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Geo-Characterisation of Highway Construction
Cost Drivers in the Niger Delta Region
Alolote Amadi I*
Department of Quantity Surveying, Rivers State University, Port Harcourt
Submission:April 27, 2020; Published: July 28, 2020
*Corresponding Author:Alolote Amadi I, Department of Quantity Surveying, Rivers State University, Port Harcourt
How to cite this article:Alolote Amadi I. Geo-Characterisation of Highway Construction Cost Drivers in the Niger Delta Region of Nigeria. Civil Eng Res
J. 2020; 10(2): 555785.DOI: 10.19080/CERJ.2020.10.555785
This study explores the geologic peculiarities of the Niger Delta region, which creates a propensity for the significantly high cost of highway development. The heterogeneous geologic configuration of the area, in terms of its landscape, geomorphology and hydrology are descriptively and techno-economically analysed, based on available local literature, supplemented by geotechnical field work data. The study discusses and analyses the construction cost implications of the inherent geo-variability of the terrain, and the financial implications as it relates to the practicalities of carrying out road construction in the Niger Delta. The analysis outlines the cost considerations associated with highway pavement and drainage construction in view of the hydro-geomorphological setting of the Niger Delta. The analysis reiterates the predominance of fined grained subgrade soils, which will require remedial re-engineering measures and significant construction costs. Furthermore, the study highlights the increased costs of drainage construction in view of the significant volume of precipitation surplus and relief of the region. The study submits that recognising the challenges and cost associated with the adverse geo-configuration of most parts of the Niger Delta, is a necessary starting point to maximise value for money in highway projects.
The Niger Delta is the 9th largest Delta in the world with an area of 19,135 square kilometres. It is one of the largest wetlands in Africa. The whole basin is dissected by numerous creeks, streams, marshes, and lagoons . The terrain of most parts of the Niger Delta is characteristically poorly drained, from the inundation of the adjoining lowlands forming swamps and coastal islands. However, the upper northern limit of the Niger Delta, is characterized by tropical rainforest woodland (Figure 1). This is the zone with the most extensive dry land, and the population distribution is denser, with far larger settlements than other wetland zones. This heterogeneous configuration of the terrain thus has correspondingly varying geotechnical implications for highway development.
The environmental condition of most parts of the Niger Delta makes the use of conventional pavement designs, materials and construction methods particularly in the swampy and coastal zones, inappropriate (Figure 2). The silted and marshy grounds of the swamps are representative of the difficulties of undertaking any
major construction in the area. Most of the soils encountered are soft and present sub-grade problems and have high water tables . Several scholars in the literature: [1-4] have consequently underscored the technical and financial undertones. This implies that pavement construction in these zones would ideally require expensive specification for works, ultimately increasing the cost of construction projects so many folds, compared to the uplands. The logical cost implications of the heterogeneous geologic setting of the Niger Delta soils for road construction can thus be viewed from the point of:
relatively higher design and specification requirements necessary to adapt the subsoil conditions in the swampy and coastal zones.
additional cost/expenditure that may likely be incurred during construction of projects situated in the swampy and coastal belts.
The subsequent subsections analyse the geo-variability of the terrain, and the financial issues that arise due to the practicalities of carrying out road construction in the Niger Delta occasioned
by adverse ground conditions. This is with a view to highlight
the impact of the geomorphology of the region on highway
The general geology of the Niger Delta consists of various
types of geologic deposits. Short & Stauble  identified three
main subsurface litho-stratigraphic units in the Niger Delta in
order their age: the Benin formations (water bearing zone);
the Agbada formations (middle); and the Akata formations
(base). In addition, these are overlain by the quaternary
deposits, representing the top 40-150m of the land surface 
corresponding the quaternary period, which is the youngest
period of the geological time scale. The rocks and sediments of
the quaternary period are of widespread occurrence on the land
surface. They tend to blanket all older deposits, except for a few
exposures. As a result, quaternary deposits lie directly within the
influence of construction works in the region and has being given
considerable scrutiny from scholars in the various interrelated
construction disciplines. For highway construction, it is the
topmost layer of the quaternary deposits, which represents the
subgrade soils, that is of significance and thus discussions on
their suitability for construction purposes are limited to this layer.
Oguara , analysed the properties of quaternary soils for road
construction with regard to their engineering application, stating
that: “Most of the soils encountered are soft and present subgrade
problems. The depth of ground water table also exists within the
reach of conventional foundation types”. Quaternary deposits in the
Niger Delta however show a wide spatial variability with frequent
irregular dispersion of sand and silt/clay intercalations with the
clay ratio increasing seawards. Its thickness is very variable and
goes up to 500m in certain places consisting of silts, clay, fine and
silty clays, sands and shale. This combines with the alternating
predominantly sands and clay profile of the Benin formation, and the high quantity of rainfall in the area to determine the distribution
and depths of groundwater in the area. Sub-soils in the riverine
areas of the Niger Delta, predominantly consists of extremely soft
deltaic marine clay locally referred to as ‘Chikoko’, which often
have CBR values less than 2, with a dark grey, dark brown to black
coloration . Fatokun and Bolarinwa in identifying the major
problem soils of Nigeria described those existing in the sub-soils
of the Niger Delta as: “Composed of ‘Peat, Organic silts and Clays
characterized by dark colours and strong odour which are highly
compressible with low strength” (pp 3). In the Niger Delta region,
these soil types occur as clayey peat over the mud plains and have
been shown to be highly undesirable in their natural forms for
road construction. This is due to their characteristic swelling and
shrinkage potential with corresponding changes in water content
as seasons change from rainy to dry. Thus, any structure including
road pavement constructed with or placed on such soils exist to
grapple with such typical geotechnical problems, in the absence
of adequate ground engineering.
Youdeowei and Nwankwala  recommended the excavation
of these soft expansive soils and replacement with suitable locally
available fill material such as sharp sand. Zumraw  opined that
this is an economical option to reduce the thickness of pavement
required in expansive soils (Figure 3). However problematic
swelling chikoko soils existing to irreplaceable extents both
laterally and vertically, within the freshwater zone of the Niger
Delta is also an established fact [7 & 10]. The views Abam et al.
 which echo those of Chukweze as well as several other earlier
scholars in the literature. Akpokodje [3 & 4]; Oguara  and Teme
, have consequently underscored that the terrain of the Niger
delta apparently has cost inducing potentials, which runs negative
to the ideal for construction works. Subsequent to these earlier
studies, several more recent works have being carried out, by
mapping out and studying specific areas in the region. Notable
works that have being published include: Omotosho & Ogboin
; Youdeowei & Nwankwala ; Otoko, ; Youdeowei 
and Ngerebara et al . Other related studies on the suitability of
sub-soils for road infrastructure development include Technical
Reports by George and Atuboyedia , Fatokun & Bolarinwa 
and the Nigerian Building and Road Research Institute (NBRRI).
These field work based studies have collectively stressed that
the geotechnical difficulties inherent in the Niger Delta call
for measures in designs and during construction to ensure
an adequate margin against the behaviour of these naturally
weak soils. The effort and methods requisite to improve the
geotechnical properties of soils, and to attain the standard
minimum engineering requirements, has direct and indirect costs
which correlates with the poorness of of the subsoils prevalent at
Various engineering parameters have been used to assess
the suitability of sub soils as bearing media for the construction
works. For road construction, the engineering properties of the
superficial sub-grade soils influence the designs and costs of
construction. The geotechnical parameters show the capacity of
the soils by providing descriptive and quantitative representation
of sub-soil strengths and constraints. Descriptive parameters
include grain size distribution used in soil classification. Grain size
analysis is used to identify the sizes of particles which make up a
soil sample to produce a grain size distribution of soil particles
. Uniform graded soils are referred to as poorly graded or
gap-graded on the basis that the ideal mix of the various size
range of particles is skewed and as such predominantly contain
one particle size all (small or all large). Soils consisting mostly of
gravel (>2 mm) and sand (0.1 –2 mm) are referred to as coarse
grained while silt (0.01 –0.1 mm), and clay (< 0.01 mm) which
are the typical grain size of Niger Delta soils, are fined grained
soils. Youdeowei  explained that poorly graded soils will have
relatively low densities and low strength, since the number of
particles per unit area and interlocking frictional forces is small, which reduces their structural stability for road construction. In
such soils, it was thus stated that modifications of the natural
properties of the sub-grade soils may be required.
To identify such soils in engineering application, two
common classification systems, the AASHTO and the Unified
Soil Classification System (USCS), are used for this purpose of
classifying soils based on grain size, with standard designations
for different soil particle size mixes. The ASSHTO classification
system has 8 major groups: A1~ A7 (with several subgroups)
and organic soils A8–. with a corresponding progressive decrease
in the suitability of the sub-grade soils from A1 to A8. Based on
available field data for 61 spatially distributed coordinates, the
approximate descriptive geotechnical properties of subgrade soils
are presented in Table 1, using the AASHTO system.
It can thus be discerned that about two thirds of the soil
types in the Niger Delta fall within the lowest range of soils (A-
6, A-7-6 and A-8) classified as fair to poor sub-grade soils in
engineering application, due to the high percentage of very fine
grained soil particles (Less than 2mm). The prevalence of finegrained
particles in the Niger delta soils, limits the applicability
of the soils as bearing media for highway pavement construction.
Although the particle size distribution and classification are useful
in providing a broader picture of the engineering applicability
of the soil types in the region, they are more descriptive. Other
more quantitative geotechnical parameters such as plasticity, free
swell and maximum dry density correlate to give more specific
geotechnical attributes of sub-soils. The index properties of the
Niger Delta soils at spatially dispersed locations are presented in
As the data shows, finer grained soils are predominant in the
Niger Delta region, particularly in the swamps and coastal areas.
The Atterberg limits (Plastic Limit and Liquid limit) as well as
the Plasticity index show that the subgrade soils in the region are
highly plastic. Highly plastic soils display high volume changes
with the introduction of moisture. The occurrence of such soils
has been noted to be problem soils in the region, as they are
associated with significant volume changes with progression
from dry season to wet [6 & 8]. The liquid limits of sub-grade soils
in the Niger Delta have a range of 70.5% varying between 0 and
71%, with a mean value of 40, with plasticity index ranging from
The free swell of soils in the region have values ranging from
0 to as high as 80%. The average free swell (50) depicts a high
swelling and compressibility potential of soils in the region Table 3. The range of values of maximum dry densities at optimum
moisture contents of between 8.2% and 22.8% respectively fall
between 1430 kg/m3 and 2400 kg/m3with a mean of 1430 kg/m3.
The plasticity index and swelling potential of the Niger Delta soils
can thus be inferred as an indication of the how much financial
constraint is posed by ground conditions to highway construction.
Higher design requirements in pavement and drainage designs as
well as adequate re-engineering of the soils is thus a pre-requisite
for highway projects, implying:
a. Requisite changes in the level of excavation or fill,
b. More compaction requiring the use of more powerful
c. Stabilisation of subsoils
d. other site-related practicalities required in managing
such soils during construction.
Typically, Otoko and Pedro , Otoko and Precious (2014)
as well as Otoko  have investigated various cost effective
stabilization alternatives suitable to improve subgrade soils in
the Niger Delta. These include cement stabilization of laterite and
chikoko soils using waste rubber fibre or with groundnut shell
The predominance of fined grained subgrade soils will
logically portend significant construction costs . This is view
of the fundamental requisite feature of sub-grade soils for highway
construction. The resistance of soils to deformation under imposed
vehicular load, underlies the philosophy of pavement design).
The California Bearing Ratio (CBR) named after the California
Highway Department, is one of the fundamental methods based on
evolving experience of sub-grade failure, adjudged to be a reliable
composite index of sub-grade strength, whereby soils have to be
tested using specified testing procedures under simulated dry and
wet conditions . High CBR vales indicate good strength while
very low CBR values are typical of highly plastic clays with volume
instability. Strength of sub-grade are determined from standard
geotechnical testing procedures and are also given in interval ranges of six CBR classes; S1 to S6. The poorest soil class being
S1 having a CBR value of 2% with the best soils having values
above 30%. The six CBR classes that are provided (S1 to S6) with
a minimum value of 2 for soft soils as follows:
a. CBR, 2.0 % (S1) .......... Very soft ground.
b. CBR, 3.0 – 4.0% (S2) .......... Soft ground.
c. CBR, 5.0 - 7.0 % (S3) .......... Medium hard ground.
d. CBR, 8.0 - 14.0% (S4) ............Hard ground.
e. CBR 15- 29% (S5) ............Hard ground.
f. CBR 30 (S6) ............Hard ground.
Using the CBR estimates of the thickness of each pavement
layer relative to material strength can be made. According to
Ngerebara et al , this approach to flexible pavement design
in the Niger Delta has gained credibility in highway designs, as
other geotechnical properties correlate very well with the CBR.
The Nigerian standard of road designs principally adopts this CBR
philosophy, from the Transportation and Road Research laboratory
(TRRL) Over Seas Road Note 31 , which serves as a guide to
the Structural Design of Bitumen-Surfaced Roads in Tropical and
Sub-Tropical countries . It is the basis for road designs which
is used by the Federal Ministry of Works (FMW) without much
modification. Based on the CBR philosophy, the thicknesses of
the cross sectional elements: surfacing; binder course; base; subbase;
and capping, in a series of standard material combination
for different traffic and subgrade classes are provided by TRRL
Over Seas Road Note 31 . The structural catalogue specifies
up to 8 variants of differing materials and standard thicknesses to
be deployed based on the relevant combination of CBR and traffic
The combined thickness of the pavement structural layers,
from sub-grade to surfacing, based on this subsoil designation
increases from 200 to 300mm for S6 (Hard ground) soils , up to
650-1000mm for S2 (Soft ground) soils. The implication of the
choice of a particular design is the influence it has on the volume
and nature of construction materials which are needed. Newcombe
and Birgisson  analysed the CBR relative to the required
thickness implied by structural relevance of the base layer in a
typical flexible pavement and the contribution of the granular base
to the overall structural capacity. As Newcombe and Birgisson
 show, high CBR values translates into a reduction in required
base layer thickness. Similarly, the United States Department of
Transportation  analysed the financial sensitivity of designs
in response to subgrade CBR properties. The study showed that
the CBR of subgrade soils is a good representative indicator of the
overall sensitivity of pavement costs to geotechnical properties
of subgrade soils Based on the analysis, at a traffic loading of 10
million ESALs and a subgrade CBR of 8, the anticipated cost per
850m2 of surface area of pavement is about $12,800 relative to
$15,600 for a poorer quality of sub-grade with a CBR value of 4.
This represents an increment of 20% of costs to for the same area
of pavement section.
This analogy directly reflects the variable cost of construction
considering the predominantly adverse soil profile in
heterogeneous terrain of the Niger Delta, which has soil types
ranging between extremes. For locations with CBR’s less than
2, which are typical of the expansive clayey peaty soils of the
swamps and coastal areas, the TRRL  guide outlines the need
for special treatment. As such problematic marine clays/peat
(Chikoko) soils, rated as very poor, with characteristic shrinkage
and swelling potential, abound in the riverine areas, will require
remedial re-engineering measures [2,13,25]. Such reengineering
will imply additional costs for either:
a. Excavation to remove such poor soils to the extent of
prevalence, and replacement with adequate fill materials
b. Compaction and preloading with locally available free
c. Stabilisation in place with chemical additives to achieve
the required minimum strength characteristic for the sub-grade.
Such measures will entail importing good quality laterites
with high CBR from burrow pits, subject to the availability
of source sheds and the haulage distance from these source
locations, often situated in the coastal plain soils of the Upper
Delta, to construction sites in the swamps and along the coast.
Table 4 shows the distances of some riverine and upland locations
in the Niger Delta from Ahoada, via Port Harcourt, situated in the
coastal plain soils of the upland regions, which has estimated
lateritic reserves in excess of 15 million tonnes. The swampy and
coastal regions which are often cut off by lack of existing roads
to transport materials, are suggestive of the prohibitive cost of
replacing such poor soils with suitable materials. The additional
costs of loading of materials at source, spreading and compacting
and backhaul (return of empty vehicle) equally push up the cost of
this subgrade improvement technique.
Road surface water drainage is principally required to drain
both the carriageway and the surrounding catchment of the
drain. 10% of the total cost of road construction, is required for
drainage. Drainage construction cost requirement is however
location specific and will to a significant extent be a determined
by the relief and precipitation parameters and ground water
levels at the location.
The relief of the Niger Delta region is that of a broadly flat,
gently steeping topography, from a height ranging from 40m to 8m
above sea level, inland to the coast (Figure 4). Rainfall is typically
high in the Niger Delta, with a progressive increase southward
towards the coastline, due to regular heavy downpours. Ngah
and Youdeowei  reveal annual rainfall isohyet values across
the Niger Delta which also shows a progressive increase seaward
from the inlands (Figure 5). The Nigerian Meteorological services
(NIMET) records reveal very high levels of annual average rainfall
(5000 mm), with a reduction towards the middle Delta, while the
upper Delta experiences the lowest values ranging between 3000
mm to 2000 mm. Table 5 shows the average rainfall amount for
some locations in the Niger Delta.
Weather pattern coupled with relief determine the quantity
of overland flow over the Niger Delta region, which evacuate
via a reticulate network of meandering system: creeks, streams
and rivers, 18 of which discharge directly into the Atlantic
Ocean. Due to the high volume of rainfall, low evaporation and
the predominantly poorly draining fine grained soils, flooding is a major issue in the region with about 50% of the landscape
regularly inundated by floods during the rainy season. Abam 
analysis of rainfall and concurrent evaporation indicate that the
amount of precipitation excess is comparatively large during the
rainy season months (Figure 6). Almost every part of the Niger
Delta is thus under water at one time of the year or another. The
northern part, which is the habitable part, remains dry after the
rains and floods except for the large swamps referred to above.
This is because since the sandy riverbed soils are porous and very
pervious, there is a rapid rise in the ground-water level, a process
which subsequently floods the lowlands.
The depth of ground water table in the Niger delta also
exhibits seasonal variability, in response to rainfall distribution:
7-10m in the uplands and 0-5m along coastal zones. Abam 
however, asserted that in the swamp zones with minimal ground
water depths (0-0.5m), this variability is more closely linked to the
topography. Groundwater recharge in the Niger Delta is primarily
by precipitation. (Figure 7) shows historical data on ground water
levels in response to seasonal precipitation levels.
The effect of ground water conditions ur is very important in
construction works, more so in coastal areas in the Niger Delta
region where the water table is always high all year round. The
nature of the effect of shallow depths of ground water lies in its
interference with other aspects of construction activities on site
which has method related cost implications. The impact of shallow
depths of ground water the usefulness of the Niger Delta soils for
construction, in terms of compaction, was noted by Akpokodije
. This due to its alteration of the geotechnical properties of soils
by causing a high in-situ moisture condition in the soils, which
then exist in a perpetual state of near saturation. The engineering
properties of the soils therefore become considerably weaker with
increasing water content, hence the maxim ‘Water is an enemy to
soils for construction purposes.
The most used techniques, to control groundwater are the
introduction of capillary cut-offs and raising embankment height
or use of sub-surface drains. The former procedure entails
dewatering of the subsoil, by extracting water from between the
saturated soil pores, and diverting of surface water to natural
drains. Muck and slush are then removed from the surface to pave
way for the embankment
As shown in (Figure 8), a capillary cut-off sand layer is then
introduced, after the placement of a geo-textile separator layer.
This serves as a preventive mechanism against the migration
of fine soil particles from underlying soft soils, which have the
propensity to clog the sand layer. The tremendous cost associated
with this construction method which may be warranted in
extremely poor soil conditions, such as that of the Chikoko
clayey peaty soils widespread in the swamps, is self-evident. As
alternative to the above procedure, lowering the water table can
be achieved by use of adequate sub-surface drainage measures.
This entails constructing deep buried drains also referred to
as ‘Trench Drains, along the length of the road pavement in the
water-logged sections to lower the natural water-table. This is
however more feasible in situations where the soil properties
are not so bad. However, the sides of trenches generally lose
the ability to retain their original vertical position in the typical
soft soils of the riverine areas, which may cause it to cave in.
This behaviour of the soils is rectified by timbering with poling
boards placed side by sides, walled and strutted along the sides
of the trench. Alternatively, the trenches may also be sheeted.
The ground excavations are done 225mm depth at a time, and on
reaching each 225mm depth in excavation; the poling boards are
placed in pairs, one on each side of the trench against the sheeting
and strutted. The associated cost of excavation, sheeting and
timbering are further additional cost, for drainage construction in
the swampy and coastal areas.
Adequate drainage design and construction, in consideration
of the intricate hydro-geotechnical dynamics of the Niger Delta
region is thus a major consideration for highway pavement
construction. The Transportation and Road Research Laboratory
 Road Note 6, Guidance Notes on Road Pavement Drainage
Design, emphasized the impact of surface drainage on the design
of road pavements and the need for simultaneous design of both
parts of a road. The design guide listed several geomorphologic
factors, in addition to estimated volume of maximum rainfall
expected in an area, which will determine the overall design of
drainage: The surrounding terrain (slope and rate of runoff):
subgrade soil (permeability and other drainage characteristics; ground water table (depth). In relating these drainage design and
construction requirements to the archetypical features of swampy
and coastal locations of the Niger Delta region, considerations
must be given to the following:
a. The coastal terrain has a generally flat topography which
does not encourage rapid run off of surface water. Particularly in
the low lying back-swamps of the fresh water and saltwater zones,
surface water can hardly be drained off by gravity. A very slight
gradient (higher than in the swampy regions) however exists in
the upland coastal plain regions, which allows faster run-off of
b. The soils are predominantly much finer grained in the
swamps than in the upland plains, which is indicative of their
very low permeability and drainage characteristics. These soils
which are generally highly plastic tend to hold water in their
pores. However, in the upland regions, the stratigraphy of the
soils reveals higher content of permeable sands intercalated with
silts and pockets of clays, resulting in their silty-sandy profile.
This implies a higher rate of infiltration into the ground upon the
occurrence of heavy downpours.
c. Water-logging of soils, from clayey sub-soils with high
ground water tables in the swamps and coastal islands, often
occur, as parts of the region generally exist in a state full or near
saturation conditions, with little or no capacity for infiltration of
the high volume of overland flow during the rainy season months.
Waterlogging is however not common in the uplands, due to the
predominantly less cohesive, more permeable soils and lower
ground water profile, that encourages rapid infiltration of rainfall
into the ground.
In view of the intricate hydrological balance between subsurface
and surface water in the Niger Delta, the cost associated
with the construction of highway drainage will depend on the
hydro-geomorphological attributes of the project location.
The inherent peculiarities of the Niger Delta terrain serve as
a strong geologic justification on the need to exploit every avenue
for ensuring value for money. The geomorphological attributes of
the Niger Delta have a significant impact in determining the design
and construction cost of highway projects. Effective practical
measures are needed to improve the engineering characteristics of
the highly compressible silty clay soils and managing construction
activities. Riverine and coastal zones require the construction of
adequate and expensive pavement and road drainage systems of
higher design standards to cater for subsoil conditions, surface
water flow and ground water. These are summarily captured in
Table 6. All these measures translate to additional cost for road
projects in coastal and swampy areas, whose prohibitive cost has
been sometimes termed as ‘unfeasible’ and to which the almost
complete absence of highway development of these areas has
being attributed. Recognising the geotechnical challenges and
cost associated with the adverse geologic configuration of most
parts of the Niger Delta, is thus considered a necessary starting
point to maximise value for money in highway projects.
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