415 .50 420.50 Max imum designed 411.50 416.50

settled tailing level (m) Spillway level (m) 412.00 4 17.00 Capacity (Iakh m) 5.25 11 .75 Expected life (years) 4-5 8

high permeability values existed underneath the original ground level. The high values of 'N' and 'K' at various depths of the bore hole have effects on the permeability values. The results of field permeability test and laboratory permeability test are comparable

Table 2 - Average minimum values observed for soil and tailing borrow area samples. foundation and bore ho le samp les

(a) Soil borrow area sall/ples Properties Standard Proctor compac tion Max. dry densi ty (MO D) ,g/mL Optimum moisture content (OMC)% excluding one sample out of 17samples Triaxial shear parameters (8 samples) Total cohesion (c), kg/cm2 Total angle of shearing resistance (<1», degrees Effective cohesion (c') , kg/cm2 Effective angle of shearing resistance (<1>'), degrees Specific gravity

(b) Tailings borrow area samples Standard proctor compactio 10 samp les Max .dry densi ty (MOD), 1.99* g/mL Optimum moislure content (OMC)% *10 sa mples (8 CUCIIMI­CI+2 SM-SC all having high specific grav ity) **5 Samples (4 GM/GM­GC, I MH-C H) Tri axial shear parameters Total cohesive (c), kg/cm Total angle of shearing resistance

18.2*

4 samples 0.20*

(<1» , degrees 29 .5' Effective cohesion (c'), 0 .12* kg/cm Effecti ve angle of shearing 32.6* resistance (<1>'), degrees *4 samples of CL group having high specific gravity **2 samples GM/GM-GC group. Specific gravity 3.8 1 * * For 4 CL group samp les ** For 2 GM-GCIMH-CH samples (c) Foundarion and bore hole samples Triaxial shear parameters Total cohesive (c) , kg/cm2 Total angle of shearing resistance (<1», degrees Effective cohesion (c'), kg/cm2 Effective angle of shearing resistance (<1>') degrees

6 samples 0.23*

28.5 * 0 . 12*

34.6*

*6 samples of ML-CUCL-MI -C I group **3 samples of CI group

A verage values 33 samples

1.44

27.4

0.24

20.9 0. 14 25.3

2.77

5 samples 1.42**

26.3**

2 samp les 0 .1 7**

21.5 ** 0.12**

27.4**

2.78"

3 samples 0.27 **

23.8** 0.17 **

29.6**

324 INDIAN J ENG. MATER. SCI., DECEMBER 2001

and found to be within the reasonable limit of variations. SPT test indicated that 'N value was below 7 in upper 6 m depth but gradually increased to 25 at a depth of 10.5 m. In view of low SPT and low coeffici ent of permeability ' K' values in upper strata, the upstream slope of heightened bound has been proposed to keep flat, i.e., 4: 1 so as to provide wider base for load transfer. Further, the top 60 cm of very loose slime will have to be removed before laying the first layer. As the slime materi al (to be disputed) were used as construction material, 2% settlement allowance has been proposed to take into consideration in designing the tailing dam to take care of settlement.

Construction material The construction material proposed for raising the

tailing dam will be mainly slime, which is already deposited in the reservoir area. A 2-3m covering of impervious soil (GM-GC type) is proposed on upstream and downstream slopes. Rock fill has been proposed on the downstream side in the deepest sec ti on for stability and drai nage consideration and in the rest of the dam (as rock toe) for drainage purpose only.

Design data used for construction of the tailing dam

In the stability analysis, the material properties for the existing dam section are the same as adopted for the firs t stage raising. The physico-mechanical properties of construction materials used in the design are given in Table 3 where the dam has to be rai sed by downstream raising method, it is founded on iron ore fines deposit. Hence, for the foundation material in this reach, the properties of iron ore fines have been used . A number of tri al sections were analysed for stabi lity before arriving at the final section . Also, a study of gravity structure was made. Geophysical

investigation was also made by CSMRS (Centre for Soil Material Research Station) to establish the firm rock level below the spillway bed21

• The geophysical investigation reveals that there are three layers having compressional 'wave velocities layer wise 600 to 650 mlsec for the first layer, 1500 to 1600 mls for the second layer and 4900 to 5600 mls for the third layer. From the results, it is known that the depth of the bed rock varies from 17 to 17.5 m and the firs t layer thickness of the bed rock varies from 2.5 m to 3.3m. The geophysical results are comparable with the bore hole data.

The computer programme used in the stability analysis was developed in CWC (Central Water Commission) and has been successfully used for a number of dams. The programme takes into account the Swedish Slip Circle Method of analysis. The stability analysis is based on the princi pl e that the shear strength at failure on any surface within an earthen dam is directly related to the normal stress on that surface22

.

S=C' + (N - U) tan <j> WhereS= Shear strength of failure surface C'=Cohesia intercept for a slice in terms of

effective stress N=Normal force acting on the failure U=Pore water pressure on the failure

slice N-U=Effective normal force

s lice <j>'=Angle of internal fricti on

for the s lice

surface of

on the

(effecti ve)

The shear strength constitutes the res isting force while the driving force is the sum of the tangential forces along the failure surface due to the self-weight of the material.

The factor of safety is given by:

Table 3 -Physico-chem ical properties of construction materials used in the design of Barsua tailin g dam

Material

I. Impervious Soil 2 . S li me (to be

deposited) 3. S lime (existing) 4 . Iron ore tines 5. Rock fill

(assumed values) 6. Foundation

Saturated unit wt (tJm3

)

1.80

2 .43 2.50 3. 15

2.25 2.00

Moist Effect ive Effec ti ve unit wt cohesion angle of (t/m) (t/m2

) fric tion (degrees)

1.75 1.0 24

2.34 0.0 29 2.45 0 .0 2 1 3.10 0 .0 32

0.0 0 .0 40 1.0 30

GHOSE & SEN: IRON ORE TAILING DAM 325

F.S. = L (N - U) tan <p + C' LT

Where T = Tangential component of the weight of a slice.

For the selec tion of the dam section at stable section, which involves the minimum overall lost, different slopes of dam section are analysed and the final section adopted with critical slip circles at different elevations for upstream and downstream slopes. The results for the minimum factor of safety for the final section are given in Table 4. The IS 7894 (1975)23 for the stability analysis for earthen dam recommends sudden drawdown as one of the critical conditions for upstream slope stability analysis of embankment dams. But in case of tailings, there is no

Table 4--The minimum factqr of safety for the final section by UIS rai si ng and DIS raising

VIS raising Condition

Steady seepage (without earthquake)

DIS raising

Sready seepage (wi thout earthquake)

Elevation

390.00

395.00

397.30 405.00 410.50 4 15.50

396.00

400.00

402 .30 406.00 406.50 410.50 415 .50

Factor of safety for U/S slope

2.5 19

1.69

2.34

3.027

Factor of safety for DIS slope

1.567

1.56 1

1.604 1.604 2.344 2.899

1.83 1

1.539

1.482 1.528

2.700 1.803 2.161

Table 5--Design feature of lind stage augmentation of Barsua tailing pond

Top of tai lings dam Deepest foundation level Maximum water level Length of tailings dam Width of spillway Type of spillway Crest level of spillway References : I.S. 7894-1975 analys is for earth dam.

420.50m 395.00m 418.50m 880.00m 25.00m

Un gated 417.00m

Code of practice for stability

question of such drawdown and hence this condition is not applicable. The drawdown from MWL to spillway crest level will be normal. For these reasons the upstream slope analysis for sudden drawdown may not necessary.

Barsua tailing pond capacity is to be augmented under two stages by increasing the height by Sm at every stage. For the design of the tailing dam the field investigations carried out for stage I and stage II have been discussed. Detail design adopted for stage IT is elaborated and given in Table S. The raising of the tailing dam at stage I was carried out by upstream construction. However, at stage II the raising of the tailing dam has been proposed partially by upstream raising and partially by downstream raising. The upstream raising will be founded on the deposited slime and maintaining the existing upstream slope. To make the section stable, loading in the form of rock fill has been provided in the downstream side upto RL 405 m. The rip rap and filter below it from the top and upstream slope of existing dam would be removed before raisi ng the dam. The existing rock toe and rock fill loading will help in draining out the seepage from the dam. A 2m thick covering of impervious soi l on both slopes and top of dam will prevent seepage through the dam. Though , earlier upstream construction was followed but in the second stage, as the general level in the dam beyond RL 490m is above the present deposited slime. It has been suggested that beyond RL 4tO m the dam should be raised by downstream raising. This will also reduce embankment earth work and create additional storage capacity for the tailings. There will be transition zone from upstream raising to downstream raising. The seepage will be drained out through rock toe. Where the dam will be raised by downstream raising the section will be made up of only slime and iron ore fines. Therefore, the impervious soi l covering has to be increased to 3 m on top and upstream (U/S) slopes. A 30 cm thick riprap over 15 cm thick gravel and IS cm coarse sand have to be provided all along the slope for protection against wave action .

Conclusions For the safe design of tailing dam detailed soil

investigation should be carried out before the design d . f '1' d . k 24 25 Th . an constructIOn 0 tal 1I1g am IS ta en up '. e In

situ coefficient of permeability K in soil samples below the present dam was found to be 3x to- to 30x 10-6 crn/sec. It was observed that a 4 m layer having high permeability values existed underneath the ground level having high K value. Such phenomena were noticed in two bore holes in the dam

326 INDIAN J ENG. MATER. SCI., DECEMBER 2001

portion. The 3rd bore hole drilled near spillway also indicated a permeable layer but maximum K value recorded between 3x 10-4 to 5x 10-4 cm/sec. The SPT indicated N value below 7 in upper layers upto 6 m depth but increased to 25 at a depth of 10.5 m. In view of low SPT value in upper layer, the upstream slope of the dam has been proposed to kept flat 4: 1 to provide wider base of load distribution. A 2% settlement allowance has been proposed to take into consideration in designing the tailing dam. For the fill material , apart from usi ng slime and iron ore fines, impervious soi l from bOITOw area near the dam site was chosen. Part of the dam on the downstream side where no obstacles are existing and on the upstream side settled tailing slime level has gone higher than general level, can be considered to be raised by downstream method to create additional volume in tailing pond. Surface and subsurface drainage arrangements should be provided. It has been estimated that thi s would give a storage capacity of 0.625 Mm3 in stage I and an additional capacity of 1.175 Mm3 in stage II augmentation.

Acknowledgements The authors are thankful to Dr L K Singhal,

Chairman-cum-Managing Director, MECON and Dr o K Paul , Director, Indian School of Mines, for extending the institutional facilities and to Mis Steel Authority of India Ltd., for providing the financial support and necessary assistance for carrying out the work.

References I Ghose M K, Environ & Ecol, 15 ( 1997) 206. 2 Mukherjee A K, Handbook of iron ore. Bull Geolog Survey

India, Govt of India. Series A, No.5 I 1988. 3 To land G. 0, Case histroy of failure and reconstruction of

Mike Horse tailing dam near Linco ln , Montuna; Proc. 15th Ann Engg Geology Soils Engg Symposium (Indaho State Un iversity) , 1977.

4 Down C G & Stocks J. Min Mag , 1977 .

5 Ghose M K & Sen P K, J Indian WaterWorks Assoc, XXXI (4) (1999) 237 -241.

6 Ghose M K & Sen P K, J IPHE, (I) (1999) 25-33. 7 Vick S G, Planning design and analysis of tailings dam

(Wiley Inter Science. John Wiley and Sons. New York), 1983.

8 Jeyapalan J, Earthquake induced mo velllellts of thickened railing slopes. Dynam'ic stability of tailing dalll (ASCE). (1982).

9 Vick S. Planning design and analysis of tailings dams (Wiley Inter Science. John Wiley and Sons, New York), 1973 .

10 Pul son K R, Th e merger of engineering aNi environll1ental

technology in modem mining and mineral processing.

Indllstry and Environment. UNEP. 1985 .

II Dan Kealy C, Min Congr J, 1973. 12 Ghose M K & Sen P K. Land Contaminatioll Reclalll,7 (2)

(1999) 143- 144.

13 Ghose M K & Sen P K. J Sci Indl Res,,18JI999) 699-704. 14 Methods for the in-situ permeability test on soil (B ureau of

Indi an Standards), IS -5529 (Part I) (1975). 15 Methods of test for soils for grain size analysis (B ureau of

Indi an Standards), IS-2720 (part IV) ( 1975). 16 Sen P K & Ghose M K. J Inst Eng, Min Eng, 80 (1999) 42-

45. 17 Methods of test fo r soil for determination of water content

(Bureau of Indi an Standards), IS-2720 (Part V II ) (1975).

18 Methods of test for soil for deterlllination of shear strong

parameters from consolidated triaxial cOlllpression test with

measurement of pore pressure (B ureau of In dian Standards),

IS-2720 (Part XII) ( 1975). 19 Methods of test for soil fo r deterlllination of consolidation

projects (Bureau of Indian Standards), IS-2720 (Part XV) ( 1975).

20 Methods of test fo r soil for deterlllination of permeability (B ureau of Indian Standards), IS-2720 (Part XV II) ( 1975).

21 Ghose M K& Sen P K, J Min. Metals Fuels , (2000) 59-62.

22 Ghose M K & Sen P K, J Inst Eng. Min Eng, 8 1 (2000) 18-22.

23 Dan Kealy C (B ureau Indian Standards), IS-7894 (1975).

24 Ghose M K& Kumar Asho k. J IA EM. 24( 1997) 63-67.

25 Sen P K & Ghose M K, Minetech. 18(4) (1997) 4 1-45.