ABSTRACT
Deltaic lateritic soils of the Niger delta region of Nigeria differ markedly fiom the other
lateritic soils because of some mode of formation related peculiarities. As the most
widely used soil materials for road earthworks in the entire region, their continual
exploitation based on properties of the more matured and widely studied lateritic soils of
the hinterland has met with mixed degrees of failure and success. In this study, samples
of this soil material fiom different locations were stabilised mechanically with or without
sand addition and chemically with controlled proportions of cement, cement-sand and
cement-geosta.
Strength of plain mechanically-stabiied (i.e. merely compacted) soil was found to
be directly dependent on the compacted density which itself is dependent on the
percentage fines, F while sand stabiiation was also found to be additionally
dependent on the optimum., s, ,a nd content (OSC) or the most effective sand content that .”I. .* , .I*
will produce the densest state of compaction. OSC is itself dependent on F and this
was used to develop a graphical model for predicting the various road design
paramete&. A graphical model was also developed to harmonise laboratory and field
compaction. Cement-related chemical stabilization was found to depend solely on the
fabric structure developed through apparent cohesion over time in course of cement
hydration. Although pbin cement stabilization significantly improve strength, soaked-
CBR of 280% required for suitability as a base course material was achieved at rather
high cement contents in excess of 12% which is at variance with the economic ceiling of
about 7% specified by first volume of FMW Specifications on roadworks. Composite
stabiliiation with sand and cement achieved this feat (i.e. 180% CBR) with about 6%
and 36% cement and sand contents respectively. Geosta addition to soilcrete was also
found to be a considerable improvement over plain cement stabilization and that even at
very low geosta content less than or equal to 2% depending on the optimum geosta
content, OGC and the percentage fines. Analytical and graphical models were also
presented to predict influence of the various stabilkation methods using as indicator
parameter the percentage fines, F obtainable fiom simple gradation tests, particularly
wet-sieving.
As a resuIt of these and other soil-property-related failures of roads in particular, it was
also recommended that all road related agencies (governmental and non-governmental)
should be more research oriented through proposal and funding.
TABLE OF CONTENTS
CERTIFICATZON
DEDICATION
ABSTRACT
ACKNOWLEDGEMENT
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
1.0 INTRODUCTION
1.1 Road and road pavement structure (1)
1.2 Lateritic soils (6)
1.3 Deltaic latetic SQ& (1 4)
1.4 Deltaic lateritic soils in road pavement design and construction (1 7)
1.5
Need for (lgj’
1.6 Goal and Objectives of study (20)
2.0 LITERATURE REVIEW …. 21
3.0 MATERIALS AND THEIR EHAkACTERISATION …. 37
3.1 Deltaic lateritic mils (37)
3.2 Cement (42)
3.3 Sl.larp sand (43)
3.4 Geosta-A (44)
4.0 EXPERIMENTAL PROCEDURES . . . ..47
Preliminary Investigations (47)
4.11 Grain size distribution test on deltaic lateritic soils (47)
4.12 Grain size distribution test for sharp river sand (47)
4.13 Liquid limit test on deltaic lateritic soil sample (48)
4.14 Plastic limit tests on deltaic lateritic soil samples
Mechanical stabit ion experiments (49)
4.2.1 Plain mechanical stu6illisation experiments (49)
4.2.2 Mechanical stabilisation with sand addition (50)
4.2.3 Experiments to hamonise laboratory andjield compaction of
deltaic lateritic soils (51)
Piain cement stabilization experiments (52)
4.3.1 Plain cement stabilisation (52)
4.3.2 Experiments to study inauence of curing aard duration (53)
4.3.3 Compressive cube strength tests (53)
4.3.4 Experiments to study influence of stabilisation on permeability
(54)
Cement-sand composite stabiion experiments (54)
Geosta-cemen-t cI,o .gppsitg stabhti on experiments (5 1)
5.0 RESULTS AND DISCUSSIONS . . . .56
5.1 Inftuence of Mechanical Stabitisation (56)
5.1.1 Direct compaction or plain mechanical stabilization test results (58)
5.1.2 Sand addition on mechanical stabhtion (61)
5.1.3 Maximum dry density, MDD and sand stabilization (63)
5.1.4 Optimum moisture content, OMC and md stabilkation (65)
5.1.5 CBR of sand-stabiied deltaic lateritic soils (66)
5.1.6 Optimum sand content, OSC (67)
5.1.7 Limitations of sand stabilization in roadworks (70)
5.1 -8 Sand stabilization and permeability (70)
5.1.9 Harrnonisii laboratory and field compaction of deltaic lateritic soils
(70)
5.2 Influence of plain Cement Stabilisation (78)
5.2.1 Maximum dry density, MDD of deStaic Wit soiicrete (8 1)
5.2.2 Optimum moisture content, OMC of deltaic hteritic soilcrete (83)
5.2.3 CBR of deltaic Writic soilcrete (84)
5.2.4 Influence of curing and duration on soilcrete (87)
5.2.5 Deltaic lateritic soilcrete as road pavement materials (90)
Compressive cube strength of deltaic iateritic soilCrete (94)
Permeability of deltaic lateritic soilcrete (96)
Influence of Cement:Sand composite Stabilisation (99)
Ceflpent-sad stabilized deltaic lateritic soils as road pavement
. ,, . . * ,. .*’ , ‘> .-
materials (203)
Comparison of cement-sand and plain cement stabilization (1 08)
Influence of ~eosta:~e&encotm posite Stabilisation (108)
MDD etnd geosta-cement stabion (1 1 1)
OMC and geosta-cement stabilization (1 12)
I 5.4.3 CBR and geosta-cement stabilization (1 13)
5.4.4 Comparison of geosta-cement and plain cement stabition (1 19)
xi
I
6.0 CONCLUSIONS AND RECOMMENDATIONS
6.1 Conclusions (1 20)
6.1.1 Mechanical Stabilisation (1 20)
6.1.2 Plain cement stabilization (122)
6.1 -3 Cement-sand stabition (1 23)
6.1.4 Geosta-cement stabilization (1 24)
6.2 Reccornrnendations (1 24)
REFERENCES
APPENDICES
A Multiple regression theory (1 37)
B Soil’s natural classification characteristics (140)
C Mechanical stabilization experiments (14 1)
D Plain cement stabilization experiments (147)
E Cement-sand stabilization experiments (1 57)
F Cement-geosta stabilization experiments (164)
xii
CHAPTER ONE
Introduction
1.1 Road and road pavement structure
Generally, a road could be described as a specially prepared wide way or path
connecting places (i.e. houses, villages, cities, states or countries) on which
pedestrians, riders and vehicles travel to such places. Preparation of a mad involves
design and controlled mixing, placing and compacting (to existing standards and
codes) of various materials malung up the various components of the road pavement
structure.
Fig. 1.1 shows a typical road pavement structure and its various components through
which vehicular loads are eventually transferred onto the foundation soil existing
kncath thc roadway. . ,. . “. .;. q
Fig. 1.1. Typical road cross-section
The strongest, most expensive and the thinnest roadway structural component is the
wearing course which is usually made of bituminous or normal concrete for flexible
and rigid pavement respectively. It is the outermost component which directly
receives the vehicular load, uniformly distributes and transfers the decayed remnant
of such load onto the base course. The base course is the next strength-wise and is
usually made of crushed rock and/or stabilised soil materials which transfers the load
remnant to the subbase. The sub-base, usually made of soil materials with or without
improvement transfers its own load remnant to the subgrade or foundation soil.
ModiJed subgrade is only necessary where the natural foundation soil possesses
strength or characteristics hfaior to the 5% to 10% soaked-CBR specified by the
codes or standards for subgrades. Thus it is obvious that remoulded soil materials
whether natural or hprovedstabilised could constitute over 75% of a typical road
pavement structure.
An engineering soit’thai”woizIa be us&l in road construction has been jointly and
severally described as a relatively thin superficial layer or blanket of soft geological
deposit. This blanket may be continuous over land (except for rocky outcrops, salty
desert basins, etc.) and even beneath the sea but overlying different types of bedrock.
Soil is formed fiom pre-existing parent or source rocks (igneous, sedimentary or
metamorphic) through geological processes. Such geological processes include
physical or mechanical weathering whereby the parent rocks are broken down or
degraded into detrital, clastic, chemical and biochemical sediments, etc. This process
may also be chemical decomposition of certain rock-forming minerals like feldspars,
biotite mica, etc. which results into formation of residual soil types (i.e. lateritic,
active, andosoils, etc.). The actual type of an engineering soil formed or found in an
area (i.e. sedimentary, residual or i3l) depends largely on the following factors:-
climate (i.e. rainfdl, heat, cold, wind, etc.) and its cyclic distribution
over time and/or seasons;
parent rock materials which are known to significantly influence
finess, mineralogical and chemical compositions depending on the soil
type formed;
geographical relief including land fonns, topography, drainage, etc.;
time or age; and sometimes
decomposed living matter (i.e. plants, micro-organisms, animals, etc.)
as relevant to organic soil deposits.
. ,. . . i. .,’ , #.
Design and construction of road pavement structure by most highway standards and
codes all over the world is usually based on some strength characteristics of the
different soil types making up the various structural components. California Bearing
Ratio (CBR) or the load intensity that will cause 2.54mm (or 0.1 inch) penetration
into a soil expressed as a percentage of that for crushed rock is one such strength
index or parameter upon which most highway design and construction codes and/or
standards are based. For instam, the design method by FMW(1973, 1997) adapted
hm Road Note 31 and its subsequent revisions adopt the soaked-CBR as the
indicator strength parameter for design. By this method, pre-determined, specified
andlor assumed values of soaked-CBR of underlying layers (starting fiom the natural
foundation soil andlor subgrade) as shown in Table 1.1 are employed to establish the
minimum requirements for structural members of the pavement layers.
on disturbed
It is not clear however why FMW(1997) specified dif€erent CBR values 80% and
180% respectively for natural and cement-stabilised soiIs especially for use as basecourse
materials.
Except in very few exceptibriW’.&s,’ soils in their natural state can hardly possess
the relevant characteristics to achieve the specifications given in Table 1.1 for road
pavement structural components. As a result, most soils have to be improved prior to
their use in order to achieve colko&ty to structural requirements. Known soil
improvement methods include among others:-
(a) direct compaction by rollers or vibro techniques through which the sol is
densified for better mechanical properties; it should be noted however that
direct compaction by rollers hardly penetrates beyond 300mm (Knight and
Dehlen, 1963);
(b) pre-loading whereby load of known and controlled magnitude is placed or
imposed on the soil over a pre-determined time interval to achieve an
equally predetermined degree of consolidation;
(c) reinforced earth whereby horizontally placed reinforcements (strip, mat,
geogrid, etc.) resist lateral earth pressure;
(d) prewetting whereby collapsible soils are made to pre-compress by
introducing water into the soil lattice before actual load is imposed;
(e) soil stabilisation by mechanical, chemical, thermal or electrical means.
Soil stabilisation can be described as any physical, chemical or biological treatment
process employed to improve certain properties of a natural soil to make it more
suitable for specific intended engineering applications. This treatment process or
processes can be achieved iii”&v& ways.
(a) Mechanical stabilisation is achieved through compaction with or without
addition of sand, vibration, etc. which significantly densifies the soil.
(b) Chemical stabilisatidn involves addition of controlled proportions (by
weight of dry soil) of certain chemicals or stabilisers to soil either insitu
through grouting or prior to compaction or remoulding. Known chemical
stabilisers include cement, sand, calcium chloride (CaClz), bitumen and
more recently geosta, all of which modify the physico-chemical properties
of the soil to improve strength.
Thermal stabilisation involves desiccation, drying or in extreme cases
dehydration which enhances development of strong internal cohesion
hence strength development.
Electrical stabilisation includes electro-osmosis suitable for dewatering
soils with permeability coefficient, k <<<lo9 metres per second.
Whichever technique is adopted, soil stabilisation may enhance positive improvement
on strength, compressibility, permeability and other natural soil characteristics.
1.2 Lateritic Soils
Lateritic soils are the most abundant tropical soil group covering over 50% of the
tropics (Bawa, 1957; Uehara, 1982). Tropics is that part of the world which Lies
between the tropics of Cancer and Capricorn, a-but latitude 22+ degrees north and
. , ,,., 3 -7. .*. ‘
south of the equator. Latentic sods are mostly yellowish to reddish brown in colour
depending on the relative proportions of iron and aluminium sesquioxides. They have
also been descrii by Buchanan(l807) and Alexander and Cady (1962) as soil
I I
materials which are highly weathered, rich in secondary oxides (or sesquioxides) of
iron and/or aluminium, nearly void of bases andlor primary silicates but may contain
large amount of kaolinite (also Queiroz de Carvalho, 1991) and quartz, may exist
either hard (as superhial “cuirasse” or lateritic rock) or capable of hardening upon
exposure to wetting and drying. A distinct product of chemical decomposition of
some of the parent rock-forming minerals in presence of rain water, lateritic soils
have been found to be predominant in areas where the necessary and sufficient
tripartite climatic conditions of (a) high rainfall, (b) rolling topography and (c)
adequate drainage jointly hold sway.
Fig.l.2 summarises the formation mode of lateritic soils through a process popularly
known as the laterisation process. As shown in this figure, the process starts with
mechanical weathering of the parent rock. Crystalline rocks like igneous and
metamorphic are formed at an extremely high temperature. Resulting sharp
temperature gradient before and during post-formation cooling process eventually
produces detrital rock materials riddled with open lissures, joints, fi-actures commonly
referred to as discontinuities. Such joint and hcture planes act as channels for
ingress of weakly acidic rainwater (subsurface) runoff. Rainwater itself is referred to
as weak electrolyte having cbnibhed’with dissolved atmospheric gases like COz, CO,
NO among others to
discontinuities, certain
chemically through the
form weak acids. As the rainwater moves through the
rock forming minerals are attacked and they decompose
process of ‘hydrolysis. Principal rock forming minerals are
feldspars (orthoclase and plagioclase), quartz and micas (muscovite and biotite)
which altogether constitute over 80% of the parent rocks with feldspars alone
constituting almost 50% (Lumb, 1965; Bowles, 1984). Except for mechanical
weathemg into smaller grains, quartz and muscovite are known to be highly resistant
to chemical decomposition hence not seriously involved in laterisation process. But
feldspars and biotite mica are jointly attacked by the electrolytic rainwater and
decomposed to form mostly sesquioxides and clay minerals.
Lateritic
Mechanical weathering
(nulural or hwnun)
Ground
1 I DeuhrMn due to natumI dryhgprocess tn
tropical dry season I
Fig. 1.2. Schematics of Laterisation Process. (after Omotosho, 1987).
Apart fiom temperature, which is quite high and common to every part of the tropics
most especially in the tropical dry season (November to May), resulting clay minerals
formed depends largely on the three necessary and dcient environmental
conditions of high rainfall, rolling topography and adequate drainage. When these
three conditions are jointly prevailing, kaolinite clay minerals and hence lateritic soils
with very low activity (Skempton, 1953; Queiroz de Carvalho, 1991; Fourie and
Palmer, 2001) predominate. On the other hand, montmorillonite clay minerals and
consequently very active soils are formed where any (even one) of the conditions is
deficient. This climatic innuence of rainfd and temperature was combined by Blight
(1 982) to define a pedology-related index, N which is given as:-
where E = evaporation during the hottest period, usually January in
most parts of the tropics; and
P = the. a ,,n n..Iu.,*a. l r,a infall.
For South African environment fiom where this index originated, when:
N < 5, kaolinitic lateritic soil formation predominates; and
N > 5, montmosillodtic active soils predominate.
hlinite is a low activity clay mineral (Queiroz de Carvalho, 1991; Fourie and
Palmer, 2001) which consists of silica and sesquioxide (gibbsite) sheets stacked
together with hydrogen bond between the sheets (Lee, 1968; Lambe and Whitman,
1969). As a result of isomorphous substitution, etc., silica gets leached out of the
mineral lattice resulting into increase in proportions of sesquioxides (Tuncer and
Lohnes, 1977). This expb why lateritic soils are very rich in sesquioxides the
abundant presence, concentration and behaviour of which are responsible for the
observed disparities between them (lateritic soils) and their temperate counterparts.
For instance, the sesquioxides acting like a cementing agent bind the naturally fine
lateritic particles together to form coarser, hard but porous micro-clusters or
micropeds found to be responsible for the observed sensitivity of lateritic soils. This
leaching process is known to be very dynamic and hence it could be responsible for
continuous deterioration of lateritic soil and gravel materials even after placement an
(road, etc.) structures – Clauss (1967), Weinert (1968). Also, the leaching has been
observed to be very intense in some parts of the tropics reding into very high intragranular
voids as high as 60% responsible for the collapsible grain phenomenon in
some lateritic soils.
– ,, , 4 w3. 3,. , .,.? .
Abundant sesquioxides combined with residual kaolinite results into immature
lateritic soils, which becomes more matured with intense heat of the tropical dry
season. The degree of maturity deper;bs on the level of heat which at some specific
locations dehydrate the sesquioxides (in aqueous suspension) near the surface to
harden irreversibly into concretionary deposits called lateritic rock or cuirasse. This
hardening potential in lateritic soils brought about by dehydration of sesquuioxides is
the hi@point of the description by Buchanan (1 807) and Alder and Cady (1 962).
Arising fiom the laterisation process described above is the fact that lateritic soils
exist insitu as heterogeneous masses commencing from the ground surface with llly
laterised or matured soil with or without cuirasse and extending downwards vertically
in the profile to immature laterites and finally to the detrital parent rock. Infact, six
inter-related weathering stages, levels or degree of laterisation have so far been
identified in such lateritic vertical profile by Little (1969), Tuncer and Lohnes (1977)
and Townsend (1985) as illustrated in Figs. 1.3, 1.4 and 1.5 respectively.
Fig. 1.3 Morphological definition of degree of
decompositiIoN n o. f… rocks [after Little, 1969)
Fig. 1.4 Variation in engineering properties of basalt-derived
lateritic soils during weathering (after Tuncer and Lohnes, 1977)
Fig. 1.5. Degree of rock weathering and environmental
inf uence(after Townsend, 1985)
From these figures, stage one comprises just the fractured or fissured parent rock
while stage two comprises the detrital rock materials with little or no chemical
alterations of the parent rock minerals. At stage 3, the hydrolysis process has started
with about 50% of the chemically degradable rock-forming minerals decomposed to
form kaolinite or montmorillonite clay minerals as the case may be. With the
lateritic zone, it should be noticed that fiom stage three in Fig.l.4, kaolinite content
increases and maximises at a point after which it starts decreasing. This is because of
the continual leaching out of silica fiom the kaolinite mineral lattice giving rise to
continually increasing sesquioxide content or proportions. Between stages four and
five, immature lateritic soils predominate with increasing void ratio. Between stages
five and six, matured soh are formed through flocculation and coagulation of fine
grains into coarser micropeds hence continual increase in internal fiiction angle 4. Of
particular note is the formation of lateritic rock or cuirasse as a result of dehydration
of sesquioxides around the ground surface of fully laterised zones. These stages or
. ,. 4 . 1 .?. degrees of laterisation are also &wn to significantly inhence the mineralogy,
structure, behaviour and engineering response or characteristics of the soils.
I’ . ..
Although final matured lateritic so& bear little or no mineralogical relationship with
the parent rock as a result of chemical decomposition of the latter’s primary minerals,
studies across the tropics over the years have revealed a somewhat strong influence of
the age of parent rock on some lateritic soils. For instance for African lateritic soils,
younger rocks like basalt, sandstone, etc. are known to produce finer soil materials
than the older ones like igneous, metamorphic, etc. Thus the age of parent rock
directly influences the proportion of fines or percentage passing No. 200 or 75pm
opening sieve (Ackroyd (1 963),
(1 982).
Madu (1977), Ola (lY80), Horn and Schweitzer
Apart &om the hardening potential which all lateritic soils exhibit (Buchanan, 1807;
Alexander and Cady, 1962), additional but peculiar behaviours of some have led to
identification of different shades or types of these soils over the years. Already
identified subgroups include collapsible lateritic soils (Brink and Kantey, 1961 ;
Knight and Dehlen, 1963; Gidigasu, 1974; Sinclair, 1980; Clemence and Finbarr,
198 1; Mashbour et al, 1999; McKnight, 1999), problem lateritic soils (Osula 1989,
1991, 1996) and deltaic lateritic soils (Akpokodje 1986.1987) among others.
Deltaic lateritic soils in the context of this study are superficial soil deposits of
varying but location-dependent thickness ranging &om 6.0m to above 30.0rn found m
the Niger delta basm of southern Nigeria. Thus they are derived from and also
overlying coastal plain sunds of the Benin formation (Short and Stauble, 1 967).
Deltaic lateritic soils have been observed to possess some markedly different andfor
distinct engineering characteristics when compared with the other lateritic soils. This
could probably be attributed to the unique or peculiar nature or their parent materials
which comprising mostly the weIl-sorted or uniformly graded, non-crystalline or
unconsolidated clastic sediments like sand, gravel, etc. These sediments possess very
low proportions of chemically degradable or decomposable rock forming minerals
like feldspars and biotite mica, the main contributors to the laterisation process. Also,
the Niger delta basin where these soil group predominate is flat or near-flat terrained
hence deficient in at least two of the aforementioned necessary and sufficient
conditions for 111 laterisation. Thus this soil group may be considerably and/or
relatively imrnatured and probably exists between stages 4 and 5 on the lateritic
vertical profile postulated by Tuncer a d Lohnes(1977). As a result of this relative
immaturity, deltaic lateritic soils may also be much more sensitive to manipulations
than other lateritic soils.
* . ,. . .-1 . I.. . . .:> ‘
Also, deltaic lateritic soils possess relatively low percentage of fines passing sieve
No. 200 which is contrary to the widely observed lateritic gradation trend whereby
younger rocks produce finer lateritic soils as illustrated in Table 1.2. From this table,
it is obvious that whereas younger consolidated rocks like basalt, sandstone, etc.
produce lateritic soils with higher percentages of fines when compared with soils
derived fiom older (including pre-cambrian) rocks, reverse is the case with deltaic
lateritic soils.
‘ab. 1.2. Influence of parent rock on lateritic soils
LOCATION I PARENT ROCK OR 1 % FINES (passing I SOURCE
\ – – – – I
Ghana I – ditto – I 10 – 40 I Gidigasu (1972, 1974)
Puerto R b I Intrusive i g n w 73 – 85 I Lohnes et a1 (1 971)
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