Open Journal of Soil Science, 2012, 2, 95-99
http://dx.doi.org/10.4236/ojss.2012.22014 Published Online June 2012 (http://www.SciRP.org/journal/ojss) 95
Evaluation of Shear Strength and Cone Penetration
Resistance Behavior of Tropical Silt Loam Soil under
Uni-Axial Compression
Seth I. Manuwa*, Omolola C. Olaiya
Department of Agricultural Engineering, School of Engineering and Engineering Technology, The Federal University of Technology,
Akure, Nigeria.
Received March 29th, 2012; revised April 30th, 2012; accepted May 14th, 2012
ABSTRACT
Laboratory investigations were conduc ted to study strength characteristics of silt loam soil of Ilorin, Kwara State, Nige-
ria, under uni-axial compression tests. The main objective of this study was to evaluate the effects of applied pressure
and moisture content on strength indices such as bulk density, penetration resistance and shear strength of the soil and
to develop relationships between the strength indices for predictive purposes necessary in soil management. The com-
pression was carried out at different moisture contents determined according to the consistency limits of the soil. The
applied pressure ranged from 75 to 600 kPa. Values of bulk density, penetration resistance and shear strength increased
with increase in moisture content up to peak values after which the values decreased with further increase in moisture
content. Regression models were used to describe the trends in the results for the soil. Results also showed that bulk
density and soil strength normally regarded as indicators of soil quality are affected by moisture content and applied
pressure and that these properties can be predicted using the models generated from the study.
Keywords: Loamy Soils; Applied Pressure; Bulk Density; Penetration Resistance; Moisture Content; Shear Strength;
Nigeria
1. Introduction
Soil strength has been regarded as important characteris-
tics that affect many aspects of agricultural soils, such as
the performance of cultivation implements, root growth,
least-limiting water range and trafficabilty [1]. They fur-
ther reported that characterization of soil strength is us-
ually made by measuring the response of a soil to a range
of applied forces.
Soil compaction may be defined as the densification of
unsaturated soil due to reduction in air volume without
change in mass wetness [2]. Soil compaction occurs in
unsaturated soils when subjected to mechanical forces [3].
While soil compaction is essential in many engineering
works (especially civil engineering) it is undesirable in
agricultural production to a large extent. Compaction re-
duces the soil permeability to water, so that run off and
erosion may occur and adequate recharge of ground wa-
ter is prevented. Compaction reduces regeneration of the
soil, so that metabolic activities of roots are impaired.
Compaction increases the mechanical strength of the soil,
so root growth is impeded. It is known that in agricultural
system, the risk of soil compaction increases with the
growth of farm size, increased mechanization and equip-
ment weight, and the drive for greater productivity. Soil
compaction also has negative effects on the environment
by increasing runoff and erosion thereby accelerating po-
tential pollution of surface water by organic wastes and
applied agrochemicals [4]. All of these effects may re-
duce the quality and quan tity of food and fiber grown on
the soil. Therefore, the knowledge of soil compaction is
increasingly important and desirable within agriculture
and environmental protection.
The state of soil compactness is expressed in several
ways: bulk density (exp ressed on a wet or dry basis), po-
rosity and apparent specific gravity [5]. Accurate com-
paction behavior equations will provide a means to pre-
dict compaction. The ability to predict compaction is the
first requirement for attaining control of compaction.
Considerable research has been performed in attempts to
develop soil compaction behaviour equations [6-12].
Others have also reported on effects of organic matter and
tractor passes on compaction and yield of crops [13,14].
The aim of this study therefore was to observe the be-
havior of Ilorin silty loam soils under uni-axial compres-
*Corresponding a uthor.

Evaluation of Shear Strength and Cone Penetration Resistance Behavior of Tropical Silt
Loam Soil under Uni-Axial Compression
96
sion as it is affected by applied pressure and water con-
tent and also to model the behavior using regression ana-
lysis for the purpose of prediction.
2. Materials and Methods
2.1. Site of Soil Sample
The soil sample was taken from the arable soils of Na-
tional Centre for Agricultural Mechanization (NCAM)
Ilorin, Kwara State, Nigeria (8.30 N 4.32 E). The soil
was Regosols (FAO). The soil samples were collected
from the first 35 cm of soil profile; each sample was dug
to a radius of 15 cm and then mixed thoroughly to get a
homogeneous mixture, and then taken to the laboratory
for further pro cessing and an alys is
2.2. Analytical Methods
Particle size analysis of the soils was performed using
hydrometer method [16]. Organic matter content of the
soils was determined using the [16] method. Other phy-
sical and chemical properties of the soils were also deter-
mined using standard methods.
2.3. Compression Test
The samples that co llected were each air-dried and ground
to pass through a 2-mm sieve. The moisture levels for
compaction tests were chosen according to the consis-
tency limits of the soils determined by the procedure de-
scribed by [15]. Compaction test was performed by fill-
ing the proctor mould with a known mass of soil and
placed under a uni-axial compression apparatus (Univer-
sal Testing Machine (UTM), manufactured by the Tes-
tometric Co. Ltd., UK). Compression was carried out at a
steady speed of 30 mm/min. Soil samples in the mould
were subjected to 75, 100, 150, 200, 300, 400, 500, 600
kPa. The soil displacement and mass were recorded for
each compaction. The mass was used to calculate bulk
density of compacted soil sample. The proctor mould
was 16.8 cm height and 10 cm diameter. A circular thick
metal plate was placed on the compression end of the
UTM to effect uniform compaction in the proctor mould.
After each compaction test, the change in depth of com-
pressed soil was measured with the aid of a digimatic
vernier caliper.
2.4. Cone Index Measurement
Cone index (CI) was determined using a Rimick CP20
recording penetrometer (model CP 20 ultrasonic, Agridry
Rimik Pty Ltd, Toowoomba), with a standard 30˚ cone of
322-mm2 base area and a penetration rate was less than
10 mm/s. Measurements were taken at two depths 5 and
10 cm of the proctor mould and the average of the read-
ings taken as the representative value of cone index at
that treatment.
2.5. Shear Strength Measurement
The shear strength of the soil was observed using a 19
mm vane of a shear vane tester. Measurements were
taken at two depths of 5 and 10 cm and the average re-
corded to represent the shear strength of the particular
treatment.
3. Results and Discussion
3.1. Soil Physical Properties
The soil studied was a silty loam soil according to the
USDA textural classification of soils. Table 1 shows
some physical and chemical properties of the soil. The
consistency limits of th e soils are also presented in Table
1. Plasticity index is an index of workability of the soil
and a large range of plasticity index implies a need for
large amounts of energy to work the soil to a desired
tilth.
3.2. Soil Strength Properties
Shear strength and cone index are indicators of soil
strength. Shear strength is the resistance of soil to shear-
ing or structural failure. The shear strength of an indi-
vidual clod decreases with wetting, but more importantly,
the strength of the bulk soil increases with increasing
Table 1. Some physical and chemical properties of experi-
mental soil.
Property Values
Sand (%) 22.6
Silt (%) 62.8
Clay (%) 14.5
Silt + clay (%) 77.3
Texture (%) Silt loam
Organic carbon (g/kg) 2.03
Organic matter ( %) 3.51
Total nitrogen (g/kg) 0.18
pH in H2O (1:2) 7.93
Ca2+ (cmol/kg) 0.17
Mg2+ (cmol/kg) 1.70
Na+ (cmol/kg) 0.17
K+ (cmol/kg) 0.29
P (mg/kg) 4.00
Plastic limit (%) 9.2
Liquid limit (%) 40
Plasticity index 30.8

Evaluation of Shear Strength and Cone Penetration Resistance Behavior of Tropical Silt
Loam Soil under Uni-Axial Compression 97
moisture up to the lower plastic limit at which each par-
ticle is surrounded by a film of water which acts as lu-
bricant. Soil strength drop s sharply from that point to the
upper plastic limit, where the soil becomes viscous.
The effects of moisture content and applied pressure
on shear strength of the experimental soils are presented
in Figure 1. Shear strength increased with increase in
moisture content up to a maximum and then decreased as
the moisture content of the soil further increased. This is
a typical soil behavior which has been reported by other
researchers. The peak value occurred at higher moisture
content as the applied pressure increased. The maximum
shear strength of the soils at applied pressure of 600 kPa
was 1025 kPa at moisture content of 9.1% (d b). Similarly,
the effects of moisture content and applied pressure on
cone index of the soils are presented in Figure 2. The
relationship is similar to that exhibited by shear strength.
The maximum cone index at applied pressure of 600 kPa
was 1325 kPa at moi st ure content of 5.0% (db ).
However, the effects of moisture content and applied
pressure on bulk density showed behavior that was dif-
ferent from those of shear strength and cone index (Fig-
ure 3).
Figure 1. Effect of moisture content and applied pressure
on shear strength.
Figure 2. Effect of moisture content and applied pressure
on cone index.
Figure 3. Effect of moisture content and applied pressure
on bulk density.
Regression models (Table 3) were also established to
show relationships between compaction indices such as
shear strength, cone index and bulk density at applied
pressures of 75, 300 and 600 kPa representing a range of
low to medium and high pressures. The relationships
vary from linear to exponential and to polynomial func-
tions.
The results also found a linear correlation between
cone index and shear strength at a low applied pressure
of 75 kPa. This agrees well with the findings of Vanags
et al. [1] who reported linear relationship between cone
index and surface shear resistance of soil.
Bulk density decreased at higher moisture content after
the peak value because further addition of water created
greater water pressure which red uced soil compressibility.
The maximum bulk density at applied pressure of 600
kPa was 2.1 Mg/m3 at 15.0 % (db). This moisture content
was significant because it was the moisture content at
which the soil reached maximum bulk density. This
agrees with other researchers’ report that soils with high
amount of fine particles (clay plus silt) are more suscep-
tible to compactability [17 ].
The regression models that describe the behavior of
soil parameters shown in Figures 1 to 3 are presented in
Table 2. The models are largely nonlinear and they agree
well with those reported by other researchers [6-12].
4. Conclusions
The following concl usi o ns can be dra w n fr o m this study.
1) The study showed that compaction behavior of silt
loam soil can be modeled after certain linear and non-
linear regression equations.
2) Cone index have good positive linear relationship
with shear strength, but can also be fitted with polyno-
mial (quadratic) function with higher coefficient of deter-
mination.
3) The effect of moisture content and applied pressure

Evaluation of Shear Strength and Cone Penetration Resistance Behavior of Tropical Silt
Loam Soil under Uni-Axial Compression
98
Table 2. Relationships between dependent and independent variables.
Dependent
variables Independent variables Predictive models R2 Applied pressure, kPa Model type
SS MC y = –0.797x2 + 21.77x + 21.63 0.9733 75 polynomial
SS MC y = –1.384x2 + 35.46x + 52.5 0.9394 150 polynomial
SS MC y = –2.54x2 + 55.09x + 191.1 0.7899 300 polynomial
SS MC 2
3.87597.17 39.08yxx 0.468 600 polynomial
CI MC y = –1.056x2 + 25.1x + 83 .75 0.8061 75 polynomial
CI MC y = –1.53x2 + 34.2x + 177.3 0.8384 150 polynomial
CI MC y = –2.63x2 + 52.79x + 34 2 .1 0.7914 300 polynomial
CI MC 2
4.54382.54 698.9yxx 0.729 600 polynomial
BD MC 0.022
1.133
x
ye 0.9661 75 exponential
BD MC y = 31.4x + 1228.7 0.9624 150 linear
BD MC y = 32.02x + 1304.5 0.8976 300 linear
BD MC 2
0.0010.059 1.395yxx 0.9442 600 Polynomial
C = cone index; BD = bulk density; MC = moisture content; SS = shear strength.
Table 3. Relationships between cone index, shear strength and bulk density.
Dependent
variables Independent variables Predic tive models R2 Applied pressure, kPa Model type
CI SS 1.18424.52yx
0.6651 75 linear
CI SS y = 0.0006x2 + 0.97x + 60.5 0.7914 150 polynomial
CI SS y = 177.59e0.0025x 0.9687 300 exponential
CI SS 2 0.0011.406866.8yx x 0.2600 600 polynomial
CI BD y = –627.3x2 + 1890x – 122 0.4711 75 polynomial
CI BD y = –0.0014x2 + 4.42x – 3108.8 0.8094 150 polynomial
CI BD y = –0.0017x2 + 5.30x – 3108.8 0.4531 300 polynomial
CI BD y = –10128x2 + 36518x – 31362 0.7790 600 polynomial
CI = cone i nd ex; BD = bulk density; MC = moisture content; SS = shea r s tr ength.
on cone index, shear strength was best fitted with poly-
nomial function of the second order.
4) The effect of applied pressure and moisture content
on bulk density of silt loam can be modeled after linear,
exponential and polynomial regression functions.
5. Acknowledgements
The authors wish to acknowledge the good gestures of
the management of the National Centre for Agricultural
Mechanization (NCAM), Ilorin, Nigeria for allowing us
to use their laboratory facility for this study.
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