Chemical Treatment of a Dispersive Clay Reservoir

Chemical Treatment of a Dispersive Clay Reservoir Traitement Chimique d'un Reservoir Constitue d'argile Dispersive 12/45 F. von M. WAGENER Director, Jones & Wegener Inc. Johannesburg, South Africa H.J. von M. HARMSE Professor, Department of Pedology, University of Potchefstroom, South Africa P- ONE Senior Geotechnical Engeer, Longworth 8>McKenzie, Sydney, Australia W. ELLIS Associate Director, Jones & Wagener Inc., Johannesburg, South Africa SYNOPSIS An off channel water storage reservoir was built usg alluvial silty clay on a foundation of similar alluvium overlyg sandstone and shale. The reservoir failed at onset of first fillg by dispersive pipg through the embankment and bas floor. The breach was repaired but further failures were anticipated. Various methods of mimisg the risk of further failures were considered and are briefly discussed. It was fally decided to treat the bas floor and uater chemically. The paper describes a rather unique chemical dosg procedure employed to successfully and economically safeguard the reservoir agast future failure. INTRODUCTION The Senekal dam was built as an off-channel storage reservoir on the Beaufort Group of the Karoo System the Orange Free State provce of South Africa. It was constructed usg alluvial silts and clays on a foundation of similar alluvium overlyg sandstone and shale at depths excess of 10 m. The dam is located on the north-eastern side of a stream from where water is pumped from behd a weir built across the stream. The topography of the area is relatively flat over the dam bas but rises steeply to a sandstone hill which forms a natural boundary to the one side of the reservoir. The embankment was built to a typical small dams specification where material, moisture content and density were checked at regular tervals. The earth wall has a maximum height of B m and is zoned as shown the cross section Figure 1. The embankment walls are some 1100 m long contag a reservoir of approximately 19 ha and 1,4 x 10^ m3 capacity. BREACH DESCRIPTION Fillg of the reservoir was started November, 1974, by pumpg water from the weir but was stopped a week later when leaks were detected on the downstream toe of the embankment. The uater level the bas at that stage uas about 3 m above the floor. A trench uas dug where the leak surfaced and revealed tuo 40 mm diameter holes at a shallow depth leadg below the dam. The flou creased rapidly and the embankment failed by pipg 4 days later. The 4 m by 5 m high breach had an irregular S shapeo path through the uall with lets and outlets offset some 25 m along the centre le. The bottom of the breach cavity was obscured by collapsed material from above. Figure 2 shows the outlet end of the breach. FIGURE 1 : TYPICAL EMBANKMENT CROSS SECTION. It consists of an ner clay core with 3 m deep cut-off, silty sand flanks and a rip-rap sandstone protection on the upstream face. A clay blanket some 20 m wide extends from the clay core to mimise seepage through the erratic alluvial foundation soils. FIGURE 2 BREACH THROUGH WAIL 50-017129 785

A g e o t e c h n ic a l v e s t i g a t i o n was i n i t i a t e d to d e te rm e th e c a u s e o f f a i l u r e (O ones & W e g en e r, 1 9 7 5 ). A p r e l i m a r y a s s e s s m e n t d ic a t e d th e p re s e n c e o f d is p e r s iv e s o i l s and th u s th e v e s t i g a t i o n was o r ie n t a t e d to d e te rm e th e t r i g g e r m echanism f o r th e i n i t i a l le a k and t o c h e c k th e s ta n d a r d o f c o n s t r u c t io n. GEOTECHNICAL INVEIGATION The c la y u sed f o r th e c o re o f th e dam was o b ta e d fro m an a l l u v i a l b o rro w a re a some 500 m a w a y. C a lc r e t e nod u le s w ere s c a t t e r e d o v e r th e s u r fa c e o f th e b o rro w and s h e e t and g u l le y e r o s io n c o u ld be seen o v e r th e e n t i r e a r e a. P o o ls o f w a te r w ere t u r b i d. Two t e s t p i t s w e re dug by hand th e em bankm ent to c h e c k g e n e r a l c o n s t r u c t io n s ta n d a r d s and t o sa m p le m a t e r ia ls. The a v e ra g e i n - s i t u d e n s ity fro m 60 t e s t s was 96,5 % P r o c to r a t a m o is tu r e c o n t e n t a b o u t 2% d r y o f o p tim u m. No s i g n i f i c a n t d e t r im e n t a l la y e r g was n o t ic e d i n th e t e s t p i t s to fo u n d a tio n l e v e l and th e c o n c lu s io n draw n t h a t th e s ta n d a r d o f w o rk was a c c e p ta b le f o r a n o n - d is p e r s iv e dam o f t h i s s iz e., H o w e ver, w it h re g a rd to d is p e r s iv e b e h a v io u r some I d e a lis e d a re a s o f th e em bankm ent e x h i b it e d m o is tu r e c o n t e n t s as lo w as 4% d ry o f o p tim u m. Such a zo ne c o u ld be th e c a u s e f o r an i n i t i a l le a k due t o i t s g r e a t e r p e r m e a b ilit y. ( S to n e, 1 9 7 7 ). The fo u n d a tio n o f th e dam was n e x t i n v e s t i g a t e d. I t c o n s is te d o f o ra n g e and g re y sa n d y to c la y e y s i l t some 1,5 m t h i c k o v e r l y g 3 m o f b ro w n s i l t y c la y f o llo w e d b y o ra n g e and g re y c la y e y sands b e fo r e e n c o u n te r g b e d ro c k a t a v a r ia b le d e p th a ro u n d 10 m. The p r o p e r t i e s o f th e f o u n d a tio n s o i l s w ere o b v io u s ly v a r ia b le b u t w ere g e n e r a lly f i n e g r a e d w it h e x p e c te d lo w p e r m e a b ili t ie s. H o w e ver, le n s e s o r la y e r s o f a m ore p e rm e a b le m a t e r ia l a re a d i s t i n c t p o s s i b i l i t y i n su ch an a llu v iu m and w ere e n c o u n te re d i n th e p r e - c o n s t r u c t i o n v e s t i g a t i o n. One o f th e t e s t p i t s e n c o u n te re d th e f o u n d a tio n s o i l a t th e e x p e c te d d e p th o f a b o u t 3 m b e lo w o r i g i n a l g ro u n d l e v e l. The m a t e r ia l was an o ra n g e s a n d y c la y w h ic h e x h i b i t e d a c e r t a p o r o s i t y due to a p p a r e n t r o o t h o le s and c h a n n e ls. S lo w se e p a g e came fro m th e s m a ll h o le s. The f o u n d a tio n c la y m a t e r ia ls w ere fo u n d t o p o s s e s s s i m i l a r p r o p e r t i e s t o th e b o rro w a re a s o i l s w it h b o th h a v g h ig h E x c h a n g e a b le Sodium P e rc e n ta g e s (E S P ). From a t o t a l o f 60 t e s t s w h ic h w e re c o n d u c te d on th e c o r e, b o rro w m a t e r ia l and f o u n d a tio n s o i l s, c o m b e d p r o p e r t ie s s i m i l a r t o th o s e shown i n T a b le 1 w ere o b t a e d. The c o n c lu s io n draw n fro m th e g e o t e c h n ic a l v e s t i g a t i o n was t h a t th e i n i t i a l le a k le a d g t o p ip g f a i l u r e by d is p e r s io n c o u ld h ave been due t o a ny o r a c o m a tio n o f th e f o l l o w i n g : ( a ) C ore l i f t s w it h m o is tu r e c o n t e n t s as lo w ae 4% b e lo w o p tim u m r e s u l t i n g i n p e r m e a b ilit ie s c o n d u c iv e t o p ip g. ( b ) An t e r c o n n e c t io n o f s h r k a g e c r a c k s a t a c e r t a l e v e l ca u s e d by a la p s e i n c o n s t r u c t io n and i n s u f f i c i e n t a t t e n t i o n t o p r e t r e a t m e n t ( s c a r i f y i n g a nd r e w e t t g ). ( c ) L a y e rs w it h h ig h e r p e r m e a b ilit y w he re b o rro w m a t e r ia l w it h a lo w e r P I was u s e d. ( d ) Seepage th r o u g h th e o b s e rv e d p o ro u s n a tu r e o f th e fo u n d a tio n s o i l s. The l a t t e r e f f e c t i s c o n s id e r e d th e m o st p r o b a b le cause (m e c h a n is m ) o f f a i l u r e. I n i t i a l seepage p a s s g th r o u g h s h r k a g e c r a c k s a t b la n k e t / c o r e t e r f a c e seeped u n d e r th e c o re c u t - o f f fo r m g th e s m a ll p ip e a o b s e rv e d i n i t i a l l y. P r o g r e s s iv e d is p e r s iv e e r o s io n a lo n g t h i s p a th le d to c o lla p s e o f th e u p p e r c o re m a t e r ia l and c o m p le te p ip g f a i l u r e. DESCRIPTION OF DISPERSI0N /FL0CCULATI0N PHENOMENA C la y i s an a s s e m b la g e o f m ic r o s c o p ic p l a t e l e t s fo rm e d by c h e m ic a l d e c o m p o s itio n o f r o c k m e r a ls. These p l a t e l e t s a re h e ld t o g e t h e r by v a r io u s f o r c e s. A h ig h ESP i n th e c la y ca u a e s th e p l a t e l e t s to be lo o s e ly b o n d e d. S h o u ld w a te r w it h a lo w c o n c e n t r a t io n o f d i s s o lv e d s a l t s ( lo w TCC e g. r a w a t e r ) f i n d an i n i t i a l m ic ro c h a n n e l i n s u c h a s o i l mass th e lo o s e ly h e ld p l a t e l e t s w i l l d is p e r s e (becom e c o l l o i d a l ) and be c a r r ie d away by th e p e r c o l a t g w a t e r. The m ic ro c h a n n e l c o u ld e n la r g e, c a u s g a p ip e w h ic h c o u ld le a d t o t o t a l dam f a i l u r e. I f th e c o n t e n t o f s w e l l g c la y i s h ig h ( P I > 3 5 ) th e c la y may s w e ll and c lo s e th e c h a n n e ls b e fo r e p ip g can ta k e p la c e. S h o u ld th e c la y h o w e ve r c o n t a a h ig h p e rc e n ta g e o f c a lc iu m ( lo w ESP) th e p l a t e l e t s w i l l be s t r o n g ly bonded and f l o c c u l a t e d. W a te r p e r c o l a t g th r o u g h su ch a c la y w i l l n o r m a lly n o t be a b le t o e ro d e i t. The c la y w i l l h ave tim e t o s w e ll as th e w a te r s e e p s c a u s g m ic ro c h a n n e ls t o c lo s e, r e d u c g s e e p a g e. G e n e r a lly w a te r s e e p g th r o u g h a d is p e r s iv e c la y w i l l TABLE 1 CORE PROPERTIES CHEMICAL PROPERTIES INDEX PROP. GRADING ph EC u ESP LL PI S ILT % CLAY % AVERAGE B,3 1 4,6 2 4,8 4 2,5 2 6,5 31 53 D. DEVIATION 0,4 5 0,4 12,5 10,1 7,7 5,1 9,8 COEFF. DEVIATION 0,0 6 0,3 5 0,5 0 0,2 4 0,2 9 0, 16 0,1 9 786

be t u r b i d w h e re a s t h a t s e e p g th r o u g h a f l o c c u l a t e d c la y w i l l be c l e a r. I t has been fo u n d t h a t i f w a te r w it h a h ig h c o n c e n t r a t io n o f c a lc iu m i s a llo w e d t o se e p s lo w ly th r o u g h a c la y w it h a h ig h c o n c e n t r a t io n o f so d iu m ( d is p e r s iv e s o i l ) io n e xchange i n th e s o i l can ta k e p la c e : S odium io n s a re r e p la c e d by c a lc iu m io n s and th e s o i l f l o c c u l a t e s i. e. becom es n o n - d is p e r s iv e. T h is phenom enon can be demons t r a t e d i n th e la b o r a t o r y by d is p e r s io n t e s t s. LATENT HAZARD OF SUBSEQUENT PIPIN G The i n i t i a l f a i l u r e o f th e dam by d is p e r s iv e p ip g o c c u r r e d u n d e r a lo w head o f 3 m and p ro v e d th e e x is te n c e o f c o n t u o u s m ic r o c h a n n e ls th r o u g h o r u n d e r th e em bankm e n t. In a d d i t io n th e dam had been e m pty f o r f o u r y e a rs b e f o r e re m e d ia l w o rk s t a r t e d and th u s m ore s h r k a g e c r a c k g c o u ld h a ve ta k e n p la c e. Hence th e p r e v io u s a d v e rs e c o n d itio n s w o u ld s t i l l be o p e r a t iv e when r e f i l l i n g v i z : ( i ) R e l a t iv e ly p u re r e s e r v o i r w a te r ( i i ) D is p e r s iv e c la y b a s and w a ll ( i i i ) P re s e n c e o f c o n t u o u s m ic r o c h a n n e ls As a co n s e q u e n c e th e r e m e d ia l w o rk f o r th e dam u n d e r f u l l o p e r a t g c o n d itio n s had to c o n s id e r th e l a t e n t h a z a rd s o f s u b s e q u e n t p ip g i n o t h e r a re a s and c o u ld n o t be l o c a l iz e d t o th e b re a c h zo ne a lo n e * POSSIBLE REMEDIAL TREATMENT The f o ll o w i n g ia m e d ia l m e a su re s w e re c o n s id e r e d and p r ic e d : ( i ) Dam r e lo c a t io n ( i i ) Dam r e c o n s t r u c t io n e m p lo y g s t r i c t c o n s t r u c t io n c o n t r o l w it h o r w it h o u t c h e m ic a l tr e a tm e n t o f th e s o i l ( i i i ) U p stre a m l i n i n g p r o t e c t io n. The ty p e o f membrane c o u ld b e : B u t y l ru b b e r P o ly th e n e s h e e t g. B e n to n ite ( i v ) C h e m ic a l tr e a tm e n t o f b a s f l o o r and em bankm ent s id e s ( v ) W a te r tr e a tm e n t A c o m a tio n o f m e th o d s ( i v ) and ( v ) was s e le c te d b eca u se i t had a re a s o n a b le ch a n ce o f s u c c e s s a nd was c o n s id e r a b l y c h e a p e r th a n any o f th e o t h e r m e th o d s. The e s tim a te d c o s t (1 9 7 9 ) f o r c h e m ic a l tr e a tm e n t was i n th e r e g io n o f 20 000 US D o lla r s w h e re a s th e o t h e r m e th o ds w o u ld c o s t a t l e a s t 15 tim e s m o re. SELECTION AND QUANTITY OF CHEMICALS The p h ilo s o p h y o f c h e m ic a l tr e a tm e n t v o lv e d tw o d i s t i n c t f a c e ts v i z : A c a lc iu m r i c h s a l t w o u ld be u sed t o e f f e c t b a se e xchange w it h th e Na r i c h s o i l and h e n ce r e n d e r th e m a t e r ia l non d is p e r s iv e. The e l e c t r o l y t e c o n c e n t r a t io n o f th e r e s e r v o i r w a te r w o u ld be m a ta e d a t a h ig h l e v e l f o r a m o n ito r e d p e r io d. T h is l a t t e r e f f e c t was c o n s i d e re d p r u d e n t t o i n h i b i t d is p e r s io n f o r a p e r io d o f tim e w h ile b ase e xchange and s w e l l g was t a k g p la c e. V a rio u s c h e m ic a ls w ere c o n s id e r e d and i t was d e c id e d to use c o m m e r c ia lly a v a i la b le gypsum (C as0^ 2H20) due t o i t s r e l a t i v e l y h ig h s o l u b i l i t y i n a h ig h ph e n v iro n m e n t, a v a i l a b i l i t y and re a s o n a b le on s i t e c o s t (1 8 US D o lla r s p e r t o n ). C h e m ic a l t e s t s on th e s o i l c o m b e d w it h th o s e done p r e v io u s ly s u g g e s te d th e f o l l o w i n g a v e ra g e v a lu e s : ESP CEC B o rro w M a t e r ia l 25 13 B a s M a t e r ia l 10 27 I t was d e c id e d t o re d u c e th e ESP o f th e c la y u sed i n th e b re a c h r e p a i r and a t h ic k n e s s o f 250 mm o f b a s f l o o r t o 5. The q u a n t i t y o f gypsum was c a lc u la t e d u s g a m e th o d p ro p o s e d i n th e U n ite d S ta te s S a l i n i t y L a b o r a to r y P u b lic a t i o n (U SSL, 1 9 6 9 ). I f i t i s r e q u ir e d t o re d u c e th e ESP fro m 25 to 5 i t w i l l be n e c e s s a ry t o r e p la c e th e f o ll o w i n g q u a n t i t y o f e x c h a n g e a b le so d iu m io n s. Na = 20 x 13 = 2,6 0 m e/100 gm 100 From T a b le 6 (U SSL, 1 9 6 9 ) and f o r a s o i l a t a d e n s ity o f 1700 k g /m 3 i t w i l l be n e c e s s a ry t o m ix 3,8 kg o f gypsum p e r c u b ic m e tre o f s o i l. In th e a bo ve p u b l ic a t io n i t i s s u g g e s te d t h a t m u l t i p l i c a t i o n f a c t o r o f 1,2 5 be u se d to co m p e n sa te f o r th e la c k o f e f f i c i e n t s o l u b i l i t y. Hence Q = 4,0 kg /m 3 f o r th e b a s m a t e r ia l a s i m i l a r c a lc u l a t i o n y ie l d s : Q = 2,5 kg /m 3 The m ethod a bo ve was c h e c k e d by u s g th e e q u iv a le n t w e ig h t o f gypsum and i d e n t i c a l q u a n t i t i e s w ere o b t a e d. W ith re g a r d to th e e le v a t io n o f e l e c t r o l y t e c o n c e n t r a t io n p h y s ic a l t e s t s w e re p e rfo rm e d t o ch e c k th e th r e s h o ld c o n c e n t r a t io n o f gypsum d i s t i l l e d w a te r f o r f l o c c u l a t i o n o f r e p r e s e n t a t iv e s o i l s u s p e n s io n s. B o th th e Em erson Crumb t e s t (E m e rs o n, 1 9 6 7 ) and a s u s p e n s io n t e s t g ave a gypsum c o n c e n t r a t io n o f 15 m e /1. The P h o le t e s t ( S h e r a r d, 1 9 7 6 ) on th e o t h e r hand g ave c o n t r a d i c t o r y r e s u l t s. A t room te m p e ra tu re w a te r w i l l h o ld no m ore th a n 28 m e /l o f gypsum i n s o l u t i o n. I t was th u s d e c id e d t o add t h i s q u a n t i t y o f gypsum t o th e i n i t i a l f i l l i n g o f 10^ o f r e s e r v o i r c a p a c ity. In sum mary th e f o ll o w i n g q u a n t i t i e s o f gypsum w ere a d d e d : ( i ) B re a c h r e p a i r a t a r a t e o f 4 kg /m 3 consum ed 26 to n s. ( i i ) 1 kg /m 2 was h a rro w e d i n t o th e b a s f l o o r and c la y b la n k e t c o n s u m g 192 to n s. ( i i i ) 0 kg /m 2 was s p re a d by hand o n to th e r i p - r a p on th e u p s tre a m fa c e u s g 125 to n s. (6 kg /m o f t h i s gypsum was sp re a d J u ly 1979. ) 787

( i v ) 2,4 kg /m 3 was added to 120 000 m3 (1 0 # o f c a p a c it y ) o f r e s e r v o i r w a te r u s g 290 to n s. A t o t a l q u a n t i t y o f 633 to n s o f gypsum was th u s u s e d. MONITORING OF LEAKS AFTER REINATEMENT The C lie n t was a d v is e d t o do a d a i l y s p e c t io n o f th a dam w h ile f i l l i n g was t a k g p la c e. Any le a k s w e re to be t r e a t e d as recom m ended i n an e m erg e n cy p ro c e d u re and r e p o r te d w it h o u t d e la y. Pum pg o f w a te r i n t o th e dam commenced i n D ece m b e r, 1970, and s to p p e d a t th e end o f J a n u a ry, 1979 when '\Q% o f th e c a p a c it y had been re a c h e d. I t was p la n n e d to c o n t u e w it h th e p u m p g o f w a te r a f t e r a b o u t 4 m o n th s. A t th e b e g n g o f May 1979 a w e t p a tc h was n o t ic e d a b o u t 20 m fro m th e to e o f th e em bankm ent tn d 200 m e a s t o f th e o r i g i n a l b r e a c h. Tha w a l l was damp o p p o s it e th e s e e p a g e. U a te r s a m p le s w e re ta k e n fro m s h a llo w p i t s th e w e t a r e a. Seepage w a te r was c le a r. Two s ta n d p ip e s w e re i n s t a l l e d, i n a t e s t h o le w h ic h was dug i n t o th e em bankm ent o p p o s it e th e w e t a r e a. No w a te r c o lle c t e d i n th e s e s ta n d p ip e s, one o f w h ic h e x te n d e d t o th e c la y c o re c u t - o f f. T h is le d t o th e c o n c lu s io n t h a t w a te r was le a k g th r o u g h th e fo u n d a tio n s o i l and n o t th e em bankm ent. Seepage c o n t u e d and c r e a s e d som ew hat b u t re m a e d c l e a r. S am ples o f w a te r w e re ta k e n a t r e g u la r i n t e r v a l s a t v a r io u s p o t s and c h e m ic a l t e s t s d o n e. The t e s t r e s u l t s a re su m m a rised i n T a b le I V. In Decem ber, 1979 pum pg a g a commenced and th e re s e r v o i r was f i l l e d t o 60% c a p a c it y. Seepage c r e a s e d and i n J a n u a ry a m arsh had d e v e lo p e d o v e r a 25 by 50 m a re a a ro u n d th e o r i g i n a l w e t p a tc h. Two s m a ll c h a n n e ls a b o u t 5 mm d ia m e te r w ere n o t ic e d is s u g a c o n t u o u s s tre a m o f c le a r w a t e r. Two b o i l s h ad d e v e lo p e d i n th e m arsh some 1,5 m i n d ia m e te r and w ere s l i g h t l y h ig h e r th a n th e s u r r o u n d g g ro u n d. I t was d e c id e d t o c o n s t r u c t an v e r t e d f i l t e r d r a o v e r th e m arsh c o n s i s t i n g o f a f i l t e r f a b r i c ( B id im U 2 4 ) f o llo w e d by 0,5 m o f s a n d s to n e g r a v e l. The o b j e c t iv e o f th e f i l t e r d r a was to p r e v e n t m e c h a n ic a l p ip g. A t th e tim e o f w r i t i n g t h i s r e p o r t th e r e s e r v o i r was 60% f u l l and seepage seems t o h ave s e t t l e d down to an a c c e p ta b le one h u n d re d l i t r e s p e r h o u r. ANALYSIS OF CHEMICAL RESULTS C h e m ic a l t e s t s w ere done on s o i l and w a te r s a m p le s and r e s u l t s a re a tta c h e d i n T a b le s I I t o V. The t e s t s on mud s a m p le s ta k e n fro m th e r e s e r v o i r b a s d ic a t e s t h a t th e ESP o f th e b a s s o i l h as d ro p p e d to b e lo w 5 as p la n n e d. T e s ts on th e r i v e r w a te r show t h a t th e TCC o f th e w a te r has c r e a s e d som ew hat fro m w h a t i t was 197 4, p e rh a p s as a r e s u l t o f se e p a g e fro m th e t r e a t e d c o n t e n t o f th e r e s e r v o i r. The SAR h as d ro p p e d to a b o u t 1,0. I t i s i n t e r e s t i n g to n o te th e h ig h v a lu e s r e g is t e r e d i n May 1979 and May 1900 when th e r i v e r i s n o t f lo w g d u r g th e d ry w t e r m o n th s. The w e ir a c ro s s th e r i v e r c o n s is t s o f a c o n c r e te s e c tio n and a c la y e m bankm ent. Seepage th r o u g h t h i s u n t r e a t e d em bankm ent ( v i a a f i l t e r ) h as been t e s t e d and th e r e s u l t s a re shown i n T a b le I I I. The c a t io n s i n th e r i v e r w a te r c r e a s e by m ore th a n 100$ on s e e p g th ro u g h th e embankm ent i n d i c a t i n g a le a c h g fro m th e s o i l l a t t i c e. The ESP o f th e w e ir em bankm ent s o i l i s th u s r e d u c g as t h i s se epage i s t a k g p la c e. I t i s i n t e r e s t i n g t o n o te how c o n s t a n t th e s e r e s u l t s h ave re m a e d th r o u g h o u t th e p e r io d o f m o n ito r g. The TCC o f th e w a te r th e dam c r e a s e d fro m 1 to 3,7 m e/1 i n th e p e r io d N ovem ber, 1974 t o A p r i l, 197 8. Soon a f t e r a d d g gypsum i t c r e a s e d t o 27 m e /1 r e d u c g r a p i d l y to 6,5 m e/1 May, 1979 and th e n s lo w ly to th e p r e s e n t v a lu e o f a b o u t 4 m e /1. Ca io n s a re r e l a t i v e l y h ig h a t 1,7 m e / l. ( T a b le V ) Seepage w a te r th r o u g h th e to e o f th e dam had a TCC as h ig h as 1 5,1 m e /1 (Na h ig h a t 1 3,0 and Ca lo w a t 0, 2 ) when i t was f i r s t m e asu re d i n May, 197 9. By S e p te m b e r th e TCC had re d u c e d to 10 m e /l (Na = 6,0 and Ca = 1, 6 ). The SAR had d ro p p e d fro m 1 2,0 t o 5,4. The w a te r has re m a e d b a s i c a l l y th e same s c e S e p te m b e r w it h one o f th e l a t e s t m e asu re m e nts b e g TCC = 1 1,3 m e /l (Na = 7,0 ; Ca = 1»B)» I t i s i n t e r e s t i n g to n o te th e d if f e r e n c e b e tw e e n th e w e ir w a l l seepage TCC = 6,0 m e /l (Na = 3,8 ; Ca = 1,3 ) and to e seepage TCC = 1 1,3 m e /l (Na = 7, 0 ; Ca = 1, 0 ), i n d i c a t i n g t h a t m ore th a n tw ic e th e Na c a t io n s a re b e g le a c h e d o u t o f th e s o i l l a t t i c e by th e t r e a t e d w a te r th a n th e u n t r e a t e d w a t e r. (T a b le IU ) CONCLUSIONS On r e f i l l i n g th e r e p a ir e d r e s e r v o i r w a te r seeped th ro u g h th e d is p e r s iv e fo u n d a tio n s o i l s i n much th e same way as p r o b a b ly o c c u r r e d d u r g f i r s t f i l l i n g i n 1 97 4. H ow e ve r, f a i l u r e d id n o t r e - o c c u r w h ic h i s c o n s id e r e d due to th e f o l l o w i n g : ( a ) The t r e a t e d w a te r h as le a c h e d Na+ io n s fro m th e ++ s o i l l a t t i c e r e p la c g them w it h Ca io n s and r e d u c g th e d is p e r s iv e p r o p e r t i e s. ( b ) The d ose d r e s e r v o i r w a te r has i n h i b i t e d p ip g g iv g th e c la y tim e t o s w e ll and c a u s g seep a g e p a th s t o c lo g g w it h f l o c c u l a t e d p a r t i c l e s. Seepage fro m th e r e s e r v o i r i s r e d u c g w it h tim e and s e epage w a te r h as re m a e d c l e a r. I t i s th e A u th o r s ' o p io n t h a t th e w a te r i n a r e s e r v o i r can be tr e a t e d c h e m ic a lly t o re d u c e th e r i s k o f f a i l u r e by d is p e r s iv e p ip g i n h ig h ESP s o i l s. APPENDIX SYMBOLS AND FORMULAE ESP = E x c h a n g e a b le Sodium P e rc e n ta g e o f S o il = E x c h a n g e a b le S odium x 100 CEC w it h u n i t s e x p re s s e d i n me/100 gm CEC = C a tio n E xchange C a p a c ity SAR = S odium A b s o rp tio n R a tio on S o il = Na P o re w a te r o r R e s e r v o ir W a te r / Ca + Mg V 2 w it h u n i t s e x p re s s e d m e / l i t r e 788

TCC = T o t a l C a tio n C o n c e n tr a tio n w it h u n it9 e x p re s s e d i n m e / l i t r e TDS = T o t a l d is s o lv e d s o lid s i n ppm EC^ = E l e c t r i c C o n d u c t iv ity i n m illis ie m e n s / m e t r e REFERENCES E m e rson, W.W. ( 1 9 6 7 ). A c l a s s i f i c a t i o n o f s o i l a g g re g a te s b ase d on t h e i r c o h e re n c e w a t e r. A u s tr a l i a n jo u r n a l o f S o il R e s e a rc h No. 5 Oones & W agener ( 1 9 7 5 ). G e o te c h n ic a l v e s t i g a t i o n on dam f a i l u r e a t S e n e k a l. C o n s u lt g E n g e e rs r e p o r t to C a h i 4 G e rte n b a c h, 3 & W R e p o rt n o. G /2 /7 5 S h e ra r d, O u n n ig a n & D e c k e r ( 1 9 7 6 ). P h o le t e s t f o r i d e n t i f y i n g d is p e r s iv e s o i l s. ASCE SMFE J a n. 1976 S to n e, P. ( 1 9 7 7 ). D e s ig n and c o n s t r u c t io n p ro c e d u re s r e q u ir e d f o r c o n s t r u c t g e a r t h dams w it h d is p e r s iv e c la y s. U n p u b lis h e d MSc t h e s is t o UNSW S ydney A u s t r a li a U n ite d S ta te s S a l i n i t y L a b o r a to r y S t a f f (USSL 1 9 6 9 ). U n ite d S ta te s D e p a rtm e n t o f A g r ic u lt u r e C a t. No. 1.7 6 :6 0 TABLE I I CHEMICAL TES ON WATER RIVER WATER APRIL 1978 I*IAY 1979 DEC. 1979 1 4 /1 /8 0 1 2 /3 /8 0 2 8 /4 /8 0 8 /5 /8 0 PH 6,5 7,1 5 7,8 7, 1 6,7 7,3 7,9 EC ms/m 2 1,4 4 0,0 2 4, 3 2 3,2 2 1,7 4 4,8 5 3,6 TDS ppm 139 260 158 151 141 291 348 Na 0,8 5 3,4 0 1,2 7 1,3 1 1,4 3 2,7 7 3,4 3 K 0,4 9 0, 15 0,1 3 0,2 4 0,2 7 0,0 7 0,0 7 Ca 0,0 5 0,20 0,68 0,5 8 0,6 0 1,3 4 1,6 4 Mg 0,1 8 0,3 5 0,3 7 0,5 1 0,4 9 0,6 5 0,8 1 TCC m e/1 1,5 7 4,1 0 2,4 5 2,6 4 2,7 9 4,8 3 5,9 5 C1 0,20 0,5 0,4 3 0,5 9 0,4 2 0,3 6 0,3 9 HC03 1,20 3,6 0 2,0 4 1,8 0 2, 16 3,5 6 4,0 8 so4-0, 10 0, 18 0,4 3 0,22 0,6 3 0,9 0 TAC me/1 1,4 0 4,2 0 2,6 5 2,8 2 2,8 0 4,5 5 5,3 7 SAR 2,5 6,5 1,8 1,8 1.9 2,8 3,1 789

TABLE I I I IdEIRUALL SEEPAGE SEPT. 1979 DEC. 1979 14/1 /SO 10/1 /B O 1 2 /3 /8 0 2 0 /4 /8 0 0 /5 /8 0 PH 7,9 0 0,2 B.4 7,6 7,9 7,0 B.2 EC ms/m 4 0,0 5 4,0 5 3,0 5 3,2 5 4,5 5 4,4 5 6,1 TDS ppm 260 356 344 345 354 353 364 Na 3,9 2 3,0 6 3,5 0 3,5 8 3, BO 3,9 3 3,8 3 K 1,1 7 0,1 4 0,0 9 0,0 9 0,0 6 0,0 5 0,0 5 Ca 0,B 3 1,10 1,20 1,1 9 1,3 3 1,3 5 1,3 4 Mg 0,11 0,8 3 0,7 3 0,8 0 0,6 0 0,8 2 0,8 2 TCC me/1 6,0 3 6,01 5,6 8 5,6 6 5,9 9 6,1 5 6,0 4 C1 0,1 7 0,5 4 0,4 9 0,5 0 0,3 1 0,3 7 0,4 3 HC03 5,1 1 5,4 7 5,2 3 4,9 0 4,9 5 4,7 B 4,6 8 S04 0,4 8 0,1 9 0,10 0,3 0 0,4 6 0,5 8 0,6 0 TAC me/1 5,7 6 6,20 5,9 0 5,7 0 5,7 2 5,7 3 5,7 1 SAR 3,9 3,9 3,6 3,6 3,7 3,0 3,7 TABLE IV TOE SEEPAGE V/OL.= 2055 5 /7 9 UOL.= 33$ 9 /7 9 V 0 L.= 56% 1 2 /7 9 V/OL. = 65% 1 4 / l/ S 0 l/0 L.= 63% 10 /1 /B O V 0 L.= 63% V 0 L.= 60% SOUTH OF 1 2 /3 /8 0 BREACH 1/BO V 0 L.= 58% 2 0 /4 /0 0 UOL.= 50$ 20/4 /B O ph 6,9 5 7,5 7 8,2 8,2 8,1 7,7 8,2 0,1 7,8 ECw ms/m 2 1 4,3 8 9,8 9 8,8 9 5,1 1 2 6,4 9 5,3 1 0 1,3 100,8 8 7,8 TDS ppm 1391 583 641 617 820 618 657 654 570 Na 1 3,0 0 6,7 6 7,0 3 7,1 0 1 0,7 0 7,0 7 7,8 2 7,3 4 5,9 3 K 0,0 5 1,5 6 0,0 3 0,0 4 0,3 6 0,0 4 0,02 0,02 0,1 8 Ca 0,1 7 1,5 6 1,5 1 1,6 4 1,7 4 1,6 0 1,7 7 1,0 7 1,8 4 Mg 1,9 0 0,0 7 1,5 3 1,5 9 1,3 1 1,5 9 1,66 1,7 6 1,1 6 TCC m e /l 1 5,1 2 9.9 5 1 0,9 0 1 0,4 5 1 4,1 1 1 0,3 1 1,2 7 1 0,9 9 9,1 1 C l 0,7 5 0,3 8 1,1 9 1,10 1,7 0 1,22 0,9 3 1,01 3,1 8 C03 - - - - - - - - - HCO3 1 3,7 0 8,10 8,0 3 7,7 5 9,8 9 7,2 9 0,20 7,7 0 5,0 7 so4 0,8 0 0,9 6 1,7 7 1,86 2,10 1,9 0 1,9 5 2,2 4 0,3 2 TAC m e /l 1 5,2 5 9,4 4 1 0,9 9 1 0,7 9 1 3,6 9 1 0,4 1 11,00 1 0,9 5 8,5 7 SAR 12,0 5,4 6,3 5,7 8,7 5,6 6,0 5,4 4,8 790

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