SOIL EXPLORATION
AND
INTERPRETATION
Erica Elice S. Uy, Ph.D.
OUTLINE OF DISCUSSION
• Soil Exploration Techniques (1hr)
• Engineering Properties of Soil (Index, Shear strength and consolidation
Properties) (1.5hrs)
• Laboratory Tests
• Computation
• Interpretation of Results (1hr)
A soil investigation/exploration program is necessary to provide
information for design and construction, environmental assessment, and
project due diligence (due diligence is the process of evaluating a
prospective project to facilitate business decisions by the owner).
SOIL EXPLORATION
The purposes of subsoil exploration include the following:
1.
Determining the nature of soil at the site and its
stratification.
2.
Obtaining disturbed and undisturbed soil samples for
visual identification and appropriate laboratory tests.
3.
Determining the depth and nature of bedrock, if and when
encountered.
4.
Performing some in situ field tests, such as permeability
tests, vane shear tests, and standard penetration tests.
5.
Observing drainage conditions from and into the site.
6.
Assessing any special construction problems with respect to
the existing structure(s) nearby.
7.
Determining the position of the water table.
SOIL EXPLORATION
The purposes of subsoil exploration include the following:
1. Determining the nature of soil at the site and its
stratification.
Soil profile
SOIL EXPLORATION
The purposes of subsoil exploration include the following:
1.
Determining the nature of soil at the site and its
stratification.
2. Obtaining disturbed and undisturbed soil samples for
visual identification and appropriate laboratory tests.
SOIL EXPLORATION
The purposes of subsoil exploration include the following:
1.
Determining the nature of soil at the site and its
stratification.
2.
Obtaining disturbed and undisturbed soil samples for
visual identification and appropriate laboratory tests.
3. Determining the depth and nature of bedrock, if and
when encountered.
SOIL EXPLORATION
The purposes of subsoil exploration include the following:
1.
Determining the nature of soil at the site and its
stratification.
2.
Obtaining disturbed and undisturbed soil samples for
visual identification and appropriate laboratory tests.
3.
Determining the depth and nature of bedrock, if and when
encountered.
4.
Performing some in situ field tests, such as permeability
tests, vane shear tests, and standard penetration tests.
Standard Penetration Test (SPT)
SOIL EXPLORATION
The purposes of subsoil exploration include the following:
1.
Determining the nature of soil at the site and its
stratification.
2.
Obtaining disturbed and undisturbed soil samples for
visual identification and appropriate laboratory tests.
3.
Determining the depth and nature of bedrock, if and when
encountered.
4.
Performing some in situ field tests, such as permeability
tests, vane shear tests, and standard penetration tests.
5. Observing drainage conditions from and into the site.
SOIL EXPLORATION
The purposes of subsoil exploration include the following:
1.
Determining the nature of soil at the site and its
stratification.
2.
Obtaining disturbed and undisturbed soil samples for
visual identification and appropriate laboratory tests.
3.
Determining the depth and nature of bedrock, if and when
encountered.
4.
Performing some in situ field tests, such as permeability
tests, vane shear tests, and standard penetration tests.
5.
Observing drainage conditions from and into the site.
6. Assessing any special construction problems with
respect to the existing structure(s) nearby.
SOIL EXPLORATION
The purposes of subsoil exploration include the following:
1.
Determining the nature of soil at the site and its
stratification.
2.
Obtaining disturbed and undisturbed soil samples for
visual identification and appropriate laboratory tests.
3.
Determining the depth and nature of bedrock, if and when
encountered.
4.
Performing some in situ field tests, such as permeability
tests, vane shear tests, and standard penetration tests.
5.
Observing drainage conditions from and into the site.
6.
Assessing any special construction problems with respect to
the existing structure(s) nearby.
7. Determining the position of the water table.
SOIL EXPLORATION
A soil exploration program for a given structure can be divided broadly into four phases:
1. Compilation of the existing information regarding the structure
2. Collection of existing information for the subsoil condition
3. Reconnaissance of the proposed construction site
4. Detailed site investigation
1.
COMPILATION
OF
THE
EXISTING
INFORMATION REGARDING THE STRUCTURE
• This phase includes gathering information such as the type of structure to be
constructed and its future use, the requirements of local building codes, and the
column and load-bearing wall loads.
• If the exploration is for the construction of a bridge foundation, one must have an
idea of the length of the span and the anticipated loads to which the piers and
abutments will be subjected.
2. COLLECTION OF EXISTING INFORMATION
FOR THE SUBSOIL CONDITION
• Considerable savings in the exploration program sometimes can be realized if the
geotechnical engineer in charge of the project thoroughly reviews the existing
information regarding the subsoil conditions at the site under consideration.
• Useful information can be obtained from the following sources:
• Geologic survey maps
• Soil manuals published by the state transportation departments
• Existing soil exploration reports prepared for the construction of nearby
structures
3. RECONNAISSANCE OF THE PROPOSED
CONSTRUCTION SITE
• The engineer visually should inspect the site and the surrounding area.
• In many cases, the information gathered from such a trip is invaluable for future
planning.
• The type of vegetation at a site, in some instances, may indicate the type of subsoil
that will be encountered.
4. DETAILED SITE INVESTIGATION
• This phase consists of making several test borings at the site and collecting
disturbed and undisturbed soil samples from various depths for visual observation
and for laboratory tests.
• No hard-and-fast rule exists for determining the number of borings or the depth to
which the test borings are to be advanced.
• For most buildings, at least one boring at each corner and one at the center should
provide a start.
4. DETAILED SITE INVESTIGATION
• National Structural Code of
FOUNDATION INVESTIGATION
the
Philippines
(NSCP)
Requirements: SECTION
303
• Foundation investigation shall be conducted and a Professional Report shall be submitted at each
building site. For structures two storeys or higher, an exhaustive geotechnical shall be performed to
evaluate in-situ soil parameters for foundation design and analysis. The minimum required numbers
of boreholes per structure based footprint is summarized in Table 303-1.
FOOTPRINT AREA OF STRUCTURES (m2)
A = footprint area of the structure in sq. m.
MINIMUM REQUIRED NUMBER OF
BOREHOLES
A < 50
1
50 < A < 500
2
A > 500
2 + (A/1000)
Rounded up to the nearest integer
4. DETAILED SITE INVESTIGATION
• All of these boreholes should fall within the footprint of the structure and should generally be
uniformly distributed throughout the building footprint.
• Unless specified by the consulting Geotechnical Engineer, all boreholes should be drilled to a depth
of at least five meters into hard strata or until a suitable bearing layer is reached.
• For buildings with basements, the depth of boring should extend to twice the least dimension of the
structure’s footprint (2B) added to the depth of the basement.
4. DETAILED SITE INVESTIGATION
• The depths of boring for a building with a width of 30 m (100 ft) will be approximately the following,
according to Sowers (1970):
4. DETAILED SITE INVESTIGATION
4. DETAILED SITE INVESTIGATION
(from Budhu, 2007)
4. DETAILED SITE INVESTIGATION
• Spacing can be increased or decreased, depending on the condition of the subsoil. If various soil
strata are more or less uniform and predictable, fewer boreholes are needed than in
nonhomogeneous soil strata.
BORING METHODS
• Rotary drilling is a procedure by which rapidly rotating drilling bits
attached to the bottom of drilling rods cut and grind the soil and advance
the borehole down.
• Several types of drilling bits are available for such work.
three-cone drill bits
Diagram of rotary drilling system
BORING METHODS
• Rotary drilling can be used in sand, clay, and rock (unless badly fissured).
• Water or drilling mud is forced down the drilling rods to the bits, and the
return flow forces the cuttings to the surface.
• Drilling mud is a slurry prepared by mixing bentonite and water
(bentonite is a montmorillonite clay formed by the weathering of
volcanic ash).
• Boreholes with diameters ranging from 50 to 200 mm (2 to 8 in.) can be
made easily by using this technique.
Diagram of rotary drilling system
BORING METHODS
• Wash boring is another method of advancing boreholes.
• In this method, a casing about 2 to 3 m (6 to 10 ft) long is driven into
the ground.
• The soil inside the casing then is removed by means of a chopping
bit that is attached to a drilling rod.
• Water is forced through the drilling rod, and it goes out at a very
high velocity through the holes at the bottom of the chopping bit.
BORING METHODS
• The water and the chopped soil particles rise upward in the drill
hole and overflow at the top of the casing through a T-connection.
• The wash water then is collected in a container.
• The casing can be extended with additional pieces as the borehole
progresses; however, such extension is not necessary if the
borehole will stay open without caving in.
BORING METHODS
• Percussion drilling is an alternative method of
advancing a borehole, particularly through hard soil and
rock.
• In this technique, a heavy drilling bit is raised and
lowered to chop the hard soil.
• Casing for this type of drilling may be required.
• The chopped soil particles are brought up by the
circulation of water.
SOIL SAMPLING
• During the advancement of the boreholes, soil samples are collected at
various depths for further analysis.
• The objective of soil sampling is to obtain soils of satisfactory size with
minimum disturbance for observations and laboratory tests.
• Soil samples are usually obtained by attaching an open-ended, thinwalled tube—called a Shelby tube or, simply, a sampling tube—to
drill rods and forcing it down into the soil.
SOIL SAMPLING
• The tube is carefully withdrawn, hopefully with the soil inside it.
• Soil disturbances occur from several sources during sampling, such as:
• friction between the soil and the sampling tube
• the wall thickness of the sampling tube
• the sharpness of the cutting edge
• the care and handling of the sample tube during transportation.
SOIL SAMPLING
• To minimize friction, the sampling tube should be pushed instead of
driven into the ground.
SOIL SAMPLING
• One common type of soil sampler is the Piston sampler.
• A thin-walled, seamless steel tube of diameter ranging from 2in.
to 4in. (50mm or 100mm, the latter most popular), wall thickness
about 0.07in. (1.75mm), and length of 2–3ft (600–1000mm, the
latter most popular) (Figure 3.11a) connected to a hydraulic
cylinder.
• This sampler is particularly useful fine-grained soils especially
soft clays
SOIL SAMPLING
• The sampler first is lowered to the bottom of the borehole, then
the thin-wall tube is pushed into the soil hydraulically—past the
piston.
• After this, the pressure is released through a hole in the piston
rod.
• The presence of the piston prevents distortion in the sample
by neither letting the soil squeeze into the sampling tube very
fast nor admitting excess soil.
• Samples obtained in this manner consequently are disturbed
less than those obtained by Shelby tubes.
SOIL SAMPLING
• Another popular sampler is the “standard” sampler, also
known as the split-spoon sampler (split-barrel sampler).
• It has an inside diameter of 1-3/8in. (35mm) and an outside
diameter of 2in. (51mm).
• The sampler has a split barrel that is held together using a
screw-on driving shoe at the bottom end and a cap at the
upper end.
SOIL SAMPLING
• In some countries, a steel liner is used inside the sampler,
but in the United States, it is standard practice not to use this
liner.
• The thicker wall of the standard sampler permits higher
driving stresses than the Shelby tube, but does so at the
expense of higher levels of soil disturbances.
• Split-spoon samples are disturbed.
• They are used for
classification tests.
visual
examination
and
for
ROCK SAMPLING (CORING)
• Rock specimens from the ground are recovered through coring, a
procedure different from sampling in soils.
• The high strength of the rock makes it necessary to use thick-walled
core barrels (tubes or pipes) with tips made of some of the hardest
materials such as diamond or tungsten carbide.
STANDARD PENETRATION TEST (SPT)
• The standard penetration test (SPT) was developed circa
1927 and it is perhaps the most popular field test.
Standard Penetration Test (SPT)
STANDARD PENETRATION TEST (SPT)
• The SPT is performed by driving a standard
split-spoon sampler into the ground by
blows from a drop hammer of mass 140lb
(63.5kg) falling 30in (760mm)
• The sampler is driven 6in. (152mm) into the
soil at the bottom of a borehole, and the
number of blows (N) required to drive it an
additional 12in. (304mm) is counted.
STANDARD PENETRATION TEST (SPT)
• The number of blows (N) is called the
standard penetration number.
• For very dense coarse grained soils or
when an obstacle is encountered, the
number of blows may exceed 50 per 1in.
penetration.
• When this occurs, the SPT record would
read “refusal.” In practice, SPT are usually
conducted at 2-ft intervals.
STANDARD PENETRATION TEST (SPT)
STANDARD PENETRATION TEST (SPT)
STANDARD
PENETRATION TEST
(SPT) OUTPUT
SPT record.
STANDARD PENETRATION TEST (SPT) (N60)
• Various corrections are applied to the N values to account for energy losses,
overburden pressure, rod length, and so on.
• It is customary to correct the N values to a rod energy ratio of 60%.
• The rod energy ratio is the ratio of the energy delivered to the split spoon sampler to
the free-falling energy of the hammer.
• The corrected N values are denoted as N60 and given as:
where ERr is the energy ratio and CE is the 60% rod energy ratio
correction factor.
STANDARD PENETRATION TEST (SPT) (N60)
• Correction factors for rod lengths, sampler type, borehole diameter, and equipment (60% rod
energy ratio correction) are the following:
The correction factor, CN, is:
We can write a composite correction
factor, CRSBEN, for the correction factors:
STANDARD PENETRATION TEST (SPT) (N60)
• The corrected N value, often written as N1,60, is:
• All corrected N values should be reported in whole numbers.
• The SPT is very useful for determining changes in stratigraphy and locating bedrock.
• Also, you can inspect the soil in the split spoon sampler to describe the soil profile and extract
disturbed samples for laboratory tests.
STANDARD PENETRATION TEST (SPT) (N60)
• The SPT hammer energy efficiency can be expressed as:
• In the field, the magnitude of Er can vary from 30 to 90%.
• The corrected N values are denoted as N60 and given as:
where
N60 = standard penetration number corrected for field conditions
N = measured penetration number
ηH = hammer efficiency (%)
ηB = correction for borehole diameter
ηS = sampler correction
ηR = correction for rod length
STANDARD PENETRATION TEST (SPT) (N60)
ηH = hammer efficiency (%)
ηB = correction for borehole diameter
ηS = sampler correction
ηR = correction for rod length
STANDARD PENETRATION TEST (SPT)
(CORING)
• Two similar parameters commonly used to ascertain the quality of rock based on the drill record are
core recovery ratio (CR) and rock quality designation (RQD).
• Core recovery ratio is defined as:
• Rock quality designation (RQD) is a modified measure of core recovery, defined as (Deere, 1964):
• The RQD is a simple and inexpensive way to recognize low-quality rock zones that may require
further investigation.
STANDARD PENETRATION TEST (SPT)
(CORING)
• The RQD, corresponding descriptions of in situ rock quality, and the allowable foundation bearing
pressures as given by Peck et al. (1974).
STANDARD PENETRATION TEST (SPT)
• SPT results have been correlated with engineering properties of soils, bearing capacity, and
settlement of foundations. Most of these correlations are, however, weak (regression coefficient, Rsquared value < 60%).
• They pertain only to soils under the conditions of the tests for which the correlations were
developed.
• There are multiple sources of errors including test performance and the use of nonstandard
equipment.
• These errors lead to N values that are not representative of the soil.
STANDARD PENETRATION TEST (SPT)
• SPT tests are unreliable for coarse gravel, boulders, soft clays, silts, and mixed soils containing
boulders, cobbles, clays, and silts.
• During an SPT, dynamic stresses are applied to the surrounding soil.
• For non–free-draining soils such as clays and mixed soils, excess pressures of unknown amount in
the water in the void spaces are created.
• Because water is incompressible, it behaves as a rigid material (like concrete) when the sampler
of the SPT is driven into non–free-draining soils.
• Consequently, the recorded N value for these soils cannot be related to soil strength or any other
soil parameter.
• Since SPTs are not performed continuously, the test may fail to capture important soil layers,
especially weak, porous soils.
ENGINEERING PROPERTIES OF SOIL
• Index Properties, Shear Strength , Consolidation Properties
• Laboratory Tests
• Computation
ENGINEERING PROPERTIES OF SOIL
• Index Properties
ENGINEERING PROPERTIES OF SOIL: GRAIN
SIZE DETERMINATION
• The grading curve is used for textural classification of soils.
• Various classification systems have evolved over the years to describe soils based on their particle
size distribution.
• Each system was developed for a specific engineering purpose.
• Unified Soil Classification System (USCS), the American Society for Testing and Materials (ASTM) system (a
modification of the USCS system), and the American Association of State Highway and Transportation
Officials (AASHTO) system.
ENGINEERING PROPERTIES OF SOIL: GRAIN
SIZE DETERMINATION
• Determination of Particle Size Of Soils (ASTM D 422)
• The distribution of particle sizes or average grain diameter of coarse-grained soils—gravels and
sands—is obtained by screening a known weight of the soil through a stack of sieves of progressively
finer mesh size.
ENGINEERING PROPERTIES OF SOIL: GRAIN
SIZE DETERMINATION
• Determination of Particle Size Of Soils (ASTM D 422)
Grain Size Distribution Curve (GSDC)
DETERMINATION OF
PARTICLE SIZE OF
SOILS (ASTM D 422):
SIEVE ANALYSIS
ENGINEERING PROPERTIES OF SOIL: GRAIN
SIZE DETERMINATION
• Determination of Particle Size Of Soils (ASTM D 422)
• The screening process cannot be used for fine-grained soils—silts and
clays—because of their extremely small size. The common laboratory
method used to determine the size distribution of fine-grained soils is a
hydrometer test.
DETERMINATION OF PARTICLE SIZE OF SOILS (ASTM D
422): HYDROMETER ANALYSIS
DETERMINATION OF PARTICLE SIZE OF SOILS (ASTM D
422): HYDROMETER ANALYSIS
DETERMINATION OF PARTICLE SIZE OF SOILS (ASTM D
422): HYDROMETER ANALYSIS
DETERMINATION OF PARTICLE SIZE OF SOILS (ASTM D
422): HYDROMETER ANALYSIS
DETERMINATION OF PARTICLE SIZE OF SOILS (ASTM D
422): HYDROMETER ANALYSIS
DETERMINATION OF PARTICLE SIZE OF SOILS (ASTM D
422): HYDROMETER ANALYSIS
DETERMINATION OF PARTICLE SIZE OF SOILS (ASTM D
422): HYDROMETER ANALYSIS
ENGINEERING PROPERTIES OF SOIL:
WATER CONTENT
• Moisture content (w) is also referred to as water content and is defined as the ratio of the weight of
water to the weight of solids in a given volume of soil:
MOISTURE
CONTENT
CONVENTIONAL
OVEN
Trial No.
Container No.
Mass of container + wet
specimen (g)
Mass of container +
oven-dried specimen (g)
Mass of container
Mass of water
Mass of solid particles
Moisture content, w (%)
1
DETERMINATION
METHOD
(ASTM
2
3
USING
2216)
ENGINEERING PROPERTIES OF SOIL:
ATTERBERG LIMITS
• When clay minerals are present in ne-grained soil, the soil can be remolded in the presence of some
moisture without crumbling.
• This cohesive nature is caused by the adsorbed water surrounding the clay particles.
• In the early 1900s, a Swedish scientist named Atterberg developed a method to describe the
consistency of fine-grained soils with varying moisture contents.
ENGINEERING PROPERTIES OF SOIL:
ATTERBERG LIMITS
• At a very low moisture content, soil behaves more like a solid.
ENGINEERING PROPERTIES OF SOIL:
ATTERBERG LIMITS
• The moisture content at the point of transition from semisolid to plastic state is the plastic limit, and
from plastic to liquid state is the liquid limit.
ENGINEERING PROPERTIES OF SOIL:
ATTERBERG LIMITS
• When the moisture content is very high, the soil and water may flow like a liquid.
ENGINEERING PROPERTIES OF SOIL:
ATTERBERG LIMITS
• The moisture content, in percent, at which the transition from solid to semisolid state takes place is
defined as the shrinkage limit.
ENGINEERING PROPERTIES OF SOIL:
ATTERBERG LIMITS
• These parameters are also known as Atterberg limits.
LIQUID LIMIT (LL) ASTM D-4318
• Percussion cup method
• The percussion method was developed by Casagrande
(1932) and used throughout the world. This is the only
method adopted by ASTM (Test Designation D-4318) to
determine the liquid limit of cohesive soils.
• This device consists of a brass cup and a hard rubber base.
• The brass cup can be dropped onto the base by a cam
operated by a crank.
• To perform the liquid limit test, one must place a soil paste
in the cup.
LIQUID LIMIT (LL) ASTM D-4318
• A groove is then cut at the center of the soil pat with the standard
grooving tool.
• By the use of the crank-operated cam, the cup is lifted and dropped
from a height of 10 mm (0.394 in.).
• It is difficult to adjust the moisture content in the soil to meet the
required 12.5 mm (0.5 in.) closure of the groove in the soil pat at 25
blows.
• Hence, at least three tests for the same soil are conducted at
varying moisture contents, with the number of blows, N, required to
achieve closure varying between 15 and 35.
LIQUID LIMIT (LL) ASTM D-4318
• The moisture content, in percent, required to close a distance of
12.5 mm (0.5 in.) along the bottom of the groove after 25 blows is
defined as the liquid limit.
before test
after test
LIQUID LIMIT (LL) ASTM D-4318
• The moisture content of the soil, in
percent, and the corresponding number
of blows are plotted on semi logarithmic
graph paper.
• The relationship between moisture
content and log N is approximated as a
straight line.
• This line is referred to as the flow curve.
LIQUID LIMIT (LL) ASTM D-4318
• The moisture content corresponding to
N = 25, determined from the flow
curve, gives the liquid limit of the soil.
• The slope of the flow line is defined as the
flow index and may be written as:
Where
IF = flow index
w1 = moisture content of soil, in percent, corresponding to N1 blows
w2 = moisture content corresponding to N2 blows
LIQUID LIMIT (LL) ASTM D-4318
• From the analysis of hundreds of liquid limit tests, the U.S. Army Corps of Engineers (1949) at the
Waterways Experiment Station in Vicksburg, Mississippi, proposed an empirical equation of the
form:
where
N = number of blows in the liquid limit device for a 12.5 mm (<0 .5 in.) groove closure
wN = corresponding moisture content
tanβ 5 = 0.121 (but note that tan is not equal to 0.121 for all soils)
This equation generally yields good results for the number of blows between 20 and 30.
LIQUID LIMIT (LL) ASTM D-4318
• For routine laboratory tests, it may be used to determine the liquid limit when only one test is run for
a soil.
• This procedure is generally referred to as the one-point method and was also adopted by ASTM
under designation D-4318.
• The reason that the one-point method yields fairly good results is that a small range of moisture
content is involved when N = 20 to N = 30.
PLASTIC LIMIT (PL) ASTM D-4318
• The plastic limit is defined as the moisture content in
percent, at which the soil crumbles, when rolled into
threads of 3.2 mm (18 in.) in diameter.
• The plastic limit is the lower limit of the plastic stage
of soil.
• The plastic limit test is simple and is performed by
repeated rolling of an ellipsoidal-sized soil mass by
hand on a ground glass plate.
ENGINEERING PROPERTIES OF SOIL:
ATTERBERG LIMITS
• The plasticity index (PI) is the difference between the liquid limit and the plastic limit of a soil, or
ENGINEERING PROPERTIES OF SOIL: SHEAR
STRENGTH
Some practical cases and laboratory tests to specify.
ENGINEERING PROPERTIES OF SOIL: SHEAR
STRENGTH
ENGINEERING PROPERTIES OF SOIL: SHEAR
STRENGTH-DIRECT SHEAR TEST
• The direct shear test is the oldest and simplest form of shear test
arrangement.
• The test equipment consists of a metal shear box in which the soil
specimen is placed.
• The soil specimens may be square or circular in shape.
• The size of the specimens generally used is about 51 mm 51 mm or 102
mm 102 mm (2 in. 2 in. or 4 in. 4 in.) across and about 25 mm (1 in.)
high.
ENGINEERING PROPERTIES OF SOIL: SHEAR
STRENGTH-DIRECT SHEAR TEST
• For a given test, the normal stress can be calculated as:
𝜎𝑧 = 𝑃ൗ𝐴
where:
𝜎𝑧 = Normal Stress
P = Normal Force
A = Cross-sectional area of the specimen
• The resisting shear stress for any shear displacement can be calculated as:
𝜏 = 𝑉ൗ𝐴
where:
𝜏 = Shear Stress
V = Resisting Shear Force
A = Cross-sectional area of the specimen
ENGINEERING PROPERTIES OF SOIL: SHEAR
STRENGTH-DIRECT SHEAR TEST
• Example results:
ENGINEERING PROPERTIES OF SOIL: SHEAR
STRENGTH-DIRECT SHEAR TEST
• Example results:
ENGINEERING PROPERTIES OF SOIL: SHEAR
STRENGTH-UNCONFINED COMPRESSION TEST
ON SATURATED CLAY
• The unconfined compression test is a special type of
unconsolidated-undrained test that is commonly used for
clay specimens.
• In this test, the confining pressure σ3 is 0.
Failure by shear
• An axial load is rapidly applied to the specimen to cause
failure.
• At failure, the total minor principal stress is zero and the
total major principal stress is σ1.
Failure by bulging
ENGINEERING PROPERTIES OF SOIL: SHEAR
STRENGTH-UNCONFINED COMPRESSION TEST
ON SATURATED CLAY
• Because the undrained shear strength is independent of the confining pressure as long as the soil is
fully saturated and fully undrained, we have:
• where qu is the unconfined compression strength.
ENGINEERING PROPERTIES OF SOIL: SHEAR
STRENGTH-UNCONFINED COMPRESSION TEST
ON SATURATED CLAY
• For each applied load, compute the corresponding axial strain, ε by dividing the specimen’s
change in height, ΔH by its initial height, Ho. The corresponding cross-sectional area can be
computed by the equation:
𝑨𝒇 =
where Ao is the initial area of the specimen.
𝑨𝒐
𝟏−𝜺
ENGINEERING PROPERTIES OF SOIL: SHEAR
STRENGTH-UNCONFINED COMPRESSION TEST
ON SATURATED CLAY
Plot the load per unit area (ordinate) versus unit strain
(abscissa).
From this graph, the unconfined compressive strength may
be evaluated as either the maximum value of load per
unit area or the load per unit area at 15% strain,
whichever occurs first.
ENGINEERING PROPERTIES OF SOIL: SHEAR
STRENGTH-UNCONFINED COMPRESSION TEST
ON SATURATED CLAY
qu =
Pf
Af
where :
qu = unconfined compressive strength
su =
Pf
2 Af
= cu
Pf = normal load at failure
where :
su = undrained shear strength
A f = cross − sec tional area at failure
cu = cohesion
TRIAXIAL TEST: THEORY AND CONCEPT
• The triaxial shear test is one of the most
reliable methods available for determining the
shear strength parameters.
• It provides information on the stress-strain
behavior of the soil that the direct shear test
does not.
Diagram of triaxial test equipment
TRIAXIAL TEST: THEORY AND CONCEPT
• It provides more uniform stress conditions than
the direct shear test does with its stress
concentration along the failure plane.
• It provides more flexibility in terms of loading
path.
Diagram of triaxial test equipment
TRIAXIAL TEST: THEORY
AND CONCEPT
• In this test, a soil specimen about 36 mm (1.4 in.)
in diameter and 76 mm (3 in.) long generally is
used.
Diagram of triaxial test equipment
TRIAXIAL TEST: THEORY
AND CONCEPT
• The specimen is encased by a thin rubber
membrane and placed inside a plastic cylindrical
chamber that usually is filled with water or
glycerine.
TRIAXIAL TEST: THEORY
AND CONCEPT
• The specimen is subjected to a confining
pressure (σ3) by compression of the fluid in the
chamber. (Note: Air is sometimes used as a
compression medium.)
σ3
Diagram of triaxial test equipment
TRIAXIAL TEST: THEORY
AND CONCEPT
• The specimen is subjected to a confining
pressure (σ3) by compression of the fluid in the
chamber. (Note: Air is sometimes used as a
compression medium.)
σ3
Diagram of triaxial test equipment
TRIAXIAL TEST: THEORY
AND CONCEPT
• To cause shear failure in the specimen, one must
apply axial stress (sometimes called deviator
stress [σd]) through a vertical loading ram.
σd
σ3
Diagram of triaxial test equipment
TRIAXIAL TEST: THEORY
AND CONCEPT
Triaxial Compression Test is divided into two stages:
• Consolidation Stage
• Shearing Stage
TRIAXIAL TEST: THEORY
AND CONCEPT
Triaxial Compression Test is divided into two stages:
• Consolidation Stage
• The specimen is subjected to a confining pressure by compression of the fluid in the chamber.
• Soil should be in a fully saturated condition before proceeding to the consolidation stage.
TRIAXIAL TEST: THEORY
AND CONCEPT
Triaxial Compression Test is divided into two stages:
• Shearing Stage
• To cause shear failure in the specimen, one must apply the deviator stress.
TRIAXIAL TEST: THEORY
AND CONCEPT
• Unconsolidated-Undrained (UU test)
• Quick Test
• Consolidated-Undrained (CU Test)
• Rapid Test
• Consolidated-Drained (CD Test)
• Slow Test
TRIAXIAL TEST: THEORY
AND CONCEPT
Consolidation Stage
Shearing Stage
UU
-
Undrained
CU

Undrained
CD

Drained
UNCONSOLIDATED-UNDRAINED
TRIAXIAL
COMPRESSION TEST
ON
COHESIVE SOILS
THEORY AND CONCEPT
• In unconsolidated-undrained tests, drainage from the soil specimen is not permitted during the
application of confining pressure (σ3).
• The test specimen is sheared to failure by the application of deviator stress and drainage is
prevented.
• Because drainage is not allowed at any stage, the test can be performed quickly.
PARAMETERS
Deviator stress, sd
P
sd =
Af
A0
Af =
1− 
Major Principal Stress, s1
Confining Pressure, s3
s1 = s 3 + s d
Total stress Mohr’s circles and failure envelope
Undrained Shear
Strength,su
 s1 − s 3 
sd 
cu = su = 
=


 2 f  2 f
109
CONSOLIDATEDUNDRAINED
TRIAXIAL
COMPRESSION
TEST
THEORY AND CONCEPT
• The consolidated-undrained test is the most common type of triaxial test.
• The test is divided into three stages:
• Saturation
• Consolidation
• Shearing
SATURATION STAGE
•
Skempton’s pore pressure parameter, B-value, (Skempton, 1954) is measured to ensure that the
sample is fully saturated.
∆𝒖
𝑩=
∆𝝈𝟑
•
σ3 = minor principal stress = confining pressure
•
u = pore water pressure
• For saturated soft soils, B is approximately equal to 1; however, for saturated stiff soils, the
magnitude of B can be less than 1.
SATURATION STAGE
• Black and Lee (1973) gave the theoretical values of B for various soils at complete saturation.
CONSOLIDATION STAGE
• In this test, the saturated soil specimen is first consolidated by an all-around chamber fluid
pressure (σ3) that results in drainage.
specimen under
chamber-conning
pressure
Volume change in specimen
caused by confining pressure
SHEARING STAGE
• After the pore water pressure generated by the application of confining pressure is dissipated,
the deviator stress on the specimen is increased to cause shear failure.
specimen under
chamber-conning
pressure
deviator stress application
SHEARING STAGE
• After the pore water pressure generated by the application of confining pressure is dissipated, the
deviator stress on the specimen is increased to cause shear failure.
Deviator stress
against axial strain
for loose sand and
normally
consolidated clay
Deviator stress against
axial strain for dense
sand and overconsolidated clay
SHEARING STAGE
• After the pore water pressure generated by the application of confining pressure is dissipated, the
deviator stress on the specimen is increased to cause shear failure.
Mode of
Failure
Deviator stress
against axial strain
for loose sand and
normally
consolidated clay
Deviator stress against
axial strain for dense
sand and
overconsolidated clay
Bulging
Shear
SHEARING STAGE
• During this phase of the test, the drainage line from the specimen is kept closed. Because drainage
is not permitted, the pore water pressure, Δud, will increase.
Variation of pore water
pressure with axial
strain for loose sand
and normally
consolidated clay
Variation of pore water
pressure with axial strain for
dense sand and overconsolidated clay
SHEARING STAGE
• During this phase of the test, the drainage line from the specimen is kept closed. Because drainage
is not permitted, the pore water pressure, Δud, will increase.
Variation of pore water
pressure with axial
strain for loose sand
and normally
consolidated clay
• In loose sand and normally consolidated clay, the pore water pressure increases with strain.
SHEARING STAGE
• In dense sand and over-consolidated clay, the pore water pressure increases with strain to a certain
limit, beyond which it decreases and becomes negative (with respect to the atmospheric pressure).
Variation of pore water
pressure with axial strain for
dense sand and overconsolidated clay
• This decrease is because of a tendency of the soil to dilate.
PARAMETERS
121
Total and effective stress failure envelopes for consolidated
undrained triaxial tests.
PARAMETERS
Total and effective angle of internal friction (ϕ,
ϕ’)
Total and effective cohesion (c, c’)
For Coarse material cohesion is zero.
122
Total and effective stress failure envelopes for consolidated
undrained triaxial tests.
CONSOLIDATEDDRAINED
TRIAXIAL
COMPRESSION
TEST
THEORY AND CONCEPT
• The consolidated-drained test is the most common type of triaxial test.
• The test is divided into three stages:
• Saturation
• Consolidation
• Shearing
THEORY AND CONCEPT
• The consolidated-drained test is the most common type of triaxial test.
• The test is divided into three stages:
• Saturation
• Consolidation
• Shearing
Similar to
CU Test
SHEARING STAGE
• After the pore water pressure generated by the application of confining pressure is dissipated, the
deviator stress on the specimen is increased very slowly to cause shear failure.
specimen under
chamber-conning
pressure
deviator stress
SHEARING STAGE
• The drainage connection is kept open, and the slow rate of deviator stress application allows
complete dissipation of any pore water pressure that developed as a result (Δud = 0).
specimen under
chamber-conning
pressure
deviator stress
SHEARING
STAGE
Deviator stress against
axial strain in the
vertical direction
loose sand
and
normally
consolidate
d clay
dense sand
and overconsolidate
d clay
Mode of
Failure
loose sand
and
normally
consolidate
Bulging d clay
dense sand
and overconsolidate
d clay
Shear
SHEARING
STAGE
Deviator stress against
axial strain in the
vertical direction
Mode of
Failure
loose sand
and
normally
consolidate
Bulging d clay
dense sand
and overconsolidate
d clay
Shear
SHEARING
STAGE
Deviator stress against
axial strain in the
vertical direction
Volume
change during deviator
stress application
PARAMETERS
131
Total and effective stress failure envelopes for consolidated
undrained triaxial tests.
PARAMETERS
Total = effective angle of internal friction (ϕ = ϕ’)
Total = effective cohesion (c = c’)
For Coarse material cohesion is zero.
132
ENGINEERING PROPERTIES OF SOIL:
CONSOLIDATION PROPERTIES
ENGINEERING PROPERTIES OF SOIL:
CONSOLIDATION PROPERTIES ASTM D 2435
• The one-dimensional consolidation testing procedure was rst suggested by Terzaghi. This test is
performed in a consolidometer (sometimes referred to as an oedometer).
This test is generally performed on fine-grained soils.
The soil specimen is placed inside a metal ring with two
porous stones, one at the top of the specimen and another at
the bottom.
ENGINEERING PROPERTIES OF SOIL:
CONSOLIDATION PROPERTIES ASTM D 2435
• A metal load platen mounted on top of the
upper porous stone transmits the applied
vertical stress (vertical total stress) to the
soil sample.
• Both the metal platen and the upper porous
stone can move vertically inside the ring as
the soil settles under the applied vertical
stress.
• The ring containing the soil sample can be
fixed to the container by a collar (fixed ring
(a) A typical consolidation apparatus, (b) a fixed ring cell, and
cell) or is unrestrained (floating ring cell).
(c) a floating ring cell.
ENGINEERING PROPERTIES OF SOIL:
CONSOLIDATION PROPERTIES ASTM D 2435
• Incremental loads, including unloading
sequences, are applied to the platen, and
the settlement of the soil at various fixed
times under each load increment is
measured by a displacement gauge.
• Each load increment is allowed to remain
on the soil until the change in settlement is
negligible, and the excess porewater
pressure developed under the current
load increment has dissipated.
(a) A typical consolidation apparatus, (b) a fixed ring cell, and
(c) a floating ring cell.
ENGINEERING PROPERTIES OF SOIL:
CONSOLIDATION PROPERTIES ASTM D 2435
• For many soils, this usually occurs within
24 hours, but longer monitoring times may
be required for exceptional soil types, for
example, montmorillonite.
• Each load increment is doubled.
(a) A typical consolidation apparatus, (b) a fixed ring cell, and
(c) a floating ring cell.
ENGINEERING PROPERTIES OF SOIL:
CONSOLIDATION PROPERTIES ASTM D 2435
The data obtained from the one-dimensional consolidation test are as follows:
1. Initial height of the soil, Ho, which is fixed by the height of the ring.
2. Current height of the soil at various time intervals under each loading (time–settlement data).
3. Water content at the beginning and at the end of the test, and the dry weight of the soil at the end of
the test.
ENGINEERING PROPERTIES OF SOIL:
CONSOLIDATION PROPERTIES ASTM D 2435
• In the one-dimensional consolidation test, the water content (w) of the soil sample is determined.
Using these data, initial height (Ho), and the specific gravity (Gs) of the soil sample, you can calculate
the void ratio for each loading step as follows:
1. Calculate the final void ratio, efin = wGs, where w is the water content determined at the end of the
test.
2. Calculate the total consolidation settlement of the soil sample during the test, (ΔH)fin = Hfin − Hi,
where Hfin is the final displacement gauge reading and Hi is the displacement gauge reading at the
start of the test.
3. Back-calculate the initial void ratio:
4. Calculate e for each loading using:
where ei is the void ratio and (ΔH)i is the
change in height for the ith loading.
ENGINEERING PROPERTIES OF SOIL:
CONSOLIDATION PROPERTIES ASTM D 2435
• In the one-dimensional consolidation test, the water content (w) of the soil sample is determined.
Using these data, initial height (Ho), and the specific gravity (Gs) of the soil sample, you can calculate
the void ratio for each loading step as follows:
1. Calculate the final void ratio, efin = wGs, where w is the water content determined at the end of the
test.
2. Calculate the total consolidation settlement of the soil sample during the test, (ΔH)fin = Hfin − Hi,
where Hfin is the final displacement gauge reading and Hi is the displacement gauge reading at the
start of the test.
3. Back-calculate the initial void ratio:
4. Calculate e for each loading using:
where ei is the void ratio and (ΔH)i is the
change in height for the ith loading.
ENGINEERING PROPERTIES OF SOIL:
CONSOLIDATION PROPERTIES ASTM D 2435
• Consolidation Curve
• e-pressure plot
• Recompression Index (Cr)
• Compression Index (Cc)
• Swell Index (Cs)
ENGINEERING PROPERTIES OF SOIL:
CONSOLIDATION PROPERTIES ASTM D 2435
Recompression Index, (Cr)
∆e
Cr = −
∆Logσ′
ec −ed
=
′
Logσz d − Logσ′z c
Compression Index, (Cc)
Cc = −
=
∆e
∆Logσ′
ea −eb
Logσ′z b − Logσ′z a
𝐂 𝐫 ≈ 𝐂𝐬
Cs
ENGINEERING PROPERTIES OF SOIL:
CONSOLIDATION PROPERTIES ASTM D 2435
Recompression Ratio, (RR)
RR =
Cr
1+e0
=
𝜀𝑧 𝑑 − 𝜀𝑧 𝑐
Logσ′z d − Logσ′z c
Compression Ratio, (CR)
CR =
Cc
1+e0
=
𝜀𝑧 𝑎 − 𝜀𝑧 𝑏
Logσ′z a − Logσ′z b
RR≈ 𝐒𝐑
Cs
INTERPRETATION OF RESULTS
• Standard Penetration Test
• Determination of Particle Size Of Soils
• Water Content
• Atterberg Limits
• Shear Strength
• Consolidation Properties
STANDARD PENETRATION TEST (SPT)
Compactness of coarse-grained soils
based on N
The consistency of fine-grained soils
based on SPT
The values in these tables are estimates to provide a preliminary assessment of the character of the
ground.
ENGINEERING PROPERTIES OF SOIL: GRAIN
SIZE DETERMINATION
• Classification of Highway Subgrade Materials
• American Association of State Highway and
Transportation Officials (AASHTO) system.
Group Index = (F-35)[0.2+0.005(LL-40)]+0.01(F-15)(PI10)
Where;
F= fines content (portion passing the #200 sieve)
expressed as a percentage (%)
LL= Liquid Limit
PL= Plastic Limit
Group Index values near 0 indicate good soils, while
values of 20 or more indicate very poor soils
ENGINEERING PROPERTIES OF SOIL: GRAIN
SIZE DETERMINATION
• Unified Soil Classification
System (USCS)
ENGINEERING PROPERTIES OF SOIL:
ATTERBERG LIMITS
Typical Values of Liquid Limit, Plastic Limit, and Activity of Some Clay Minerals
Activity is used as an index for identifying the swelling potential of clay soils.
ENGINEERING PROPERTIES OF SOIL:
ATTERBERG LIMITS
• The plasticity index (PI) is the difference between the liquid limit and the plastic limit of a soil, or
• Burmister (1949) classied the plasticity index in a qualitative manner as follows:
The plasticity index is important in
classifying fine-grained soils.
It is fundamental to the Casagrande
plasticity chart, which is currently
the basis for the Unified Soil
Classification System.
ENGINEERING PROPERTIES OF SOIL: SHEAR
STRENGTH-UNCONFINED COMPRESSION TEST
ON SATURATED CLAY
General Relationship of Consistency and Unconfined
Compression Strength of Clays
ENGINEERING PROPERTIES OF SOIL:
CONSOLIDATION PROPERTIES ASTM D 2435
Cr
1+e0
or
Cc
1+e0
Classification
0-0.05
Very Slightly Compressible
0.05-0.10
Slightly Compressible
0.10-0.20
Moderately Compressible
0.20-0.35
Highly Compressible
>0.35
Very Highly Compressible