Geotechnical Properties of Soils From Wukari Taraba State
Transcript of 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