documentFrame (1)

Chapter 34

WASTE DUMP STABILITY AT FORDING COAL LIMITED I N B.C.

Robert S. Nichols

Sen ior Planning Engineer , Fording Coal Limited Elkford , B.C.

ABSTRACT

Fording Coal L imi ted ' s mine i n t h e Rocky Mountains n e a r E l k f o r d , B.C. has produced 21.8 m i l l i o n c l e a n tonnes o f m e t a l l u r g i c a l c o a l from 1971 t o 1980, i n c l u s i v e . This p roduc t ion h a s come from s e v e r a l p i t a r e a s and r e q u i r e d t h e removal o f 149.1 m i l l i o n bank cubic meters o f waste rock and overburden by t ruck-shovel and d r a g l i n e opera t ions .

Waste dumps from 30m t o 200111 i n h e i g h t have been s u c c e s s f u l l y cons t ruc ted on n a t u r a l s l o p e s , g e n e r a l l y between 100 and 26O. Design cons idera t ions f o r t h e dumps i n c l u d e founda t ion and s o i l condi t ions , n a t u r a l s l o p e a n g l e s and containment of weak overburden o r rehandle m a t e r i a l s .

Dump c o n t r o l i s main ta ined by monitor ing t h e c r e s t and t h e f a c e o f an a c t i v e dump a r e a . C r e s t movement i s measured by ex tensometers l o c a l l y termed "Spoi l Monitors" . When c r e s t movement r a t e s pro- g r e s s i v e l y i n c r e a s e above normal, dumping o p e r a t i o n s a r e t e m p o r a r i l y r e l o c a t e d . Some u n s t a b l e c o n d i t i o n s and mass f a i l u r e s have o c c u r r e d a s t h e r e s u l t of n a t u r a l founda t ion s l o p e s be ing i n excess o f 26O, f a i l u r e of weak water s a t u r a t e d founda t ion s o i l s o r weak m a t e r i a l being p laced on t h e dump.

INTRODUCTION

The Fording River Mine i s l o c a t e d i n t h e Rocky Mountains of sou theas te rn B r i t i s h Columbia a s shown i n F i g u r e 1.

A l l product ion i s from multi-seam open p i t mining a t e l e v a t i o n s ranging from 1600m t o 2200111.

795

3RD STABILITY IN SURFACE MINING

Figure 1. Fording Coal Location Map

The mine i s l o c a t e d i n a c o n t i n e n t a l c l i m a t i c zone w i t h t empera tures rang ing from -40 C i n January t o +35O i n J u l y . Annual p r e c i p i t a t i o n c o n s i s t s o f r a i n f a l l between 220 t o 350mm p l u s s n o w f a l l i n t h e range of 240 t o 680 cm.

Cleaned c o a l p roduc t ion a t an annua l r a t e o f 3.0 m i l l i o n tonnes began i n 1972. A t o t a l of 149.1 m i l l i o n bank cubic metres (bcm) of w a s t e rock , overburden and r e h a n d l e h a s been moved by t ruck-shovel o p e r a t i o n s and a 46 m3 d r a g l i n e .

I n 1980, a major expansion program was i n i t i a t e d t o i n c r e a s e annua l p r o d u c t i o n t o 5.0 m i l l i o n tonnes . This w i l l r e q u i r e 40.0 m i l l i o n bcm of was te t o be removed p e r year .

P lanning , c o n s t r u c t i n g and m a i n t a i n i n g s t a b l e waste p i l e s along t h e v a l l e y s l o p e s h a s been a s i g n i f i c a n t f a c t o r i n t h e success of

this operation. This paper describes the design considerations, development and control used at Fording to maximize waste dump stability.

GENERAL MINE LAYOUT

Multi-seam open pit mining is done in pit areas on the east and west sides of the Fording River. The pits are designed to an over- all average strip ratio of 7 bcm waste : 1 bcm raw coal. The Eagle Mountain side east of the river with Clode, Turnbull, Taylor and Blackwood truck-shovel pits has produced 53% of the waste from 1971-1980 inclusive. The Greenhills truck-shovel pits on the west side of the river, have produced 13% of the waste. The remaining 34% waste, including rehandle, was moved by a 46 m3 dragline.

Ten mineable coal seams occur in the lower Cretaceous Coal Bearing Member of the Kootenay Formation. The 4SOm thick sequence consists of interbedded sandstone, siltstone, mudstone, shale and sub- bituminous coal.

DETERMINING WASTE DUMP LOCATIONS

Placement of the large quantities of waste material removed to meet coal targets is a major concern to mine planning, environmental and production departments. Figure 2 outlines the areas which cannot be readily used for developing waste dumps because they are within resource areas, plantsite and facilities areas, areas of steep slopes and environmentally sensitive areas. Note that dumps are numbered on the Eagle Mountain side and lettered on the Greenhills side.

Coal Reserves

It is desireable to locate waste dumps outside areas of potential economic open pit coal reserves. The exceptions to this are relatively small dumps which are planned to be rehandled at a later date. No. 1 Spoil, Turnbull Spoils and K Spoil were developed during early stages of mining within reserve areas.

Pit development has not sufficiently advanced to begin back- filling except for the dragline pits.

Plant Site and Facilities

This area occupies a major portion of the valley floor. Included in the facilities are the tailings pond areas, coarse reject storage area and the railway and road access. The plant site area includes the washplant complex, offices, warehouse, shops and dry.

798 3RD STABILITY IN SURFACE MINING

F i g u r e 2. P lan Showing P i t s and Dumps R e l a t i v e t o R e s t r i c t e d Areas

S t e e p S lopes

The v a l l e y w a l l s i n some a r e a s , a r e t o o s t e e p on which t o develop s t a b l e waste dumps. I n g e n e r a l , a waste dump i s n o t developed on a founda t ion s l o p e i n excess of 24O. Some except ions t o t h i s a r e i n a r e a s where t o e s u p p o r t i s e s t a b l i s h e d f i r s t on shal low s l o p e s o r where c o n t r o l l e d development t o reach shal lower s l o p e s is necessary t o minimize haulage d i s t a n c e s .

Environmental

The envi ronmenta l ly s e n s i t i v e a r e a s a r e a s s o c i a t e d wi th major d r a i n a g e p a t t e r n s . P r e s e n t p lann ing a l lows f o r a 50 - l O O m d i s t a n c e between t h e 1,000 y e a r f l o o d p l a i n l i m i t and a 26O res loped dump t o e . This d i s t a n c e p r o v i d e s a long term w i l d l i f e c o r r i d o r f o r m i g r a t i n g animals i n t h e v a l l e y .

Other Concerns

Other f a c t o r s c o n s i d e r e d when choosing a dump s i t e a r e haulage d i s t a n c e s , hau l road g r a d e s , and d r a i n a g e c o n t r o l .


Waste Dump Materials

Dump materials can be o f three types: waste rock, overburden and rehandle.

Waste rock i s estimated t o comprise 80% o f the t o t a l property waste (excluding dragline rehandle). The waste rock i s a combin- a t ion o f 5 5 % s i l t s t o n e , 28% sandstone, 15% carbonaceous mudstone and shale and 2 % non-recoverable coal. The quant i t ies o f each rock type delivered t o a particular dump depend on what s t r a t i - graphic l eve l a particular bench i s a t . This material has a dry densi ty o f 1.76 kg/m3 and a f r i c t i o n angle o f 37O.

The sandstone i s t y p i c a l l y coarse grained, durable, blocky and not s i g n i f i c a n t l y a f f e c t e d by weathering. The s i l t s t o n e may grade t o f ine grained sandstone and can a l so be blocky depending on i t s sand content. S i l t s t o n e usually weathers wel l , although t h i s i s a l so a function o f the sand content. The carbonaceous mudstone and shale i s usually f i n e , f r iab le and readi ly breaks down t o f ine part ic les when weathered.

Overburden i s a mixture o f glacial till and weathered rock. This material i s not f ree draining and contains an average 15% moisture a t a densi ty o f 1.72 kg/m3. The undrained shear strength was determined t o be 55 .2 kPa wi th a O0 f r i c t i o n angle. Therefore, the height o f a dump developed wi th t h i s material i s c r i t i c a l .

In Greenhills K-Spoil for example, t h i s material, a t a height o f 24m, was found t o slump from an i n i t i a l face angle o f 40 down t o 26O over a period o f several months. For planning purposes, the material must be kept t o a minimum height and be contained.

Rehandle cons i s t s o f a mixture estimated t o be 85% waste rock and 15% overburden which has been subjected t o weathering for 5 - 10 years. This material i s not f ree draining but does drain s i g n i f - i c a n t l y be t t e r than overburden. The f r i c t i o n angle i s 33O. Samples o f rehandle from Taylor P i t show a s i z e d is tr ibut ion o f 68% gravel, 27% sand and 5 % s i l t and c lay . I t i s in teres t ing t o note that a f t e r compaction t e s t i n g , there i s a s h i f t i n the s i ze d i s t r ibu t ion between 15 and 20% t o the f i n e r s ide . This material has caused un- s table conditions i n No. 2 Spo i l , which i s 200m high, both a t the time o f dumping and many months l a t e r a f t e r it had been covered by waste rock. During i n i t i a l dumping, the rehandle did not free-roll t o the dump bottom. This created an oversteepened face o f 40. For planning purposes, t h i s material should be contained and kept t o a minimum height .

Dump Height

For waste rock, there i s no r e s t r i c t i o n on the dump height. Where overburden i s t o be placed i n an unconfined dump area, the maximum height designed i s 25m. An estimated height o f 60m i s used for

FORDING COAL LIMITED 80 1

rehandle. This es t imate i s based on % of t h e f r ee - ro l l i ng he igh t of the rehandle observed on No. 2 Spo i l .

Development of the Waste Dumps

Three main types of dumps developed a t Fording a r e f r e e , wrap- around and formed p i l e s a s i l l u s t r a t e d i n Figure 3. Dump a reas a r e b u i l t using a combination of these types. The type of dump chosen depends on phys ica l condi t ions of t h e s i t e , t h e quan t i t y of p a r t i c - u l a r mater ia l s expected from t h e p i t and t h e i r de l ive ry schedule.

Free dumping involves t h e placement of ma te r i a l s from a s p e c i f i c e levat ion i n one l i f t only. These dumps a r e genera l ly a t l e a s t 90m high. The he ight of these dumps does no t al low f o r t h e placement of s i g n i f i c a n t q u a n t i t i e s of overburden o r rehandle. Free dumps on t h e Eagle Mountain s ide were e s t ab l i shed t o accommodate s h o r t waste hauls from the higher e l eva t ions i n Clode and Taylor P i t s ( s ee Figures 2 and 8 ) .

Wrap-around dumps involve t h e cons t ruc t ion of long dumps a t successively lower e l eva t ions . They gene ra l ly p a r a l l e l t h e s lope contours of t h e va l l ey wal l s . This type of dump i s used f o r road cons t ruc t ion and containment of weak ma te r i a l . The lower po r t i on of t he wrap-around a l s o provides t o e suppor t t o t h e higher dumps. Re- sloping of t he se dumps f o r reclamation, r equ i r e s t h e l e a s t amount of work. There a r e two ways i n which t h e wrap-around method can be used t o conta in t he weak ma te r i a l s a s shown i n Figure 4. The b a r r i e r type is used where t h e r e is s u f f i c i e n t quan t i t y of waste rock ava i l ab l e t o precede de l ive ry of t h e weak ma te r i a l . The waste rock t o e type i s used when t h e weak ma te r i a l is re leased be fo re t h e waste rock i s ava i l ab l e . The waste rock wrap-around i s b u i l t a s soon a s poss ib l e (wi th in months) a f t e r t h e weak ma te r i a l i s i n p l ace .

Formed dumps a r e developed by t h e depos i t i on of waste i n l a y e r s o r l i f t s . This type of dump i s used i n t h e Greenhi l l s p i t a r e a s t o minimize uph i l l waste haulage ( s ee Figure 9 ) . This type of dump w i l l accommodate overburden m a t e r i a l s because of t h e r e l a t i v e l y low l i f t height .

WASTE DUW MONITORING AND CONTROL

Monitoring

The two types of monitoring a r e v i s u a l and measured. The v i s u a l examination i s done by r egu la r observat ion of t h e dump face f o r bulges and the dump su r f ace f o r c racks . S i g n i f i c a n t bulges a r e those which i n t e r r u p t t h e l i n e of s i g h t t o t h e dump toe . Tension and shear cracks a r e common on a l l dumps. Large tens ion c r acks p a r a l l e l t o t h e face , can be 30 t o 60m long and up t o l O O m back from t h e face. These cracks do no t appear t o be r e l a t e d t o dump


WASTE DUMP TYPES AT FORDING

2. Wrap-Around Dump n

Figure 3. Idealized Section Showing Dump Type and Development Sequence

stability. They are believed to be a result of differential compaction due to increased dump height toward the crest. Shear cracks are most common near the dump crest. These cracks indicate the amount of crest movement which is directly related to dump stability.

The rate of movement at the dump crest is measured on a routine basis using an extensometer, locally termed a "spoil-monitor." The monitors consist of two stands with pulleys, a steel pin, a weight and durable light wire. The set-up is illustrated in Figure 5. Figure 6 shows a monitor set-up on No. 2 Spoil.

As the face of the pile settles, the relative displacement between the pin and weight is measured. The monitors are placed 30 to

FORDING COAL LIMITED

CONTAINMENT METHODS AT FORDING

1 Waste Rock Barrier

2 Waste Rock Toe Support

, WEAK MATERIAL

Figure 4. Sketch Showing Methods Used to Contain Weak Material

lOOm beside the active face. Readings are taken once per shift under normal circumstances. If the daily displacement increases and exceeds 0.5m, then readings are taken more frequently.

The steepness of the foundation slope has an influence on monitor rates. On No. 2 Spoil, crest movement decreased from 0.6 - 0.8m/day to less than 0.2m/day as the toe advanced over a 27O slope to a 14O slope. Precipitation has not influenced the rate of movement.

3RD STABILIW IN SURFACE MINING

FORDING WASTE PILE MONITORING 1- 3 0 - I W ~ -1 7 C R E S T MOVEMENT

Figure 5. Ideal ized Diagram Showing Monitoring Done on Waste Dumps

Dump Control Dump control is maintained by monitoring the c r e s t movement and

developing a long o r a l t e r n a t e dump area . Most dumps a r e b u i l t with a 300m long face t o enable development of several dump areas. When c r e s t movement is acce le ra t ing and exceeds 0.5m/day, dumping i n the a rea is temporarily stopped. This 0.5m/day l i m i t is considered conservative. Rates of movement i n excess of 1.5m/day have occurred on No. 2 Spoi l without any subsequent mass f a i l u r e .

Figure 7 shows a recent example of dump control on No. 2 Spoil . I t is evident t h a t shor t ly a f t e r times of "no dump", the c r e s t movements decreased r ap id ly back t o normal. The unstable conditions occurred d i r e c t l y a s the r e s u l t of rehandle being placed i n the dump. Shor t ly a f t e r it had been b u i l t up, the rehandle area beqan t o s e t t l e rapidly u n t i l it reached a s t a b l e condition. Once c r e s t movements had decreased and s t a b i l i z e d , waste rock was dumped over it. The rehandle beqan t o y i e l d again a f t e r e igh t months. Rapidly increas ing c r e s t movements slowed back t o normal r a t e s a f t e r dumpinq was stopped.

Figure 7 a l s o shows t h a t p r e c i p i t a t i o n does not s ign i f i can t ly inf luence c r e s t movement. There was a l s o no e f f e c t when the average


-WASTE ROCK- lll~rthsno d u m p l d r d dump swlh l o m t h - p 1.5 J IREH'LI ..--- NORTH MONITOR 3 2 I N D ~ ! ~ ) ? 5 5 (Soulhl 6 - - SOUTH MONITOR -

I- i = 1.0- x E w ;I > 0

O.L--- - - . 0 w I-

C

Figure 7 . Chart showing r a t e of c r e s t movement r e l a t ed t o dump a c t i v i t y , d a i l y r a t e of waste production from Taylor P i t and d a i l y p rec ip i t a t ion . Note t h a t during periods of "no dump", waste was hauled t o an a l t e rna te dump.

Tab

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FORDING COAL LIMITED 81 1

i n p a r t i c u l a r , D.B. Campbell. The e n g i n e e r i n g s t a f f a t Ford ing were very h e l p f u l i n c r i t i c a l l y reviewing t h i s paper .

REFERENCES

Campbell, D.B., and Shaw, W . H . , 1978 "Performance of a Waste Rock Dump on Moderate t o S t e e p l y S lop ing Foundat ions", S t a b i l i t y i n Coal Mining, proceedings of t h e f i r s t I n t e r n a t i o n a l Symposium on S t a b i l i t y i n Coal Mining, Vancouver, B.C., A p r i l , 1978,

Question Is dump movement o c c u r r i n g a long t h e a r e a of c o n t a c t w i t h t h e foundat ion, i n t h e founda t ion s u b s o i l s o r confined t o t h e was te m a t e r i a l .

Answer

The dump movement t h a t we moni tor o c c u r s a s a r e s u l t of s e t t l e m e n t of t h e m a t e r i a l wi th in t h e p i l e s n e a r t h e c r e s t . Movement of t h e dump a t t h e foundat ion o n l y o c c u r s i n l o c a l a r e a s where weak o r g a n i c s o i l s g r e a t e r than 0.3m t h i c k have n o t been removed. G e n e r a l l y , t h e r e is no dump movement along t h e founda t ion c o n t a c t because t h e compact- ed till s o i l i s s t r o n g e r t h a n t h e was te m a t e r i a l s .

Question I n l each ing dumps, t ime l a g s between wate r on-flow r a t e s and recovery r a t e s a t t h e dump t o e may b e s e v e r a l weeks. Have you cons id- e r e d t h i s i n your p r e c i p i t a t i o n c o r r e l a t i o n .

Answer

No. The extreme case of t h i s would b e s n o w f a l l where t h e run-off o r "recovery" could be months a f t e r t h e p r e c i p i t a t i o n . However, it h a s been o u r experience t h a t r a i n f a l l does n o t immediately ( w i t h i n one week) a f f e c t t h e dump. I t can a f f e c t t h e s t r e n g t h of o r g a n i c found- a t i o n s o i l s and cause them t o f a i l .

Question What p o s s i b i l i t y i s t h e r e t h a t w i t h t i m e , t h e r e a d i l y d e g r a d a b l e s h a l e s w i l l e v e n t u a l l y s e t t l e t o t h e lower p o r t i o n o f t h e dumps, t h u s l y reducing t h e p e r m e a b i l i t y and i n c r e a s e dump weigh t due t o increased wate r .


Answer

The key p o i n t s h e r e a r e ' t i m e ' and ' s l o p e ' . We d o n ' t know how long it would t a k e f o r t h i s mechanism t o occur i f it w i l l occur . Our exper ience is t h a t No. 1 dump, which was developed over e i g h t ( 8 ) y e a r s ago, s t i l l d r a i n s e f f e c t i v e l y a t t h e toe . You may r e c a l l from my p r e s e n t a t i o n , a s l i d e which showed a l a r g e "bowl" above No. 1 dump. Snowfal l accumulat ions i n t h i s bowl can be s i x metres p e r y e a r . A l l t h e run-off from t h a t a r e a goes through No. 1 dump.

The second p o i n t is t h e founda t ion s l o p e . Water flowing on a r e l a t i v e l y s t e e p s l o p e w i l l be a b l e t o c a r r y a g r e a t e r sediment l o a d than on v e r y sha l low o r f l a t s l o p e s because of t h e h y d r a u l i c g r a d i e n t . No. 1 dump h a s founda t ion s l o p e s from lo0 t o 17O.

Quest ion Have you managed t o c o r r e l a t e dump c r e s t i n s t a b i l i t y w i t h r ,a te of dump advance. A t B o u g a i n v i l l e , exper ience h a s i n d i c a t e d t h a t 1 metre/ day is c r i t i c a l f o r t h i s s i t u a t i o n . Answer

The r a t e of dump advance a t Ford ing depends on t h e r a t e of loading and t h e dump h e i g h t . We do n o t measure t h e r a t e of advance b u t use t h e " S p o i l Monitors" t o measure t h e r a t e of c r e s t movement. The c r i t e r i a t h a t we c o n s i d e r r e p r e s e n t a t i v e of an uns tab le condi t ion developing i s when t h e moni tors i n d i c a t e an a c c e l e r a t i n g c r e s t movement above a r a t e of 0.5 m/day.

A s shown i n F i g u r e 7, t h e normal c r e s t movement f o r o u r h i g h e s t dump, No. 2 dump, does n o t change when t h e r a t e of loading is increased from 8,000 bcm/day t o 16,000 bcm/day.

Quest ion Are t h e dumps l o c a t e d i n a s e i s m i c l y a c t i v e a r e a . I f s o , has t h i s been i n c o r p o r a t e d i n t h e d e s i g n .

Answer

The p r o p e r t y i s n o t l o c a t e d i n a s e i s m i c l y a c t i v e a r e a and we do not i n c o r p o r a t e t h i s a s p e c t i n t o o u r des ign .

I should n o t e t h a t l a r g e b l a s t s (approaching 500,000 Kg of exp los ives ) i n nearby Clode and T a y l o r P i t s have n o t had any e f f e c t on dump s t a b i l i t y .

Ques t ion What p r e p a r a t i o n s a r e made on n a t u r a l ground s u r f a c e p r i o r t o was t ing on t h a t s u r f a c e .

Answer

G e n e r a l l y , where weak o r g a n i c s o i l s a r e i n excess of 0.3m t h i c k , they

FORDING COAL LIMITED

a r e removed. The g l a c i a l till below t h i s is not removed because i t i s s t r o n g e r than t h e waste m a t e r i a l s p l a c e d on it.

Question Did you e s t a b l i s h a mathematical model t o e x p l a i n t h e type of f a i l u r e s you've got .

Answer

No. However, a l l f a i l u r e s have been a combination of c i r c u l a r a r c type through t h e dump and p l a n e f a i l u r e along t h e foundat ion.

Question I t would appear t h a t was te dumps a t Fording a r e c o n s t r u c t e d by g u i d e l i n e s t h a t a r e f a r from t h o s e t h a t would be allowed by O.S.M. i n t h e United S t a t e s . Could you comment on t h e P r o v i n c i a l r e g u l a t i o n s t h a t govern t h e s t a b i l i t y o f was te dumps i n B.C.

Answer

B a s i c a l l y , f o u r a r e a s must be addressed when applying f o r Government approval t o b u i l d dumps:

a . A geo technica l assessment of t h e dump s t a b i l i t y must be made f o r t h e development s t a g e and f i n a l dump. At F o r d i n g , we use c o n s u l t a n t s t o a n a l y s e t h e s t a b i l i t y o f a p roposed dump and submit t h e i r r e p o r t w i t h t h e permi t a p p l i c a t i o n .

b . The e f f e c t s on wate r q u a l i t y a r e p r e d i c t e d . T h i s work i s u s u a l l y done in-house a t Fording.

c . The land d i s t u r b a n c e must be o u t l i n e d . T h i s i n v o l v e s a s s e s s i n g t h e e c o l o g i c a l e f f e c t o f b u i l d i n g a was te dump i n a c e r t a i n l o c a t i o n . T h i s work is done in-house a t Fording.

d. A p l a n f o r rec lamat ion is a l s o inc luded . The long term des ign must a l low f o r r e s l o p i n g and r e v e g e t a t i n g t h e dumps.

Once t h e dump is completed, t h e fo l lowing t h r e e t h i n g s must b e done a s follow-up:

a . The dump must be r e s l o p e d t o 26O.

b. The 26' s l o p e s must be r e v e g e t a t e d t o t h e s a t i s f a c t i o n of t h e M i n i s t e r of Energy, Mines & Petroleum Resources.

c . Drainage i n t h e a r e a must b e r e - e s t a b l i s h e d t o t h e s a t i s f a c t i o n o f t h e M i n i s t e r of Environment

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