Assessment Of Geo-Technical Competence Of …...40 Assessment Of Geo-Technical Competence Of Tertiary Sedimentary Deposits In Umudike Area, Southern Nigeria *Nwokoma E.U. & Chukwu

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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).

Nwokoma & Chukwu …. Int. J. Inno. Scientific & Eng. Tech. Res. 4(4):40-62, 2016

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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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http://en.wikipedia.org/wiki/Soil

http://en.wikipedia.org/wiki/Water_content

http://en.wikipedia.org/wiki/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

Nwokoma & Chukwu…. Int. J. Inno. Scientific & Eng. Tech. Res. 4(4):40-62, 2016

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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

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on

ten

t (%

)

Sample No. Incr

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soil

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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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