THE EFFECT OF DEGREE OF SATURATION ON THE RESULTS OF ...

jTHE EFFECT OF DEGREE OF SATURATION ON THE RESULTS OF DIRECT
SHEAR TESTS OF C-Φ SOIL
M.Sc. THESIS
SEPTEMBER, 2017
THE EFFECT OF DEGREE OF SATURATION ON THE RESULTS OF DIRECT
SHEAR TESTS OF C-Φ SOIL
HABTAMU KEFYALEW MOLLA
SCHOOL OF CIVIL ENGINEERING,
GRADUATE STUDIES
HAWASSA UNIVERSITY
HAWASSA, ETHIOPIA
REQUIREMENTS FOR THE
(SPECIALIZATION: GEOTECHNICAL ENGINEERING)
A MASTER OF THESIS APPROVAL
This thesis entitled with “The Effect of Degree of Saturation on the Results of Direct Shear
Tests of C-Φ Soil” has been approved by the advisors, examiners and school in partial
fulfillment of the requirement for the degree of Master of Science in Civil Engineering
Department Geotechnical Engineering stream.
5. _________________ ___________________ ___________________
i
Acknowledgements
I would like to express my gratitude to my supervisor Dr. Yoseph Birru for initiating an
interesting study during his lecturing class, his personal commitment, interesting
discussion and valuable advice. His encourage and guidance leads me to follow his foot
step and to do this interested research. He has been continuously inspiring me throughout
the work and contributing with valuable assistance and supervision.
I express my sincere appreciation to the Ethiopian Road Authority for providing financial
support in the course of this research.
I would like to thank Ethiopian Construction Design & Supervision Works Corporation for
their Geotechnical Laboratory Facility permission to do my Laboratory works. Also
Grateful thanks go to all Ethiopian Construction Design & Supervision Works
Corporation, Geotechnical Laboratory staffs for their technical support in the laboratory.
I also thank my friends and colleagues for sharing knowledge and helping throughout this
research.
Finally, I would like to thank the love and support of my parents and the Almighty, whose
blessings gave me the strength to finish my research.
ii
e - Void ratio;
LL - Liquid limit;
V - Volume of total mass of the sample;
Vm - Volume of mold;
VS - Volume of soil;
VV - Volume of void;
WS - Weight of soil;
WW - Weight of water;
d - Dry unit weight;
σ - Total stress;
τf - Shear stress on the failure plane at failure.
iv
Abstract ........................................................................................................................... xii
1.1 Background of Problems and Significance of the Study ..................................... 1
1.2 Research Objectives ............................................................................................. 2
1.3 Research Methodology ........................................................................................ 3
2. Literature Review ................................................................................................ 5
2.1 General ................................................................................................................. 5
2.2 Effect of Degree of Saturation Variation on Direct Shear Test Result ................ 6
2.3 Shear Strength for Soils ....................................................................................... 8
2.3.1 Friction Angle .................................................................................................. 9
2.6.1 A History of the Direct Shear Box Test ......................................................... 13
2.6.2 Significance and Use of Direct shear Test ..................................................... 14
3. Research Methodology ...................................................................................... 16
3.2.1 Water Content ................................................................................................ 17
3.2.2 Specific Gravity ............................................................................................. 18
3.2.4 Free Swell ...................................................................................................... 21
3.3 Compaction Curve ............................................................................................. 23
3.5 Experimental Work ............................................................................................ 28
3.5.2 Degree of Saturation Controlled Soil Samples Preparation........................... 28
3.6 Correlation of Cohesion (C) and Friction Angle (Φ) With Degree of
Saturation… ..................................................................................................................... 29
4.1 General ............................................................................................................... 30
vi
4.2.4 Free Swell ...................................................................................................... 32
4.3 Compaction Curve ............................................................................................. 33
4.4.1 Shear Strength Parameters on Different Degree of Saturation. ..................... 34
4.4.2 Relationships between Cohesion (c), Angle of Internal Friction () and
Degree of Saturation (S) ............................................................................................... 45
4.4.3 Analysis of the Effects of Degree of Saturation on the Cohesion (c) ............ 50
4.4.4 Analysis of the Effects of Moisture Content on the Angle of Internal Friction
() ....................................................................................................................... 50
5.1 Summary ............................................................................................................ 52
5.2 Conclusion ......................................................................................................... 52
5.3 Recommendation ............................................................................................... 53
vii
List of Table
Table 4-1: Weight of water added under different degree of saturation .............................. 36
Table 4-2: Direct Shear Test Data at 30% of Degree of Saturation .................................... 36
Table 4-3: Direct Shear Test Result at 30 % Degree of Saturation ..................................... 37
Table 4-7: Direct Shear Test Result at 73.64 % Degree of Saturation ................................ 41
Table 4-8: Direct Shear Test Result at 80% Degree of Saturation ...................................... 42
Table 4-9: Direct Shear Test Result at 90% Degree of Saturation ...................................... 43
Table 4-10: Direct Shear Test Result at 100% Degree of Saturation .................................. 44
Table 4-11: Shear Strength Parameters Value at Different Degree of Saturation ............... 45
viii
List of Figure
Figure 2-1: The Great Mosque of Djenne In Mali Built in Adobe . ...................................... 6
Figure 3-1: Laboratory Investigation Program .................................................................... 17
Figure 4-1: The Relationship of Percent of Finer With Grained Size Distributions ........... 32
Figure 4-2: Liquid Limit Flow Chart ................................................................................... 33
Figure 4-3: Compaction Curve for the Investigated Soil ..................................................... 34
Figure 4-4: Shear Vs Horizontal Displacement Due to 30 % Degree of Saturation. .......... 37
Figure 4-5: Maximum Shear Vs Normal Stress Curve Due To 30 % Degree of Saturation.
...................................................................................................................................... 38
Figure 4-6: Maximum Shear Vs Normal Stress Curve Due to 40 % Degree of Saturation. 39
Figure 4-9: Maximum Shear Vs Normal Stress Curve Due to 73.64 % Degree of
Saturation. ..................................................................................................................... 42
Figure 4-10: Maximum Shear Vs Normal Stress Curve Due to 80% Degree of Saturation.
...................................................................................................................................... 43
Figure 4-11: Maximum Shear Vs Normal Stress Curve Due to 90% Degree of Saturation 44
Figure 4-12: Maximum Shear Vs Normal Stress Curve Due to 100% Degree of Saturation
...................................................................................................................................... 45
Figure 4-13: Degree of Saturation (S) Vs Cohesion (C) Relationship ................................ 46
Figure 4-14: Incremental Cohesion (C) Vs Degree of Saturation (S) Correlation graph ... 47
Figure 4-15: Decremental Cohesion (C) Vs Degree of Saturation (S) Correlation graph .. 47
Figure 4-16: Degree of Saturation (S) Vs Angle of Internal Friction () Relationship ...... 48
Figure 4-17: Incremental Angle of Internal Friction () Vs Degree of Saturation (S)
Correlation graph .......................................................................................................... 49
ix
Figure 4-18: Decremental Angle of Internal Friction () Vs Degree of Saturation (S)
Correlation graph .......................................................................................................... 49
Table A. 2: Specific Gravity ................................................................................................ 58
Table A. 3: Free Swell ......................................................................................................... 59
Table A. 4: Liquid Limit ...................................................................................................... 59
Table A. 5: Compaction ....................................................................................................... 60
Table A. 6: Direct Shear Test Data at 40 % Degree of Saturation ...................................... 60
Table A. 9: Direct Shear Test Data at 73.64 % Degree of Saturation ................................. 62
Table A. 10: Direct Shear Test Data at 80 % Degree of Saturation .................................... 62
Table A. 11: Direct Shear Test Data at 90 % Degree of Saturation .................................... 63
Table A. 12: Direct Shear Test Data at 100 % Degree of Saturation .................................. 63
Table A. 13: Unified Soil Classification Systems ............................................................... 64
Table A. 14: Sieve Number and Sieve Opening ................................................................. 65
Table A. 15: Grain Size Distribution (Sieve Analysis) of Clay-Sand Mixtures Soil .......... 66
Table A. 16: Hydrometer Analysis of Clay-Sand Mixtures Soil ......................................... 66
xi
List of Figure (Appendix)
Figure B. 1: Shear Vs Horizontal Displacement Due To 40 % Degree of Saturation. ........ 68
Figure B. 2: Shear Vs Horizontal Displacement Due to 50 % Degree of Saturation. ......... 68
Figure B. 4: Shear Vs Horizontal Displacement Due to 73.64 % Degree of Saturation. .... 69
Figure B. 7: Shear Vs Horizontal Displacement Due to 100 % Degree of Saturation. ....... 71
Figure B. 8: Sample Preparation of Clay and Sand ............................................................. 71
Figure B. 9: Liquid Limit Test Using Cone Penetration Apparatus .................................... 72
Figure B. 10: Specific Gravity, Free Swell and Hydrometer Soak...................................... 72
Figure B. 11: Sample Preparation for Compaction Test ...................................................... 73
Figure B. 12: Direct Shear Test Sample Preparation and Testing ....................................... 73
xii
Abstract
A series of direct shear test have been performed on a compacted clayey sands soil with
different degree of saturation to analyze the effect of degree of saturation on the shear
strength parameters of c- soil. A comprehensive research methodology was developed to
determine shear strength properties of c- soil in controlled degree of saturation with in
fixed void ratio and sand clay mixture based on compaction result. Clayey sands mixture
was prepared from disturbed 70% sand and 30% clay. The clay and sand sample were
collected from Gullele, Addis Ababa and Langano, near Adama. The index properties and
compaction test were conducted to figure out the properties and maximum dry densities of
the mixed soil. Also the direct shear tests were conducted by varying the degree of
saturation. The soil sample of preparation for direct shear test has sieved passing sieve no.4
and was compacted as required by ASTM D standard. The shearing displacement is
constant at 1 mm/minutes and the normal stresses are 100 kPa, 200 kPa and 400 kPa. The
investigated clay-sand mixture soil, its specific gravity was found to be 2.69 with 26%
liquid limit and categorized in non-plastic zone. The maximum dry density was found to be
1.942 g/cm 3 at the optimum water content of 10.4%. The increase in degree of saturation
causes the increase in cohesion and angle of friction which is similar to the compaction
curve (Figure 4-13 and 4-16). After the optimum moisture content point as the degree of
saturation continue its increase but the cohesion and angle of internal friction decrease. The
correlations were developed to show the effect of degree of saturation on shear strength
parameters of c - soil of compacted soils.
Key words: Cohesion, internal friction angle, degree of saturation, direct shear test.
Hawassa University, | Hawassa
The Effect of Degree of Saturation…… Summary, Conclusions and Recommendation
1. Introduction
1.1 Background of problems and significance of the study
Mechanical properties of soil are necessary for the design and analysis of earth structures,
soil slope, retaining wall, soil foundation. Soil strength indicates the ability of the soil to
carry load. Direct shear testing is one of the oldest strength tests and popular is determining
shear strength of soil. The method has been standardized by the American Society for
Testing of Materials (ASTM D).
Conventional soil mechanics principles are commonly used in engineering practice
assuming soils are typically found in a state of saturated condition in nature. However,
soils typically have other fluids in the voids (e.g., air) along with water.
The variation of moisture content stored in the ground and earth structures under varying
environmental conditions is an important aspect closely related to the mechanical behavior
of partially saturated soils. Change in the degree of saturation can cause significant
changes in volume and shear strength.
The volume change properties of soils (water addition causing swelling and water removal
causing shrinkage) lead to severe management issues related to the integrity of civil
infrastructure that generally involves soil compaction. Most of the infrastructure has been
constructed on/with compacted soils. Shear strength of compacted soil is an important part
of geotechnical engineering because of the role it plays in: (i) the evaluation of bearing
capacity of foundations for residential and commercial facilities, (ii) the evaluation of
stability of the slope for highway embankments, earth dams, canals, excavations and (iii)
the design of earth retaining structures like retaining walls, sheet piles and coffer dams.
Direct shear test device is commonly used on diameter of about 60 mm and thickness of
about 20 mm (ASTM 3080-98). The standard method has limited the maximum particle
size to one-tenth of the mould diameter or one-sixth of the mould thickness. The soil
samples however may have different degree of saturation. For soil mass with only one
degree of saturation, the obtained result of the direct shear test therefore may not truly
represent the actual in-situ properties dry and wetting season. Hence its significant to
develop some relationship to understand the impact of degree of saturation. Therefore this
research has been undertaken the direct shear test on c- soil sample with different degree
of saturation.
The study investigate the effect of degree of saturation on shear strength of c- soil which
is a function of the apparent angle of internal friction and of the apparent cohesion as
evaluated by direct shear tests.
1.2 Research Objectives
The main objective of this research is to investigate the effect of degree of saturation on the
results of shear strength of compacted c- soils.
Specific objective:
To determine the shear strength parameters, i.e., - friction angle ('), and cohesion
(c') of compacted samples under different degree of saturation using direct shear
testing.
To develop some correlation between shear strength parameters and degree of
saturation (s).
1.3 Research Methodology
This section presents a basis for a comprehensive laboratory investigation program. The
whole laboratory investigation program that would be divided into three stages:
geotechnical index properties, compaction curve and shear strength properties.
The sample of Clay and sand was obtained from Gullele, Addis Ababa and Langano, near
Adama. Geotechnical index properties would be done to determine the index properties of
investigated soil. The compaction curve of investigated soil would be done to determine
the optimum water content and maximum dry density, and to obtain the compacted sample
to investigate the behavior of compacted soil. The shear strength properties would be
determined for compacted different degree of saturation samples using consolidated drain
direct shear testing (by using perforated metal plate to let the water out). Total eight sets of
tests would be conducted before and after the optimum moisture content point based on the
compaction test result. The research will focus on understanding the shear strength
properties of compacted soils through the laboratory test and parametric study. Overall, the
output would be expected to introduce the impact of degree of saturation on the results of
direct shear tests of c- soil and a correlation to determine the cohesion and internal
friction angle with relationships of degree of saturation. Generally, the research
methodology comprises; literature review, index properties and standard proctor
compaction test, direct shear tests, determination of cohesion (c) and friction angle , and
discussions and conclusions.
1.4 Scope and limitations of the Study
Even though there are many influential properties should be studied in conjunction with
degree of saturation but due to the absence of laboratory apparatus and small budget, this
study focus only to investigate the effect of degree of saturation on the results of direct
shear strength of c- soil using direct shear test in correlated degree of saturation with a
predetermined void ratio and sand-clay proportion.
The scope of the research include as follows.
1. Laboratory test were conducted on c- soil specimens with a fixed ration of clay
and sand mixture.
3. Determining the properties of soil, including water content, specific gravity,
Atterberges limits, grained size analysis (sieve and hydrometer analysis), and
compaction test.
4. Direct shear testing using small-scale direct shear box (60x60x20mm).
5. Normal stresses used in the direct shear were 100 kPa, 200 kPa, and 400 kPa with a
constant shear displacement rate of 1 mm/minute.
2. Literature Review
2.1 General
The influence of degree of saturation on both shear strength parameters cohesion „c and
angle of internal friction „ is very significant. Formulating some relationship and
correlation leads to understand the effect and to express in equation and graphs. This
provides the general trends expected in direct shear tests for clay-sand mixtures of variable
degree of saturations.
In ancient civilization, in the time where there was no cement, sand and clay were used to
form brick called adobe means mud brick in Spanish which is a building material made
from earth. Even though many years were passed, there are a lot of structures exist as a
testimony which stood stand still today, like the great mosque of Djenne in Mali and Poeh
Museum tower in USA (THJ Marchand, 2016). It is still in use around the world in
southern United States and South America and Europe even in Africa. To form a good
adobe brick and other geotechnical structure like earth bag, the best ratio was mixing clay
and sand in 30 to 70 present respectively (James R. Clifton, 1979).
clay-sand mixtures were considered by design geotechnical and environmental engineers
for use as hydraulic barriers. Adding clay to the sand helps in achieving low hydraulic
conductivity. The ASTM standards classify the material as clay when the percentage of
material passing sieve no. 200 is greater than 50%, and the consistency tests indicate CL
classification. Also sand is soil that is smaller than 4.75 mm sieve and retained 0.075 mm
sieve. Direct shear test has long been used to estimate the shear strength parameters for the
analysis of slope stability, retaining wall, and bearing capacity problems. Shear strength is
the ultimate resistance force to limit damage when the soil was sheared by force, which is
one of the most significant indicators of soil physical properties. The main effects of shear
strength are the species of soil structure, moisture content, void ratio and the amount of
sand (Li-chang Wang, 2014).
Shear strength parameters are the key parameter required in the design of foundations,
dams, retaining walls, bridge abutments, and temporary support structures. This chapter
compiles background information related to direct shear test and its result of shear strength
properties of soils due to the effect of variation of degree of saturations.
Figure 2-1: The Great Mosque of Djenne In Mali Built in Adobe (THJ Marchand, 2016).
2.2 Effect of Degree of Saturation Variation on Direct Shear Test Result
Kim (2011) studied the variation of shear strength of weathered granite soil with water
content. This study investigates the effects of initial water content and disturbance on the
strength reduction for both disturbed and undisturbed samples of weathered granite soil in
Korea using direct shear test. Several series of direct shear tests were carried out on
undisturbed or disturbed samples with various water contents under normal stress ranging
from 30 KPa to 140 KPa. He found out that cohesion and friction angle of weathered
granite soils linearly decrease with an increase in degree of saturation.
Blazejczak et al. (1995) investigated the effect of soil water conditions and soil compaction
on the age hardening process of loamy sand and silty loamy sand in relation to the tensile
strength. Soil samples were moulded at water contents 10%, 15%, and 20% and
compacted upto1.35, 1.45, 1.55 g/cm 3 . At intervals after moulding, the tensile strengths of
the moist samples were measured with the indirect tension test. High water content had a
negative effect on the tensile strength of soil at constant bulk density. High bulk density,
however, had a positive effect on tensile strength at constant water content.
The Terzaghis definition of effective stresses for saturated soils implies that the changes
in volume and the shear strength of a soil element are entirely due to the change in
effective stresses; in other words, shear strength and void ratio are unique functions of
effective stresses. Therefore the state of a saturated soil is considered as completely
specified by one stress state variable (total stress minus pore water stress) and a volumetric
variable (the void ratio or the water content). Since in partially saturated soils also the air
component has to be taken into account in the description of the mechanical behaviour,
additional parameters are required to describe the state of partial saturation. On the other
hand, to describe the volumetric behavior it is necessary to define a volumetric variable
that identifies the amount of total pore volume (Vv) that is occupied by water (i.e. ew = Vw
/ VS , Toll, 1995). Thus, as far as the volumetric behavior is concerned, the degree of
saturation S is considered by many authors (Gallipoli et al., 2003, Wheeler and Sivakumar,
1995, Wheeler, 1996,) the more suitable to describe the state of partial saturation since it is
related to both the water content and the void ratio by the following expression:
S = ew / e = Vw / Vv = WGs / e (2.1)
Where W is the water content, Gs is the specific gravity and e is the void ratio.
2.3 Shear Strength for Soils
Shear strength of soils is highly affected by moisture conditions, especially if the soil
contains clay materials. Several landslides were caused by a sudden drop in the mechanical
properties of the material associated with an increase in the water content. This was the
case, for example, in the catastrophic events of the Vaiont Dam failure, where a landslide
caused sudden emptying of the reservoir (Hendron Jr.,A. J. and Patton, F. D., 1987).
An understanding of the shear strength of soil is essential in foundation engineering. This
is because most geotechnical failure involves a shear type failure of the soil. This is due to
the nature of soil, which is composed of individual soil particles that slide when the soil is
loaded. Shear strength of soil is characterized by cohesion (c), and friction angle (). The
parameters, define the soil maximum ability to resist shear stress under defined load. The
shear strength of soil is required for many different types of engineering analyses (K
Bláhová, 2013). Cohesion mobilizes at the beginning of stress conditions and reaches
maximum values around the plastic limit, i.e. at the beginning of structural collapse
(Mencl, V., 1997).
Internal friction is generally defined as resistance of two planes moving against each other,
determined by their grading. Friction increases with increase in normal load, provided that
the soil specimen is allowed to consolidate (Mencl, V., 1997). It is expected for the shear
strength to grow with the decrease in water content. This assumption is in accordance with
Toll (2000), who says that clayey materials compacted drier than optimum moisture
content behave in a coarser fashion, due to aggregation, than would be justified by the
grading. Therefore reduction of water content in clayey soils results in higher friction
angle, due to the fact, that clay particles group into aggregates which have larger effective
particle size, as proposed in Brackley (1973, 1975).
From Terzaghis definition the concept of effective stress will first be introduced. The
effective stress is defined as
(2.2)
Where,
u is pore water pressure.
In shear strength testing, the total stress acting on the soil specimen can be determined as
the load divided by the area over which it acts. The pore water pressure and the air pressure
in the soil is typically assumed to be equal to zero that is slowly sheared in direct shear
apparatus. The shear strength of soil can be defined as,
f = c + nftan
c' = effective cohesion,
σnf = effective normal stress on the failure plane at failure,
' = effective friction angle.
2.3.1 Friction Angle
Friction angle for a given soil is the angle on the graph (Mohr's Circle) of the shear stress
and normal effective stresses at which shear failure occurs. Friction angle of soil is
generally denoted by "". Gravels with some sand typically have a friction angle of 34 ο to
48 ο , loose to dense sand have 30
ο to 45
ο to 35
ο and clay
have around 20 ο . Well graded soils have high values of friction angle. (Palossy et
al.,1993).
The friction angle is a function of the characteristics like particle size, compaction effort
and applied stress level (Hawley, 2001; Holtz and Kovacs, 2003). Friction angle increases
with the increase in particle size (Holtz, 1960) whereas Kirkpatrick (1965) made it more
specific by indicating that the friction angle increases as the maximum particle size
increases. Friction angle also increases with the increase in angularity and surface
roughness (Cho et al, 2006). With an increase of density or decrease in void ratio, friction
angle increases (Bishop, 1996). Bhandary and Yatabe (2007) reported that friction angle
decreased with the increasing values of expansive mineral ratio (relative amount of
expansive clay mineral to non-expansive clay mineral).
2.3.2 Cohesion
Cohesion of soil is usually denoted by "c" and is one of the important components of shear
strength soil mainly for fine materials. Cohesion is the attraction by which soil particles are
united throughout the mass. Cohesion is the strength of soil which behaves like glue that
binds the grains together. Rock has a cohesion value of 10,000 kPa, whereas silt has 75
kPa and clay has 10 to 20 kPa. Depending on the stiffness of the clay soft to high, cohesion
varies from 0 to 76 kPa. Natural minerals that have been leached into the soil, such as
caliches and salts, can provide a very strong cohesion. Heat fusion and long term
overburden pressure will tend to fuse the soil grains together, producing significant
cohesion (R. H. Chowdhury, 2013).
2.4 Compaction Curve
In geotechnical engineering, compaction is defined as the densification of soils by the
application of mechanical energy (Holtz et al., 2011). Soil compaction is a general practice
in geotechnical engineering to construction road, dams, landfills, airfields, foundations,
hydraulic barriers, and ground improvements.
Compaction is applied to the soil, with the purpose of finding optimum water content in
order to maximize its dry density which eventually decreases long term compressibility,
increases shear strength, and sometimes reduces permeability. Proper compaction of
materials ensures the durability and stability of earthen constructions (A Maher, and L
Gucunski., 1998). A typical compaction curve presents different densification stages when
the soil is compacted with the same apparent energy input but different water contents. The
water content at the peak of the curve is called the optimum water content (OWC) and
represents the water content at which dry density is maximized for a given compaction
energy.
Several different methods are used to compact the soil in the field, such as tamping,
kneading, vibration, and static load compaction. However, laboratory tests employ the
tamping or impact compaction method using the type of equipment and methodology
developed by Proctor (1933). This is because, the test is known as the proctor test. Two
types of compaction tests are commonly used in laboratory tests, (i) The Standard Proctor
Test, and (ii) The Modified Proctor Test (WV Ping, 2003).
Zein (2000) showed that, compacted materials are highly aggregated on the dry side of
optimum moisture content, albeit aggregation does not exist on the wet of optimum and
also noted that there were no aggregations at optimum water content (and also wet of
optimum). However, the degree of aggregation increased as the water content reduced
below optimum moisture content. At moisture contents below 70% of OWC the material
was completely aggregated with no matrix material. The effect of compaction water
content (at three points of the compaction curve: dry, OWC, and wet) on the microstructure
of Jossigny silt (the clay fraction is 34%) was studied by Delage et al. (1996). At optimum
water content, a more massive structure with less obvious aggregates occurred. The higher
density is a result of lower resistance to deformation of the aggregates, which deform and
break down more easily; reducing in particular the interaggregate pores. On the wet side,
due to hydration, the clay particles volume is much larger and forms a clay paste
surrounding the silt grains.
According to Toll (2000) fabric plays a vital role in determining the engineering behavior
of compacted soils. Clayey materials compacted dry of optimum moisture content develop
an aggregated or „packet fabric. The presence of aggregations causes the soil to behave in
a coarser fashion that would be justified by the grading. For soils, compacted to degrees of
saturation of 90% and over, the material would be expected to be non-aggregated. As the
degree of saturation drops, the amount of aggregation increases rapidly and reaches a fully
aggregated condition for degrees of saturation below 50%.
2.5 Soil Classifications
Based on ASTM (D 2487 00) soil classification is the method used to purpose the soil type
and predict the soil behavior introductory. In the soil classification method is mostly used
the Unified Soil Classification System (USCS). The basic element of the USCS is the
determination of the amount and distribution of particle size larger than 0.075 mm
(retained sieve no.200) is determined by sieving and the distribution of particle size smaller
than 0.075 mm by the hydrometer analysis. For the USCS, the rocks fragments and soil
particles versus size are defined as Boulders is rock that have an average diameter greater
than 300 mm, Cobbles is rock that is smaller than 300 mm and retained on 75 mm sieve
(USCS standard sieve), Gravel is rock or soil that is smaller than 75 mm sieve and retained
4.75 mm sieve, Sand is soil that is smaller than 4.75 mm sieve and retained 0.075 mm
sieve, Silt is the fine soil that is passing 0.075 mm sieve and larger than 0.002 mm, Clay is
the fine soil that is passing 0.075 mm sieve and smaller than 0.002 mm, It is important to
separate between the size of soil particle and the classification of the soil.
The basis of the USCS is that the engineering behavior of coarse-grained soils is based on
their grain size distributions and the engineering behavior of fine-grained soil is related to
their plasticity characteristics. The USCS summary is shown in Table A.13 (Appendix).
2.6 Direct Shear Testing
2.6.1 A History of the Direct Shear Box Test
The direct shear box test is a conceptually simple test that apparently was used for soil
testing as early as 1776 by Coulomb (Lambe & Whitman, 1969) and was featured
prominently by French engineer Alexandre Collin in 1846 (Skempton, 1984). He used a
split box, 350 mm long, in which a sample of clay 40 x 40 mm section was subjected to
double shear under a load applied by hanging weights.
In Britain, Bell (1915) made the earliest measurements who constructed a device which
was to be the prototype for subsequent developments of the shear box. Bell was the first to
carry out and publish result practical of shear tests on various types of soil (Skempton,
1958).
A simple shear box with a single plane of shear was designed in 1934, using the stress
control' principle where the load was applied in increments by progressively adding
weights to a pan. This required considerable care and judgment on the part of the operator
in order to ascertain the load at which failure occurred.
A modern shear box was designed by A.Casagrande at Harvard (USA) in 1932. Four years
later, Gilboy at MIT, developed a constant rate of displacement machine which applies the
'strain control' principle, using a fixed speed motor. In 1946, Bishop at imperial College
introduced the improvements of design using this principle in details. Most commercial
shear box machines are still based on the displacement control principle. These machines
provide a wide range of displacement speeds, from a few millimeters per minute to about
10000 times slower. The stress-control method has certain advantages in some long-term
tests in which increments of stress must be applied very slowly, and in tests for the study of
the effect of 'creep' under constant shear stress. However, for routine testing applications
the displacement-control method is the one now normally used.
2.6.2 Significance and Use of Direct shear Test
The direct shear test is suited to the relatively rapid determination of consolidated drained
strength properties because the drainage paths through the test specimen are short, thereby
allowing excess pore pressure to be dissipated more rapidly than with other drained stress
tests. The test can be made on all soil materials and undisturbed, remolded or compacted
materials. There is however, a limitation on maximum particle size (T Boonklung, 2013).
During the direct shear test, there is rotation of principal stresses, which may or may not
model field conditions. Moreover, failure may not occur on the weak plane since failure is
forced to occur on or near a horizontal plane at the middle of the specimen (R.F. RF.
Craig., 1992.)
Shear stresses and displacements are non-uniformly distributed within the specimen, and
an appropriate height is not defined for calculating shear strains or any associated
engineering quantity. The slow rate of displacement provides for dissipation of excess pore
pressures, but it also permits plastic flow of soft cohesive soils (R. H. Chowdhury, 2013)
Generally, advantages of the shear box test pointed out as follows:
2.6.2.1 Advantages
1) The test is relatively quick and simple to carry out.
2) The basic principle is easily understood.
3) The principle can be extended to gravelly soils and other materials containing large
particles, which would be more expensive to test by another means.
4) Preparation of re-compacted test specimens is not difficult.
5) Friction between rocks and the angle of friction between soils and many other
engineering materials can be measured.
6) The apparatus can be used for drained tests and for the measurement of residual
shear strength by the multi-reversal process.
3. Research Methodology
3.1 General
The laboratory testing program was focused on determination of the shear strength
properties of local c- soil in different degree of saturations. The research methodology
was divided into two parts. As a Figure 3.1 shows, in part one, index properties and
compaction curve were conducted and in part two, based on the result of geotechnical
index properties and compaction curve the direct shear test was performed.
clay-sand mixtures were prepared in proportion of 30% clay (with some slit) to 70% sand.
Sand and clay were used to form brick called adobe brick. To form a good adobe brick and
other geotechnical structure like earth bag, the best ratio was mixing clay and sand in 30 to
70 present respectively. The sample of Clay and sand was obtained from Gullele, Addis
Ababa and Langano, near Adama. These sample of clay were retrieved from a depth of 1.5
m in accordance with ASTM D1452-07a. Then samples were transported to the Ethiopian
Construction, Design and Super Vision Works geotechnical laboratory facility near to
emperial, Addis abeba.
Figure 3.1 shows the laboratory investigation program. Each of the test procedures is
described in this section whereas the test data and example calculations are given in the
Appendix.
Tests performed on investigated soil consisted of the determination of
Geotechnical index properties,
Standard proctor test,
Direct shear test.
Figure 3-1: Laboratory Investigation Program
3.2 Geotechnical Index Properties
3.2.1 Water Content
Water content (w) is the amount of water present in the soil and is represented as
percentage. Water content was determined according to ASTM D2216-05. This test
method is determination the water (moisture) content by mass of soil, rock and similar
materials where the reduction in mass by drying is due to loss of water. This test can be
performed on disturbed and undisturbed sample. The water content (w) is the ratio,
expressed as percentage of a weight of water (ww) in a given weight of soil (ws) to the
weight of dry soil (wd). The water content test consists of determining the weight of wet
soil (ws) specimen and then drying the soil in an oven about 12 to 16 hours at a
temperature of 110 °C ± 5 °C in order to determine the weight of dry soil (wd). The loss of
mass due to drying is considered to be water. The following equation was used to
determine the water content:
Where,
Mc = mass of moisture can
3.2.2 Specific Gravity
Specific gravity (Gs) is the ratio of the mass of soil solid to the mass of an equal volume of
distilled water at 4 ºC. The specific gravity was determined by ASTM D854-10. The test
method are cover the determination of the specific gravity of the soil solids that passing
sieve no.4. A clean and dry pycnometer was weighed to the nearest 0.01 g. Distilled water
was de-aired using the vacuum pump and that was kept overnight to remove all the air
bubbles. Then the distilled and de-aired water was added to the pycnometer up to the
calibration mark of 500 ml. The mass of pycnometer and water and temperature were
measured. Around 100g of soil was dispersed and with the distilled water soil was made to
slurry. Slurry was poured into the pycnometer and remaining soil particle was carefully
washed with spray squirt bottle to pour into the pycnometer. More water was added to
make around two third volume of the pycnometer. Then vacuum pump was connected to
the pycnomter and operated for 4 hours to remove entrapped air from the soil slurry. After
the de-airing process was completed, the pycnometer was filled with de-aired distilled
water to the calibration mark and weight of the pycnomter was measured. The soil slurry
was transferred to the evaporating dish and it was kept in an oven that maintained the
temperature at 110°C. The following relationship was used to measure the specific gravity
of investigated soil:
( ) ( )
Where:
Mrw,t = mass of the pycnometer and water at the test temperature (Tt), g,
Mp = the average calibrated mass of the dry pycnometer, g,
Vp = the average calibrated volume of the pycnometer, mL, and
pw,t = the density of water at the test temperature (Tt), g/mL from Table 2(ASTM)
To calculate the specific gravity at soil solids the test temperature, Gt:
( ( )) (3.3)
Where:
ps = the density of the soil solids Mg/m 3 or g/cm
3 ,
pw,t = the density of water at the test temperature (Tt), from Table 2(ASTM), g/mL
. or g/cm 3 .
Ms = the mass of the oven dry soil solids (g), and
Mpws,t = the mass of pycnometer, water, and soil solids at the test temperature, (Tt), g.
Finally, calculate the specific gravity of soil solids at 20°C:
(3.4)
Where:
K = the temperature coefficient given in Table 2(ASTM). Or K is the ratio of the
density of water (or t) at the test temperature t and at 20 °C.
( )
The specific gravity of a soil solid is used in calculating the phase relationships of soils,
such as void ratio and degree of saturation.
3.2.3 Grain Size Distribution
The grain size distribution (GSD) was determined in accordance with ASTM D422-63
(2007). GSD was done in two phases. In the first phase particle sizes larger than 75 μm
(retained on the No. 200 sieve) was determined by sieve analysis and then in the second
phase the distribution of particle sizes smaller than 75μm was determined by a
sedimentation process, using a hydrometer.
3.2.3.1 Sieve Analysis
Around 500g soil specimen was taken and distilled water was added to the sample to make
it slurry. The slurry was allowed to passed through the Sieve No. 200 (opening size =
0.075 mm). The soil retained and passing from the sieve was transferred to the evaporating
dishes and kept in oven at the temperature of 110 °C ± 5 °C. The specimens were taken out
of oven after drying and weighed. The fines content (%) was calculated from retained soil.
Sieve properties following ASTM standard is shown in Table A.14 (Appendix)
3.2.3.2 Hydrometer Analysis
The finer soil, mainly the clay fraction (the percent finer than 0.002 mm) which cannot be
analyzed by sieve, is usually done by hydrometer analysis. After sieve analysis, the soil
retained on the pan was dried and around 100 g of soil was taken for the hydrometer
analysis. This sample was mixed with 125 mL of 4% NaPO3 solution in a small
evaporating dish and the dish was covered by wet paper towel to minimize evaporation.
The mixture was kept for 16 hours to soak. After soaking, the mixture was transferred to a
dispersion cup and water was added until the cup was about two-thirds full. Then the
mixture was transferred to the sedimentation cylinder and agitated carefully for about 1
minute to make the mixture uniform. Then the cylinder was set for the hydrometer test and
first reading was taken at an elapsed time of 2 minutes. At the same time water temperature
was recorded. At least 15 seconds before the reading taken, the hydrometer was placed on
the cylinder so that it can be settled down.
Hydrometer and temperature readings were continued at approximate elapsed times of 5,
15, 30, 60, 250 and 1440 minutes. The density of the suspension at the level of hydrometer
can be computed by means of Stocks law, whereas the weight of the particles finer than
that size can be computed from the density of the suspension at the same level. The results
are represented by cumulative curve plotted on semi-logarithm graph.
3.2.4 Free Swell
Free Swell Index is the increase in volume of a soil, without any external constraints, on
submergence in water.
(3.6)
Where:
Vf = final volume of soil specimen read after 24 hours from the graduated
cylinder containing distilled water.
Vi = initial volume of soil specimen read from the graduated cylinder.
3.2.5 Atterberges (Consistency) Limits
The liquid limit is the water content at which soil changes from the liquid state to a plastic
state or the minimum moisture content at which a soil flows upon application of very small
shear force. Liquid limit (wl) is the water content, in percent, of a soil at the arbitrarily
defined boundary between the semi-liquid and plastic states whereas the plastic limit (wp)
is the water content, in percent, of a soil at the boundary between the plastic and semi-solid
states.
The liquid limit, plastic limit and plasticity index were determined according to ASTM
D4318–10. The liquid limit and plastic limits are used for soil identification and
classification and for strength correlation. The specimen was processed to remove any
material retained on a 425-m (No. 40) sieve. The liquid limit was determined by
performing trials in which a portion of the specimen was spread in a brass cup, divided in
two by a grooving tool, and then allowed to flow together from the shocks caused by
repeatedly dropping the cup in a standard mechanical device. But due to the amount of
sand compared to clay was high, conducting this procedure was failed hence to determine
the liquid limit cone penetrometer was used. This is based on penetration of cone shaped
metal object into a homogeneously prepared soil mud with free fall. 300 g sample of soil is
taken and mixed with water then by pushing a portion of mixed soil into the cap with a
palette knife taking care not to trap air. Strike off excess soil with the straightedge to give a
smooth level surface. With the penetration cone locked in the raised position lower the
supporting assembly so that the tip of the cone touches the surface of the soil. When the
cone is in the correct position a slight movement of the cup will just mark the soil surface.
Lower the stem of the dial gauge to contact the cone shaft and recording of the dial gauge
to the nearest 0.1 mm in 5+1 seconds. Take a moisture content sample about 10 g from the
area penetrated by the cone and determine the moisture content. Repeat the step at least
three more times using the same sample of soil to which further increments of distilled
water have been added. Lastly, water content corresponding to 25 blows was determined.
(See cone penetration apparatus used for liquid limit test in appendix b)
About 20 g portion of soil was taken from the material prepared for the liquid limit test to
determine the plastic limit of soil. The water content of the soil was reduced to a
consistency at which it can be rolled without sticking to the hands by spreading on the
glass plate. The mass is rolled between the palm or fingers and the ground-glass plate with
just sufficient pressure to roll the mass into a thread of uniform diameter throughout its
length. The thread was further deformed on each stroke so that its diameter reached 3.2
mm (1/8 in). Two trials were done for plastic limit test and the average value was taken for
plastic limit.
Finally, the difference between liquid limits and plastic limits were taken as plastic index.
The study of plasticity index, in combination with liquid limit, gives information about the
type of clay. Plasticity chart, which is a plot between the plasticity index and liquid limit, is
extremely useful for classification of fine-grained soils. In fact, the main use of consistency
limits is in classification of soils.
The standard proctor compaction tests were done on the investigated soil according to
ASTM D1557-09. Total five numbers of samples have compacted to get a proper
compaction curve. Air dried sample was used for each compaction test. All the lumps of
the soil were broken and sieved through a 4.75 mm opening sieve (sieve No. 4) and collect
the entire passed sample and stored in the container. Enough water was added to the soil
sample (passed through 4.75 mm opening sieve) and mixed thoroughly to bring the water
content up desired quantity. Weight of the proctor mold and base plate were measured.
After attaching the mold top extension soil sample will be poured into the mold in three
equal layers. Each layer will be compacted with standard proctor compaction effort by 25
times before the next layer of loose soil was poured into the mold. After compaction of
each three layers, by removing the top extension, excess soil above the mold was trimmed.
Weight of the (proctor mold + base plate + compacted moist soil) was measured.
Compacted soil was removed from mold carefully. Small amount of soil was kept for the
water content determination after removing the sample from the mold. To determine dry
( )
Mmd = mass of compaction mold, kg, and
V = volume of compaction mold, m 3
To determine dry density:
w = water content, %.
To determine dry unit weight:

d = dry unit weight of compacted specimen.
To calculate points for plotting the 100 % saturation curve or zero air voids curve
select values of dry unit weight, calculate corresponding values of water content
corresponding to the condition of 100 % saturation as follows:
( )( )
wsat = water content for complete saturation, %,
w = unit weight of water, 9.789 kN/m 3 at 20°C,
d = dry unit weight of soil, kN/m 3 , and
Gs = specific gravity of soil.
Soil placed as engineering fill (embankments, foundation pads, road bases) is compacted to
a dense state to obtain satisfactory engineering properties such as, shear strength.
Laboratory compaction tests provide the basis for determining the percent compaction and
water content needed to achieve the required engineering properties, and for controlling
construction to assure that the required compaction and water contents are achieved.
During design of shear or other tests require preparation of test specimens by compacting
at some water content to some unit weight. It is common practice to first determine the
optimum water content (wo) and maximum dry unit weight (dmax) by means of a
compaction test.
3.4 Direct Shear Test
In this study, based on the compaction result eight sets of direct shear test had conducted 3
in wetting and 4 in drying side with controlled degree of saturation in predetermine void
ratio (from maximum dry density with optimum moisture content) and c- soil proportion.
The consolidated drain test was conducted on different degree of saturation by varying the
moisture content of the samples.
Direct shear test are performed to determine the shear strength parameters of soil. The test
procedure follows the relevant ASTM standard (ASTM D3080). This test method covers
the determination of the consolidated drained shear strength of a soil material in direct
shear test. The test is performed by deforming a specimen at a controlled strain rate on or
near a single shear plane determined by the configuration of the apparatus. Generally, three
or more specimens are tested, each under a difference normal load, to determine the effect
upon shear resistance and displacement, and strength properties such as Mohr strength
envelopes. Shear stress and displacement are non-uniformly distributed within the
specimen. An appropriate height cannot be defined for calculation of shear strains.
Therefore, stress-strain relationships or any associated quantity such as modulus cannot be
defined from this test. The test condition including normal stress and moisture content are
selected by varying the degree of saturation which represent the field conditions being
investigated.
Rectangular direct shear ring was used in the shear box assembly which is 60x60x20mm
size. Normal stresses which applied are 100 kPa, 200 kPa, and 400 kPa. The shearing rate
is using as low speed about 1mm/minutes and 0.001376 N/div proving ring calibration,
which is the occurrence exceed pore water pressure. This test method consists of placing
the test specimen in the direct shear device, applying a predetermined normal stress,
providing for wetting or draining of the test specimens consolidating the specimen under
the normal stress, unlocking the frames that hold the test specimen, and displacing one
frame horizontally with respect to the other at a constant rate of shearing deformation and
measuring the shearing force and horizontal displacements as the specimen is sheared.
The peak strength is calculated and plotted the corresponding normal stresses with shear
strength. The test results are summarized the shear strength parameters of plotted friction
angle with water content and cohesion with water content and in relation with degree of
saturation.
To determine nominal shear stress, acting on the specimen is,
(3.11)
Where:
F = shear force (N),
Normal stress acting on the specimen is,
(3.12)
Where:
N = normal vertical force acting on the specimen (N).
3.5 Experimental Work
3.5.1 Construction of Test and Preparation of Sample
The tests were performed on disturbed compacted c- soil samples. The air-dried sand and
clay has been crushed and sieved passing sieve no.4 for the preparation of soil samples.
The physical characteristics then determined. Using conventional strain controlled direct
shear apparatus according to three classes of vertical load (100 kPa, 200 kPa and 400 kPa)
as a rapid method to determine the shear strength of soils.
The degree of saturation of the soil samples have been controlled accurately through
adjusting the amount and proportion of air- dried soil, water and sand on the experimental
study.
3.5.2 Degree of Saturation Controlled Soil Samples Preparation
There are eight soil samples have been prepared by compacting air-dried soil and the
certain amount of water to study the effects of degree of saturation on the shear strength
parameters (c, ). The properties of prepared samples are as follows: void ratio e = 0.385
(from compaction test result), degree of saturation s = 30%, 40%, 50%, 60%, 73%, 80%,
90% and 100%.

( )
( ) ( ) (3.16)
The formulas for calculating the weight of air-dried soil and the amount of water added on

( ) (3.18)
Note:
(3.19)
But
Where; e = void ratio, n = porosity, S = degree of saturation, w = water content, GS = the
specific gravity of soil particles, ρd = dry density, w = water density (1g/ml, or 1000kg/m 3 ,
or 1mg/m 3 ), Vv = volume of void, Vw = volume of water, V = Volume, mds = weight of dry
soil, w0 = water content of air-dried soil, mwo = weight of air-dried soil, mw = weight of
water and mw(add) = the additional water to be added on the sample.
3.6 Correlation of Cohesion (C) and Friction Angle (Φ) With
Degree of Saturation
Analysis of the effects of degree of saturation on the shear strength parameters (the
cohesion c, the angle of internal friction ) has been worked out by using a single factor
analysis test which is a statistical method used to describe variability among observed,
correlated variables in terms of a potentially lower number of unobserved variable called
factors.
4. Results and Discussions
4.1 General
This chapter presents and discusses the results obtained from the study. The test results
were water content, specific gravity, grained size distribution, compaction test,
Atterberges limit, and direct shear strength. These are believed to be important parameters
to determine the properties of soils which are essential for the design of foundation and
other geotechnical related structures.
4.2 Geotechnical Index Properties
4.2.1 Water Content
As the soil sample was clay-sand mixtures, the initial moisture content for disturbed soil
was determined. The initial water content of 3 samples are: 1.32%, 1.21%, and 1.21%.
Accordingly, the average initial (air dried) water content is 1.25%.
4.2.2 Specific Gravity
The specific gravity relates the density of the soil particles to the density of water. The
determination of the dry mass of the soil is using a pycnometer to obtain the volume of the
soil solids (ASTM D854). Specific gravity tests carried out on 2 samples were 2.679 and
2.700. Hence, the average specific gravity is 2.69.
4.2.3 Grained size distributions
The grained size analysis is the methodology to present the grained size distributions of
soil. The relationship of percent finer with grained size distributions have plotted on the
semi-log graph as showing in Figure 4.1. The soil fabrication has containing sand 70%, silt
7% and clay 23% (from sieve and hydrometer result). Soil classification is following
Unified Soil Classification System (USCS). According to the USCS, this soil sample group
name is clayey sands and can be classified as low plasticity (CL).
4.2.3.1 Sieve Analysis
The grain size distribution (sieve analysis) of this soil sample has been shown in Figure 4.1
below in conjunction with hydrometer analysis (Table A.15).
4.2.3.2 Hydrometer analysis
The hydrometer analysis of soil sample has been shown in Figure 4.1 in combination with
sieve analysis result (Table A.16). After sieve analysis, the soil pass to the pan was dried
and around 50 g of soil was taken for the hydrometer analysis.
Generally,
Silt 0.075-0.002mm = 7%
Figure 4-1: The Relationship of Percent of Finer With Grained Size Distributions
4.2.4 Free Swell
Free Swell Index is the increase in volume of a soil, without any external constraints,
submergence in water. The free swell indexes obtained from the test using two soil sample
were 14 % and 15 %. The average value of free swell is 14.5%.
4.2.5 Atterberges (Consistency) Limits
Liquid limit obtained from the test using 300 g soil sample. From cone penetration test the
liquid limit test result was 26 % as shown in the figure 4.2. As the sand amount was 70 %,
even though there is small plasticity due to the addition of clay but the sample cant be
rolled with the hand on the glass plate. Hence, the plastic limit wasnt determined.
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
110.00
0.00100.01000.10001.000010.0000
% P
Figure 4-2: Liquid Limit Flow Chart
As a general, the specific gravity of the soil was found to be 2.69. Materials finer than
0.075 mm and finer than 0.002 mm were respectively found to be 7% and 23%, thereby,
which is more dominate by sandy soil which was 70% confirming the coarse grained
nature of the soil. The clayey sands mixture had 26% liquid limit and with small plasticity
due to the addition of clay even if it was categorized in non-plastic zone. Hence, the soil
mixture was classified as CL.
Figure 4.3 shows the compaction curve for investigated soil from the result of standard
proctor compaction test. The curve consisted of the five proctor points as shown in the
figure. The maximum dry density was found to be 1.942 g/cm 3 at the optimum water
content of 10.4%. The increase in d with an increase in water content on dry side of
optimum is due to expulsion of air from the pore space and re-arrangement of particles that
decreases the pore space. Conversely, an increase in water content on wet side of optimum
10.00
12.00
14.00
16.00
18.00
20.00
22.00
24.00
26.00
28.00
30.00
results in an increased volume of water (w = 1 g/cm 3 ) which replaces the soil particles (Gs
= 2.69 g/cm 3 ).
4.4 Direct Shear Test
Even though there are many factors, the variation of degree of saturation is one of the main
factors that affect the nature of soil, which is composed of individual soil particles that
slide when the soil is loaded. Hence, understanding of the shear strength of soil is
necessary in the geotechnical engineering.
4.4.1 Shear Strength Parameters on Different Degree of Saturation.
In process of founding cohesion and internal friction angle, the first step is determining the
weight of air-dried soil and the amount of water added on air-dried soil samples based on

( )
D ry
D en
si ty
( )
( )
Using equation (3.17) and (3.18), the weight of air-dried soil and the amount of water
added for preparing soil samples for conducting direct shear test under different degree of
saturation were worked out as shown in Table 4.1.
A relationship was developed from the compaction test result to fix the value of void ratio.
Therefore, at maximum dry density

( )
( ) ( )
The amount of water should be added to develop the desire degree of saturation was also
based on the compaction test result.
Sample calculation: to meet 40 % degree of saturation, the water content of the sample
should be,
( )
( )
( ) ( )
Table 4-1: Weight of water added under different degree of saturation
Number
1 30 4.29 141.59 4.25
2 40 5.72 141.59 6.25
3 50 7.16 141.59 8.26
4 60 8.59 141.59 10.26
5 73.64 10.54 141.59 12.99
6 80 11.45 141.59 14.26
7 90 12.88 141.59 16.26
8 100 14.31 141.59 18.26
The shear strength parameters were determined from the graph of normal stress verses
shear stress and other direct shear test results were found by plotting the shear stresses with
shear displacement. Therefore, based on the effect of degree of saturation the cohesion and
the internal friction angle were determined.
Table 4-2: Direct Shear Test Data at 30% of Degree of Saturation
0.01
0.001376 0.004
in Div in Kpa in Div in Kpa in Div in Kpa
0 0 0 0.00 0 0 0 0.00 0 0 0 0.00
25 0.25 143 54.66 25 0.25 192 73.39 25 0.25 311 118.87
50 0.5 177 67.65 50 0.5 350 133.78 50 0.5 500 191.11
75 0.75 205 78.36 75 0.75 478 182.70 75 0.75 675 258.00
100 1 231 88.29 100 1 573 219.01 100 1 784 299.66
150 1.5 252 96.32 150 1.5 601 229.72 150 1.5 936 357.76
200 2 265 101.29 200 2 588 224.75 200 2 989 378.02
250 2.5 272 103.96 250 2.5 560 214.04 250 2.5 971 371.14
300 3 261 99.76 300 3 300 3 947 361.96
350 3.5 245 93.64 350 3.5 350 3.5
Proving Ring
Readingh ,div
Horizontal displacement
Readingh ,div
Table 4-3: Direct Shear Test Result at 30 % Degree of Saturation
Figure 4-4: Shear Vs Horizontal Displacement Due to 30 % Degree of Saturation.
Sample No. 1
Sample Condition DISTURBED
Initial Volume (cm 3 ) 72.00
Normal Stress (KPa) 100 200 400 C(KPa) φ (Degrees)
Shear Stress (KPa) 103.96 229.72 378.02 29.43 40.94
30 % SATURATION SHEAR RESULT
Figure 4-5: Maximum Shear Vs Normal Stress Curve Due To 30 % Degree of Saturation.
Sample No. 1
Shear Stress (KPa) 114.28 221.69 384.90 32.68 41.32
Figure 4-6: Maximum Shear Vs Normal Stress Curve Due to 40 % Degree of Saturation.
Sample No. 1
Shear Stress (KPa) 118.11 227.80 390.63 36.69 41.45
Sample No. 1
Shear Stress (KPa) 122.69 237.36 400.19 41.28 41.83
Table 4-7: Direct Shear Test Result at 73.64 % Degree of Saturation
Sample No. 1
Shear Stress (KPa) 126.52 245.00 407.07 45.48 42.02
73.64 % SATURATION SHEAR RESULT
Figure 4-9: Maximum Shear Vs Normal Stress Curve Due to 73.64 % Degree of
Saturation.
Table 4-8: Direct Shear Test Result at 80% Degree of Saturation
Sample No. 1
Shear Stress (KPa) 129.19 228.95 402.10 42.62 41.93
Figure 4-10: Maximum Shear Vs Normal Stress Curve Due to 80% Degree of Saturation.
Sample No. 1
Shear Stress (KPa) 122.69 197.61 366.55 38.22 39.37
Sample No. 1
Shear Stress (KPa) 110.84 193.02 347.44 33.64 38.11
4.4.2 Relationships between Cohesion (c), Angle of Internal Friction () and
Degree of Saturation (S)
The shear strength parameters cohesion (c) and angle of internal friction () of soil samples
have been obtained by the shear strength tests of compacted c- soil.
Table 4-11: Shear Strength Parameters Value at Different Degree of Saturation
Number
3 50 41.45 36.69
4 60 41.83 41.28
5 73.64 42.02 45.48
6 80 41.93 42.62
7 90 39.37 38.22
8 100 38.11 33.64
4.4.2.1 Correlation between degree of saturation (s) and cohesion (c)
Figure 4-13: Degree of Saturation (S) Vs Cohesion (C) Relationship
Figure 4-14: Incremental Cohesion (C) Vs Degree of Saturation (S) Correlation graph
Hence, the correlative equation between incremental cohesion and degree of saturation is:
( ) (4.1)
Figure 4-15: Decremental Cohesion (C) Vs Degree of Saturation (S) Correlation graph
Hence, the correlative equation between decremental cohesion and degree of saturation is:
( ) (4.2)
4.4.2.2 Correlation between Degree of Saturation (s) and Angle of Internal Friction
()
Figure 4-16: Degree of Saturation (S) Vs Angle of Internal Friction () Relationship
Figure 4-17: Incremental Angle of Internal Friction () Vs Degree of Saturation (S)
Correlation graph
Hence, the correlative equation between angle of internal friction () and degree of
saturation is:
( ) (4.3)
Figure 4-18: Decremental Angle of Internal Friction () Vs Degree of Saturation (S)
Correlation graph
Hence, the correlative equation between angle of internal friction () and degree of
saturation is:
( ) (4.4)
4.4.3 Analysis of the Effects of Degree of Saturation on the Cohesion (c)
Fig. 4.13 gives the relationship between cohesion and degree of saturation. The cohesion
increase with increase of degree of saturation similar to the shape of the compaction curve,
after the point where maximum dry density attained with optimum moisture content, even
though the degree of saturation increase but the value of cohesion decrease.
Cohesion was found lower at the drier side of optimum due to the presence of clay
aggregate which made the soil mass more granular. Then cohesion value increases with the
water content and reached maximum value at around optimum due to reduction of the size
of clay aggregate. This is similar to the work done by Cokca et al. (2004). They mentioned
that the cohesion at the drier side of optimum will be lesser than that at optimum water
content due to the „clay aggregation phenomenon where the soil mass exhibits a granular
texture.
Increase in water content above optimum moisture content reduced cohesion as excess
water might develop thicker water film around the clay particle and thus increased the
distance between particles. Seed et al. (1961) observed that cohesion on the wetter side of
optimum is lesser than that at optimum water content due to the formation of thicker water
films around clay particles in the „clay-water system.
4.4.4 Analysis of the Effects of Moisture Content on the Angle of Internal
Friction (Φ)
Fig. 4.16 shows the relationship between internal friction angle and degree of saturation.
As it shows, there is a slight variation which is less than 5°. The internal friction angle
increase with increase of degree of saturation until 73.64% of degree of saturation and
decrease as degree of saturation continue its increase.
On the dry side of optimum, the values of friction angle are not much varied with respect
to the maximum friction angle; this is due to clay aggregates formed on the dry side of
optimum (Cokca et al., 2004). The granular structure and large size of aggregates within
soil mass increased the interlocking between the particles and this generates a resistance to
slippage at the contacts between the particles (or aggregates) as the moisture content
decreases on the dry of optimum.
On the wet side of the optimum; the reduction was due to the increased lubrication of the
soil paste following water addition causing soil particles to slip and slide, resulting in a
reduced friction angle. For higher water content, the soil particles dominated the behaviour
of the soil mixture and the water acts as a lubricant, which decreases the friction angle as
the degree of saturation increases. The lubrication occurs when the surface of the soil
particle is wetted, causing the mobility of the absorbed film to increase due to increased
thickness and greater surface ion hydration and dissociation (Mitchell, 1993).
5. Summary, Conclusions and Recommendation
5.1 Summary
The purpose of this project work was to explore the effect of degree of saturation on the
shear strength parameters of c- soil by using direct shear test and to establish a clear
understanding of the shear strength behavior of compacted clayey sands mixture. The
result of direct shear tests has been discussed on the previous chapter.
A clay-sand mixture was considered by design geotechnical and environmental engineers
for use as hydraulic barriers. Adding clay to the sand helps in achieving low hydraulic
conductivity. Most geotechnical structures (like, earth bag and adobe brick) mixing 30%
clay with 70% sand were best ratio. The variation of degree of saturation in this clayey
sands mixture affects cohesion and internal friction angle. This study shows the effect of
degree of saturation on cohesion and internal friction angle.
A comprehensive research methodology was developed to determine shear strength
properties of c- soil in controlled degree of saturation with in fixed void ratio and sand
clay mixture. Hence, a relationship was developed between shear strength parameters (c,
υ) of disturbed compacted clayey sands and degree of saturation (s).
5.2 Conclusion
From the results and discussions presented earlier, following conclusions are drawn:
The investigated clay-sand mixture soil, its specific gravity was found to be 2.69 and a
materials finer than 0.075 mm and finer than 0.002 mm were respectively found to be 7%
and 23%, thereby, which is more dominate by sandy soil which was 70% confirming the
coarse grained nature of the soil. This clayey sands mixture had 26% liquid limit and with
small plasticity even if the category lays in none plastic. The maximum dry density was
found to be 1.942 g/cm3 at the optimum water content of 10.4%.
The cohesion (c) followed the compaction curve with a maximum value of 45.48 kPa at the
optimum water content. The cohesion increase with increase of degree of saturation, after
the point where maximum dry density attained with optimum moisture content, even
though the degree of saturation increase but the value of cohesion decrease.
Likewise, the friction angle increase with increase of degree of saturation until 73.64% of
degree of saturation and decrease as degree of saturation continue its increase.
Even though the soil mixture would exhibit properties as discussed in discussion part, but
the shear strength test shows the compacted mixed clayey sands soil (like, Adobe brick)
wouldnt much affected due to the saturation. This is why the structures which were
constructed in ancient time still exist.
During the degree of saturation variation, the cohesive values much more vary than the
values of friction angle. This shows the clay with optimum water content create more
strong bonding that increase the cohesion force between the particles. Hence, this made the
material and the structure stable and strong.
5.3 Recommendation
Suction controlled direct shear test is highly recommended for accurate measurement of
friction angle due to suction.
Large-scale direct shear test should be performed on the disturbed sample at various
gradations to understand the actual situation.
Other shear strength properties test such as unconfined compression and triaxial test should
be conducted in addition to direct shear test, for more satisfactory and acceptable result and
comparison.
As the shear strength parameters are the key parameter required in the design of
foundations, dams, retaining walls, bridge abutments, temporary support and earth
structures, but degree of saturation is not the only effects of shear strength. Therefore, the
study should be broad to include the effects of the species of soil structure, void ratio and
the amount of sand proportion.
References
ASTM D1452-09 (2009) Standard Practice for Soil Exploration and Sampling by Auger
Borings. Annual Book of ASTM Standards, West Conshohocken, PA.
ASTM D2216-10 (2010) Standard Test Methods for Laboratory Determination of Water
(Moisture) Content of Soil and Rock by Mass. Annual Book of ASTM Standards,
West Conshohocken, PA.
ASTM D2487-11 (2011) Standard Practice for Classification of Soils for Engineering
Purposes (Unified Soil Classification System. Annual Book of ASTM
Standards, West Conshohocken, PA.
ASTM D422-63 (2007) Standard Test Method for Particle-Size Analysis of Soils. Annual
Book of ASTM Standards, West Conshohocken, PA.
ASTM D4318-10 (2010) Standard Test Methods for Liquid Limit, Plastic Limit, and
Plasticity Index of Soils. Annual Book of ASTM Standards, West Conshohocken,
PA.
ASTM D698-12 (2012) Standard Test Methods for Laboratory Compaction
Characteristics of Soil Using Standard Effort. Annual Book of ASTM Standards,
West Conshohocken, PA.
ASTM D854-10 (2010) Standard Test Methods for Specific Gravity of Soil Solids by
Water Pycnometer. Annual Book of ASTM Standards, West Conshohocken, PA.
ASTM D3080-04. Standard Test Method for Direct Shear Test of Soil Under Consolidated
Drained Condition. In Annual Book Of ASTM Standards (Vol. 04.08) Philadelphia:
USA.
Bareither, C. A., and Benson, C. H., and Edil, T. B. (2008). Comparison of shear strength of
sand backfills measured in small-scale and large-scale direct shear tests. Canadian
Geotechnical Journal 45: 1224-1236.
Cerato, A. B., and Lutenegger, A. J. (2006). Specimen Size and Scale Effects of Direct Shear
Box Tests of Sands. Geotechnical testing Jounal. Vol. 29, No.6.
Chinkulkijniwat, A., and Man-Koksung, and E., Uchaipichat, A., and Horpibulsuk, S. (2010).
Compaction Characteristics of N

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