Page 1 of 12
European Journal of Business &
Social Sciences
Available at https://ejbss.org/
ISSN: 2235-767X
Volume 07 Issue 02
February 2019
Available online: https://ejbss.org/ P a g e | 11
An Experimental Study on Effects of Saturation on Soil
Subgrade Strength
1K.Siva Rama Krishna, 2Modi.Musalaiah
1
(M.Tech, Transportation Engineering, MVR College of Engineering and Technology)
2
(Assistant Professor, MVR College of Engineering and Technology)
ABSTRACT
The design of the pavement layers to
be laid over sub grade soil starts off with the
estimation of sub grade strength and the
volume of traffic to be carried. Design of the
various pavement layers are very much
dependent on the strength of the sub grade soil
over which they are going to be laid. Sub
grade strength is mostly expressed in terms of
CBR (California Bearing Ratio). Weaker sub
grade essentially requires thicker layers
whereas stronger sub grade goes well with
thinner pavement layers. The sub grade is
always subjected to change in saturation level
due to precipitation, capillary action, flood or
abrupt rise or subsidal of water table. Change
in moisture level in sub grade causes change in
the sub grade strength. And it becomes quite
essential for an engineer to understand the
exact nature of dependence of sub grade
strength on moisture variation. An
understanding of the dependence of the CBR
strength of local soils on water content will
contribute towards better design and
maintenance practices. Normally CBR test is
an easy and well adopted method conducted
on soil samples to measure the strength of sub
grade. However, many other tests are also
considered for assessing the sub grade
strength.
The strength of soil, used for sub
grade may vary largely on the amount of
saturation in it, i.e. amount of water exposed to
the soil. Hence, in this study an attempt has
been made to vary the degree of soaking and
hence the saturation level in various types of
soils and study the engineering properties of
soils including CBR at different saturation
levels.
It is observed that for coarse grained soil,
worst engineering properties are observed after
three days of soaking and for fine grained
soils, the same is found at the end of four days.
Keywords: Sub grade soil, Moisture content ,
Compaction, Degree of saturation, CBR
Introduction
SUBGRADE
The crust of a pavement, whether
flexible or rigid, rests on a soil foundation on
an embankment or cutting, normally known as
subgrade. Subgrade can be defined as a
compacted layer, generally of naturally
occurring local soil, assumed to be 500/300
mm in thickness, just beneath the pavement
crust, providing a suitable foundation for the
pavement.
Subgrade Performance
A subgrade’s performance generally
depends on three of the basic characteristics,
which are briefly described below:
Load bearing capacity: The subgrade must be
able to support loads transmitted from the
pavement structure. This load bearing capacity
is often affected by degree of compaction,
moisture content, and soil type. A subgrade
that can support a high amount of loading
without excessive deformation is considered
good.Moisture content: Moisture tends to
affect a number of subgrade properties
including load bearing capacity, shrinkage and
swelling. Moisture content can be influenced
by a number of things such as drainage,
groundwater table elevation, infiltration, or
pavement porosity (which can be assisted by
cracks in the pavement).
Desirable Properties
The desirable properties of subgrade
soil as a highway material are
Withstand capability (Stability)
Ease of compaction
Strength permanency
Low change in volume during adverse
conditions of weather and ground
water table
Superior drainage
Incompressibility
California Bearing Ratio Test
The California Bearing Ratio Test (CBR Test)
Page 2 of 12
European Journal of Business &
Social Sciences
Available at https://ejbss.org/
ISSN: 2235-767X
Volume 07 Issue 02
February 2019
Available online: https://ejbss.org/ P a g e | 12
is a penetration test developed by California
State Highway Department (U.S.A.) for
evaluating the bearing capacity of subgrade
soil. The CBR test was first introduced or
developed by O.J. Porter at California
Highway Department in 1920. It is otherwise
called as load-deformation test which is
conducted in the laboratory or in the fields and
these results are generally used to find the
thickness of pavement layers, base course and
other layers of a given traffic loading by the
use of empirical design chart. Initially it
practiced for the design of surfaced and un- surfaced airfields which is still based upon
CBR today.
Organization Of Thesis
The thesis consists of six chapters as described
below:
Chapter 1 gives a brief introduction of the
project
Chapter 2 deals with a review of previous
work on laboratory studies.
Chapter 3 deals with methodology and
experimental investigation.
Analysis of the results and discussion on the
experimental investigations is discussed in
Chapter 4.
Methodology and Experimental
Investigations
Introduction
Initially experiments were conducted to find
out different properties of soil such as index
properties, grain size distribution and
differential free swell index. Later on heavy
compaction tests were conducted to find out
the optimum moisture content &
corresponding maximum dry density. Then
CBR tests were made at different moisture
contents including OMC and analysis made to
investigate the variation of CBR with respect
to different days of soaking, i.e. from
unsoaked (day 0) to soaked (day 5).
Experimental Investigations
Soils are classified with different engineering
properties which affect the behavior of soil
under different conditions. These properties
are described briefly here.
Liquid Limit
The liquid limit (LL) is the water content at
which a soil changes from plastic to liquid
behavior. At this limit, the soil possesses a
small value of shear strength, losing its ability
to flow as a liquid. In other words, the liquid
limit is the minimum moisture content at
which the soil tends to flow as a liquid.
Plastic Limit
Plastic limit (PL) is the arbitrary limit of water
content at which the soil tends to pass from the
plastic state to the semi-solid state of
consistency. Thus, this is the minimum water
content, at which the change in shape of the
soil is accompanied by visible cracks, i.e.,
when worked upon, the soil crumbles.
Plasticity Index
Plasticity Index (PI) is the range of water
content within which the soil exhibits plastic
properties, that is, it is the difference between
liquid and plastic limits.
Plasticity Index (IP ) = Liquid Limit(WL) -
Plastic Limit (WP)
Differential Free Swell
Free Swell Index is the increase in volume of a
soil, without any external constraints, on
submergence in water.
Where, Vd = volume of soil specimen read
from the graduated cylinder containing
distilled water.
Vk = volume of soil specimen read from the
graduated cylinder containing kerosene.
Specific Gravity
Specific gravity of soil solids is defined as the
ratio of unit weight of solids to the unit weight
of water at the standard temperature (4°C).
Sieve Analysis
About 1kg of soil was taken and it was washed
thoroughly with water on 75 micron sieve
,soilretained on sieve was dried and weighed
and used for sieve analysis .These dried soils
werepassed through stack of sieves like
4.75mm,2.36mm,1.18mm,600μm,300 μm,150
μm, 0.75 μm.
Modified Proctor Test
The Proctor compaction test is a laboratory
method of experimentally determining the
optimal moisture content at which a given soil
type will become most dense and achieve its
maximum dry density. The term Proctor is in
honor of R. R. Proctor, who in 1933 showed
that the dry density of a soil for a given
compactive effort depends on the amount of
water the soil contains during soil compaction.
His original test is most commonly referred to
as the standard Proctor compaction test; later
Page 3 of 12
European Journal of Business &
Social Sciences
Available at https://ejbss.org/
ISSN: 2235-767X
Volume 07 Issue 02
February 2019
Available online: https://ejbss.org/ P a g e | 13
on, his test was updated to create the modified
Proctor compaction test.
California Bearing Ratio Test
The CBR is a measure of resistance of a
material to penetration of standard plunger
under controlled density and moisture
conditions. The test procedure should be
strictly adhered if high degree of
reproducibility is desired. The CBR test may
be conducted in re-moulded or undisturbed
specimens in the laboratory. The test has been
extensively investigated for field correlation of
flexible pavement thickness requirement.
Briefly, the test consists of causing a
cylindrical plunger of 50mm diameter to
penetrate a pavement component material at
1.25mm/minute. The loads, for 2.5mm and
5mm are recorded. This load is expressed as a
percentage of standard load value at a
respective deformation level to obtain CBR
value.
Direct Shear Test:
A direct shear test is a laboratory or field test
used by geotechnical engineers to measure the
shear strength properties of soil or rock
material, or of discontinuities in soil or rock
masses.
RESULTS AND DISCUSSIONS
TYPE 1 SOIL
Index Properties
The index properties such as Liquid limit,
Plastic limit, Plasticity Index value are
presented in Table4.1 as follows:
Table-4.1Index properties of type – 1 soil
Index property Experimental Value
Liquid Limit 55.29%
Plastic Limit 34.04%
Plasticity Index 21.25%
Specific Gravity 2.65
Differential Swell
Index 57%
PARTICLE SIZE DISTRIBUTION
The grain size distribution of this soil sample
has been shown in Table 4.2.
Table-4.2Grain size distribution of type - 1
soil
I.S Sieve
no.
Weight
retained in
gm
Percentage
Weight
retained
Percentage
weight
passing
.75mm 4.6 0.46 99.54
2mm 14.5 1.45 98.09
1mm 20.2 2.02 96.07
0.6mm 5 0.5 95.57
0.425mm 9.8 0.98 94.59
0.3mm 5.1 0.51 94.08
.212mm 21.1 2.11 91.17
.015mm 15.6 1.56 90.41
0.075mm 30.49 3.049 87.361
Based on the above properties the IS Soil
Classification for the soil sample under test is
‘OH’.
Modified Proctor Compaction Test
The results of modified proctor compaction
test are represented in figure 4.1:
Figure 4.1: Modified proctor compaction test
for type – 1 soil
California Bearing Ratio Test Results
Test-1(conducted under OMC (16.4%) and
MDD (1.84g/cc)
Figure 4.2 Load vs penetration graph for test –
1, type – 1 soil for unsoaked condition
