Geotechnical Properties of Soils From Wukari Taraba State

7/28/2019 Geotechnical Properties of Soils From Wukari Taraba State

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Evaluation of geotechnical properties of

soils from Wukari, Taraba state

ABSTRACT

This study was aimed at evaluating the geotechnical properties of soils within

Wukari , Taraba state. Disturbed samples were collected from and

subjected to engineering classification and strength tests. Results of the index

tests suggests that the soil

INTRODUCTION

The geotechnical properties of soils play a significant role in all construction

projects ranging from buildings, roads or earthen structures such as dams and

embankments. These soils serve as engineering materials and are used to

support structures. In order to use soils, an understanding of its behavior

under various loadings and condition is required. Soil behavior also varies

widely from region to region as well as within similar patches of land. The

effective use of soils as engineering materials requires a detailed knowledge of

the geotechnical properties of soils, which are naturally complex and

heterogenous.

This study therefore is aimed at evaluating the geotechnical properties of soils

within Wukari, Taraba


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STUDY AREA

Soils samples used for this study were obtained from borrow pits in Wukari

between latitudes; 070 50 25.2N , 070 51 06.50N and longitudes 0090

45 51.12E, 0090 46 46.48E. Geologically, Wukari lies in the middle of the

Benue trough which is a cretaceous sedimentary basin. A review of literature

shows that layer is underlain by very coarse, conglomeratic, gritty, arkosic

sandstone. The outcrops of this sandstone are very rare in the southern part

of the area as they are mostly covered by weathered, loosed lateritic sediment

derived from their weathering, but in the north part, most especially towards

the north-eastern part, the sandstone was seen to be outcropping into a highly

compacted lateritic or ferruginized hill measuring about 2 to 3 m in thickness.

The overburden thickness around this area is also very thin and rarely

exceeds 0.5 m in thickness.

Like most parts of northern Nigeria, Taraba State has a wet

and dry climate. The wet season lasts, on the average, from April to October.

Mean annual rainfall varies between 1058mm in the north around Jalingo and

Zing, to over 1300mm in the South around Serti and Takum. The wettest

monthsare August

and September.

The dry season lasts from November to March. The driest months are

Decemberand January with relative humidity dropping to about 15 percent. Mean

annual

temperature around Jalingo is about 28C with maximum temperatures

varying between 30C and 39.4C. The minimum temperatures range between


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15C to 23C. The Mambilla

plateau has climatic characteristics typical of a temperate climate.

Temperatures are tow throughout the year and the rainy season lasts from

February to November with a mean annual rainfall of over 1850mm.

Vegetation: Rainfall distribution and topogra phy are the most important

factors influencing the pattern of vegetation in Taraba State. The vegetation

may be classified into

three broad types: the Northern Guinea, the Southern Guinea and the

Mountain

Grassland and forest vegetation.

The study area is located on a fairly level terrain with elevations ranging from

158m to 176m a.s.l. The area is characterized by an uneven spread of

vegetation comprising mainly of shrubs and isolated trees. Some areas of the

site were observed to have stunted trees and shrubs while others revealed a

lot of shrubs and trees. The study area falls within the savannah belt of the

northern part of Nigeria and soil types in this area are typically sands and

sandy clays.

MATERIALS AND METHODS

Field Investigation

During the field investigation, representative soil samples were collected in a

grid network at an average interval of 100m between points. A total number

of samples were collected in order to ascertain the lateral variation of soil

across the study area. Undisturbed samples were collected at a depth of 1.m

from the surface.


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Laboratory Investigation

Samples were air-dried and specimens were prepared in accordance with BS

1377: 1990

Particle Size Distribution:

This test is performed to determine the percentage of different grain sizes

contained within a soil. The mechanical or sieve analysis is performed to

determine the distribution of the coarser, larger-sized particles, and the

hydrometer method is used to determine the distribution of the finer

particles. This test is performed in accordance with BS 1377: Part 2: 1990. The

distribution of different grain sizes affects the engineering properties of soil.

Grain size analysis provides the grain size distribution, and it is required in

classifying the soil.

Atterberg Limits

The Atterberg limits are based on the moisture content of the soil. This test is

performed to determine the plastic and liquid limits of a fine grained soil. The

liquid limit (LL) is arbitrarily defined as the water content, in percent, at

which the soil changes from a plastic to a viscous fluid state The plastic limit

on the other hand, is the moisture content that defines where the soil changes

from a semi-solid to a plastic (flexible) state. The shrinkage limit is the

moisture content that defines where the soil volume will not reduce further if

the moisture content is reduced. A wide variety of soil engineering properties


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have been correlated to the liquid and plastic limits, and these Atterberg limits

are also used to classify a fine-grained soil according to the Unified Soil

Classification system or AASHTO system.

This test was performed in accordance with BS 1377: Part 2: 1990.

Specific Gravity Test

Specific gravity is the ratio of the mass of unit volume of soil at a stated

temperature to the mass of the same volume of gas-free distilled water at a

stated temperature. The specific gravity of a soil is used in the phase

relationship of air, water, and solids in a given volume of the soil.

This test was performed to determine the specific gravity of soil by using a

pycnometer and is performed in accordance with BS 1377: Part 2: 1990.

Determination of specific gravity of soil using pycnometer.

Natural Moisture Content :

The water content is the ratio, expressed as a percentage, of the mass of water

in a given mass of soil to the mass of the dry soil solids. For many soils, the

water content may be an extremely important index used for establishing the

relationship between the way a soil behaves and its properties. Theconsistency of a fine-grained soil largely depends on its water content. The

water content is also used in expressing the phase relationships of air, water,

and solids in a given volume of soil. This test was performed in accordance

with BS 1377: Part 2: 1990 using the oven-drying method.


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Direct Shear Test

The direct shear test is used for carrying out drained shear tests for the

determination of effective shear strength parameters in cohesionless soils.

The test was carried out using the small shearbox apparatus on undisturbed

specimens obtained from the trial pit. This test was carried out in accordance

with clause 4 of BS 1377: Part 7: 1990.

ConsolidationTest

One-dimensional consolidation tests were carried out to determine the

amount of compression and the rate at which this will take place. The test was

carried out on undisturbed samples from the trial pits and performed in

accordance with clause 3 of BS 1377: Part 5: 1990.

Analysis of Laboratory test results

Grain Size Analysis

The subsoil investigation at the proposed site shows that the topsoil consists

basically of brownish to light brown sandy soils. The grain size distribution of

the soil showed more than 50% of each soil sample tested was retained on

sieve No 200, indicating the soils are generally coarse grained. They were

further classified as sandy soils since a greater portion passed through sieve

No 4.

The grain size analysis shows that the soil within the study area are

predominantly sandy soils with some silt and clay. The gravel content is

relatively low to non-existent. However, percentage of fines (i.e materials


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finer than 75microns) varies between 1.2 to 66.7% with a corresponding

mean of and standard deviation of..

The relationship between plasticity index and the fines is presented in figure

Atterberg Limits

The atterberg limits of the soils (Liquid Limit and Plasticity Index) plotted

above the A-line on the Cassagrande plasticity chart. Based on the USCS soil

classification system, these soils fall into the category of silty or clayey fine

sands with slight plasticity. This group of soils tends to have slight to medium

degree of compressibility and expansion with poor drainage characteristics.

However, about 10% of the trial pits were classified as fine grained soils: both

clays and silts with generally low plasticity. The plasticity index and linear

shrinkage values obtained show that the soil would exhibit a medium degree

of expansion.

Direct Shear

The results from the direct shear test further confirmed the soils to be

generally cohesive with varying degrees of cohesion throughout the site.

Allowable bearing pressures ranged from 89 250 kN/m2.

Compaction and California Bearing Ratio


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Standard compaction and California Bearing ratio tests were performed on

some samples collected from the trial pits. The table below summarizes the

results obtained.

Table 2: Compaction and C.B.R test results

Trial Pit Depth m M.D.D

Kg/m3

O.M.C % C.B.R %

TP22 1.5 2.12 9.2 21.0

TP16 1.5 2.33 17.0 3.2

TP153 1.5 2.43 14.5 6.9

Consolidation

The results from the one-dimensional consolidation test at 250 KN/m2

incremental pressure are presented in the table below:

Table 3: Consolidation test results


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Trial Pit Depth Coefficient of

Consolidation

m2/yr

Coefficient of

Volume

Compressibility

m2/MN

TP24 1.5 13 0.0049

TP28 1.5 17 0.015

TP44 1.5 21 0.0004

TP56 1.5 9.4 0.024

TP85 1.5 14 0.015

TP168 1.5 13 0.014

TP180 1.5 16 0.015

Geotechnical Properties of Soils From Wukari Taraba State

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