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Assessment Of Geo-Technical Competence Of Tertiary
Sedimentary Deposits In Umudike Area, Southern Nigeria
*Nwokoma E.U. & Chukwu G.U.
Department of Physics, Michael Okpara University of Agriculture, Umudike,
P.M.B. 7267, Umuahia, Abia State, Nigeria.
*corresponding author’s Phone, Email: +2347039846930, [email protected]
ABSTRACT
In order to determine the competence of the near-surface formation as foundation material of
Umudike/Ikwuano area of Abia State, geo-technical survey was conducted within the study area. The
survey was focused at evaluating the stratigraphy and competence of the shallow formation material.
Geotechnical laboratory result shows that the soils are generally of low natural moisture content. It has
relatively low clay content as revealed by the percentage passing 0.07mm sieve mesh. Since the Plastic
Index of the soil within the area is less than 20%, the soil can be adjudged to be low to medium plasticity;
hence, the soils are expected to exhibit low to medium swelling potential. The result also shows that the
linear shrinkage of all the tested soil samples were above 8% except sample C at ABSU; therefore for the
construction of high rising building with sample A (Timber market), sample B (GCU), sample D
(NRCRI) and sample E (MOUAU) are recommended. Based on the results of the survey, the soils within
which engineering structures will be founded within the study area are competent.
Keywords: competence, stratigraphy, swelling potential, linear shrinkage, soil, plasticity.
INTRODUCTION
The statistics of failures of building structures throughout the nation has increased geometrically. These
failures have been attributed to a number of factors such as inadequate information about the soil, poor
foundation design and poor building materials as well as handling problems.
Soils have been attributed to factors causing the failures of buildings simply because some earth
materials, due to their nature, cannot support solid and rigid structures. Among these materials are clays
and clay-bearing earth contrarity, earth materials such as sands and fresh basement rock provide firm
support for solid foundation for roads, buildings, dam sites, bridges, etc.
High rising buildings are among large civil engineering structures that are subjected to strong dynamic
and static loads. Thus, design and construction should be preceded by adequate investigation in order to
prevent the collapse. Since structural failure ranges from settlement, differential settlement, upthrust and
total collapse; therefore geological observations, geophysical measurements, soil explorations, in-situ
tests and laboratory tests are needed to provide information of the subsurface sequence and structural
disposition necessary for foundation design.
An essential part of site recommendation and foundation design for high rising buildings is to devise a
foundation type and size that will result in acceptable values of deformation (settlement) and an adequate
margin of safety to failure. Therefore, before a foundation design can be embarked upon; the associated
soil profile must be well established and the results of sub-surface information adequately evaluated.
International Journal of Innovative Scientific & Engineering
Technologies Research 4(4):40-62, Oct-Dec. 2016
© SEAHI PUBLICATIONS, 2016 www.seahipaj.org ISSN: 2360-896X
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Many other parameters such as stability studies, depth to bedrock, stratigraphic continuity, structural
mapping, etc are also necessary (Hunter et al., 2011).
Presented here is geotechnical survey carried out in Umudike area and its environs. Foundation study
usually provides subsurface information that normally assists civil engineers in designing the foundation
of civil engineering structures. To this end, geotechnical approaches are routinely used for foundation
investigation.
In Abia State of Nigeria, there are about eleven different geologic formations and cases of erosion menace
and failure of boreholes have been frequently reported especially in the northern and central parts of the
state than in the southern parts (Fig. 1). These have been attributed to a combination of distinct
geological, morphological and pedological characteristics. Umudike area and its environs are located
within a transition zone of two different geologic formations namely: Bende-Ameki Formation and Benin
Formation. Within a transition zone, there are at times abrupt or gradual changes in lithology; therefore a
complex overall situation with respect to defining the competence of near-surface formation as foundation
materials could arise as a result of attempts in the construction of high rising buildings.
Location of the Study Area
The chosen study area (Umudike and its environs) is located within the central parts of Ikwuano-Umuahia
area which lies within latitudes 5o28‟645N and 5o34‟645N and longitudes 7o31‟602E and 7o34‟661E.
Five geotechnical sampling test pits were used in the study.
Institutions and research centres like Forestry Research Institute, New Industrial Market, Soil and Water
Department of the Federal Ministry of Agriculture and Rural Development and Government College,
Umuahia are situated within the area. Others are Abia State University campus, National Root Crops
Research Institute (NRCRI) and Michael Okpara University of Agriculture, Umudike (MOUAU) (Figure
2).
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Figure 1: Geologic map of Abia State of Nigeria showing the study area
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Fig. 2: Map of Ikwuano-Umuahia area of Abia State showing the study area.
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The coastal plain sands which are the constituents of Benin Formation are predominantly yellow and
white sands alternating with pebbly layers and clay beds (Reyment, 1965). The formation comprises of
shale/sand sediments with intercalation of thin clay beds (Asseez, 1976; Murat, 1972).
The sands are mostly medium to coarse grained, pebbly, moderately sorted with local lenses of poorly
cemented sands and clays. Petrographic analysis indicates that the composition of the rocks is as follows:
95-99% Quartz grains, 1-2.5% of Na+K-mica (Onyeagocha, 1982).
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Table 1. Stratigraphic correlation chart of post-Santonian to Recent Niger delta outcrops and their subsurface equivalents.
Age Surface outcrop equivalent formation Subsurface formation Mega depositional
environment
Pleistocene – Recent
Alluvial Plain deposits
Alluvial Plain deposits
B e
n i
n
F o
r m
a t
i o
n
Continental
Pliocene –Recent
Miocene – Recent Coastal Plain Sands Afam Clay Member Paralic Continental
Oligocene - Recent
Ijebu Formation
Ogwashi-Asaba Formation
Continental Delta Plain
Eocene – Recent
Ilaro Formation
Agulu-Nanka Sands
Bende-Ameki
Formation
Agbada Formation
Paralic Delta Front
Oshoshun Formation
Paleocene – Recent
Ewekoro Formation Imo Formation Akata Formation Marine Pro-Delta
Maastrichtian – Paleocene
Nsukka Formation
U n
k n
o w
n
E
q
u i
v a
l e
n t
s
P r
o-
D e
l t
a
S u
c c
e s
s i
o n
s
Maastrichtian
N k
p o
r o
S
h a
l e
Enugu
Shale
Ajalli Sandstone
N k
p o
r o
S
h a
l e
Campanian
Mamu Formation
Owelli
sandstone
Nkporo Shale
Enugu Shale
Santonian
Orogenic complex crust
*Modified after (Short and Stauble, 1967), Petters (1982); Amajor (1986); Edet et al., (2011) and (Amos-Uhegbu et al., 2012).
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Regional Geology And Physiography Of The Study Area
Abia State the study area is located within the tropical rainforest belt. Climate of the area is characterized
by two main seasons: the rainy season and the dry season. The dry season originates from the dry
northeasterly air mass of Sahara desert (Harmattan), while the rainy season originates from humid
maritime air mass of Atlantic Ocean.
The rainy season spans from Mid-April to Mid-November while the dry season spans from Mid-
November to Mid-April. The rainy season is characterized by double maxima rainfall peaks in July and
September, with a short dry season of about three weeks between the peaks known as the “August break”.
The mean monthly rainfall in the rainy season in the area ranges from about 320mm to 335mm while that
of the dry season is about 65mm, thus the annual average rainfall ranges from about 2000mm to 2400mm
with high relative humidity values over 70% (Adeleke and Leong, 1978).
Abia State is characterized by a great variety of landscapes ranging from dissected escarpments to
rolling hills, and has principal geomorphologic regions ( plains and lowlands) such as the Niger River
Basin and the Delta; the coastal plain and the Cross River basin; the plateau and the escarpment.
Geologically, present Nigeria was probably broad regional basement uplift (upwarp), with no major basin
subsidence and sediment accumulation during the Paleozoic to Early Mesozoic, simply because older
Phanerozoic deposits were not preserved, but around this region Paleozoic deposits accumulated
northwards in the Northern Iullemeden Basin in Niger, westwards in Coastal Ghana, and Southward in
Brazil, South America (Petters, 1982).
A triple-R junction (rift system) developed during the break-up of Gondwana leading to the separation of
the continents of South America and Africa in the Late Jurassic. The third arm of the rift after extending
to about 1000 km northeast from the Gulf of Guinea to Lake Chad failed (aulacogen), thus forming the
Benue Trough. A rapid subsidence of the trough ensued (aulacogen - failed continental margins) as a
result of the cooling of the newly created oceanic lithosphere. Subsequently, sediments from weathering
of the basement uplift were deposited into the trough through rivers and lakes by Early Cretaceous.
By Mid-Cretaceous onwards marine sedimentation took place in the Benue Trough; thus making it
possible in conjunction with other geologic events for Nigeria to be presently underlain by sedimentary
basins. The Benue Trough is arbitrarily divided into Lower, Middle and Upper Benue Trough; and by
Santonian times the area underwent intense folding and compression whereby over 100 anticlines and
synclines were formed.
After the Santonian-Campanian tectonism which formed the Abakiliki anticlinorium, the western margin
of the Lower Benue Trough subsided, and the corresponding synclinorium became the Anambra basin
where over 2500 m of deltaic complexes accumulated. However by Eocene, the inception of Tertiary
Niger Delta Basin commenced. Thus, the Late Cretaceous deltaic sedimentation in the Anambra Basin
was followed by the shift in deltaic deposition southward and consequently the construction or
outbuilding of the Niger Delta took place.
Geologically, the study area falls within the Benin Formation of the Cenozoic Niger-Delta province of
Nigeria.
The Niger-Delta started to evolve in Early Tertiary times when clastic river depositions increased leading
to the delta progradation over the subsiding continental-oceanic lithospheric transition zone, and
subsequently prograded on to oceanic crust of the Gulf of Guinea during the Oligocene. The sediments
were sourced through the weathering flanks of the continental basement outcrops via the Benue-Niger
drainage basin. The delta has since Paleocene epoch prograded a distance of more than 250 km from the
Benin and Calabar flanks to the present delta front. The interplay between subsidence and deposition
arising from a succession of sea transgressions and regressions (Hospers, 1965) gave rise to the
deposition of three lithostratigraphic units in the Niger Delta (Short and Stauble, 1967).These units are
marine Akata Formation, paralic Agbada Formation, and the continental Benin Formation. The overall
thickness of these sediments is about 12,000 meters covering a total area of about 140,000 km2 (Obaje,
2009).
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Previous Geophysical Work In The Area
As stated earlier on, the study area is within the Bende-Ameki Formation and the Benin Formation of the
Coastal Plain sands (see insert Figure 1).
Mbonu et al., (1991); carried out a geoelectrical determination of aquifer characteristics of the coastal
plain sands of the area. Okolo (2004) also used available geoelectrical data in correlating boreholes within
the study area and concluded that the boreholes fall within same lithological sequence but with differing
thicknesses. Chukwu (2010) investigated causes of borehole failures in Ikwuano-Umuahia area (Imo
Formation, Bende-Ameki Formation and Benin Formation) using geoelectrical methods.
Amos-Uhegbu et al., (2014) used geological, drill log, geophysical and hydrogeo-chemical techniques in
the delineation and characterization of the aquifer systems in the study area. Nwokoma et al., (2015) used
geoelectric method to investigate the soils as foundation material. Little or nothing has been done on
geotechnical foundation studies, thus this is probably the first geotechnical investigation of the study area.
METHODOLOGY
The first exercise was the gathering of relevant literature materials of the area under investigation
including maps.
A reconnaissance survey of the study area was carried out in which the determination of surface
elevations and co-ordinates were done.
A general inventory of the geological parameters (geological formation, surface run-off, climatic factors
and types of lithology) were done.
This inventory was carried out using the following instruments: hammer, sample bags, measuring tape,
Global Positioning system (GPS) and map of the study area.
Garmin 72 Global Positioning system (GPS) was used in the determination of elevation and coordinates,
which further aided in gridding of the area.
Geotechnical soil sampling points were chosen, the types of data collected at each locality are listed in
Table 2.
Table 2: Data localities and type of data collected for the study
Data
Number
Data Location GPS Reading Type of Data
Collected Elevation (m)
a.m.s.l
Latitude °N Longitude
°E
1 Umuohu-Azueke (Ministry
of Agriculture)
(186.5m) 5034.623
! N 7
034.661
! E
2 Umuohu-Azueke (Timber
Market) (135.4m) 5
030.558
! N 7
032.004
! E
3 Umuohu-Azueke (Timber
Market)
(148.9m) 5030.318
! N 7
031.602
! E GTP
4 Umuohu-Azueke (Behind
GCU)
(131.5m) 5030.134
! N 7
032.233
! E
5 Umuohu-Azueke (Inside
GCU)
(151.2m) 5030.070
! N 7
032.268
! E
6 Umuohu-Azueke (Igbugbo
Opposite GCU)
(162.5m)
5034.645
! N 7
032.564
! E GTP
7 Umudike (Ihiuzo
American Quarters
Plantation)
(147.0m)
5029.560
! N 7
032.323
! E
8 Umuohu-Azueke
(ABSUPAC)
(137.9m)
5028.645
! N 7
033.721
! E GTP
9 Umuohu-Azueke (Behind
ABSUPAC)
(123.0m)
5029.732
! N 7
032.334
! E
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10 Umudike (Behind NRCRI) (98.3m)
5028.877
! N 7
032.411
! E GTP
11 Umudike (Inside NRCRI) (107.5m)
5028.859
! N 7
032.432
! E
12 Umudike (V.C‟S Lodge) (126.3m)
5029.312
! N 7
032.761
! E
13 Umuariaga (Opposite
MOUAU) (129.4m)
5028.881
! N 7
033.052
! E
14 Umudike (Inside
MOUAU) (113.3m)
5028.793
! N 7
032.433
! E GTP
15 Umudike (Behind
MOUAU)
(159.3m)
5029.521
! N 7
032.445
! E
16 Amaoba I (199.4m)
5029.421
! N 7
032.445
! E
17 Amaoba II (172.7m)
5029.633
! N 7
032.544
! E
18 Amaoba III (190.1m)
5029.655
! N 7
032.632
! E
GTP = Geotechnical Test Pits, MOUAU = Michael Okpara University of Agriculture Umudike, NRCRI=National
Root Crops Research Institute.
Five test pits were excavated and soil samples collected at different locations within the study area. These
samples were preserved in polythene bags and transported to the Geology laboratory of University of
Port-Harcourt within three hours of collection. The natural moisture content of the samples collected from
the field was determined in the laboratory within a period of 24 hours after collection. Further
determination of other parameters was followed by air drying of the samples by spreading them out on
trays in a fairly warm room for four days. Large soil particles (clods) in the samples were broken with a
wooden mallet. Care was taken not to crush the individual particles. Methods of testing soils for
engineering purposes were conducted in accordance with British Standard 1377 for all the soil samples
collected, the tests include specific gravity, grain size analysis, liquid limit, and plastic limit.
Determination of Water Content
Water content can be directly measured using a known volume of the material and a drying oven.
A specified mass of wet sample ‟ was put in an oven pan and weighed immediately after collection
on a scale and mass recorded. After weighing, the wet sample „ ‟was heated in an electric oven at a
uniform temperature of 110ºC for about 1hr 6000 secs, and then allowed to cool.
Upon cooling, the sample is reweighed on the scale and labeled „ ‟.
Geotechnics requires the moisture content to be expressed as a fraction of the sample's dry weight
i.e. % moisture content = u * 100%
where … 1
Determination of Specific Gravity
Specific gravity in this context could be defined as the ratio of the unit weight of a given material to the
unit weight of water. This was determined in the laboratory as follows:
The dry density bottle was weighed on the weighing balance to obtain W1.
25.0g of the oven dried soil specimen was obtained and transfered into the density bottle; the stopper was
replaced and the bottle and contents weighed to obtain W2.
The bottle was half-filled with distilled water and stirred. After about 5minutes; the bottle was then fully
filled, corked and carefully shaken to remove any remaining air and subsequently weighed to obtain W3.
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The contents of the bottle were emptied; washed thoroughly and filled with water, then weigh to obtain
W4.
The formula below was used to obtain the specific gravity (Sp) of the soil:
… 2
Mechanical Sieve Analysis
To conduct the sieve analysis, the soil samples were first oven-dried and then all lumps broken into
smaller particles. The soil is then shaken through a stack of sieves with openings of decreasing size from
top to bottom (a pan is placed below the stack); the smallest size sieve that was used for this type of test is
the BS 0.075mm sieve while the largest is 200mm. After the soil is shaken, the mass of soil retained on
each sieve is determined:
Mass of soil retained % ... 3
The graph of percentage passing the sieve (mass of soil retained) is calculated from the formula above;
and is plotted on a semi-logarithm graph paper against sieve opening size (abscissa). The logarithmic
scale is called particle –size distribution curve (Fig.3).
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Fig 3 A display of the particle – size distribution chart
Atterberg Limits and Derived Limits
The Atterberg limits measure the critical water contents of fine-grained soils. A dry, clayey soil
undergoes changes in behavior and consistency as its water content increases. The changes maybe in four
states: solid, semi-solid, plastic and liquid depending on its water content.
As a hard, rigid solid in the dry state, soil becomes a crumbly (friable) semi solid when certain moisture
content, termed the shrinkage limit, is reached. If it is an expansive soil, this soil will also begin to swell
in volume as this moisture content is exceeded. Increasing the water content beyond the soil's plastic limit
will transform it into a malleable, plastic mass, which causes additional swelling. The soil will remain in
this plastic state until its liquid limit is exceeded, which causes it to transform into a viscous liquid (Fig.
20).
Since the consistency and behavior of a soil is different and consequently so are its engineering
properties. Therefore, Atterberg limits are used in soil's classification and in engineering purposes
because a close relationship exist between Atterberg limits and soil properties such as compressibility,
permeability and strength. These tests are used in the preliminary stages of designing any structure to
ensure that the soil will have the correct amount of shear strength.
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Fig.4: The consistency of soils according to Atterberg limits
Shrinkage Limit
The shrinkage limit (SL) is the water content where further loss of moisture will not result in any more
volume reduction, but where the degree of saturation is still essentially 100 % (Holtz and Kovacs, 1981).
The Shrinkage Limit test calculates the volumetric shrinkage and the Linear Shrinkage test is used to
calculate one-dimensional shrinkage, although the volumetric shrinkage may be calculated.
Plastic Limit The plastic limit (PL) is the lowest water content at which soil behaves like a plastic material. In the
laboratory, it is the water content, in percentage, at which a soil can no longer be deformed by rolling into
3.2 mm diameter threads without crumbling.
Liquid Limit
The liquid limit (LL) is arbitrarily defined as the lowest water content above which soil behaves like
liquid. In the laboratory, it is the water content, in percentage, at which a part of soil in a standard cup and
cut by a groove of standard dimensions will flow together at the base of the groove for a distance of 13
mm when subjected to 45 shocks from the cup being dropped 10 mm in a standard liquid limit apparatus
operated at a rate of two shocks per second.
Plasticity Index
The plasticity index (PI) is a measure of the plasticity of a soil in respect of its water contents. It is the
difference between the liquid limit and the plastic limit (PI = LL-PL).
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RESULTS AND DISCUSSION
Test-Pit Logs
The practice of making a detailed record of the geologic formations penetrated by boring a hole is called
logging. The log may be based either on visual inspection of samples brought to the surface (lithological
logs) or on physical measurements made by instruments lowered into the borehole (geophysical logs).
The lithologic deductions of the five test-pits are in Table 3.
Table 3: A profile of VES data of the various sounding stations in the study area
Depth (m) Lithology Test pit
1 Loose silty or clayey fine-grained brown sand A (Timber Market)
1 Loose silty or clayey fine-grained brown sand B (GCU)
1 Loose fine to coarse-grained light brown sand C (ABSU)
1 Fine-grained brownish red sand D (NRCRI)
1 Loose but gritty reddish sand E (MOUAU)
Geotechnical Soil Classification Tests
Soil classification for civil engineering purposes is primarily on the basis of particle size (notional particle
diameter) for coarser particles, but also on the basis of mineralogy (plasticity) for finer material.
Classification Systems vary from country to country, but most are based on the US system (The Unified
Soil Classification System, USCS), or the British Standard Soil Classification System.
Geotechnical soil classification tests for samples A, B, C, D, and E were carried out in accordance with
the relevant British Standards “BS 1377:1990 Method of soil test for civil engineering purpose”. The
summary of the results as obtained from the laboratory experiment are shown in Table 4.
Table 4: A summary of the results of the geotechnical soil classification tests
Sample
Location/
Pit No.
Natural
Moisture
Content
(%)
Percentage
Passing
0.075mm
(%)
Liquid
Limit
(%)
Plastic
Limit
(%)
Plastic
index
(%)
LS
(%)
MDD
g/cm3
OMC
(%)
(A)
Timber
Market
19.5 20 43.2 37.7 5.5 9.5 1.53 19.1
(B)
GCU
(C)
14.9 18 47.0 40.0 7.0 31.0 1.53 14.6
ABSU
(D)
0.9 25 17.1 13.3 3.8 7.0 2.07 0.7
NRCRI
(E)
6.1 22 25.4 20.2 5.2 12.6 1.79 6.0
MOUAU
14.0 28 38.0 33.6 4.4 26.5 1.82 14.2
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Water Content
The natural moisture content of the tested soil samples ranges from 0.9% - 19.5% (Table 4). Soil that falls
within the range of 5 to 15% is described as sandy soil (Terzaghi et al., (1996). Therefore the samples B,
D and E are sandy soils.
Moisture variation is generally determined by the intensity of rain, depth of collection of sample and
texture of the soil (Jegede, 2000); and since the samples are almost within same geographical locality,
therefore soil texture have played a major role in the water content of each sample as indicated in the
variation in bulk densities (Table 5.).
Table 5: The bulk density of the tested soil samples
Soil sample with bulk density within the range of 1.60 to 2.0 is classified as sand (Hillel 1980a, b); and
since the bulk density of all the samples fall within the range, the samples are therefore classified as sand.
Mechanical Sieve Analysis
Soils that are largely made up of fine particles are likely to have poor geotechnical properties as
foundation materials than soils that are largely made up of coarse particle.
Grain size distribution analysis shows that the tested soils range from 18-28% passing the 0.075 mm sieve
(Table 4). The finer particles that passed through the 0.075mm sieve were subjected to Atterberg limit
tests. A display of grain size distribution curves for some locations in the study area is as shown in
Figures 5 to 7.
Sample Location
Bulk density (g/cm3)
(A) Timber Market
1.68
(B) GCU
1.74
(C) ABSU
2.00
(D) NRCRI
1.88
(E) MOUAU
1.76
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Fig.5: Grain size distribution curve of sample A at Timber market
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Fig. 6: Grain size distribution curve of sample C at ABSU
Fig.7: Grain size distribution curve of sample E at MOUAU
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Atterberg Limits
Soil consistency is a measure of the degree and kind of cohesion and adhesion between the soil particles
in relation to its resistance to deformation. Soil consistence varies with moisture content, and largely depends on soil minerals and the water content.
The Soil classification based on the Atterberg Limits (liquid limit) and grain size is as shown in Table 5.
This is used in combination with the plasticity chart and the Unified Soil Classification System (USCS) to
classify the tested soil samples (Fig. 8, Table 7).
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Table 6: The British Soil Classification System for coarse-grained soils (Dumbleton 1981)
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Table 7: The Unified Soil Classification System (Modified after Bowles, 1990)
Major Division Group
Symbols
Typical Name
Silts and clays
Liquid limit <
50%
ML Silts and very fine sands, rock flour, silt or clay fine sands or
clayey silts with slight plasticity
CL Inorganic clays of very low plasticity, gravelly clays, sandy
clays, silty clays, lean clays
CI Medium plastic inorganic clays
OL Organic silts and organic silty clays of low plasticity
MI Silts and silty clays of medium plasticity; rock flour; silty or
clayey fine sands
Silts and clays
Liquid Limit >
50%
MH Micaceous or diatomaceous fine sandy or silty soils, elastic
silts
CH Inorganic clays or high plasticity
OH Organic clays of medium to high plasticity, organic silt
Highly organic
Soils
Pt Peat and other high organic soils
The plasticity chart indicates that samples A, B and E fall within MI or OL (Fig. 8). In the classifications
used, they are either “organic or inorganic silts and silty clays of medium plasticity; rock flour; silty or
clayey fine sands”. Thus, from the description of the test pit logs, the plasticity chart, and the
classification systems; samples A, B and E are deduced as silty or clayey fine sands of medium or
intermediate plasticity.
While on the other hand, samples C and D fall within ML (Silts and very fine sands, rock flour, silt or
clay fine sands or clayey silts with slight plasticity); they are silty fine sands of low plasticity.
The result is consistent with most of the sample description presented in the test pit log.
It is worthy to note that the A-line (Fig. 8) generally separates the more clay-like materials from silty
materials, and the organics from the inorganics. While the U-line indicates the upper bound for general
soils. If the measured limits of soils are on the left of U-line, they are incorrect and should be rechecked.
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Fig. 8: The plasticity chart (Developed from Casagrande, 1948; Howard, 1977; and Modified after Holtz and
Kovacs, 1981)
Soils with high plasticity index (PI) tend to be clay, those with a lower PI tend to be silt, and those with a
PI of 0 (non-plastic) tend to have little or no silt or clay (Table 8).
Table 8: Plastic indices and their corresponding state of plasticity (Modified after Sowers and
Sowers, 1979)
Plasticity Index State of plasticity
0-3 Non-plastic
3-15 Slightly plastic
15-30 Medium plastic
>30 Highly plastic
From the above indication in Table 8, all the tested samples are slightly plastic since their plasticity
indices ranged from 3.8 to 7.0
Subsurface Engineering Evaluation
Excavation for footings or foundation walls shall extend below depth of soil subjected to seasonal or
characteristic volume change to undisturbed soil that provides adequate bearing capacity. So, topsoil is
normally removed and variations in ground level corrected. Therefore, the best recommended depth of
foundation is from 1.0 m to 1.5 m from original ground level (NHBC, 2011).
The depth of foundation depends on some factors such as the availability of soil with adequate bearing
capacity, depth of shrinkage and swelling as in the case of clayey soils, due to seasonal changes which
Note: SILT (M) plots below A-line.
CLAY ( C) plots above A-line.
The letter O is added to the symbol
if it has a significant amount of
organic matter.
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may cause appreciable movements; and the depth of frost penetration in case of fine sand and silt. Also,
proximity of excavation and depth of ground water-table are considered.
Soil Geotechnical Foundation Engineering Evaluation Water content affects properties of fine-grained soils (silts and clays) unlike sand and gravel and the
strength of soils decreases as water content increases.Therefore the most competent soil sample studied
based on water content is sample C at ABSUPAC which is 0.9%, while the least is that of sample A at
Timber Market that is 19.5% (Fig. 10).
Generally, the water content of all the test samples are low and are considered good for foundation
engineering purposes.
Fig. 10: A histogram of the foundation competence of the tested soils samples based on water content.
Soils having high values of liquid and plastic limits are considered as poor foundation materials. The
plastic index of all the soil samples is lower than 20% maximum limit as recommended by Federal
Ministry of Works and Housing (FMWH) (1972).
The Liquid Limit of the soil samples ranges from 17.1% to 47.0%, the Plastic Limit ranges from 13% to
40%, and the Plasticity Index ranges from 3.8% to 7.0%. The tested soil samples are of low to medium
consistency limits indicating low percentage of clay content in the soil samples. Since the higher the
plastic index of a soil, the less the competency of the soil as a foundation material, therefore all the soil
samples have good engineering property.
The percentage of the tested soils passing through the 0.075 mm sieve mesh ranges from 18% to 28%
which is far below the 35% limit recommended by Federal Ministry of Works and Housing (FMWH)
(1972) for a foundation material, hence; the soils can be generally rated as good foundation materials. The
linear shrinkage value of the tested soils ranges from 7.0 to 31.0% (Table 4).
Shrinkage is one of the major causes for volume change associated with variations of water content in soil
when the water content is reduced from a given value to the shrinkage limit. Since linear shrinkage is a
one-dimensional decrease in soil mass expressed as a percentage of the original dimension, therefore soils
with linear shrinkage below 8% are suggested to be good foundation materials (Brink et al., 1992). From
the indication, only sample C at ABSUPAC met the criteria.
CONCLUSION AND RECOMMENDATIONS
The result of the investigation carried out within the study area using laboratory geotechnical method has
provided thorough information on the subsurface conditions of soils of Umudike area of Abia state,
A
Mo
istu
re c
on
ten
t (%
)
Sample No. Incr
ea
sin
g
soil
co
mp
ete
ncy
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southeastern Nigeria. Geotechnical studies were useful in the determination of the conditions and
suitability of soils of Umudike area and its environs as foundation materials.
The geotechnical laboratory results show that the soils are generally of low natural moisture content. It
has relatively low clay content as revealed by the percentage passing 0.075mm sieve mesh. Since the
Plastic Index of the soils within the area are less than 20%, the soil can be adjudged to be low to medium
plasticity, hence, the soils are expected to exhibit low to medium swelling potential.
Since the linear shrinkage of all the tested soil samples were above 8% the maximum limit suggested by
Brink et al., 1992, except sample C at ABSUPAC; therefore for the construction of high rising buildings
in the area, consolidation and compaction tests are hereby recommended.
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