BULLETIN

of the International Association of ENGINEERING GEOLOGY

de I'Association Internatienale de Gg:OLOGIE DE I'INGI~NIEUR

Paris - No

54

- Octobre 1996

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 C A R T O G R A P H I E G E O T E C H N I Q U E EN T E R R A I N S T R O P I C A U X 9 L ' E X P E R I E N C E DU G H A N A

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 laterites 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 appraisea the methodology used in the evaluation, highlights the fieldwork, including sampling processes and limits of extrapolation ol 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. Rdsum6 Les latdrites et autres produits de l'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 latdrites, les sols latdritiques et autres sols ,~ 5 probl~mes ,~ ont dtd entreprises. Cet article prdsente la mdthodologie utilisde, mezt en exergue le travail de terrain, incluant les procddures d'dchantillonnage dvaluant les limites d'extrapolation qui conduisent ~ 2 types de cartes : (a) cartographie des sols dans des secteurs sdlectionn6s; (b) cartographic gdotechnique globale des sols du Ghana : i) cartographic des latdrites et sols latdritiques, ii) cartographic des sols sapro[ithiques. Une dvaluation quantitative est ensuhe 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 Its facteurs gdndtiques qui permettent de prdvoir et de cartographier les diffdrents sols sous l'angle gdotecbnique. Le cofit et les ddlais de rdalisation des dtudes sont dgalemem dvoquds.

1. I n t r o d u c t i o n

laterization. Within a given climatic zone the distribution of laterites and lateritic materials is dependent almost exclusively on topography and drainage while the distribution

Over the past two decades during which Ghana has been accelerating communication improvement by undertaking design, rehabilitation and construction of roads and highways and their maintenan.ce, the need was feh to find a way to predict the occurrence of various construction materials, evaluate and map them. The formation of materials is primarily dependent on geology, topography and climate. Their engineering and physical properties are mainly dependent on relative position within the catenary.

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 p a r e n t rock, t o p o g r a p h y a n d d r a i n a g e .

The formation of these materials is dependent on factors of weathering which in the tropics is carried further through

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 and 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 c o n s t r u c t i o n a n d w h i l e e x p l o i t a t i o n has g o n e on f o r s o m e time, i n f o r m a t i o n f r o m the field has 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 the 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 and a b o v e 40 % on m a p s c a l e s o f 1 : 1,000,000. 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 on the b a s e m a p the r e s u l t o f the m a t e r i a l m a p p i n g b e c o m e s v e r y a c c u r a t e .

* Chief Research Officer, Geotechnical Engineering Division, Building 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. F i e l d o c c u r r e n c e of c o n s t r u c t i o n m a t e r i a l s

3. E v a l u a t i o n m e t h o d o l o g i e s

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 (B RRI/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.

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.

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 1 : 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.

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 borehotes 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 a n d 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.

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)

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 :

Groove((y tater[re (Pisolitic)

Distribution general except along water courses. Later te her zan thick with groove[ layer generootiy tram lm ani:i above.

Gravelly Iooterife {quartz[tic}

Distribution found even ooton(] water cqurses. Only very th h quarrziflc groove~ly"klfeF'ite horizons found in profiles,

C{ayey to sandy [~ lqferile

Distribution sfricHy distinctly hiqh ground. Absent on tow ground w,th poor drair.a~e. Loter~te grnve! forms very thir horizon of tess than lm in orofile oond even disooppeooring towards lowest ground

Profi{e very variabieNith hoFizons generally dot eoosllyo~scermote Laterifes opart from vooriable horizons of with gravel ~1}@1 Interite groveLThese maybe quartz pebbte boulder gravel peDbtes from sandstones admixture or conglomerates and bou!ders from the receding Vottaian as wet[ as quartz veins Profi[e rather generally not well Sandy c [ a y ~ developed. Area generally lowon!y s[ighf[ yl:i!,:i tying oond predominanftysandand laterihzed cictyey sands. L a t e r [ r e grove[ scanty with horizon thickness well below lm Clay toorerite

Heavy clays

Sands I (Un[aferifized) i

Profile poorty developed. Looferitic gdroovet scanty with practically no efinite horizon. Clay very dispersive in places. PreCominanfty heavy c[c1ys onty slighHy l a t e r i t i z e d on high ground which are very m fed n the aaron where t ~ t e r i t l c p r o f i l e is deve[aped. It is not thick and grave[ horizon is very thin. Sands~ cohesion[ess

Fig. lu : Engineering soils map [ : laterite & Ia/eritic soils.

11c Pebbly fo gravelly sandy soils r ~ 10c Sandy to silty d a y s ~

Soils formed over Volta[an sandstones with conglomerates.Recedmg Vnffooian scarp provides quartz pebbles and boulders in matrix of clay and/or sand. Profuse quartz veins also produce grave[(y clays and sands. Soils formed over Volta[an shales and Bit[mien phytlite%schists~fuffs and greywoockes.

9~ Clayey to Soils predominantly over granite silty s a n d s ~ complexes and sandstones. Soils very deep ranging from a few metres ,near water courses to over mumonmgn ground

8o Oefritool Soils confined to Toqo and Tarkwaian quartz(tic ~ ' ~ quartzites and isoTafed gneisses. gravels Produce generally shallow quartz graveUy soils over Mils and hilislopes. Metamorohosed Invas and puroctastic rocks a[so produce l'hese 'soils but 7o with decreased quartz gravel content. Heavy clays

San ds 5~ Quarl'zitic gravelly sands -

-

Predominantly dark grey heavy clays r ~ l formed over acid ond basic gneisses in Accm plains Unconsolidated s(~nds developed on some sandstones End gmmtes {Is we[[ as on other recent and tertiary formations

Predominantly gravelly soil generally over high ground derived from Tarkwaian quQrfzites, Upper Birimio,n phyLlites and greywoockes

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

Legend fir oveHy lateri te Earthy

IQferi fe

~ od /~o~

-1

, .. --" In ternafion~l

boundary

5

10

15

20k:

l

I

I

I

Scale

Fig. lc : Bolgatanga area soils map.

Legend 6ot4 [~

6ravelly Io~eri l"e

[]

Ear[hy [~feri te

m

Cloy

~5~nd

So35

Scale

$~

I0 Fig. ld : Kumasi area soils map.

37

0

3

6

Rippabt e m

0

!2

IS

H e t r e per s e c o n d x l O 0 13 21 2~. 27 30 33

HQroincL [

,

36

39

z,2

(,5

~.8

so,ooo

2opoc

E=200BSR

lol)oc

/ /

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Non-Rlppabte ~

% 500 1000 1500 2000 25003000 3500 G000g500 5000

cm J~ ta.i

200 zone

ripped usino 3 - 0 wr~:;Q t

( b )

RiapQbilit),

be rlppe(I

r

Seismic ~,elocity m / s e { x 10013 0 1 t(zbour~r with pick ~nd s h o v ~ t ~ Tractor~ sc~per : no ripPinO etc Tr~r : a f t e r rlpQing ~ L o a d i n shovet'-no bLQstln0 _ ~ Bu c keel-chain excavator ~ ~ ~ B u c k e t - whee{ e x c a ~ t o r Dr~0tine{crIlwler) no bL~stln0 ~ ~ Walkin~ drogline : no b l a s t i n g ' Strippino shovel :no blasting

I

r

~

a -

;~=50 9 /

,

.,~

110CBF CBR

=

//

/ #,"a : / ,//, .r /"

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[0[1 Relatlon bel-ween ripoab[li,i'y and v e l o { l b / C~t~r~I[(lr S e r i e s B Ripper

VeLocity

//

E =

/

A./"jE~/" .," A"/" ./ 7 / / J'/- " ,

~Li~l

5

A Heasurements of R, 9 E from wove vetocity measuremen fs 0 E from stiffness m e ~ . s u r e I

10

meritS,,,,,1

20 C'B-R'

50

,

100

200

~

L

500

Fig. 3 : Relationship between the dynamic modulus of elasticity and the CBR value of various soils (Heukelom and Foster, 1960).

--

---~

Possible ,,- . . . . . ~ M(lr Oih o.L , , I ~poSSlbte

m

(c } Seismic velocihes, f o r determining diog:bihty

Fig. 2 : Rippabilityand diggability-compressionalwave velocity relationship.

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

Horizcnl'ai 0 30

~176176

60 90 150150,~

,.~

.~ "~ . 9 1 7 6 1 7 ~176 6

..--<

..%

2.

. o -"

....

~. . . .

~ 9176176

q

......

i E. ~, t ~-rl t,,4 ~ :tC H A l N,z~.G-E

Z3Z

:Z2,1 9- e,cx~

+oo~

S~,,-t besc~'*n

--

d 0

--

--

do

--

--

do

-

--

do

--

--

d o

-

do

-

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 and 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.

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

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

The durability, specific gravity and water absorption of lateritic gravels are influenced significantly by the iron oxide content (Millard, t 962, B hatia and Hammond', 1970).

39

i~

o

~

~

o

~

o

~4

.... " ~

~

' o~

rv7

~__

~

~,4

m

""

74

r

0 3 6 9 12 15m I l i~ i I i Horizonk~l 0 30 60 90 120 150t~ Vertical

'......... ,...,...,ii1.,.,-----"c~m~

~A~E q~ +

s~e

500 ~e~e~'o~,

st, ~]

1so 2 s

~3 t r,~.-io.61~,..~,.,~

~ , ,e~

v ~

~o ~s ss i

•

25

sl I 2 [ 27 : s'

~ gaLr b.,,~m R ~ a.;..4, bn,,~n

J

Cfk

IcH r,:, ~ + .~oo

dO--

t 35 19 46

3

39 ] 5g

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-

H ~ r izon

Hu-nlterous Loyer St~ ~"

" Sii "Siii

'

S~.n~y m SiDy Soil Gr•velly '.o.,"eritic Soil

Pre~omna7fly iron coaf~C ~ r a v e l

LATERITI C

Grcve',ly La feri t i c Soil { M c i n l y C~.crWziFIC gr~vel)

"Sir

Ear~'hy L~'eri!'ic Soil 'wi~'hItttl~

,, S~:

S~il ~etem~esed rock

SOIL

Orave~

(~ r~n{ ,hc rock)

Rh)~ R(i

!~

R

H~xturt of Roc~ (Ind Sci:

i

Week,ned c.nd a a r q y disin tegr~ted Soil

i

SAPSOIL RC'..ITl E

100% Rock dlsco!aured Mare ~hcn 50% Rock ~iscoloured Less

l'ho,n 50% RQck disc~ioured

~isccn~inul

~'ies ~t~ined

gresh Rock

Fig. 5 : Schematic.sequenceof laterisationand massweatheringgrades in a profileover granite (modifiedfrom Bayneset al., 19"/81.

40

"," ~ ~Go

~

{ ,$

262m

~4~.,~ I I I I I I . ~ f . ~ l

Im

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Ilfllll III1111 III]III ; Imll' IIIIHI i

~/J "/

IllIIll

~

III1111

I i ~ nI IllliTi t Ill I H I H I

( Y A ~ I I]..-411

~I It II ~E2-Jq,,, ,I , I1[i ,,H, "7 I I11111 IIIIItl

ea=-~l/A~l ~'~ q~ |L/;/,,"ll ~.1 3 ~ I ~ ' l ~1 : Z ~ 4 I i } tI

256

.~ ,2M C ~ '1 .1~ ~

I I I It I I I I I I I I I I { II

I I I I

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I I I I

III I11 Iit !l[

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m

9

9

~--

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9

.

9. - - ' v " - ~

~

~

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~

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250m

PhyUife

2/~L

m

1r

8230m

16t. 6 0 m

2t, 690m

Fig. 6 : Variationof grading in a profile developedover phyllite along a slope.

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

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

The bearing strengths (CBR) of laterites and lateritic soils have also been found to be sensitive to soaking 9 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 water9 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 9

C o n c l u s i o n s

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 9 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 C'jl:ICED) Cc Description ~,~'~.(~/LI (Po/I.}~[,,

&

.7i~

:

Dark brown

,,

laterife highly pia i

-1 "8

rite

-3-6 ::~!

.1

i

Plas fic fol'

44

I

,

(%llk~/~ild~ t.c

Reddish brown clo wi th I~ ferl

.... irey mofft Mottled "~-'-5---.L:~, sandy Cl~) lCy with ~__~.,~with pea l.Bt 26 : 15 i~-81 17 0453 h~gh. pla ~, .~ c/ravel i -----'- shc~ty '

i

13

re sidu un{ 1"571 5&

I

___.

10

-~2~ 1

26

-6'0-~ .6 -63,..--St'ICy high y ! . micaceousl l 7.8 ~ laferite 1'55 61+ 22 20"91 6 0 " 2 Z ~ -%.~ residuum I

Highly 2.02 33 i 11 3 . ~ weatheredl ~ - ~ micaceous [ gneiss !

Highly w e a t h e r e d 1" 17 52 shale

29

37,6

12 0 219

I I

12.08

-N-P

Do.homeyer gneiss

t3 "8 32

.o.I~7

I

soft

J

.o~d

i ~

clayshalr _ , 51 2 ~ I i

~ ~ ~-~

i I

with ' horizontal

t~g

I

12 r 10

i

bedding;

i

I

------4..

_x.,...,

I N,iP -16t~7 23

[

II

I

-~

p~rtially wea fh ered

I

53

21 0 ' 1 % 4 -

m

i

.9"0 ~ Highlywe~ 9.6 ~ thered -I02 C~." g r i n te /2,S with sfrucfur~ 11%I0"t]{,\~_~,r e c o g n l s - | l ' 7 1 ~ 2'0 'Lq" able |

LOCATION BLACK STAR SI%-ACC~A

B~D ~OCK ACCRAI~. SHALE

I%

I J"~

.~,.~,~_~

.t.~: .?T.

i

nous Icztel' ~, ~7

-2./* -3 '0

LOCATION' ACCRA-rIODOWA ROAD BED R O C K GNEISS Oescripfi ~ ' ~ I ) ( ) .........

I Fig. 7 1 : Typical laterite profiles over different geological formations.

I

[

LOEATION-AIRPDRT ROAD~KUI~ASI BED ROEK-PHYLLITE

~'J

i

C

9

I

I:.i~ 16

- ' I

I brown

/

3 61 = I l a t e r i t i c

-4.71

a [lateritir

"'"~-Ig~~

LOCATION-PAI3HORE ROAD- ACCRA JBED ROCK- ACCRAIAN SANDSTONE ')d

LL

~l~y ] _

26

9 i _

,

/

/

~

I

I

I

| I

~,

I

-g.,~HN'- y . I

I

- I -

--

,brown

16 10.2011,, Isandstonb [

[

Igravels[

I

I

/

['~'-".'.'il send

/

~silry

I I

'

~S ~ ~

,~ ,g ~,.,,

'','"~pflyldll-ei17" 4

.,':, ~z. t~."t

Cc

I

brown

_

~.i

J

A-~::~ "'I ] ~I~ Cc

]

~.~..

:,,1...1

C ~

~I ~' I~ ,.~ ~ o.~

-s',r x'IH~rd .~'0L~-~-Im~

,.,i

P

LOCATION'.K~asl-'~A~J~e BED ROCK- SCHIST 9 . Yd s LL P I C ~

t

'~

10,~i.

o

~6

I

I

I

I

I" , ICoarse. 0.I~01" " Is,~y~

_

ron'.t0nl

,,t~cto

--?. :.~ ~ch~,t

116 ~4 '

1

~.~

"I$o.nd.stor

I I

, _L__L___

~!3 ~ PI = Plasticity Index

C~= Effective

--I

cm

I, /, l,Ac~,o,-,|

=Ory Density LL = Liquid Limit

-

I:':;":'~4F"~ ''

I

'

--

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

I

I

I

I I

I I

I

I

I

|

I

I

''--,-,-"~-' I

,0 I

__

Cohes~'on ~'= Effective Angle of Internal Friction

Cc = Compression Index (All tests performed on undisturbed samples except LL ~ PI )

I - I

_| I

T

42 L06 SETTIIINGVELOs

'"

~/s~

8.S

SIEVES

IIIlli I I l~l{II t l I!II~

.=_,o

I

I!i I!1 i!il

/!

I Sandy fo s

imHC-A4~/

laferltic

Lateri fi{ Soil

illlll_-~D/~k

~e

0.0001

.

:.'.~-',

Earthy la terific ,-~--', soil with little

,i i IZ/1,~'~kl 1 F I I I I-9SL v I 12"/fA,.F-~.l

1"7"~[~,~'~]"~ i I

r:7-~7

l'S~i

9

10

100

;oil decomposed rock ( granitic rocks)

k OucrtziL- - Accra {Hillside]

C~;(~rrz Schist-Accro (Upper mid sloee ) C Peq'~ctite - Mouri (Upper mid slape] 't Amisian Bed-- Saltpond (Lower m!d's!epe( E C'ercenlin{*~- Aliku'na Bosu ( Up~,ee mid s!ope ( F N~ta S(EIS~+- S"~Itpcn~ (Upoer mid ilo~e]

I ~

9 .~:

gravel

t'"'I:'"7 "'I'oodI'o4o" t oR"E< t

l

soil

...... f | VYXNI 1 I t ! I I i

zon

r tzifi c gravel}

I II11111 II!I ~;4~Li~ ~/t'<1111~",%),P11111 li t Illllil~_~ ' .i~" I'rlllll~ ~ IIIItl II ~d-ij~L.--b ~~'ItNHtl Illl ~-"hL-_~--4-Hi~il t1111tli I lll 0-001 0.01 0.1 1 ? PARTICLE SIZE- millimet'res _ Fine Modiu"Courts Fine Nellu Coarse

=~,"--~ f

;lfirllvell y IQterlhC 4~.oil ( predomfna | n t l y iron coated | gravel) Oravelly InteritiC soil I mainT~ qua

~50

-:~~

2'5

F m;uniiterous Idyer

I I i I I l ki

i'1~t

II

<' t r ,t

f

~/f\ l hiixtu:ro of rock and soil

G Gneiss - Ha (Upper mid s logo ; H Sandstone-- Mpraeso (Upper mid slope; 1 5ranil-e-- Kumasi(Up~ermid slope', 1 P h y l h f e - Bekwoi (Upper mid slope }

,

'

. 9';,Tf,~';.'

i

Water

canton f - %

Riv

~7'.;'~':

Saproliflc soils Weakened and

~a~'ly disingrafe

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

Riii

00% focal disco lou red

Hiv

More than 50=/o

llih:

rock discolou re d

Less thenSO% 'roll Ilii

discoloured Oisonhhuities

Rii

lli

~.._stoin e d

I

Fresh rock

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

- ,

'5l I 'I: Depth ~.'5 m 9~

o! *'

75

k,7

=

70

":o >

1-0

7.6m

O ecom~p osed

granite

17m

? 0.9

a

9

~

0.8

65

20m ~

0'7

SO

28m O'l

55 t

01 pressure

i

i

1'0 less/m

10

100

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 ) .

01 Pressure-

with

depth

in

10

10

kJq/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

t h e r e f o r e r e q u i r e d w h e n e v a l u a t i n g lateritic g r a v e l s in t h e laboratory, so as to o b t a i n r e p r o d u c i b l e r e s u l t s . The construction material occurrence prediction methods in G h a n a h a v e b e e n l a r g e l y e f f i c i e n t , i n h e r e n t l y f a s t a n d e c o n o m i c a l . I n f o r m a t i o n f r o m t h e field h a s e n a b l e d an a p p r a i s a l o f the m a t e r i a l m a p p i n g a n d e v a l u a t i o n m e t h o d o l o g i e s to be m a d e . 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 : L,000,000. T h e G h a n a e x p e r i e n c e , t h o u g h little t e s t e d , h a s p r o v e d l a r g e l y s u c c e s s f u l a n d h a s b e e n the b a s i s for m a t e r i a l m a p p i n g for h i g h w a y p l a n n i n g , d e s i g n , c o n struction, rehabilitation and maintenance.

References American Association of State Highway and Transportation Officials (AASI'TTO). 1978 : Standard specifications for transportation materials and methods of sampling and testing. Part 2 AASHTO : 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..L. DEARMAN W.R. and IRFAN T.Y.. i978: Practical assessment of grade in a weathering granite. International Association of Engineering Geology {IAEG), Bulletin n~ 18.

BHATIA I-t.S. and HAMMOND A.A., 1970: Durability and strength properties of laterite aggregates of Ghana. Building and Road Research Institute, Kumasi, Ghana. Project Report SM 9 : [5 p. British Standards Institution (BSIL 1990 : Methods of test of soil for civil engineering purposes. British Standards 1377 : 1990; London. Building and Road Research Institute / Ghana Highway Authority, 1986 : Engineering geology and highway geotechnics of lateritic and saprolitic soils. BRRI TP HM/1 Papers 2.4, 5 : 89 p. (113). 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-2L64 : 289 p. HEUKELDM W and FOSTER C.R., L960 : Dynamic testing of pa;ements. J~mrnal of the Soil Mechanics and Foundation Division. Proceedings of American Society of Civil Engineers (ASCE), Voh 86, SMI : 1-28. MACKECHNIE W.R.. !967: Some consolidation characteristics of a residual mica-schist. Prec. 4th African RegionaL Conterence 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 LTD.. 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.