I ' I

BULLETIN

of the lnternational Association of ENGINEERING GEOLOGY de I'Association Internatienale de Gg:OLOGIE DE I'ING#NIEUR

Paris

- No

54

- Octobre 1996

: I

E N G I N E E R I N G S O I L S M A P P I N G IN T H E T R O P I C A L T E R R A I N : T H E G H A N A E X P E R I E N C E CARTOGRAPHIE G E O T E C H N I Q U E EN TERRAINS TROPICAUX 9 L'EXPER[ENCE DU GHANA

AYETEY

J.K.*, FREMPONG

E.M.**

Abstract Laterites and other products of weathering in the tropics are an important source of material for highway construction. For any extensive use such as has happened over the past two decades when there has been a lot of development, rehabilitation and maintenance of roads throughout Ghana, it became necessary to evaluate and map these materials. Research into lateritus and lateritic soils and other problem soils of Africa has been undertaken. Highway materials evaluation and mapping in Ghana has also been undertaken. This paper appraisez the methodology used in the evaluation, highlights the fieldwork, including sampling processes and limits of extrapolation of point profiles for the development of line profiles leading to the preparation of two types of maps : (a) Area soils maps of selected areas ; (b) General engineering soils maps of Ghana : i Laterite and lateritic soils maps, ii Saprolitic soils maps. The quantitative evaluation is then undertaken using the map and various profiles, and problems encountered in doing this are discussed. I.aboratory testing methods are discussed with problems, some of which are unique to the tropical situation, highlighted. The relevance of geology and soil forming factors to the prediction of occurrence and mapping of engineering soils is emphasized. The speed and economy inherent in the method is also shown. R~sum6 Les late3rites et autres produits de t'ahdration dans les rdgions tropicales sont une source importante de matdriaux pour la construction routi~re. Pendant les deux derni~res ddcennies l'extension et l'entretien du rdseau routier se sont beaucoup ddveloppds au Ghana et il est apparu ndcessaire d'dvaluer et de cartographier ces matdriaux. Des rccherches sur les tatdrites, les sols latdritiques et autres sols ,, b. probl~mes ,* ont dtd entreprises. Cut anicte prdsente la mdthodologie utilisde, mezt en exergue le travail de terrain, incluant les proo3dures d'6chantillonnage dvaluant les limites d'extrapolation qui conduisent ~t2 types de cartes : (a) cartographic des sols dans des secteurs sdlectionnds ; (b) cartographie gdotechnique globale des sols du Ghana : i) cartographic des latdrites et sols latdritiques, ii) cartographic des sols saprotithiques. Une dvaluation quantitative est ensuite entreprise, en utilisant les cartes et des coupes, et l'article discute les probl~mes rencontrds, de m~.me que ceux lids aux essais de laboratoire, parfois spdcifiques des rdgions tropicales. On insiste sur la gdologie de base et les facteurs gdndtiques qui pcrmettent de prdvoir et de cartographier les diffdrents sols sous l'angle gdotechnique. Le cofit et les ddlais de rdalisation des dtudes sore dgalement dvoquds.

1. I n t r o d u c t i o n O v e r the p a s t t w o d e c a d e s d u r i n g w h i c h G h a n a h a s b e e n accelerating communication improvement by undertaking design, rehabilitation and construction of roads and highw a y s a n d t h e i r m a i n t e n a n c e , t h e n e e d w a s felt to find a w a y to p r e d i c t t h e o c c u r r e n c e o f v a r i o u s c o n s t r u c t i o n m a t e r i a l s , e v a l u a t e a n d m a p t h e m . T h e f o r m a t i o n o f m a t e r i a l s is primarily dependent on geology, topography and climate. T h e i r e n g i n e e r i n g a n d p h y s i c a l p r o p e r t i e s are m a i n l y d e p e n d e n t on r e l a t i v e p o s i t i o n w i t h i n the c a t e n a r y . T h e f o r m a t i o n o f t h e s e m a t e r i a l s is d e p e n d e n t on f a c t o r s o f w e a t h e r i n g w h i c h in t h e t r o p i c s is c a r r i e d f u r t h e r t h r o u g h

l a t e r i z a t i o n . W i t h i n a g i v e n c l i m a t i c z o n e the d i s t r i b u t i o n o f l a t e r i t e s a n d lateritic m a t e r i a l s is d e p e n d e n t a l m o s t e x clusively on topography and drainage while the distribution o f the n o n - l a t e r i t i c w e a t h e r e d p r o d u c t s is d e p e n d e n t on parent rock, topography and drainage. W i t h this b a c k g r o u n d k n o w l e d g e m a t e r i a l s h a v e b e e n e v a l u a t e d a n d m a p p e d in G h a n a p r i m a r i l y for h i g h w a y construction and while exploitation has gone on for some t i m e , i n f o r m a t i o n f r o m t h e f i e l d h a s also c o m e to e n a b l e a p p r a i s a l o f the m e t h o d o l o g y . T h e s u c c e s s o f t h e m e t h o d is a b o v e 80 % on m a p s c a l e s o f 1 : 6 3 3 6 0 a n d a b o v e 4 0 % o n m a p s c a l e s o f 1 : 1 , 0 0 0 , 0 0 0 . It m u s t be p o i n t e d o u t that w h e r e the r e l i e f is a c c u r a t e l y s h o w n o n t h e b a s e m a p t h e result of the material mapping becomes very accurate.

* Chief Research Officer, Geotechnical Engineering Division, Buitding and Road Research Institute (Council for Scientific and Industrial Research), University P.O. Box 40, UST-Kumasi, Ghana. ** Research officer, Geological Engineer, same address.

34 2.

Field

occurrence

of construction

materials

Weathering in the tropics, which is usually variably influenced by the type of parent rock, topographical and drainage conditions is carried through to laterization. This leads to the formation of laterites, lateritic soils and saprolites. The wide variety of tropically weathered soils, red or reddish brown in colour, having slightly different chemistry and morphological and physical properties and with silica : sesquioxide ratio between 1.33 and 2.00 constitute the laterites and lateritic soils (BRRI/Lyon Associates 1971). The laterites and lateritic soils are very different from the parent rock and all structural and textural features from the parent rocks are completely obliterated. In the tropics, lateritic soils are found over almost all types of rocks and in climates ranging from the savanna areas through the humid rainfall regions. They occur in well-defined and regular sequence of the soil catena which is related to relief. Efficient drainage coupled with factors like pH and oxidation potential of the environment determine the extent of leaching. It however appears that, whatever the material from which laterites and lateritic soils are derived it is essential to obtain an adequate supply of sesquioxides. It has generally been observed that due to the ease of alteration of basic rocks, laterization processes on them are more intense and widespread than on acid rocks. Drainage is the most important factor in the formation of laterites and lateritic soils and is also very closely related to topography. With these two factors as the basis for field reconnaissance, traverses are run mainly from the bottom up the hillside to the hillcrest. Sampling along such a line reveals the boundaries between the laterites and lateritic soils and the saprolites. First, the saprolitic-lateritic soil boundary is marked. This point is then marked on the field sheets of I : 63360 maps. Two or three such uphill traverses are run. When the points thus obtained are marked on the map and joined as far as possible along contours the boundary is obtained between the laterites and lateritic soils on one side and the nonlateritic material which is invariably saprolite. The height of the point of the boundary is transferred on to a good 1 : 1,000,000 map. This is done in different climatic regions and in different topographical areas covering the country. The most important consideration at this stage, when the heights of the boundaries on the 1 : 63360 maps above become established, is to complete the 1 : 1,000,000 map. This is completed on the basis of about 18 field sheets of 1 : 63360 maps. When the map is drawn for the whole country on the basis of 1 : 1,000,000 scale, then detailed work could be done at 1 : 63360. Three (3) types of maps are produced, namely : (a) Laterite and lateritic soils maps 1 : 1,000,000 (fig. la) (b) Saprolitic soils maps 1:1,000,000 (fig. Ib) (c) Area soils maps 1 : 63360 (fig. lc & ld)

3. E v a l u a t i o n m e t h o d o l o g i e s For mapping and the evaluation o f the chemical and physical properties, both disturbed and undisturbed samples were taken from the field. Where serial photographs existed these were studied before site visits to ensure selection of preliminary construction materials which would be checked in the field. Auger holes and pits were made during the study and it was from these that the representative soil samples were taken. Outcrops were also sampled for testing in the laboratory. The quantitative and qualitative evaluation of construction materials is made by means of geophysical (seismic) traverses, boreholes, pitting and augering, examination of road cuts, borrow pits and laboratory analyses. From the geophysical traverses, where forward and reverse traverses are run, depth to bedrock could be ascertained and a correlation of the compressional wave velocities obtained with information from boreholes or pits made along or close to traverse lines. It is thus possible to delineate the lateritic and saprolitic horizons from the bedrock. From the compressional wave velocities the rippability could also be ascertained (Fig. 2). It has been found that the lateritic soils have compressional wave velocities of around 305 m/s which increased up to 460 m/s as the lateritic gravels become more compact, and well beyond this where induration had produced hardpans. When the dynamic modulus is obtained from geophysical traverses a first approximation of California Bearing Ratio (CBR) is obtained from correlations of dynamic modulus and CBR values of soils (Fig. 3). From the pits and road cuts, visual examinations of materials are made and samples taken. The vertical variation of materials in profiles with respect to texture and suitability for construction purposes are ascertained. The quantitative and qualitative evaluation of lateritic gravels for construction have been based on simplified economic geology methods. Field traverses are run along grids. From point profiles (derived from these field traverses) extrapolations are made laterally and longitudinally to obtain line profiles, from which vertical and areal distribution and variability are determined through field traversing (Fig. 4a, 4b) so that area soils maps and general engineering soils maps are produced. The general engineering soils maps may be either a laterite and lateritic soils map or saprolitic soils map. Area materials maps showing clays, sands, gravels and quarriable rocks may also be produced. The mode of occurrence of laterites and lateritic and saprolitic soils has been found to be the same in almost all areas of occurrence and the principles applied during exploitation of materials were as follows : --

the principle of integrated exploration,

--

the principle of gradual accumulation of data, and

--

the principle of uniform degree of exploration.

This decision is based on the relatively simple structure, extensive areal distribution and the relatively low material depth variation of laterites, lateritic soils and saprolitic soils. A combination of four (4) main exploratory grids are used. namely :

o

11c

Grave((y taterite

{Pisolitic )

Distribution general except along [;-TI water courses Later te horizon

~

thick with g r a y e [ foyer g e n e r a t [ y from /m an0 aoove.

Gravetty ~ Ic1terife 10c (quarfzific}

Distribution found even atonq wafer c0urses. 0F~IY very fhl~ quartziflc gravelly/late~'ife horizons found in pretties.

C(ayey fo sandy ~ lqterile ~

Distribution s t r i c t l y distinctly hiclh ground. Absent on tow ground with poor drainage, Laterife gr~vel forms very fhir horizon of tess than lm in profile and even disappearing towards lowest ground

Laterites ~ 8~ with g r a v e i ~ e I pebb[e boulder admixture

Profile very vari~bte with horizons generally dot easllyd~scermote apart fromvariabte horizons of t~terite graver. These maybe quartz grovel pebbles from sandstones or conglomerates and boulders from the receding Vo[taian as wet[ as quartz veins

9~

7~

5o

Profile rather generally notwell Sandy cLayl-r~ developed. Area generally lowonly slighfiyl:ii~.] tying and predominantly sand and aterihzed~ cto.yey sands. L a t e r i t e gravel scanty with horizon thickness wet[ below Im Clay taterite

Heavy c lclys

Profile poorly developed. Laferitic graver scanty with practicatty no definite horizon, troy very di'spersive in places. Predominanfty heavy clays onty slightly t a t e r i f i z e d on high ground wh ch are very limited in the area where taterifEc p r o f i l e is dave[aped. It is not thick and graver horizon is very thin.

Sands ~ Sands~ cohesiontess (Un[afe ritized)mlm Fig, la : Engineering soils map [ : laterite & Iateritic soils.

-

-

11~ Pebbly to gravelly ~ sandy soils ~

~oiIs formed over ~[taian sandstones with conglomerates.Receding Voltaian sc~.rpprovides quartz pebbles and boulders in matrix of cIc1y and/or sand. Profuse quartz veins also produce grave[ly c(ays and sands.

I0~ Sandy to silty c|ays1~.:~1Soils formed over Voltaian shales and Birimian phytlites~schists~fuffs and greywackes. ~(, Clayey to silty sandsF" ~

Soils predominantlyover granite complexesand sandstones. Soils very deep ranging from a few^ metres near water courses to over lure on h~gn ground

Defrifal r----jSoils confined to ToQo and Tarkwaian quarfzitic J-_.-~ quertzites and isolated gneisses. gravels : P r o d u c e generally shatlowquartz gravelly soils over hills and hiUs[opes Metamorohosed,lavas~,and p~roctastic rocks also prooucernese soils but with decreased quc~rfz gravel content Heavy cloys

Son ds 5 ~ Quartzitic

'

~Predominantly dark grey heavy clays J~__.m~formed over acid and basic gneisses in Acorn ptains N

gravelly ~ ~ sands

Unconsolidmted sands dove{aped on some sandstones and granites as welt as on other recent and tertiary formaHons Predominantly gravelly soil gener(Mly over high ground derived from Tarkwaian quarfzifesj UoI~er Birimian phyitites and-~jreywackes

Fig. 94 : Engineering soils map II : saprolitic soils.

Legend fir clvetly toPeri te Ect r f h y

--I

[cI feri te

S~nd /~t~y

,, 9 --" In t'ernctfion~i bound=ry

5

10

15

20kin

f

I

~

T

Scctte

Fig. lc : B o l g a t a n g a area soils map.

S'o~.

36~~s

6~ 4

6o z,.O

Legend ['~

Gr'av etty Io.i'eri ~'e

~

Ear[hy [e.Peri te

~

Ctcty--Sand

~Ctay So.n d

6o3!

6~

Scale

1%5

1o 40

1"35 Fig. ld : K u m a s i area soils map.

6~

1~30

37

O

3

6

g

12

15

Metre oer s e c o n d x10O 18 21 2/* 2"7 30 33

36

39

Z~2

L5

5o,ooo

L,B

20p0(

E=200BSR

1op0c

/

//

mQrg n~

~

,

Non-Rl~ooblel

.

110CBF

a / ~ = 5 0 CB R "/ 9 /

,//~" y ~ Ripp(ib~ e I

E=

,'

s,ooo

t v t/rl

[Q) Relation ~l~rw~en

rl;~p(IbLIiI'y ~ n d velOClh/C~,tar~tI~,r S e r i e s B Ripper

% Velocity

500 1000 1500 200025003000 3500 L,000kSO05000

~ %00C 50(

200 elppe(~ ~asing 0 - g t'mttor { bI

zo ne

RIQ~abilify

chart

Seismic velo~it~ m SeCX 1000 0 1 2 L(ibour~r wi~h p$cK und s h o v ~ l ~ Tractor~ sc~per :. rlo ripping ei'c ~ ~ Tr~r : after rlpt~ing ~-~LoadJn shovel'-rio b h l s h n g _ ~ - Bu c k e~,'-chain excavator ~ - ~ ~ - ~ B u c k e t - wheel exco,vutor Dragtine(crcIwl~r) no b l a s t i n g ~ W a l k i n g d r a g t i n e : no blasting ' - - ~ Stripping shove :no blos~ing I r ~ ~ I Possible ,,........, M a r g i h a

,

, I rnoosslble

{c ) Seismic velocities for d e t e r m i n i n g

;/',,/,

~,000

n',is

diggo, Oltlry

Fig. 2 : Rippabilityand diggability-compressionalwave velocityrelationship.

a//

./

/ / a / = a ~ '~ * ' . / ~/- o " A'/" ./ Y // , , ,Libel

5

10

// I"

A. He saurements of Ro 9 E from w~.ve veracity measurements

[/ E from stiffness mectsure'

I

pepf~.,,,,t

20 C'B.R"

50

100

,

200

, L

500

Fig. 3 : Relationshipbetween the dynamic modulusat elasticity and the CBR valueof various soils (Heukelomand Foster, 1960). are too widely spaced at this stage to permit the establishment in detail of the quantity and quality of the construction material. Results could, however, be used for alignment selection and preliminary design. Measured or proved construction material reserve is the material quantity computed from very detailed sampling from high density pitting grid, All variations in the material quality are now delineated.

4. E n g i n e e r i n g a n d p h y s i c a l p r o p e r t i e s - - exploratory grid of regular geometric shape consisting of discontinuous profiles of drill holes and test pits, - - exploratory grids of regular shapes consisting of two continuous or discontinuous profiles, - - exploratory grids consisting of one system of profiIes, and - - irregular exploratory' grid whose distribution depends on where the material occurs in a terrain where the deposits occur few and far between. When appropriate, optimum density of exploratory grids is used to give the areal distribution and thickness of the construction materials and the accuracy of the results is checked with actual borrow pit quantity computations. Based on the broad knowledge of the geological setting of areas of occurrence of materials, construction materials reserves are inferred from field sheets and geological maps. Very few to no samples or measurements are taken in such a computation but pits, trenches, road cuts and borehole logs are examined before computation. The computed, inferred or possible reserves base is used for general road network planning. Indicated or probable reserves are computed partly from projections for a reasonable distance based on geologic evidence. The sites which are available for inspection, measurement and sampling (point profiles)

Whereas all the rock types produce fairly well graded laterites and lateritic soils, these soils grade from silty to sandy laterites through gravelly to earthy laterites with more or less gravel. A typical lateritic profile is shown in Fig. 5. The thicknesses of these horizons depend to a great extent on the point in the terrain with respect to topography. The grading characteristics of the lateritic horizons developed over different rock types differ. For a particular rock type, laterites and lateritic soils are generally earthy at lower altitudes and more gravelly at higher altitudes (Fig. 6). Where vegetation is sparse, considerable amount of fines is eroded leading to concentration of gravels. Furthermore, the sparse vegetation cover leads to exposure of the laterites and lateritic soils and subsequent hardening into hardpans of irregular thickness and extent. The grading of saprolitic soils is related to the texture of the parent rock. Saprolitic soils derived from granites show a grading which is predominantly sandy with some clay and silt. Those from the phyllites show a grading predominantly clayey with some silt. The fines of lateritic gravels are sensitive to moisture and drying. Pretest sample preparation and testing procedures influence laboratory test results of some tropical soils used in road construction and the current internationally adopted standards for identification and classification tests i.e. ASTM(1989), BSI(1975), AASHTO(1978), etc. need to be modified to evaluate them.

38

m

m ~lm

r

r-4

r

r

Horizont~l ~ S O ' ~

!

.%

,,~

L

cW a ~ - + ' ~ o o

I c~,,4~ca

......

,

.-.

.~

~ ....

,..-'"

~le~ zaT_+ooo

fsRd,

~'fN

I

2 0 20 60

7 37 56 1"2j*,~.1 u ~-s

b.~.122

|

"21,~a~bo.rk b,-ouam

~<

2 ./, _=~-2 - m

--

do do

--

0

10 90

--

Fig. 4a : Line profile (Wamfie). It has been established from previous study (Building and Road Research Institute and Lyon Associates, 1971) that the nature of the predominant clay mineral and hence the levels of plasticity of soils containing these minerals is influenced by the parent material and climatic conditions. Laterites and lateritic soils formed over different geological formations throughout the country show variations in plastic limits and liquid limit both laterally and longitudinally (Fig. 7a and Fig. 7b). In fact, local experience has shown that the plasticity characteristics with depth in laterites and lateritic soils in two similar profiles on different topographical sites cannot be predicted. However, generally, laterites and lateritic soils with high liquid limit and plasticity index values are highly plastic. Those with low values are lean. Laterites and lateritic soils with water contents close to the liquid limit are usually much softer than those with moisture contents close to the plastic limit.

(Fig. 8) and the various degrees of induration due to iron hydroxides and the predominant clay minerals, the shear parameters, cohesion and internal friction vary greatly not only in different topographical positions in the field but also within a profile at a point. Very fine-grained, soft, lateritic clays give very low cohesion and low internal friction angles. However, these values increase either with induration or increased sand a n d gravel fractions. It is important to stress that for laterites derived from highly foliated rocks, the cohesion a n d internal friction angles could be directional depending on the present layering in the profile as well as on the intrinsic foliation of parent rock. The durability, specific gravity and water absorption of lateritic gravels are influenced significantly by the iron oxide content (Millard, t962, Bhatia and Hammond', 1970).

Due to the great variability in physical composition of laterites which is evident in particle size distribution

Coarse particles of laterites and lateritic soils vary in terms of durability, strength, flakiness index, water absorption

39

m

m

~-,

o.x

Verticml Horizon~rnl

r

~

t-.,4

pr~

0

3 .1

6

I

0

30

60

I

9

12

15m

. L __.k,~

gO 120 i50~

"

e~J

<:Ta

.-.

~ ~ ....

t- 9~ "

LdVs

el4 ro ~ § .~oc~ t~v~L ~

c,~6~

~adt.~

-- d o - -

do Fig. 4b :

Line profile (Wamfie).

and resistance to breakage during laboratory compaction. The compaction characteristics of tropical soils vary within profiles, the maximum dry densities decreasing with depth with a corresponding increase in optimum moisture content (Fig. 9). This is understandable since the gravel horizon generally reduces to earthy lateritic soils with depth. Large samples must be obtained and several tests would require the use of fresh samples in the determination of compaction characteristics due to the high degree of breakdown of coarse particles during compaction. The method of pretest drying and degree of desiccation of the soil which is dependent on position in the profile, influence the moisturedensity relations. This makes the definition of compaction characteristics of laterites and lateritic soils quite difficult, Lateritic gravels are sensitive to compactive efforts and studies are envisaged to find out whether correlations exist between the compaction standards being used locally (i.e. Proctor, Ghana, Modified AASHTO, etc.) to convert values from one standard to another. This could contribute ira-

Horizon

~.

Hu miterou,f Layer

"Siii

Sandy fo Si]l'ySoil Gravelly Laferitif: Soil Predomin~n,ly iron coct~-~d gravel Gr~ve y L a t e r tic Soi iH~.inly Q.UO.r f Z I f j c qro.vell

"Sir

Eo.rthy Lc~'eri~'ic Soil

" Sv

SoiI decomposed

Sr~ Sii

9

LATERITIE SOIL

~ith IiHqe

Graver

rock

(G rclni i'iC rock]

Mixture of

Riz)~ RLi,

!

R

Fig.

5 : Schematic

Weakened

Rock

end

disJn tegr~ted

o~nd

Soil I SAPROLITIC SOIL

p~rfLy Soil

100% Rock' dJsco~oured More i'h~n50~ Rock dis~otoured Le.sSfhc~n50~ Rock disr Oiscanl'inui I'~es ~l:~in e d ~=resh Rock

sequence

of laterisation and mass weathering

a profile over granite (modified

from Baynes

et

aL,

g r a d e s in

19781.

40

,~

~'~,',i ""' IIIII 111 II]ll

IIIll

262m

~J':~71 I I I I 11/~--4.-,fM'l I I/1- Ill 1111171

~1 I t l l I ~lrl~ f _I2I - l{N , I, ,I , I ,,,ININI ~"7 I I I IJ I1[ IIIIHI ,

e~"--~l/All

~,1 3 ~ f~l

I I I I t I I

iJ~f-'ff

II I l l l l l

I i

tl

t i

.6o.7. 256

I I I i I I i

,o~

I

Ill IIII1[I Ill IIIIlll

[ I lit :~

i ill

-

flllll,

~t,

\s

m

\

50

,

"i1" L...., .

.

250m

Phyltife

2t~t~ m

lg_ 9 8230rn Fig.

6 : Variation

t

L

16t+ 60 rn of grading

in a p r o f i l e d e v e l o p e d

over phyllite

2 t+ 6 9 0 rn along

a slope.

mensely to substantial savings in cost of sampling and transportation of construction gravels as the volume of materials would be reduced 9

- - a knowledge of soil forming factors, terrain evaluation principles and the broad local geology of areas of occurfence,

The bearing strengths (CBR) of laterites and lateritic soils have also been found to be sensitive to soaking. Whereas some have been found to lose bearing strength on soaking after compaction, others gain in strength due to the cementation which is caused by the change of ferrous oxide to ferric oxide due to the presence of water. Due to the sensitivity of lateritic gravels to compactive efforts and moulding moisture contents and consequently the CBR values, extreme caution is required during testing in order to obtain meaningful results.

- - a v a i l a b l e geological memoirs, topographical maps, geological maps, aerial photographs, etc.

The consolidation characteristics of most laterites and lateritic soils depend on the nature of the soil, position of the sample in the profile and the parent rock type (Mackechnie, 1967) (Fig. 10). Due to the unavailability of efficient methods for the rapid and inexpensive determination of engineering properties of soils, over wide areas within a given terrain or those with similar climatic, geologic and physiographic conditions, an appreciable degree of uniformity with respect to engineering properties and behaviour is usually assumed.

Conclusions The prediction of the occurrence and mapping of engineering soils in Ghana is greatly aided by 9

The chemical and physico-chemical weathering processes in the tropical situation may be greatly influenced by the parent rock type, topography and drainage conditions. Consequently, completely different soil types may be developed over the same parent rock depending on the drainage conditions and topographical location. Denudation of laterites and lateritic soils could also lead to desiccation with some consequent modifications in geotechnical characteristics. The mode of occurrence of laterites and lateritic and saprolitic soils has been found to be the same in almost all areas of occurrence and by means of hand augering and pitting on a combination of exploratory grids the areal and lateral distributions of materials are delineated. The qualitative and quantitative evaluation of lateritic gravels for road construction have been based on simplified economic geology methods. For lateritic road construction gravels the sensitivity of fines (which may be in appreciable quantities) to drying and moisture and their highly plastic nature, vulnerability to pretest preparation and testing procedures, possible particle breakdown during laboratory compaction and sensitivity of bearing strengths'(CBR) to soaking constitute some of the major problems involved in laboratory testing. Pretest sample preparations and testing procedures appropriate to the tropical situation are

41

&

LOCATION;

U.S.T-KUMASI

BED ROCK:

GRANITE

9

.

-

~d

.

L

Pl

" Oark brown I [al'erlfe I

C' a

I

LOCATION: ACCRA-OODOWA R O A D

j '

e

Cc

/ I

I

LOCATION:BLACK STAR SO.- A~CR~ BED ROCK ; ACCRAIAN SHALE " 9 ~" LL I P I C ' s ~" Cc

BED ROCK: GNEISS "

"

LL

Vd-~

PI

~ ::'~" Ferru~inouJl,g- ' 20 6 ~ sand;r.la% ~ 1

'

U~I

0.6,

Cc

l

' " ' ~Reddish 0-16I .t.~.~., ~.~.. 'wifh br0wnla fetidci~ .~-..~

21

=1"8 ~

62

-2.~ --~-.3'0 ~ ' ~ -3 "6 = ~ -

39

30"01111 . 0431

53 '31

~-5-3!15 i0.1~,

~-~ 2 ~--~ .5,x+ =~-xz_-i - 6 , 0 ~--~ ---T -6 -6 .~..-- S~'Ity highly

i : :

'

laf,erife 1'55 residuum

-7 '8

:'

64

I ~

!

22 20.91 6

~ ~

i

-9.6 . ~ ] High ly w ~ thered -102 C~2 granite wi t h ! structur~ 6/+'5 recognlsI"71 -N,P 23 -11"~ , , . 12' 0 ".~-,ll

~

~

9.167~

13"8"32

partially ered

'

i

~ ~ ....

I

horizontal L . . ~ bedding:

wea th

,0~ !/;

k

I (

i

~ 'g Y ~ ~ w e ~ t h e r e d ~. I~ 52 ~ shale

O'2~C!-~-'-~. .2.0,J-N(.P Oc~hometal: | i"~ gneiss soft

j

----9.0 ["~- *

m caceous gneiss ! i

29 37,6 12 0219

I

i

Hard i ! 2~, t ~ 7 i cloy shalr 2t, 1 51 [ ] 12 0 10~

with

~ r

--,~.~

Fig, 7a : Typical laterite profiles over different geological formations.

LoC~,T!0N-AIRPORT ROAD~KUHASt BED R O C K - P H Y L L I T E

~C

LOCATION-PADMORE BED ROCK- ACCRAIAN 9 IYd ! L L I p

,

, ~ " , Black I "~" . . . . . . l r/-' ~ " " ' g , . ..~ sandy( ' II...~ clay - i26 - 0 ' 6 a ocrlgaa) n f c i -I.2 : I ' " " I Hard Ireddish 1'9~i31 -1,8 ^ ,Hard . , . I~" r Ibrown ! -2'4 ~ reddishl 51 62 25 ~,6,I 17 u,mo11__._lsandycla) !brown i.-iwith I'~'132 .3,0 ~ micaceou:.1 ~,9 71 41 30.0 16 0.201[~__Isandstom: -3.6 ;iateri#ic ,~___Iclcy with ] ~]gravels -/+'2 ~ l~ferific -t~.e . ~ i g r a v e ( s I 12 3/+ 83 1t. 5 13 0.21t,~1~

_S.z, .6'0

XjHard

_

~4~ mottled

_6.0 7 , 2 ~X

'

imlcaceou s !si'Ify ctmJ

-7,B ~

.8,/+ ~

19 75

-9,0 ~ -9'6 '"'tlHighty -10"2 / ~ ,~eoJ'here~ phytlite ~b~ I 17 -10,8 ~~ . ~

1 153 28

16 3/* 6 17

~

I

11

0

26

Hir s i l ~ cl~y

_ %..~.' ~..,~

sandstone

0

~.,-

~-."^ Index

C'= Effective

Index (All t e s t s performed on u n d i s t u r b e d

Eohesfon

samples e x c e p t LL

I~'= E f f e c t i v e ~ PI )

Fig. 7b : Typical laterite profiles over different geological formations.

Angle of Internal

17 2-83

{ 19.2 26

~:~.

0-25/,

P I = Plasticity

3

~ . : ~ Mica ~.~ schist ~.~ -,..~

~:~

LL = Liquid Limit

~1~ JEc

i 51 ~,00 . 179 - N - P -

~..~

12"0 ,-.~

Cc = Compression

tt~4-o

" , 'Coarse

25 6 2/+ 0-t2( 9 ~ sand qa ar~"rzifl

.11.~ C-:

"5'd=Dry Density

ILOCATION-'K=~as]-~,~u~cr BED ROCK- SCHIST yd LL PI C ~

~ Reddish 9 . . . . ' . - . .~ l o r o w n i ~ jsandy 1.79 /+3 2& 12.0116 i 0"168 cl~y w fn 2 26-0 17 0 ' l L ' 3 1 ~ = : l a t e r l t i c ' ! :, -'= g r o v e l 16 2g'~ 21 0.195::,<': .. .... rtne -N P- 0 i 8 :"..'-".":: sand ~ 5ray 70 391 12 1L. t+ It, ~ siH'y cloy :,"s Fine s,ithj ; :~ :': < SO, n g I

~'.)~iStiff : " ~ : imedium- 1 9/+ -N P .~''Jg rained - ~'.~." sand and 29 l&.~ lO 0,231 & - sandstone

: /,2

J - ,

R O A D - ACCRA SANDSTONE

Friction

--

42 LOG 100 ~0 .-=70

8.S. SIEVES

IIII!1, I tllill !~

I I1 1!11

I i !

~ /~1

21s I

ill 1~

""I

I a t e ri fie 50il

ii~

ii i i J

IA//~--~,\l i J I I I i V/i/T//r",'~c,t 111 I I

,,I

1~./~/!1

I Fi

~0

.=]o

,I t IZ/1,~'~kl

I ! i I I

9Siv

0.0001

0"001

0.01

0.1

1

l . s , ~ I I I I I l F'I ) I " , L ] /

10

2

PARTICLE

SIZE- m i ( l i m e ~ ' r e s . I F ne IMcdiur~Co~rsAFin e }M~un~Coerse/

b

A Oucrtzi~.- - Accra { H i l l s i d e ) ( G E F G N

Iti J/I JZI~L.] kl I I ? I /L.~,'-F'~I k} I I .

C~orrz 5 c h i s t - A c c r ( l ( U p p e r m~d s l o g e ( PeqmcHte - Mouri ( U p p e r mid s l o p e ! Amisian Bed-- S a l t p m n d ( L ~ w e r m ! d ' s ! c ~ e ) Sercentini-~ - Astku~ct 9csu ( Upper mid slope ) ~tc~ Sc~'iS~- S=ftpcn~ ( U ~ p e r mid s~a~e~ Gneiss - He ( ' J p p e r mi~ slooe', Sc{nds~ome-- H~raeso ( U p p e r mid s l o p e ;

I [ !.~l

FA---,~I~ I

,510 I L5 I 10 "ll i/rl ] \l I I 5 70 2b &O Wofer

r

f _ %

Saprolific

soils

I 5r~ni~'e-- Kumasi(U~;ermid slope; ] P h y l h t e - Bekwr (Upper mid sl9 }

Fig. 8 : G r a d i n g c u r v e s of lateritic soils on various parent rocks 9

Fig. 9 : S o m e c o m p a c t i o n c h a r a c t e r i s t i c s of lateritic and saprolitic horizons.

3"&m

90-

8S]--

Sm

1 '1;

Depth ~'5~,~m

70

Decorr~posedgr~,nite

1-0

7~

!2-~

,

0'9

o

z. 0'8 65 o

0'7

2

50

8

~

~

01

55 I-

, 0,5

I

01 pressure

I'0 less/m 2

Fig. 10a : Variation of c o n s o l i d a t i o n c h a r a c t e r i s t i c s lateritic profile o v e r phyllite ( K u m a s i ) .

10

|00

0'I Pressure-

with

depth

in

I 0

IC

k.lq/ m 2

Fig. 10b : V a r i a t i o n of c o n s o l i d a t i o n c h a r a c t e r i s t i c s of laterite soil o v e r granite ( m o d i f i e d after R u d d o c k . 1967).

43 therefore required when evaluating lateritic gravels in the laborato~ so as to obtain reproducible results~ The construction material occurrence prediction methods in Ghana have been largely efficient, inherently fast and economical. Information from the field has enabled an appraisal of the material mapping and evaluation methodologies to be made. The success of the method is above 80 % on map scales of 1 : 63360 and above 40 % on map scales of 1 : t,000,000. The Ghana experience, though little tested, has proved largely successful and has been the basis for material mapping for highway planning, design, construction, rehabilitation and maintenance.

References American Association of State Highway and Transportation Officials (AASt-|TO), 1978 : Standard specifications for transportation materials and methods of sampling and testing, Part 2 AASItTO : Washington. American Society for Testing and Materials (ASTM), 1989 : Annual Book of ASTM Standards Part 15 : Road Paving. Bituminous Materials, Skid Resistance : 1260 p. BAYNES F.J., DEARMAN W.R. and IRFAN T.Y.. 1978: Practical assessment of grade in a weathering granite. International Association of Engineering Geology (IAEG), Bulletin n ~ 18.

BHATIA H.S, and HAMMOND A.A,, 1970: Durabilky and strength properties of taterite aggregates of Ghana. Building and Road Research Institute, Kumasi, Ghana. Project Report SM 9 : 15 p. British Standards Institution (BSt). 1990 : Methods of test of soil for civil engineering purposes. British Standards 1377 : 1990; London. Building and Road Research Institute I Ghana Highway Authority, 1986 : Engineering geology and highway geotechnics of lateritic and saprolitic soils. BRRI TP HM/1 Papers 2.4, 5 : 89 p. (I 13). Building and Road Research Institute / Lyon Associates. 1971 : Laterites and lateritic soils and other problem soils of Africa, An engineering study for United States Agency for International Development. AID/csd-2164 : 289 p. HEUKELDM W. and FOSTER C.R. 1960 : Dynamic testing of pavements. Jour'aal of the Soil Mechanics and, Foundation Division. Proceedings of American Society of Civil Engineers (ASCE), Vol. 86, SMI : 1-28. MACKECHNIE W.R.. !967: Some consolidation characteristics of a residual mica-schist. Proc. 4th African Regionat Cont)rence on Soil Mechanics and Foundation Engineering, Cape Town, South Africa. Vol. 1, p. 235-239. MEIDAV T., 1965 : Dynamic testing of soil with the seismic method. HUNTEC I+TD. Toronto. Canada. MILLARD R.S., 1962 : Road building in the tropics. Journal of Applied Chemistry, 12 : 342-357, RUDDOCK E.C., 1967 : Residual soils of the Kumasi District in Ghana. Geotechnique 17 : 359-377.