CHAP - 1 SOIL INVESTIGATION.ppt civil engineering

DEBRE MARKOS UNIVERSITY
DEBRE MARKOS INSTITUTE OF TECHNOLOGY
(DMiT)
SCHOOL OF CIVIL ENGINEERING
FOUNDATION ENGINEERING
(CEng-3141)
By: Yoseph M.
1

CHAPTER 1
SOIL INVESTIGATION
2

1.1 INTRODUCTION
 Site Investigation is the first step in solving most geotechnical engineering
problems.
 Adequate knowledge of ground conditions is very important for the analysis,
design and construction of geosystems.
 Inappropriate or poor site investigation leads to:-
 Project delays
 Generate great expenses
 Soil failures
 Leads to litigation
3

• The field and laboratory investigations required to obtain the essential information on the
subsoil is called Soil Exploration or Soil Investigation.
• The stability of the foundation of a building, a bridge, an embankment or any other
structure built on soil depends on the strength and compressibility characteristics of the
subsoil.
• Soil exploration happens to be one of the most important parts of Foundation Engineering
and at the same time the most neglected part of it.
• Terzaghi in 1951 (Bjerrum,etal.,1960) had rightly remarked, that "Building foundations
have always been treated as step children". His remarks are relevant even to day.
4

• Site investigations or subsurface explorations are done for obtaining the information
about sub-surface at the site of proposed construction.
• Soil exploration consist of determining the profile of the natural soil deposits at the
site, taking the soil samples and determining the engineering properties of the
soils. It also includes in-situ testing of the soils.
• “The process of exploring to characterize or define small scale properties of substrata
at construction sites is unique to geotechnical engineering. In other engineering
disciplines, material properties are specified during design, or before construction or
manufacture, and then controlled to meet the specification. Unfortunately, subsurface
properties cannot be specified; they must be deduced through exploration.”
Charles H. Dowding (1979).
5

1.2 PURPOSE OF EXPLORATION
• The purpose of soil exploration is to find out strength characteristics of the
sub-soil over which the structure has to be built.
• Soil characteristics vary both with respect to depth from the ground surface
and stretch in the horizontal direction.
• It is, therefore, the prime objective of soil exploration for a building, bridge
or other civil Engineering works, to analyze the nature of soil in all respects.
6

The main Purpose of Soil exploration are:
To evaluate the general suitability of the site for the proposed project.
To enable adequate and economical design.
To obtain properties of soils.
To obtain ground water condition.
To select suitable materials for the construction.
To choose alternative types or depth of foundation,
To select alternative methods of construction,
To evaluate the safety of existing structure,
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 Soil parameters and properties of different layers (e.g. for classification,
bearing capacity or settlement calculation),
 Thickness of soil layers and depth to bedrock (stratification of soil),
 Location of ground water level and important groundwater related issues.
The soil exploration should provide the following data:
(Outcomes)
8

9
Commercial Bank of Ethiopia, Head Quarter Building
Commercial Bank of Ethiopia, Head Quarter
Building Foundation soil log

1.3 PLANNING AN EXPLORATION PROGRAM
A timely and planned site exploration should be considered before design
and construction.
A proper planning program of soil investigation for a given project depends on:
• The type of project;
• The importance of the project
• The nature of the subsoil involved.
The site investigation program should be so planned that maximum amount of
information is obtained at minimum cost.
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1.4 PHASES OF SITE INVESTIGATION
1. Collection of available information about the site (maps and previous records)
2. Reconnaissance of the site (site visit, take photos, etc.)
3. A preliminary site investigation(a few borings done to explore)
4. Detailed site investigation (soil sampling lab tests and in situ test.)
5. Write report
[Please read the details from Chapter 3 of the Ethiopia Roads Authority – ERA,
Site Investigation Manual - 2013]
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i. Collection of Preliminary Information:
• Assembly of all available information on type and use of the structure, and also of the
general topographic and geological character of the site.
For example:
Building:
• Appropriate column loads.
• Spacing of columns.
• Code requirements.
Bridge:
• Span length.
• Loading on piers and abutments.
12

ii. Reconnaissance of the Area:
 consists of site visit and visually assessing the local condition, inspection of behavior of
adjacent structures, rock projections, cuts, etc.
 Reconnaissance includes the assessment of;
Any previous development on site,
Any previous grading on site,
Any potential landslide or other stability problems,
Condition of nearby structure
13

iii. A preliminary Site Investigation:
 This is usually in the form of a few borings or a test pit to establish the types of materials,
Stratification (vertical profile) of the soil, and possibly the location of the ground water
level.
For small projects this step may be sufficient to establish foundation criteria, in which case
the exploration program is finished.
iv. A detailed Site Investigation:
 For complex projects or where the soil is of poor quality and/or erratic, a more detailed
investigation may be undertaken. This may involve sinking several boreholes, taking soil
samples for laboratory investigations, conducting sounding and other field tests.
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2.4.1 Number, Location and Depth of borings and in situ tests
Depends on factors, including the type of geotechnical project, the
arrangement of the site, the soil type, and the water table conditions.
15

1.5 METHODS OF EXPLORATION
Methods of determining the stratification and engineering characteristics of
sub-surface are:
1. Test pits
2. Boring and sampling
3. Field tests/Insitu tests/
4. Geophysical methods
5. Laboratory tests
17

1.5.1. Test Pits
• The simplest and cheapest method of shallow soil exploration is to sink test
pit to depths of 3 to 4 m.
• The use of Test pits enables the in-situ soil conditions to be examined
visually.
• It is relatively easy to obtain disturbed or undisturbed soil samples.
• In cohesive soils block samples can be cut by hand from the bottom of the
pit and tube samples can be obtained from the sides of the pit.
19

1.5.2 Boring and Sampling
• This is the most widely used method.
• It provides samples from shallow to deeper depths for visual inspection
as well as laboratory tests.
• The most commonly used methods of boring are:
A. Auger boring
B. Wash boring
C. Percussion drilling
D. Rotary drilling
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A. Auger boring
• An auger boring is a boring tool similar to one used by a carpenter for boring
holes in wood. The augers can be operated manually or mechanically. The hand
augers are suitable for advancing holes up to a depth of 3 to 6m in soft soils.
• Mechanical augers (helical) are driven by power. These can be used for making
holes in hard stratum to a great depth, even to 30m.
• Auger borings are particularly useful for subsurface investigations of highways,
railways and air fields, where the depth of exploration is small.
22

23
Hand Augers (a) Helical (b) Post hole

solid-stem (left) and hollow-stem (right) auger flights.
24

B. Wash boring
Power operated
• This method best suits in sandy and clayey soils and not in very hard soil strata
(i.e. boulders) and rocks. Depth of boring could be up to 60m or more. Changes in
soil strata are indicated by changes in the rate of progress of boring, examination
of out coming slurry and cutting in the slurry. Undisturbed samples whenever
needed can be obtained by use of proper samplers.
25

C. Percussion Drilling
Power operated.
• Because of the deep disturbance of the soil this method of boring is not favored. Casing is
generally required.
Advantage:
• It can be used for all types of materials.
Disadvantages:
• The material at the bottom of the hole is disturbed by heavy blows of the chisel. It is not
possible to get good quality undisturbed samples. Furthermore, it becomes difficult to detect
minor changes in the properties of the strata penetrated.
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Percussion Drilling at Site
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D. Rotary Drilling
• Rotary drilling is the most rapid method of advancing holes
in rock unless it is badly fissured; however, it can also be used
for any type of soil.
• In this method undisturbed samples can be obtained at
desired depths by using suitable samplers.
30

Soil Sampling
• There are two main types of soil samples which can be recovered from bore holes or
trial pits. These are: - Disturbed and Undisturbed samples.
• Laboratory test results are mainly dependent on the quality of soil samples.
• The objective in sampling a soil is to obtain samples that have least disturbance.
• The sample disturbances comes from change in:-
 stress,
 water content and porosity,
 mechanical disturbances etc.
32

Disturbed Samples
• These are samples where the structure of the natural soil has been disturbed to a
considerable degree by the action of the boring tools or excavation equipment.
• Disturbed samples are satisfactory for performing classification tests such as, sieve
analysis, Specific gravity of soil solids, determination of organic content, mineral
content test, Atterberg limits etc.
• But disturbed soil samples cannot be used for consolidation, hydraulic conductivity,
or shear tests, because these tests must be performed on the same soil of the field
without any disturbance (to be representative).
33

Undisturbed Samples
• These are samples, which represent as closely as is practicable, the true in-situ structure
and water content of the soil.
• Undisturbed samples are required for determining reliable information on the shearing
resistance and stress-deformation characteristics of a deposit.
• It is practically impossible to obtain totally undisturbed samples.
• Samples should be taken only from a newly-drilled or newly extended hole, with care
being taken to avoid contact with water.
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• These types of samples are used for the following types of laboratory soil tests:
Consolidation test, Hydraulic Conductivity test, Shear Strength tests.
• These samples are more complex and expensive, and it’s suitable for clay, however in
sand, it is very difficult to obtain undisturbed samples. The major equipment used to obtain
undisturbed sample is Thin-Walled Tube.
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• As soon as they are brought to the surface:
Core tube ends should be sealed with wax and
capped to preserve the loss of moisture content.
Core tubes should properly be labeled to
indicate the number of bore holes and the depth
at which they are taken and then stored away
from extremes of heat or cold and vibration.
36

The conditions that contribute for the disturbance of samples and unreliable test
results are:
1. Distortion of samples during pushing/driving of sampling tubes into the natural strata.
2. Relief of in-situ pressure leading to surface cracks when samples are extracted from
sampling tubes for laboratory tests. This is particularly applicable to over consolidated
clay soils.
3. Disturbance caused to the samples during extraction from sampling tubes.
4. Disturbance to the samples during handling and transporting from the site to the
laboratory.
5. Evaporation of moisture from the sample due to improper sealing.
6. Carelessness during sampling and testing.
37

Types of tube samplers
a. Split Spoon Sampler
b. Thin-Walled Tube Sampler
c. Piston Samplers
38

40
• A standard split spoon sampler has a 2 inch (50.8mm) outside diameter, 1⅜ inch
(35mm) inside diameter and a length of 24 inch (600m).
• The tube is split longitudinally in the middle.
• While the sample is being taken, the two halves of the spoon are held together at the
ends by short pieces of threaded pipe, one of which couples, into the drill rod and the
other serves as the cutting edge.
• The sampler is forced or driven into the soil to obtain a sample and is then removed
from the hole.
• With this sampler, disturbed samples of soft rock, cohesive and cohesionless soils are
obtained.
• This sampler is used for making standard penetration test.

b. Thin-Walled Tube Sampler:
• It is a thin walled continuous brass or steel tubing, with common outside diameter of 2
to 3 inch and length of 30 to 3 6 inch.
• The lower end is chamfered to form a cutting edge and it can be slightly tapered to
reduce the wall friction and the upper end fitted for attachment to the drill rod.
• In order to take a sample, the sampler is pushed downward into the soil by static force
instead of being driven by a hammer.
• This sampler is used to take undisturbed samples from cohesive soils.
42

c. Piston Samplers:
• They are very thin tube samplers with pistons fitted at their cutting ends.
• While taking sample, the piston is held in positions and the tube pushed
down.
• The piston aids the retention of the soil in the tube during withdrawal.
• Piston samples provide best undisturbed samples of cohesive soils.
44

1.5.3. FIELD TESTS /IN-SITU TESTS
The main problems associated with the laboratory testing of soils are disturbance
during sampling and the difficulty of testing samples large enough to be representative
of the soil in the field, where the effects of structure and fabric can be highly significant.
In addition to avoiding sample disturbance problems, in-situ tests have the following
advantages:
They are usually less expensive, so greater number of tests can be performed,
 Characterizing the soil in more detail.
The test results are available immediately.
46

However, they also have disadvantages, including:
 Often no sample is obtained, thus making soil classification more difficult.
 The engineer has less control over confining stresses and drainage.
• These tests are valuable means of determining the relative densities; shear strengths
and bearing capacities of soils directly without disturbing effects of boring and
sampling.
• The most commonly used field tests are:
 Penetration or sounding tests
 Vane shear test
 Plate loading test
 Pile loading test
47

General Considerations in Tests :
1) Objective of the Test
2) Main Theories and Principles Considered in the Test
3) References of the Equipment
4) Required Equipments
5) Test Procedure
6) Calculation
7) Conclusion
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Standard Penetration Test(SPT)
Objective of the Test:- to obtain an approximate measure of the soil resistance to dynamic penetration and a
disturbed sample of the soil.
Basic Theories:- This test is developed around 1927. Its the most popular and economical test. It’s estimated
that (85-90)% of conventional foundation design in North and South America is made using SPT.
 The method has been standardized as ASTM D 1586 since 1958 and AASHTO T-206.
The test consists of the following:-
 Driving the standard split-barrel sampler of standard dimensions a distance of 460 mm into the soil at the
bottom of the boring.
 Counting the number of blows to drive the sampler the last two 150 mm distances (total = 300 mm) to obtain
the N number.
 Using a 63.5-kg driving mass (or hammer) falling "free" from a height of 760 mm.
49

The boring log shows refusal and the test is stopped if:-
1. 50 blows are required for any 150-mm increment.
2. 100 blows are obtained (to drive the required 300 mm).
3. 10 successive blows produce no advance.
Factors Contributing for SPT result discrepancy:-
1) Equipment from different manufacturers
2) Drive hammer configurations
3) Whether or not a liner is used inside the split-barrel sampler
4) Overburden pressure
5) Length of drill rod
52

SPT CORRELATIONS
• The SPT has been used in correlations for unit weight , relative
density Dr, angle of internal friction ɸ, and undrained compressive
strength qu. Examples:-
NB:- This test method is best suitable for sandy soils.
(Meyerhof, 1959)
53

Correlation between Number of blows (N), Angle of Internal Friction
and Relative Density of Frictional Soils(Terzaghi and Peck).
Correlation between Number of blows (N), Unconfined Compressive
Strength and Consistency of Cohesive Soils. (Terzaghi and Peck).
54

Cone Penetration Test (CPT)
 This test method is used in most of Scandinavia, some countries in eastern
Europe, Japan and China.
 CPT soundings are typically performed in conjunction with borings and
sampling, and often represent an economical means of reducing the number of
borings.
 The test is standardized as
• ASTM D3441(Mechanical CPT) and
• ASTM D5778 (Electronic friction CPT)
 The test seems suited only for very soft silt or soft clay deposits (Bowles)
 Basically, the test consists in pushing the device to some depth and then adding
sufficient mass until the screw tip begins to self-turn and move downward.
55

There are at least five configurations of CPT equipment currently being used (Bowles, 1988):
1. Mechanical: This is the earliest type and is often referred to as the "Dutch Cone" since
it originated in the Netherlands.
2. Electric friction: This represents one of the first modifications using strain gauges to
measure qc and qs'
3. Electric piezocone: In this modification of the electric friction cone, the pore water pressure
at the cone tip can be measured.
4. Electric piezo/friction: This reflects a further modification to measure point resistance,
sleeve friction and pore pressure.
5. Seismic cone: This is a recent modification to include a vibration pickup to obtain data to
compute shear wave velocity from a surface shock so that the dynamic shear modulus can be
computed.
57

Field Vane Shear Test
►The field vane shear test VST is a substantially used method to estimate the in situ
undrained shear strength of very soft, sensitive, fine-grained soil deposits
(cohesive soils).
►The Vane Shear Test standardized as AASHTO T 223, ASTM 2573.
►It also has considerable application in offshore soil exploration, particularly when
used with sample recovery equipment.
►The test is performed by inserting the vane into the soil and applying a torque
after a short time lapse, on the order of 5 to 10 minutes.
60

62
• The test procedure requires pushing a four-bladed
vane into undisturbed soil layers and rotating it until
a cylindrical volume of the soil, theoretically, having
height and diameter dimensions the same as the vane
fails in shear.
• The torque required to cause the failure is read
and translated into force applied to the total
surface area of the failed cylinder. This value is
used as the strength of the soil.
• Correlations with unconfined compressive or triaxial
strength of soils exist for areas where this test method
has been used extensively.

• In most cases a hole is drilled to the desired depth, where the vane shear
test is planned to be performed and the vane is carefully pushed into the
soil.
• A torque necessary to shear the cylinder of soil defined by the blades of
the vane is applied by rotating the arm of the apparatus with a constant
speed of 0.5 degree/sec.
• The maximum torque is then measured from which the shearing strength
is determined.
• From the measured maximum torque one may estimate the shearing
resistance of the tested clay from the following formula.
63

Cu
D
H
D
T








12
2
3
2

 =
where : T = Torque
D = Diameter of Vane
H = Height
Since for quick condition τ = Cu, one ultimately arrived the in-situ
value of cohesion
65

Pressure Meter Test(PMT)
 Pressuremeters are used to measure the in situ deformation
(compressibility) and strength properties of a wide variety of soil
types, weathered rock and low to moderate strength intact rock.
 The borehole pressuremeter test, which was developed around 1956
(Menard, 1956), is conducted in a carefully prepared borehole which is
about 10 percent oversized.
66

Dilatometer Test(DMT)
The flat dilatometer was developed by Marchetti in the later 1970's in Italy (Riaund
and Miran, 1992).
►The dilatometer consists of a stainless steel probe. It has a sharp cutting edge. A 60-mm
diameter stainless steel membrane is centered on and flush with one side of the blade.
►The dilatometer is attached to a string of drill rods and pressed, or at times driven,
into the ground.
► A single, combination gas and electrical line extends through the drill rods and
down to the blade from a surface control and pressure readout box.
►The dilatometer is pressed into the ground in 15 to 30 cm increments. The force
or blows required to cause penetration provide information similar to that of CPT and
SPT.
68

Plate Loading Test
 The plate-load test has been a traditional in-situ method for
estimating the bearing capacity of foundations on soil.
 Plate-load tests involve measuring the applied load and penetration of
a plate being pushed into a soil or rock mass.
 The test is most commonly carried out in shallow pits or trenches
but is also undertaken at depth in the bottom of a borehole or pit.
 In soils, the test is carried out to determine the shear strength and
deformation characteristics of the material beneath the loaded plate.
 The test is standardized as ASTM D1194.
70

Plate Loading Test Procedure:
• In this test a gradually increasing static load is applied to the soil through a steel plate, and
readings of the settlement and applied load are recorded, from which a relationship between
bearing pressure and settlement for the soil can be obtained.
1. Pit for the test must be at least 5 times the size of the plate.
2. The plate should be properly placed in the soil. In the case of cohessionless soil (to prevent
early displacement of soil under the edges of the plate), the plate must be positioned in cast
in-situ concrete.
3. Loading platform should be properly erected.
4. Loading of the soil is conducted in steps (loading increment is kept constant).
5. Once completion of the test, the plate is unloaded in the same incremental steps (to draw the
expansion curve).
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• Bearing capacity of non-cohesive soil is determined from settlement consideration.
• If the maximum permissible settlement, S, of a footing of width Bf is given, the
settlement, Sp, of a plate of width, Bp under the same intensity of loading is given
by.
2
)
(
2
)
2
(
p
B
f
B
Bf
Sp
S


• Using the value Sp, computed from the above equation, the loading intensity
under the footing could be read from the load settlement curve.
• The settlement of footing in clay is normally determined from principles of
consolidation. However, from plate load test, the approximate settlement of
footing of width B can be determined using the following expression.
p
f
p
B
B
S
S 
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Limitation of Plate Loading Test:
• Plate loading test is of short duration. Hence consolidation settlement
does not fully occur during the test.
• For settlement consideration, its use is restricted to sandy soils, and to
partially saturated or rather unsaturated clayey soils.
• Plate loading test can give very misleading information If the soil is not
homogeneous within the effective depth (depth of stress influence) of the
prototype foundation.
• Plate loading test should not be recommended in soils which are not
homogeneous at least to depth of 1½ to 2 times the width of the prototype
foundation.
76

Borehole Shear Test(BST)
This test consists in carefully drilling a 76-mm diameter hole (usually vertical but may be
inclined or horizontal) to a depth somewhat greater than the location of interest.
Next the shear head is carefully inserted into the hole to the point where the shear strength
is to be measured.
The test proceeds by expanding the serrated cylinder halves into the soil by applying
pressure from the surface through a piping system. Next the cylinder is pulled with the
pulling load and displacements recorded. The expansion pressure is σn and the pulling load
can be converted to the shear strength s to make a plot to obtain the in situ strength
parameters ɸ and c.
77

The BST is applicable for
all fine-grained soils and
may be done even where a
trace of gravel is present.
It has particular appeal if a
good-quality borehole can
be produced and for modest
depths in lieu of
"undisturbed" sample
recovery and laboratory
testing.
78

1.5.4. GEOPHYSICAL METHODS
GENERAL:-
Geophysical methods of exploration provide a rapid and economical means of
supplementing information obtained by direct exploratory methods such as
borings, test pits and test trenches from which samples of soil and rock are
retrieved for visual classification and laboratory testing and direct
measurements of the depth to groundwater can be made.
The geophysical methods are basically indirect methods of exploration in which
changes in certain physical characteristics such as magnetism, density,
electrical resistivity, elasticity or a combination of these are used as an aid in
developing subsurface information.
79

Geophysical methods are particularly useful in subsurface exploration for the
design of projects, such as highways, that have large longitudinal extent
compared to lateral extent and in explorations for the rehabilitation of existing
highway structures that have been subject to general deterioration or scour, for
which foundation information is unavailable, or whose foundations are not
accessible for inspection.
It should be noted that data from geophysical exploration must always be
correlated with the information from direct methods of exploration which
permit visual classification of materials, direct measurement of the depth to
groundwater, and laboratory testing to obtain the geotechnical parameters
necessary for design.
80

• This method can be used for the location of different strata (soil stratification) and for a
rapid evaluation of the sub-soil characteristics. However, these methods are very
approximate.
• The utility of these methods in the field of foundation engineering is very limited since
the methods do not quantify the characteristics of the various substrata.
• Geophysical methods at best provide some missing information between widely spaced
boreholes but they cannot replace boreholes.
• The method can be broadly divided into two categories;
1. Seismic Refraction method and
2. Electrical Resistivity method
81

1. Seismic Refraction Method:
• These methods are based on the principle that the elastic shock waves have
different velocities in different materials. i.e. sound (shock) waves travel faster
through rocks than through soils.
• The shock wave is created by a hammer blow or by a small explosive charge as
shown in the figure below.
82

Wave velocity through different materials:
where, v = velocity of the shock wave,
E = modulus of elasticity of the soil,
g = acceleration due to gravity,
γ = density of the soil, and
C = a dimensionless constant involving Poisson’s ratio.
(IS: 1892-1979 Appendix B)
83

• The geophones convert the ground vibration into electrical impulses and transmit them to a
recording apparatus.
• The basic equations of the refraction survey are derived based on the assumption that the
velocity of the shock wave increases as the depth increases. i.e. V3 > V2 > V1
• At geophones located close to the point of impact, such as point A, the direct waves with
velocity V1 reach first.
• At point B, the refracted waves reach earlier than the direct waves.
• At points further away from the point of impact, such as point C, the waves which are
refracted twice, once at the interface of the layers Ι and ΙΙ, and once at the interface of the
layers ΙΙ and ΙΙΙ, reach earlier.
• For the determination of the thickness of different layers, a distance-time graph is plotted.
85

Up to certain distance X1, the direct waves in the layer Ι reach first. At this
point, the first two lines in above figure intersect, which indicates that the direct
wave travelling a distance X1 with a velocity V1 and the refracted wave
travelling with a velocity V1 in distance 2H1 and with a velocity V2 in distance
X1 reach simultaneously, where H1 is the thickness of the layer I.
86

3. If the area contains some underground features, such as
buried conduits, irregularly dipping strata and irregular water
table, the interpretation of the results becomes very difficult.
4. If the surface is layer is frozen, the method cannot be
successfully used, as it corresponds to a case of harder
overlying a softer layer.
5. The methods require sophisticated and costly equipment.
6. For proper interpretations of the seismic survey results, the
services of an expert are required.
89

2. Electrical Resistivity:
• In this method four metallic spikes to serve as electrodes are driven in to the
ground at equal intervals along a line. A known potential is then applied
between the outermost electrodes and potential drop is measured between
the innermost electrodes.
• The resistivity method makes use of the fact some soils (e.g. soft clays)
have low electrical resistivity than others (e.g. sand or gravel).
• Seismic refraction and electrical resistivity methods are normally employed
as preliminary or supplementary to other methods of exploration.
90

The common laboratory tests that concern the foundation
engineers are:
Grain size analysis
Atterberg limits
Natural moisture content
Unit weight
Unconfined compression test
Direct shear test
Triaxial compression test
Consolidation test
Compaction test
Chemical analysis
1.5.5. Laboratory Tests
92

They are typically classified in the following main categories:
1. Tests for index properties (e.g., water content, unit weight, particle size, Atterberg
limits)
2. Tests for strength properties (e.g., direct shear, unconfined compression, triaxial, lab
vane)
3. Tests for deformation properties (e.g., consolidation, triaxial, simple shear, resonant
column)
4. Tests for flow properties (e.g., constant head permeability, falling head permeameability,
erosion tests)
93

Laboratory Index Tests for Soils
• Information obtained from index tests is used to select samples for engineering property
testing as well as to provide an indicator of general engineering behavior.
• Common index tests are moisture content, unit weight (wet density), Atterberg limits,
particle size distribution, visual classification, specific gravity and organic content.
• Information from index tests should be assessed prior to a final decision regarding the
specimens selected for subsequent performance testing.
94

1. Moisture Content
• Purpose:- To determine the amount of water present in a quantity of soil in terms of its
dry weight and to provide general correlations with strength, settlement, workability and
other properties.
• Moisture contents of soils as determined from in-situ moisture content tests may be
altered during sampling, sample handling, and sample storage. Because the top end of
the sample tube may contain water or collapse material from the borehole, moisture
content tests should not be performed on material near the top of the tube.
95

2. Particle Size Distribution
97
• Particle size distributions by mechanical sieve and hydrometer analyses are useful for soil classification
purposes.
a. Sieve Analysis
• Purpose: To determine the percentage of various grain sizes. The grain size distribution is used to
determine the textural classification of soils (i.e., sand, silty clay, etc.) which in turn is useful in
evaluating the engineering characteristics of soils such as permeability and susceptibility to frost action.
b. Hydrometer Analysis
• Purpose: To determine distribution (percentage) of particle sizes smaller than No. 200 sieve and identify
the silt, clay and colloids percentages in the soil.

3. Atterberg Limits
• Purpose: To describe the consistency and plasticity of fine-grained soils with varying degrees of moisture.
• The Atterberg limits generally refer to the liquid limit (LL), plastic limit (PL), and shrinkage limit (SL).
• Liquid limit:- minimum water content at which the soil will start to flow under the application of a
standard shearing force (dynamic loading).
• Plastic limit:-the moisture content at which the soil begins to fracture when rolled into a 3mm diameter
thread.
• Shrinkage limit:- is the maximum moisture content after which further reduction in water content does
not cause reduction in volume.
100

Plasticity index (PI=LL-PL) is the numerical difference between the liquid and plastic
limits.
102

4. Specific Gravity
• The specific gravity of solids (Gs) is a measure of solid particle density and is
referenced to an equivalent volume of water.
• Purpose: To determine the specific gravity of the soil grains.
• Specific gravity of solids is defined as Gs = (Ms/Vs)/ ρd .
• This can be rewritten as Gs = ρs / ρd.
• The typical values of specific gravity of most soils lie within the narrow range of Gs =
2.7 ± 0.1.
103

5. Compaction Test
• Purpose: To determine the maximum dry density attainable for a given soil and the
(optimum) moisture content corresponding to this density.
• In the construction of highway embankments, earth dams, structure foundations and
many other facilities, loose soils must be compacted to increase their densities.
• Compaction increases the strength characteristics of soils, there by increasing
the bearing capacity of foundations constructed over them. Compaction also
decreases the amount of undesirable settlement of structures and increases the
stability of slopes and embankments.
105

6. Unit Weight
• In the laboratory, soil unit weight and mass density are easily measured on tube
(undisturbed) samples of natural soils.
• The moist unit weight is γt = Wt/Vt,
• Similarly, the dry unit weight is defined as γd = Ws/Vt .
• The relationship between total and dry unit weight is given by
107

7. Organic Content
• Laboratory test should be used to evaluate the percentage of organic material in a specimen
where the presence of organic material is suspected based on field information or from previous
experience at a site.
• The test involves weighing and heating a previously dried sample to a temperature of 824°F
(440°C) and holding this temperature until no further change in weight occurs.
• Soils with relatively high organic contents have the ability to retain water. Water retention may
result in higher moisture content, higher primary and secondary compressibility, and potentially
higher corrosion potential.
109

Strength Tests
• Design of shallow or deep foundations, stability analysis of earthen structures and cuts
and fills require a thorough understanding of the strength properties of soils.
• The selection of soil strength information needed and types of tests to be performed
vary depending on the type of construction, the foundation design, the intensity, type
and duration of loads to be imposed, and soil types existing at the site.
• Both undisturbed and remolded or compacted samples are used for strength tests.
• Commonly used laboratory tests are unconfined compression, triaxial, direct shear,
and CBR tests. Resonant column tests are used where dynamic or earthquake loads
are expected.
111

1. Unconfined Compressive Strength of Soils
• Purpose:-To determine the undrained-cohesive strength (cu) of cohesive soils.
• The determination of unconfined compressive strength of undisturbed, remolded or
compacted soils is limited to cohesive or naturally or artificially cemented soils. Since
the angle of internal friction, is inherently zero, application of this test to non-
cohesive soils may result in underestimation of the shear strength.
112

2. Direct Shear
• Purpose:-To determine the shear strength of soils along a defined planar surface.
• The direct shear test is particularly applicable to those foundation design problems
where it is necessary to determine the angle of friction between the soil and the
material of which the foundation is constructed, e.g., the friction between the base of a
concrete footing and underneath soil. In such cases, the lower box is filled with soil
and the upper box contains the foundation material.
114

4. Triaxial Strength
• Purpose:-To determine strength characteristics of soils including detailed
information on the effects of confinement and lateral stresses, pore pressure,
drainage and consolidation. Triaxial tests may also be used to determine the angle
of internal friction of mixed soils (i.e. sandy clay) or to compensate for the
effects of various intrusions and fissures and Young's Modulus and damping
coefficient of soils for foundations subjected to dynamic loads.
• In general, there are five types of triaxial tests: Undrained unconsolidated (UU),
Consolidated undrained (CU), Consolidated drained (CD), Consolidated
undrained with pore pressure measurement (CU) and Cyclic loading. 116

5. Vane Shear Test
• Purpose:-To determine the undrained shear strength of saturated clays.
• The test assumes that the stresses applied are limited to the cylindrical surface
represented by the diameter and the height of the vane. This is hardly the case in reality.
• Depending on the cohesion and stiffness, the soils in an area radiating outward from the
surface of the idealized cylindrical zone are also disturbed by the shearing action of the
vane. A portion of the torque therefore is used to mobilize this zone.
• The laboratory vane shear test should not be used as strength test. It is an index test
and it will only provide relative values and not true strength.
117

6. California Bearing Ratio (CBR)
• Purpose:-To determine the bearing capacity of a soil under controlled moisture and
density conditions.
• CBR is a practical bearing capacity test. Various well-established, tried and proven
pavement design methods are based on the CBR value derived from field or
laboratory tests. The test results are used for highway, airport, parking lot and
other pavement designs using empirical local or agency-specific methods.
119

Deformation Tests
1. Consolidation
• Purpose:- For the determination of stress-strain-time properties of saturated soils under sustained loads.
• When saturated soil masses are subjected to incremental loads, they undergo various degrees of dimensional
change.
• Depending on the permeability and the availability of drainage layer(s) in contact with the soil, the liquids in the
voids begin draining and continue to do so until the excess hydrostatic pressure is dissipated. As the hydrostatic
pressure decreases, a proportional amount of the incremental load is transferred to the solid portion of the soil.
When the excess hydrostatic pressure reaches zero, all of the new load is carried by the soil's solids.
• This gradual transfer of stresses causes the soil particles to reorient themselves. Soils thus go through plastic
deformation called primary consolidation.
120

2. Simple shear test
• A disk of soil is placed in a flexible membrane with a porous stone on the top and on
the bottom of the disk. A vertical load is applied to the top of the sample and maintained
constant during the test. This vertical load creates a total normal stress σ. Then the soil
is sheared by holding one of the two platens and pushing the other one horizontally.
• The major difference between the direct shear test and the simple shear test is that in the
direct shear test, the shearing takes place along a predetermined thin band of soil near
the middle of the sample. In the simple shear test, the shearing takes places over the
entire height of the sample. Therefore, the shearing strain γ can be measured in the
simple shear test as:
122

3. Resonant column
• Purpose:-To determine the shear modulus, shear damping and Young's moduli of
soils for cases where dynamic forces are involved.
• The test is time-consuming and requires special test and laboratory setup. Ambient
vibrations, if not known, substantially alter the test results. When performed by
experienced specialists, the test is reliable. Similar results can be obtained from a
series of field seismic tests.
124

Permeability Tests
1. Constant Head Permeability Test
• The constant head Permeability is used to obtain the coefficient of hydraulic
conductivity k of saturated coarse-grained soils.
• The soil sample is placed in a cylinder about 75mm in diameter and 150mm high,
with one filter stone at the top and another at the bottom.
• The top of the sample is connected by tubing to a container in which the water level is
kept constant through an overflow regulator. The bottom of the sample is connected to
another container in which the water level is also kept constant.
• The bottom container is kept lower than the top container and the flow Q (m3/s) out
of the bottom container is measured.
125

2. Falling head Permeability test for saturated soils
• The falling head permeability (FHP) is used to obtain the hydraulic conductivity k of
saturated fine-grained soils.
• The soil sample is placed in a cylinder about 75mm in diameter and 150mm high with
one filter stone at the top and another at the bottom. The top of the sample is connected
to a tube with a much smaller diameter (say, 10 mm) filled with water.
• Unlike the constant head Permeability, the water level in this tube goes down with
time. The bottom of the sample is connected to a container where the water level is kept
constant through an overflow. The measurements consist of recording the time on one
hand and the drop in height in the small tube on the other.
127

Chapter Questions
1. What are the advantages and disadvantages of cone penetration (CPT) test over standard
penetration test(SPT)?
2. One of the drawbacks of sampling for laboratory test is scale effect. What does it mean? How
do you do to avoid it?
3. Disturbed samples are not recommended for strength and consolidation tests. Why do you
think so?
4. What is the reason behind standardizing soil tests?
5. During standard penetration test, the first 15cm is usually ignored. Why?
6. What are the advantages and disadvantages of geophysical methods over Test pits and
borings? Discuss briefly.
7. The blow counts for a standard penetration test in a coarse-grained soil at every 0.15m is 11,
13, 19.
a. What is the N number?
b. Correct the N number for an energy ratio of 60%.
c. Make a preliminary description of the compactness of the soil
8. What are the common causes of foundation failures?
129

CHAP - 1 SOIL INVESTIGATION.ppt civil engineering

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CHAP - 1 SOIL INVESTIGATION.ppt civil engineering