The geology of the Lesotho Highlands Water Project with special reference to the durability of construction materials

Journal ofAfrican Earth Sciences, Vol. 16, No. 1/2, pp. 181-192, 1993.

0899-5362/93 $6.00+0.00 © 1993 Pergamon Press Ltd

Printed in Great Britain

The geology of the Lesotho Highlands Water Project with special reference to the durability of construction materials J. L. VANRooY and A. VANSCHALKWYK Department of Geology, Universityof Pretoria, Pretoria 0002, Republic of South Africa

Abstract- The LesothoHighlandsWater Projectis situated in the Kingdomof Lesothoand the adjoining north-eastern part of the Orange Free State Province of the RSA in an area underlain by Triassic and Jurassic basalts of the LesothoFormationand Triassic sandstones and mudrocksof the Karoo Sequence. The Project will consist of a series of dams and tunnels to convey water from the Lesotho Highlands to the industrial centre of the Republic of South Africa. The geologicalsettingof the project areaand someengineeringgeologicalpropertiesof the underlying stratigraphic horizons is discussed. The geotechnicalpropertiesof the differentrock types are discussedwith specialreferenceto their use as concrete aggregate. The durability of basalt is primarily determined by the amount of secondary smectite clays present in the rock. These clays occur as discrete grains or as intergrowths with other secondarymineralsdisseminatedthroughoutthe rock. The clays orig~ate from the deuteric alterationof primary glass, pyroxene, olivine and rarely plagioclase and also occur as infillings of amygdales. The durability of the sedimentaryrocks is a functionof their dimensionalchange with variability in moisture content and also the degree of cementationbetween grains.

INTRODUCTION

T h e K i n g d o m of L e s o t h o is a m o u n t a i n o u s c o u n t r y in s o u t h e r n Africa a n d is c o m p l e t e l y s u r r o u n d e d b y land. T h e l ar ge s t r e s o u r c e is t h e a b u n d a n c e of w a t e r a s a r e s u l t of a n n u a l rain- a n d snowfalls of a p p r o x i m a t e l y 1 0 0 0 m m . T h i s cont r i b u t e s to h a l f (150 m 3 s ~) of t h e flow of t h e S e n q u - / O r a n g e River, w h i c h is t h e l ong est a n d l a r g e s t river in S o u t h Africa. T h e m i n i n g of gold in t h e T r a n s v a a l h a s led to the development of the so-called PretoriaW i t w a t e r s r a n d - V e r e e n i g i n g (PWV) c o m p l e x as t h e i n d u s t r i a l c e n t r e of t h e R epubl i c of S o u t h Africa. A p p r o x i m a t e l y 60 p e r c e n t of t h e i n d u s t r i a l prod u c t i o n of S o u t h Africa is s o u r c e d f r o m the PWV area. T h e w a t e r d e m a n d of t h i s a r e a h a s b e e n satisfied b y t h e Vaal River s u p p l e m e n t e d b y w a t e r f r o m the Tugela basin and other water transfer s c h e m e s f r o m 1974. A m a j o r u n t a p p e d s o u r c e w a s t h e O r a n g e River w h i c h o r i g i n a t e s in t h e L e s o t h o H i g h l a n d s , a p p r o x i m a t e l y 3 0 0 k m to t h e s o u t h of t h e PWV area. P r o p o s a l s for t h e e x t r a c t i o n a n d gravity t r a n s f e r of w a t e r f r o m t h e u p p e r r e a c h e s of t h e O r a n g e River in L e s o t h o to t h e R e p u b l i c of S o u t h Africa (RSA) h a v e b e e n m a d e s i nc e t h e 1950's. In 1978 t h e two c o u n t r i e s a g r e e d on a j o i n t feasibility i n v e s t i g a t i o n for t h e L e s o t h o H i g h l a n d s W a t e r Project (LHWP).

T h e r e c o m m e n d a t i o n of t h i s i n v e s t i g a t i o n w a s to c o n s t r u c t t h e p r o j e c t in f o u r p h a s e s , over a p e r i o d of 25 y e a r s , d e p e n d i n g o n t h e w a t e r d e m a n d of t h e RSA. A final t r e a t y w a s s i g n e d b y t h e K i n g d o m of L e s o t h o a n d t h e R e p u b l i c of S o u t h Africa o n 2 4 O c t o b e r 1986 w h i c h a p p r o v e d t h e LHWP a n d r e s u l t e d in t h e c o m m e n c e m e n t of c o n s t r u c t i o n w o r k o n p h a s e IA of t h e project. T h e r e will be five big d a m s with a c o m b i n e d s t o r a g e c a p a c i t y of 6.5 k m 3, a h y d r o p o w e r s t a t i o n with a c a p a c i t y of 110 MW, t u n n e l s w i t h a c o m b i n e d l e n g t h of 2 2 5 k m a n d n e w a n d u p g r a d e d r o a d s totalling 6 5 0 k m at t h e c o m p l e t i o n of t h e proj ect (JPTC, 1991). T h e p r o j e c t a r e a is c o n f i n e d to t h e n o r t h e r n p a r t of t h e K i n g d o m of I_~sotho a n d t h e n o r t h e a s t e r n p a r t of t h e O r a n g e Free St at e, as i n d i c a t e d o n Fig. 1. T h e P h a s e IA desi gn s t a r t e d in 1986 a n d will i n c l u d e t h e c o n s t r u c t i o n of t h e K a t s e D a m in t h e M a l i b a m a t s o River, a 45 k m long T r a n s f e r T u n n e l 4. 95 m in d i a m e t e r f r o m K at se D a m to t h e Muela Hydro-electric p o w e r station, t h e M u e l a D a m w h i c h will be p a r t of t h e H y d r o - e l e c t r i c s c h e m e a n d a Delivery T u n n e l , 3 6 k m long f r o m t h e M u e l a D a m to t h e A sh River a t r i b u t a r y of t h e Vaal River. T h e c r e a t i o n of n e w i n f r a s t r u c t u r e , a s well a s t h e u p g r a d i n g of existing r o a d s will also be u n d e r t a k e n d u r i n g t hi s p h a s e . T h e K a t s e D a m will be o n e of t h e l argest d a m s in t h e S o u t h e m H e m i s p h e r e a n d ,

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Fig. 1. Locality plan of the LHWP area. with a height of 180m, it will be the highest in Africa. The level of interest in the geotechnical properties and b e h a v i o u r of the Karoo Sequence rocks h a s increased dramatically since the LHWP became a reality. Concern a b o u t the durability of the Lesotho basalts, Karoo dolerites, s a n d s t o n e s and m u d r o c k s existed, due the k n o w n presence of secondary clay minerals in these rocks and w a s further underlined due to (a) the observation of hair-line cracks in the basalts, which sometimes r u n through the amygdaloidal zones of the specimens causing a reduction in strength, (b) the previous experience with slaking m u d r o c k s during the construction of t h e O r a n g e - F i s h t u n n e l a n d (c) i n s u f f i c i e n t knowledge of the geotechnical properties of the sandstones. GEOLOGY OF T H E LHWP

The project area is located in the Great Karoo Basin, a large shallow b a s i n of mainly sedimentary rocks that were deposited from 200 million years to a b o u t 160 million years ago. The formation of the b a s i n took place while Africa w a s still part of the supercontinent, Gondwanaland, which included m o s t of the present day l a n d m a s s e s of Africa, South America, India, Australia, New Zealand and Antarctica.

During the formation of the Karoo b a s i n the climate changed markedly from glacial conditions w h e n the Dwyka tillites were deposited through temperate to desert conditions w h e n the Clarens Formation of wind blown s a n d w a s deposited in extensive s a n d y d e s e r t s (Eriksson et al., this issue). Sedimentary

Rocks

The present day r e m n a n t of the Karoo Basin underlies the whole of Lesotho as well as a b o u t 75 per cent of the surface area of the RSA. This basin is a near horizontal layered sequence of sedimentary rocks of continental origin capped by thick flood b a s a l t s of the Lesotho Formation, also referred to as the D r a k e n s b e r g Basalts (Fig. 2). Only the u p p e r m o s t formations of the sedimentary rocks of the Karoo Sequence are of importance here, since m o s t of the delivery t u n n e l s and the Muela Dam and Hydro-electric Scheme will be constructed in these horizons. These strata are referred to, from the top downwards, as the Clarens, Elliot and Molteno Formations and the Tarkastad S u b g r o u p of the Beaufort Group. A general~ed stratigraphic column is presented in Fig. 3. During the delivery tunnel investigations of the sedimentary rock sequences, the m u d r o c k s were classified as siltstones, clayey siltstones, silty clay-

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s t o n e s a n d c l a y s t o n e s d e p e n d i n g on the percentages of silt a n d clay (Table I). The s a n d s t o n e s are g e n e r a l l y c l o s e l y interc a l a t e d with m u d r o c k s . The T a r k a s t a d S u b g r o u p is m o r e argillaceous t h a n t h e overlying Molteno F o r m a t i o n a n d t h e c o n t a c t a p p e a r s to b e u n c o n f o r m a b l e . Two s o u r c e s were active, a delta in t h e s o u t h n e a r E a s t L o n d o n which d e p o s i t e d the s a n d s t o n e facies, a n d a n o t h e r s o u r c e to the e a s t of Mooi River in Natal (Dingle et al., 1983). This latter s o u r c e w a s less significant a n d p r e d o m i n a n t l y argillaceous s e d i m e n t a c c u m u l a t e d b e t w e e n t h e two areas. The T a r k a s t a d S u b g r o u p s a n d s t o n e s were deposited in fluvial a n d deltaic e n v i r o n m e n t s a n d t h e m u d r o c k s r e p r e s e n t low energy alluvial fiat sedim e n t s (Dingle et al., 1983). The high s a n d s t o n e to m u d r o c k ratio e n c o u n t e r ed n e a r the D e l i v e r y T u n n e l outfall, p o s s i b l y reflect interfmgering b e t w e e n the Katberg s a n d s t o n e facies a n d t h e B u r g e r s d o r p m u d s t o n e facies of t h e T a r k a s t a d S u b g r o u p . In this zone of interflngering, both continuous and discontinuous sandstones, m a y be present. Particularly d i s c o n t i n u o u s s a n d s t o n e s d e p o s i t e d b y s t r e a m s , in t h e zone of transition from b r a i d e d to m e a n d e r i n g s t r e a m s might b e expected. The Molteno F o r m a t i o n c o n s i s t s of r o c k t y p e s ranging from m u d s t o n e s to c o a r s e s a n d s t o n e s a n d is c h a r a c t e r i z e d b y m e d i u m a n d c o u r s e , b e d d e d / c r o s s - b e d d e d a n d l a m i n a t e d s a n d s t o n e s with s u b -

J. L. VANRooy and A. VANSCHALKWYK

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Table 1. Classification of m u d r o c k s Mudrock Type

%Clay % Silt

Siltstone 0 - 33 100-67

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(After HDTC, 1990 ordinate m u d s t o n e s . This f o r m a t i o n f o r m s a pred o m i n a n t l y s a n d s t o n e w e d g e with a t h i c k n e s s of 4 8 0 m in t h e s o u t h , t h i n n i n g to a b o u t 30 to 40 m in t h e Delivery T u n n e l outfall a r e a (HDTC, 1990). T u m e r (1980) s t a t e d t h a t t h e c h a r a c t e r i s t i c s of this s e q u e n c e are c o n s i s t e n t with d e p o s i t s of braided s t r e a m s draining a n alluvial plain on the distal s l o p e s of a w i d e s p r e a d alluvial f a n s y s t e m or series of coalescing alluvial fans. E r i k s s o n (1984) s u p p o r t e d t h e above c o n c l u s i o n a l t h o u g h he envis a g e d t h e b r a i d e d s t r e a m s to b e c h a r a c t e r i z e d b y migrating b a r s w h i c h w e r e often r e w o r k e d into s a n d fiats during low river flow stages, in c o n t r a s t to t h e d o m i n a n c e of i n - c h a n n e l d u n e b e d f o r m s d e s c r i b e d b y Turner. The generally finer grained nature and different bedforms described by E r i k s s o n , reflect a generally lower energy environm e n t in t h e Delivery t u n n e l area, w h i c h w o u l d b e e x p e c t e d n e a r t h e distal m a r g i n of t h e p r o p o s e d alluvial p l a i n e n v i r o n m e n t . The finer g r a i n e d nature and interbedded mudrocks described by E r i k s s o n are in c o n t r a s t to the findings b y Turner, b u t c o n s i s t e n t with t h e findings of the Delivery T u n n e l investigations (HDTC, 1990). The Elliot F o r m a t i o n lies c o n f o r m a b l y above the Molteno F o r m a t i o n a n d c o n s i s t s m a i n l y of m u d r o c k s i n t e r b e d d e d with fine a n d m e d i u m grained s a n d s t o n e s . It is often difficult to define the b o u n d ary b e t w e e n t h e Molteno a n d Elliot F o r m a t i o n s . Visser a n d B o t h a (1980) identified t h r e e different depositional e n v i r o n m e n t s a n d classified the s a n d s t o n e s into f o u r different types, e a c h c h a r a c t e r izing a different m o d e of deposition. T h e y interp r e t e d a lower zone, r e p r e s e n t i n g a m e a n d e r i n g c h a n n e l a n d flood plain facies, a middle flood b a s i n facies a n d a n u p p e r flood fan a n d d u n e facies. This s u c c e s s i o n p r o b a b l y i n d i c a t e s a d e c r e a s e in depositional energy with time a n d t h e e s t a b l i s h m e n t n e a r t h e top of p r e d o m i n a n t l y aeolian conditions. There are s o m e s t r o n g similarities in the s u c c e s sion, in t h e Delivery T u n n e l area, as far a s t h e a l t e r n a t i o n of m u d r o c k a n d s a n d s t o n e rich facies are c o n c e r n e d (HDTC, 1990). There are s o m e c o m m o n e l e m e n t s in the p o s t u lated d e p o s i t i o n a l e n v i r o n m e n t s p r o p o s e d b y Kitching a n d R a a t h (1984) a n d E r i k s s o n (1985) with t h a t of Visser a n d B o t h a (1980). T h e y all indicate a c h a n g e from a fluvial d o m i n a t e d to

f l u v i a l / a e o l i a n mixed facies a n d reflect a n increasingly arid climate with t h e o n s e t of t h e deposition of the C l a r e n s Formation. The Clarens F o r m a t i o n again c o n f o r m a b l y overlies the Elliot F o r m a t i o n with a s o m e t i m e s o b s c u r e b o u n d a r y d u e to a g r a d a t i o n a l c h a n g e in r o c k types. It c o n s i s t s p r e d o m i n a n t l y of fine grained s a n d s t o n e s with s u b o r d i n a t e , b u t still significant siltstone and medium grained sandstone. Occasional mudstone horizons occur towards the b a s e of the Formation. B e u k e s (1970) divided t h e C l a r e n s F o r m a t i o n into t h r e e zones, n a m e l y a b a s a l zone d e p o s i t e d u n d e r s e m i - a r i d c o n d i t i o n s , t h e m i d d l e zone formed u n d e r p r e d o m i n a n t l y arid c o n d i t i o n s a n d the u p p e r zone f o r m e d d u r i n g a t r a n s i t i o n from arid to semi-arid conditions. Analysis of grain size, s h a p e a n d sorting s u g g e s t s t h a t t h e C l a r e n s s a n d s t o n e is of aeolian origin. E r i k s s o n (1981; 1986) d o e s n o t d i s t i n g u i s h the three z o n e s d e s c r i b e d b y B e u k e s b u t identifies four s e d i m e n t a r y facies. The p a l a e o - e n v i r o n m e n t in t h e Delivery T u n n e l area s e e m s to c o r r e s p o n d with Facies 4 (HDTC, 1990). This facies i n c l u d e s r o c k t y p e s varying from s i l t s t o n e s to m e d i u m - g r a i n e d s a n d s t o n e s with c o a r s e material a n d m u d s t o n e s c o n s p i c u o u s l y a b s e n t . Massive b e d s a n d s u b o r d i n a t e p l a n a r stratification are i n t i m a t e l y a s s o ciated. An aeolian d u n e i n t e r p r e t a t i o n is f a v o u r e d for Facies 4. E r i k s s o n (1984) r e g a r d e d the p l a n a r b e d d i n g a s r e p r e s e n t i n g t h e b o u n d i n g s u r f a c e s of c r o s s - b e d s e t s in w h i c h t h e internal c r o s s - s t r a t a are not visible or were d e s t r o y e d b y s u b s e q u e n t rainfall. F r o m p e t r o g r a p h i c w o r k b y B e u k e s (1970) a n d E r i k s s o n (1981) it c a n b e c o n c l u d e d that t h e s a n d s t o n e s of t h e C l a r e n s F o r m a t i o n c o n t a i n b e t w e e n 35 - 90 p e r c e n t quartz, u p to 20 p e r c e n t feldspar a n d a m a t r i x w h i c h is p r e s e n t in variable a m o u n t s from 10 to 60 p e r cent. The m a i n c o m p o sitional difference is therefore t h e q u a r t z / m a t r i x ratio. Calcite is p r e s e n t a s a c e m e n t i n g agent a n d the m a t r i x is r e p o r t e d to b e f e r r u g i n o u s chloritic m u d (Eriksson, 1981) or illite with m i n o r a m o u n t s of kaolinite, m o n t m o r i l l o n i t e a n d chlorite (Beukes, 1970). During t h e t u n n e l investigations it w a s found that interstratified clay minerals with swelling c o m p o n e n t s are p r e s e n t in m o s t s a m p l e s . Poor durability a n d high erodibility c a n therefore

The geology of the Lesotho Highlands Water Project with special reference to the durability ... be expected although the massive erosion resistant cliffs In this Formation contradicts this conclusion. There was a period w h e n desert and lava flow environments existed side by side. Volcanic tufts a n d a g g l o m e r a t e s are also c o m m o n l y f o u n d between or interbedded with the u p p e r Clarens Formation a n d the lower basalt flows.

Igneous Rocks Katse Dam, the upgraded and new infrastructure, as well as the t r a n s f e r tunnel will be situated in the Drakensberg basalts and associated dolerite dykes. During the period extending from the late Triassic, t h r o u g h the J u r a s s i c a n d into early Cretaceous, the fragmenting Gondwanaland s u p e r c o n t i n e n t was subjected to an igneous event of huge proportions (Irwin et aL, 1980; Hawkesworth eto3., 1983; Bristow and Saggerson, 1983; Eales et aL, 1984; Melvill et al., 1989 and Eriksson et al., this issue). This is referred to as the Karoo flood basalt province. The D r a k e n s b e r g basalts are intersected by dolerite dykes which formed the c h a n n e l s along which the lava was fed to the surface. They are t h u s about the s a m e age as the basalts. Some of the dykes extend to relatively high elevations in the basalt. Other features that transect the strata and occur in the region, are diatremes, (which are roughly circular vents which formed feeders for the lava), and the younger, diamond-bearing kimberlite pipes. The Drakensberg lavas consist almost exclusively of basalt, petrographically very similar to the Karoo dolerites. Many of the lavas of the thicker flows are often coarse grained and can often be distinguished from Karoo dolerites only by the presence of small amygdales in their upper and lower portions, although some dolerite sills might also contain amygdales. The basalt in Lesotho is referred to by some as the Drakensberg Formation or Group (SACS, 1980) and by others as the Lesotho Formation. Following the s e p a r a t i o n of the continental plates, periodic uplift and erosion has occurred and a n u m b e r of erosional cycles can be identified along the e s c a r p m e n t of the Lesotho plateau, m u c h of which is above 2500 m a m s l (Melvill et al., 1989). The plateau is dissected by a n u m b e r of narrow river valleys up to 500 m deep, that drain southw a r d s a n d join up to become the Senqu River, k n o w n as the Orange River in South Africa. The highest part of the plateau h a s been identified as part of the G o n d w a n a planation, the earliest erosional cycle in Africa. Other phases of the erosional process can be identified as abandoned

185

local floodplains, oxbow loops a n d bedrock ledges. Some of these features contain relatively thick deposits of gravels and s a n d y clays. The progress of the erosional cycles up the river valleys h a s in some cases c a u s e d the formation of spectacular waterfalls (Melvill et al., 1989). Despite having b e e n eroded laterally, about 200km in 140 million years (an average of 10 m m every 6.5 years), the Drakensberg lavas are relatively resistant to erosion (Irwin et al., 1980). The present scenery of the Drakensberg h a s b e e n carved and sculptured primarily by flowing water. The area is dissected and well drained by n u m e rous streams and the surface run-off is very high because of the steep relief. The disintegrated rock and weathered minerals are removed practically as fast as they are formed. In present times, ice plays a role only as far as the alternating freezing and thawing action in cracks and other openings is concerned. The area is extremely cold during the winter months. The m o u n t a i n tops, where black clays occur, are covered by snow for long periods during winter. The s u m m e r s have a mild temperate climate. The precipitation varies from 896 to 1152 m m per annum.

The depth of weathering of the basalt varies from less t h a n ten metres on the u p p e r slopes, to more t h a n 25 metres at the bottom of the valleys. The more advanced state of weathering in the lower slopes is partly due to the more frequent occurrence of inclined open stress- relief joints, c a u s e d by the removal of load as the river valley cuts downward, and to the increased water seepage n e a r river level. In the m a i n part of the lava pile, the basalt flows are mostly evenly superposed. The average thickness of the flows is about 6 metres (OSC (Olivier S h a n d Lahmeyer MacDonald Consortium) 1986) although the median value of 3.2 m a n d the upper and lower quartile values of 6.2 m and 1.4 m are more indicative of the u s u a l flow thickness (Galliers et al., 1991). The flows exhibit t h e s t r u c t u r a l f e a t u r e s characteristic of intermediate a n d basic volcanic outpourings. The base of nearly every flow is m a r k e d by tubular, coalescing pipe amygdales produced by the flow of bubbles of gas from the chilled surface of the previous flow through the viscous cooling material. The zoning within each lava flow h a s been described in detail by a n u m b e r of a u t h o r s (du Toit, 1939; Bristow et al., 1983; OSC, 1986; Melvill et aL, 1989; Lesotho Highlands Consultants, 1989; Van Rooy, 1992; Galliers etal., 1991). In aUbut the thinnest flows, a vertical zonation based on the f u n d a m e n t a l division of the basalts into amygdaloidal and non-amygdaloidal varieties is evident.

186

J. L. VANRoov and A. VANSCHALKWYK

Thin flows are typically amygdaloidal t h r o u g h o u t b u t the thick ones consist typically of a massive, fine to relatively coarse grained, central zone b o u n d e d at the top and b o t t o m by amygdaloidal zones. These non-amygdaloidal basalt zones show the m o s t p r o m i n e n t d e v e l o p m e n t of d a r k disseminated clayminerals, consisting, at least partly, of smectite clay minerals. These clay minerals originate from the deuteric alteration of primary interstitial glass and pyroxene (Van Rooy and Nixon, 1990) or the filling of small angular voids in the rock as a result of secondary deposition from circulating g r o u n d water. Five different types of basalt have b e e n identified b y Van Rooy (1992) b a s e d on the visible amygdale content and the presence of dark disseminated clay m i n e r a l s . The a m y g d a l o i d a l t y p e s were further subdivided into slightly and moderately to highly amygdaloidal b a s a l t s (Table 2). The D r a k e n s b e r g b a s a l t s are typically tholeiitic in appearance, containing little olivine and up to 25 per cent of interstitial glass. Although the rocks rarely appear porphyritic in the field, microphenocrysts ofplagioclase and altered olivine are almost always present in the chill phase at the b a s e of flows. Augite is the dominant pyroxene and is accompanied in m a n y rocks by pigeonite which m a k e s up a b o u t a quarter of the total pyroxene. Plagioclase is the m o s t a b u n d a n t mineral. Zeolites are very widely present in amygdales. Deuteric alteration includes the p s e u d o m o r p h o u s replacement of olivine, the localized b u t intensive attack on the plagioclase, and the replacement partial in some places, complete in others - of the original glass. Chlorite is a c o m m o n product of primary mineral alteration with accompanying carbonates, and montmorillonite is a c o m m o n product in vesicles of basalt which have been subjected to the actions of solutions and gases originating from a cooling basaltic magma. The minerals p r o d u c e d by the post-consolidation p r o c e s s e s are of key importance to the durability of the basalts.

Basic m a g m a which solidified within the fissures along which the basaltic lavas were emplaced form the present day dolerite dykes, sills a n d plugs. Both the b a s a l t s and dolerites are of the tholeiitic m a g m a type. The dykes range in t h i c k n e s s from 2 to 6 m and m a y be traceable for several kilometres (HDTC, 1990). GEOTECHNICAL PROBLEMS ASSOCIATED WITH THE DIFFERENT ROCK TYPES Mudrocks

The m u d r o c k s of the Karoo Sequence are renowned for their rapid deterioration on exposure. The most obvious c a u s e of poor durability is usually the mineralogical composition of a rock. Research by Olivier (1979a) and Venter (1980) indicated that these rocks contain predominantly stable lattice illite, quartz a n d feldspar a n d only small quantities of chlorite, hematite, calcite and mixed layered montmorillonite-illite. The swelling clays representing less t h a n 2 per cent of the clay sized material. Venter found, contrary to Olivier, that quartz is the m o s t a b u n d a n t mineral. The mineralogical analyses of the m u d r o c k s indicate that expansive clay minerals are likely to be a b s e n t or present in insignificant amounts. Therefore no correlation b e t w e e n the mineralogy and durability w a s found by the researchers as the m u d r o c k s are expected to be of the non-expandable group of rocks. Venter and Olivier also indicated that m u d r o c k s with almost identical mineralogy and grain size can display markedly different durabilities. Most of the m u d r o c k s , which a p p e a r m a s s i v e and showing no colour or grain size laminations, m a y have a very well-developed fabric due to the orientation of platy minerals and flattening of flocs during compaction. The m u d r o c k s show an anisotropic free swell behaviour with the swelling not the result of the presence of swelling clays. When either through natural erosion or m a n - m a d e excavations, a rock

Table 2. Basalt types TYPE

DESCRIPTION

DARK CLAYMINERALS

AMYGDALES

I

Dense basalt

Not present

Not visible

II

Dense basalt

Present

Not visible

III

Amygdaloidal

Not present

Amygdales present

IV

Slightly amygdaloidal

Present

< 10% amygdales

V

Moderately to highly

Present

> 10% amygdales

amygdaloidal (After Van Rooy, 1992)

The geology of the Lesotho Highlands Water Project with special reference to the durability ... m a s s is unloaded, a volume increase or swelling occurs (Duncan et al., 1968). The m e c h a n i s m of b r e a k d o w n of the m u d r o c k s is not yet fully understood. The process of "air breakage" or slaking is usually described as the c a u s e of disintegration in m u d r o c k s (Brink, 1983). This process r e d u c e s the rock to silt- and clay-sized particles. Venter recognized that m a n y m u d r o c k s also b r e a k down to hard gravel-sized fragments and suggested t h a t this m a y be c a u s e d by the development of micro-cracks as a result of moisture loss or stress relief. When this exposed and air dried materials come into contact with water and again expand to varying degrees, they b r e a k along these w e a k n e s s planes. Olivier also considered changes in t e m p e r a t u r e and humidity to be the main c a u s e s of disintegration of mudrock. He concluded that the large variation in the durability w a s mainly due to the textural differences as well as the presence of micro-cracks and incipient slickensided micro-joints (Olivier, 1973). At the conclusion of the feasibility studies OSC (1986) concluded that the propensity for deterioration of m u d r o c k s increases with clay content and the degree of drying and that the rate and degree of drying and that the rate and degree of deterioration is related to the free swell potential of the rock. Following extensive studies during the design stage, HDTC (1989) found that the free swelling behaviour of m u d r o c k s is almost entirely a function of initial moisture content which, in turn, can be correlated with the clay content. It was suggested (HDTC, 1989) that initial (field) moisture content o f m u d r o c k s be u s e d as a rapid indication p a r a m e t e r of its potential for deterioration. Sandstones

S a n d s t o n e s are u s u a l l y more durable t h a n m u d r o c k s and problems such as slaking is uncommon. Karoo s a n d s t o n e s are not usually suitable for use in concrete or as road building material and some problems have arisen in s a n d s t o n e s u s e d as building stone. As the s a n d s t o n e s might be exposed in unlined t u n n e l s or u s e d as concrete or road aggregates, as filters, rip-rap and rockftll, their durability is of importance. Mineralogy is evidently the m o s t important factor influencing the durability of sandstones. Olivier (1979b) f o u n d the fine grained, well c e m e n t e d s a n d s t o n e s a n d s i l t s t o n e s of t h e T a r k a s t a d S u b g r o u p to be of good quality. These rocks consist of detrital quartz and feldspar in a matrix of fine silt and partially recrystallized clay material. The poorer durability s a n d s t o n e s were usually coarse-grained, friable, weakly cemented with again quartz and feldspar as the main constituents b u t with the important difference that

187

the feldspar w a s more a b u n d a n t and often altered to kaolinite and sericite. Interstitial clay and calcite form the matrix. The s a n d s t o n e s of the Karoo sequence, with some exceptions in the Molteno Formation, are characterized b y having a high matrix clay content. The clay is mostly s e c o n d a r y derived from the b r e a k d o w n of rock fragments and alteration of feldspars. The clays are usually non-expansive but, like in the mudrocks, the clays will absorb water resulting in small volume changes. The process is also reversible, resulting in shrinkage. Excessive shrinkage occurs in concrete containing Karoo s a n d s t o n e s due to the dimensional changes occurring within the aggregate. Roper (1959) postulates that the s a n d s t o n e aggregates, having a high specific surface, absorb water from the concrete mix and expand. The workability of the mix is usually affected by this absorption and an excess of water is added. When drying occurs, shrinkage of the cement and the aggregate results in a very high overall shrinkage. S u b s e q u e n t wetting and drying takes place from the surface inwards and c a u s e s differential strains which result in cracking. The deterioration of s a n d s t o n e building stone can be attributed to the dimensional changes r e s u l t i n g from v a r y i n g m o i s t u r e c o n d i t i o n s . Swelling is u s u a l l y anisotropic a n d slabbing occurs parallel to bedding. If the bedding fabric is weak and the matrix h a s a high clay matrix content and little cementation, the surface grains become loose. This is sometimes accompanied by shrinkage cracking (OSC, 1986). Detailed petrographic and geotechnical testing of Clarens s a n d s t o n e h a s b e e n u n d e r t a k e n to a s s e s s its durability in an u n l i n e d t u n n e l (Lesotho Highlands Tunnel Partnership LHTP, 1992). Wellsorted, even grained s a n d s t o n e s with dense grain packing are characterized by fusion along grain b o u n d a r i e s and Uniaxial Compressive Strengths greater than 40 MPa. These rocks proved to be highly resistant to various durability tests including wetting and drying cycles, ethylene glycol soaking, brushing and water spraying. In a test adit where the s a n d s t o n e s were found to have deteriorated, the grain packing was found to be loose and with point contact or no contact at all. These s a n d s t o n e s occupy less t h a n 6 per cent of the tunnel length and will require lining. All the above processes can be expected to contribute to the rapid weathering and deterioration of the Karoo s a n d s t o n e s w h e n u s e d as rip-rap, rockfill, concrete aggregate and in road building. High durability s a n d s t o n e h a s less clay content a n d / o r stronger cementation to resist the tendency to swell while s a n d s t o n e with significant a m o u n t s of clay m a k e s unsatisfactory concrete aggregate due

188

J. L. VANRooY and A. VANSCHALKWYK

to the excessive water d e m a n d and drying shrinkage. Igneous rocks The geotechnical problems in basalt and dolerite are largely related to their mineralogical compositions a n d textures. Swelling clay minerals of the montmoriUonite group occur in m o s t of the Drakensberg basalts and some dolerites, b u t in widely varying concentrations and with several m o d e s of occurrence. The d o m i n a n t clay mineral is montmorillonite, which, in the basalts, is formed mainly by deuteric alteration of olivine and interstitial glass (probably immediately following the solidification of the rock) and by deposition in vesicular cavities and gasfilled voids from fluids percolating through the cooling rock. Ethylene glycol soak tests indicated that the expansive n a t u r e of the montmorillonite would c a u s e degradation of m a n y of the basalts, although m a n y rocks were apparently unaffected even by prolonged glycolation. The mineralogical investigation of both basalts and dolerites revealed the presence of minerals and a m o r p h o u s p h a s e s considered to be potentially deleterious to concrete made of high-alkali cement (OSC, 1986; Melvill et al., 1989). Oberholster (1978) noted that basalt with chalcedony, opal and palagonite are potentially deleteriously reactive with alkalies in cement. The CSIR (1987) u n d e r t o o k accelerated tests which indicated that although the presence of volcanic glass usually implied potential alkali- reactivity, the basalt samples tested were innocuous to the reaction. The zeolites present in the basalts m a y however effloresce on the concrete surface. The following materials, identified in the basalts, are also listed in the s t a n d a r d specification of the South African Bureau of S t a n d a r d s (SABS, 1083-

1976) as potentially deleterious to concrete aggregate: Clay minerals (both expansive and nonexpansive) Glass (in its original state or devitrified) Calcite Zeolites Silica minerals (quartz, opal, chalcedony. although fairly rare in the basalts). The distinction between dolerite a n d basalt is of importance in the selection of aggregate. It appears that the reliance on a basic petrographic description (i.e. dolerite or basalt) as the principle m e a n s of distinguishing between s o u n d and u n s o u n d aggregate is unwise a n d that the a s s u m p t i o n that dolerites constitute high quality aggregates is not necessarily valid (Orr, 1979). Orr (1979) described rapid-weathering SouthAfrican dolerites which behave similarly to dolerites in t h e U n i t e d K i n g d o m , L e s o t h o a n d Mauritius and to smectite-bearing basalts and gabbros in the Western USA. Black and d a r k g r e e n soft clay infilling of joints and within ill-defined patches caused rapid weathering of rock masses. Dolerite core samples were also found to b r e a k up along joints with smectite and chlorite infllling and later disintegrated to a powder in the core boxes, while surrounding portions of unaltered dolerite showed no visible signs of deterioration. SOME TYPICAL TEST RESULTS FROM LHWP INVESTIGATIONS Sedimentary Rocks

The results from the feasibility stage investigations showed that consistent engineering properties could be assigned to specific identifiable geologic horizons. Some test results are summarized in Table 3.

Table 3. Typical test results for sedimentary rocks TEST

CLARENS Sl

% Swelling Clay UCS {MPa} Braz. Tensile S t r e n g t h (MPa) E-Mod (GPa) Density ( k g / m 3) Apparent c o h e s i o n (MPa) Friction angle Schmidt Hammer Hardness

ELLIOT

S

C

SIC

MOLTENO BEAUFORT Tarkastad} CSl

Sl

I

S

S

C

I SIC

ICSl

C = 3.4 - 26.3; SIC = 5.7 SI = 0.3 - 7.3; S = 1.6 (average). 52.6 4.4

95.7 6

13.3 1.8

18.1 2.5

17.6 5.2

10.6 2503 0.05

17.0 2403 0

2.9 2529

3.5 2563 0.047

2.1 2342 0.033

26

24

32

34 3O

Abbreviations: UCS - U n c o n f i n e d c o m p r e s s i v e s t r e n g t h CSI - clayey s i l t s t o n e (After HDTC, 1990)

28.9

2580

35.1

I 1.7 1.6

15.2 2465

10.3 2251

2.6 2530

23 30

S - sandstone SIC - silty elaystone

66.7 4.4

40

23

Sl - siltstone C - claystone

29.2 2.0 4.3 2580 0.0540 32

18.3 2.9 4.4 2630

The geology of the Lesotho Highlands Water Project with special reference to the durability ... As c a n be expected t h e more a r e n a c e o u s rocks s h o w h i g h e r s t r e n g t h s a n d lower clay p e r c e n t a g e s t h a n t h e argillaceous rocks. The r e s u l t s of s o m e of t h e concrete aggregate t e s t s are p r e s e n t e d in Table 4. The fine-grained sandstones from the Clarens and Elliot F o r m a t i o n s were f o u n d to be u n s u i t a b l e a n d t h e s e s o u r c e s were not investigated in detail. Prelimi n a r y investigation of t h e coarse g r a i n e d Molteno F o r m a t i o n s a n d s t o n e s indic ated promising r e s u l t s a n d detailed t e s t s were done on two s o u r c e s in t h i s formation. HDTC (1990) r e c o m m e n d e d t h a t t h e c r u s h e r s a n d from t h e Molteno F o r m a t i o n be u s e d only as a b l e n d i n g s a n d after w a s h i n g .

Table 4. Typical test results from washed crusher sand MOLTENO CRUSHER SAND

TEST Grading % Passing:

4.750 mm

100

2.360 mm

92

1.180 mm

87

0.600 mm

79

0.300 mm

47

0.150 mm

14

0.075 mm

3.6

Fineness modulus

1.82

Relative density

2.65 (unwashed)

Drying shrinkage: % shrinkage

0.079

% of control

96

Water absorption % (4.75 mm) 0.7 Water demand (I/m3, c/w ratio = 1.7) 225

The g r a d i n g t e s t s indicate t h a t t h e s a n d s are s u i t a b l e for u s e in concrete a l t h o u g h t h e f i n e n e s s m o d u l i are low (HDTC, 1990) a n d t h e s a n d s h o u l d r a t h e r not be u s e d on its own. The s h r i n k a g e v a l u e s fall w i t h i n t h e r e c o m m e n d e d limit of 175 per cent. The coarse aggregate to be u s e d will in m o s t c a s e s be a Type I b a s a l t or dolerite a n d t h e c r u s h e d s a n d s t o n e will be b l e n d e d with c r u s h e d dolerite or basalt. Igneous rocks

A s u m m a r y of test r e s u l t s from a n u m b e r of different t e s t s w h i c h were d o n e o n t h e D r a k e n s berg b a s a l t s , s a m p l e d t h r o u g h o u t t h e n o r t h e r n part of Lesotho, are p r e s e n t e d i n T a b l e 5. These are typical test r e s u l t s a n d only t e s t s d e t e r m i n i n g t h e durability a n d u s e a b i l i t y of t h e b a s a l t s as aggregates are given. The average v a l u e s do not refer to a n y specific q u a r r y site investigated for t h e LHWP. If the t e s t r e s u l t s are e v a l u a t e d it is evident t h a t the r e s u l t s follow c e r t a i n t r e n d s d e p e n d i n g on t h e mineralogical c o m p o s i t i o n (primarily t h e secondary smectite clay content) of t h e different types. F r o m Table 5 it c a n be s e e n t h a t t h e Type I b a s a l t s c o n s i s t e n t l y s h o w the m o s t acceptable r e s u l t s in all the t e s t s for u s e as concrete aggregate. It c a n for i n s t a n c e be s e e n t h a t the b a s a l t t y p e s c o n t a i n i n g h i g h e r p e r c e n t a g e s of s e c o n d a r y clay m i n e r a l s also t e n d to have h i g h e r w a t e r a b s o r p t i o n , lower specific gravity a n d h i g h e r p e r c e n t a g e losses in t h e s u l p h a t e test t h a n the Type I a n d Ill b a s a l t s with lower clay content. The b a s a l t s were also s u b j e c t e d to t h e ethylene glycol s o a k t e s t from w h i c h it w a s d e t e r m i n e d t h a t a high p e r c e n t a g e (>30 per cent) of s m e c t i t e clay m i n e r a l s result in a low specific gravity a n d a n extensive b r e a k - u p of t h e rock. Low w a t e r absorption m a y be one of the m o s t i m p o r t a n t properties c o n c e r n i n g concrete aggregate. Only t h e d e n s e b a s a l t (Type I) a n d the slightly a m y g d a l o i d a l b a s a l t (percentage a m y g d a l e s less t h a n I 0 %), w i t h o u t visible d a r k clay m i n e r a l s (Type Ill), s e e m to be suitable for u s e in roads, concrete a n d as rip-rap (Van Rooy, 1992). The properties of b a s a l t concrete aggregate u s e d in existing s t r u c t u r e s are s u m m a r i z e d in Table 6.

Table 5. Average index test results of some a~regate properties of the Drakensberg basalts TYPE % CLAY (smectite)

WATER ABSORPTION

S.G.

8-10

0.9

2.88

II

33 -44

2.2

III

10-26

IV V

I

189

DENSITY

WET/DRY % loss

MgSO4

UCS (MPa)

2844

0. I

5.5

189

2.75

2684

1.2

9.5

124

2.5

2.48

2545

I.I

5.3

129

21-49

2.3

2.6

2792

0.65

4.7

151

15-45

3.0

2.46

2696

12

10.8

122

190

J.L. VANRooY and A. VANSCHALKWYK Table 6. Aggregate properties of Malibamatso bridge (after Melvill et al., 1989) TESTS

Acceptance Feasibility Study

Marakabei Weir

Limits BASALT TYPE Smectite clay %

max 20

Relative density

I

II

V

II

Ill

IV

4

22

24

14

35

23

19mm

Crusher

Stone

Stone

I

I

3.07

2.91

2.68

2.93

2.74

2.84

3.0

2.93

Water absorption (%)

max 2

0.7

2.3

3.1

1.3

2.6

2.3

1.2

1.3

Soundness (%)

max 18

3.0

24.8

44.0

18.2

47.0

74.5

0.6

8.4

loose BS 812 (kg/m ")

1480

1490

cons. SABS 845 (kglm,)

1600

1630

Bulk densities (air dry)

Flakiness index (%)

28

L. A. Abrasion wear by mass (%)

15.7

10% FACT wet (kN)

250

dry (kN)

285

ACV

wet (%)

15.3

dry (%)

14.2

Water soluble sulphate content (%)

0.0051

chloride content

0.0007

Accelerated test potential reactivity 12d. expans. (%)

0.021

Drying shrinkage - % % control

max200

0.094

0.104

0.129

0.095

0.118

0.125

165

182

226

167

207

219

T h e s e s p e c i m e n s w e r e t a k e n f r o m t h e 30 y e a r old M a r a k a b e i bridge, w h i c h is still in a s o u n d condition. T h e t e s t s s h o w e d t h a t t h e aggregate w a s r a n d o m l y t a k e n f r o m t h e br i dge s u r r o u n d i n g s a n d t h a t t h e c o n c r e t e w a s s o u n d a n d d u r a b l e aft er t h e service p e r i o d of 30 y e a r s . SUMMARY T h e Kats e D a m a n d t r a n s f e r t u n n e l will be situa t e d in t h e b a s a l t i c r o c k s a n d t h e Delivery T u n n e l a n d M u e l a D a m in t h e s e d i m e n t a r y rocks. B o t h these rock types pose certain durability problems r e g a r d i n g t h e i r u s e as aggregate, r i p - r a p a n d rockfill. T h e a m o u n t of s e c o n d a r y s m e c t i t e clay p r e s e n t in a r o c k is p r o b a b l y one of t h e m o s t i m p o r t a n t f a c t o r s c o n t r o l l i n g its d u r a b i l i t y as a n aggregate, a l t h o u g h t h i s is n o t t h e c a s e with t h e m u d r o c k s

w h e r e air b r e a k a g e c a u s e s sl aki ng of t h e r o c k a n d n o t n e c e s s a r i l y t h e swelling of clay m i n e r a l s . If t h e a c c e p t e d limits for u s e of r o c k as aggregate are t a k e n into a c c o u n t it is s u g g e s t e d b y V a n Rooy (1992) t h a t only t h e T ype I a n d t h e slightly a m y g daloidal T y p e Ill b a s a l t will be s u i t a b l e for u s e a s r o a d a n d c o n c r e t e aggregat e or rip-rap. If t h e clay c o n t e n t of t he T y p e W b a s a l t is w i t h i n a c c e p t a b l e limits, t hi s b a s a l t t ype will also be s u i t a b l e for u s e as aggregate. Only t he c o a r s e g r a i n e d s a n d s t o n e from t h e Molteno F o r m a t i o n will be m a r g i n a l l y a c c e p t a b l e a s a fine a g g r e g a t e a f t e r c r u s h i n g a n d w a s h i n g , a n d t h e HDTC (1990) h a s s u g g e s t e d t h a t t h i s s a n d be u s e d onl y as a b l e n d with c r u s h e d i g n e o u s m at eri al . T he t u n n e l s will be e x c a v a t e d with t u n n e l b o r i n g m a c h i n e s a n d only s h o r t d i s t a n c e s will b e lined. T h e K at se D a m wall will be c o n s t r u c t e d with Type W b a s a l t aggregat e c o n t a i n i n g sm al l a m o u n t s of clay m i n e r a l s a n d a m y g d a l e s .

The geology of the Lesotho Highlands Water Project with special reference to the durability ...

Acknowledgements - The

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