SOILS
Identify Basic Soils Properties
TEC C03-A01
Why is Soil important ?
It is the foundation for
all types of pavement
systems and building
structures.
What is Soil ...
is the entire unconsolidated
earthen material that overlies
and excludes bedrock; plus
the water, air, organic
matter, and other substances
that may be contained therein.
BASIC PHYSICAL PROPERTIES
Engineers evaluate soils by the following Basic
Physical Properties:
 GRADATION of sizes of the different
particles
 BEARING CAPACITY as reflected by
soils density
 PARTICLE SHAPES interlock according
to shapes; this affects strength
An engineer will also consider the effect of water on
various types of soil for the project.
PHYSICAL PROPERTIES
 GRAIN SIZE
 PARTICAL SHAPE
 GRADATION
 DENSITY
 SPECIFIC GRAVITY
 MOISTURE
 CONSISTENCY
 ORGANIC SOIL
….a soil’s
properties help
determine the
engineering
characteristics
.
GRAIN / PARTICLE SIZE
+ 3”
COBBLES
- 3”
+ #4
}
GRAVELS
- #4
+ #200
}
SANDS
- #200
FINES (CLAY OR SILT)
GRAIN/PARTICLE SHAPE
The shape of soil particles influences
the strength and stability of a soil.
BULKY
Gravels, sands and silt fall into the bulky
shape ...relatively equal in all three dimensions.
Bulky shapes are further subdivided based
upon the effects of weathering. They may be
angular, subangular, subrounded, or rounded.
WHAT ARE THE 2 TYPES?
PLATY
Particles of clay exhibit a platy shape ... one
dimension is very small compared to the other
two.
The shape of soil particles influences
the strength and stability of a soil.
bulky shapes
angular
subangular subrounded
platy shape
rounded
GRADATION
The distribution of the different size
particles in a soil mass is the gradation.
Well-graded soils have a good range of all
representative particle sizes between the largest
and the smallest.
WHAT
ARE
THE
2
TYPES?
Poorly-graded soils contain either a narrow range
of particle sizes or lack some intermediate sizes.
• Uniformly graded have a narrow range of sizes.
• Gap, step, or skip graded have some
intermediate sizes missing or not well
represented.
GRADATION
Well-graded
Uniformly graded
Gap, step, or skip graded
DENSITY
• Density is how closely packed
or loose the soil structure is.
• A density structure provides
interlocking of larger particles
with smaller particles filling the
voids between larger particles.
SPECIFIC GRAVITY
• Ratio between the weight-per-unit volume of
a given soil sample and the weight-per-unit
volume of a similar volume of water at a stated
temperature.
• Specific Gravity of water is 1, Gold is 18.
• If a soil has a Specific Gravity of 2.72, then
it is 2.72 times heavier than water.
MOISTURE
Moisture Content is often the
most important factor affecting
a soils behavior.
It is the proportion of the weight
of water to the weight of the solid
(dry) mineral grains in the soil.
MOISTURE
Moisture has its greatest effect on the behavior of
fine-grained soils such as silts and clays.
Coarse-grained soils with larger voids
• Are less susceptible to capillary action
• Hold less water
• Tend to drain more freely
Clay soil particles retard movement of water
• Vary from liquid to brick hard
• Impervious to passage of free moisture
Soil Moisture occurs in five types:
1)
SURFACE WATER
2) SUBSURFACE
WATER
3) GRAVITATIONAL
PULL
4) CAPILLARY ACTION
5) ABSORBED WATER
& HYGROSCOPIC
MOISTURE
… water from
precipitation or
… collected or held
runoff.
in pools or layers
beneath surface
… by
seeks
lower layer
a restricting
& moves
through
layer
of soil
or
voids
… rock.
voids until
in soilitmay
reaches
a
form tunnels
&
restriction.
tubes and cause
… water that may
water to rise in
be present as
the tubes
thin films.
PLASTICITY & COHESION
• Plasticity is a property of fine-grained
soils that allows it to be deformed without
cracking or breaking.
• The Plasticity Index (PI) is used to
determine weather soil is cohesive. Not all
plastic soils are cohesive. (PURE SILT)
• Soil plasticity is determined by observing
the different physical states a soil passes
through as the moisture content changes.
Physical States and Atterberg Limits
In 1911, a Swedish soil scientist, Albert Atterberg described the
effects of changing water content on the consistency of fine-grained
soil. He also proposed a series of tests to determine soil properties,
which define the water content ranges of soil states or phases.
The change from one phase to the next is not observable as a
precise boundary, but takes place as a gradual transition.
Nevertheless arbitrary but specific boundaries have been
established empirically and are universally recognized.
The moisture contents at these boundaries are known as the
Atterberg limits or consistency limits.
In 1932, A. Casagrande designed a device, the cup in use today, to
standardize the liquid limit test. He incorporated Atterberg limits in a
procedure, the Unified Soil Classification System (USCS), to identify
the soils used in construction.
CONSISTANCY
 Consistency- four states of consistency for
fined grained soils (liquid, plastic, semisolid
and solid).
 The dividing line between these states of
consistency are called the Liquid Limit (LL)
and the Plastic Limit (PL). All are quantified
in terms of water content (w).
SOIL MOISTURE STATES
%W INCREASING
%W
%W
LOW %W
HIGH %W
SOLID
SEMI-SOLID
STATE
STATE
PLASTIC
LIQUID
STATE
STATE
PI
PL
PI = LL - PL
LL
60
PLASTICITY INDEX (PI)
50
CH OR OH
40
30
20
CL OR OL
MH OR OH
10
7
4
CL - ML
0
10
16 20
LL-PL = PI
ML OR OL
30
40
50
60
LIQUID LIMIT (LL)
70
PLASTICITY CHART
80
90
ORGANICS
Soil having a high content of organic
material is described as organic soil.
Organics are typically very
compressible and has poor loadmaintaining properties.
Why classify Soil ...
purpose of a soil classification
system is to be able to predict
the engineering properties of
the soil.
WHAT SYSTEM DO
WE USE?
Unified Soil Classification System
The USCS looks at properties such as:
 Percentages of gravel, sand, and fines.
 Shape of the grain-size-distribution
curve.
 Plasticity and compressibility
characteristics
Soil Classification gives a
good indication of …
•Strength
•Drainage
•Resistance to Frost Action
•Compressibility
(Volume Change)
•Compaction
(Constructability)
USCS Letter Symbols
Soils are divided into major groups and
further subdivided by characteristics.
Soil Groups
G
Gravel
S
Sand
M
Silt
C
Clay
Soil Characteristics
W
Well graded
P
Poorly graded
H
High compressibility
L
Low compressibility
Pt
Organic (Peat)
O
Organic (Silts and clays)
L
Liquid limit under 50
H
Liquid limit over 50
USCS
Soil Classification Chart
If Less than 50 % of Total Sample Determine %
It is a Coarse-grained Soil
Passing #200
If Less than 50 % of Coarse Fraction
Determine %
It is a Gravel
Passing #4
If 50 % or More of Coarse Fraction
Determine %
Passing #200
YES
Classify
as
GW
Cu > 4
&
1 < Cc < 3
Determine
Cu & Cc
NO
Cu > 4
&
1 < Cc < 3
NO
Plot Atterberg
Test Data
Plot Atterberg
Test Data
YES
Classify
as
ML or MH CL or CH CL-ML SW
Fines
Fines
Fines
Classify
as
GP
YES
Classify
as
GM
Classify
as
GC
GW-GM-GC
Classify
as
GW-GM
Classify
as
GW-GC
Classify
as
GM-GC
Plot Atterberg
Test Data
Classify
as
GP-GM
Classify
as
GP-GC
Determine
Cu & Cc
Classify with
Dual Symbol
Cu > 6
&
1 < Cc < 3
Determine
Cu & Cc
NO
Cu > 6
&
1 < Cc < 3
Plot Atterberg
Test Data
Classify
as
SM
Classify
as
SC
Classify
as
GP-GM-GC
SW-SM-SC
PI < 4
or Plots
below
A Line
Limits
PI
Plot in
Plots on
Hatched or above
Area
A Line
PI
Plots
below
A Line
Classify
as
CL
Classify
as
ML
Classify
as
CL-ML
Classify
as
MH
If
Liquid Limit (oven dried)
< 0.75
Liquid Limit (not dried)
Classify
as
OL
Classify
as
SM-SC
Classify
as
CH
or
Classify
as
OH
60
NO
YES
Plot Atterberg
Test Data
CL-ML ML or MH CL or CH ML or MH CL or CH CL-ML
Fines
Fines
Fines
Fines
Fines
Fines
Classify
as
PI > 7 &
Plots on
or above
A Line
ML or MH CL or CH CL-ML
Fines
Fines
Fines
Classify
as
SP
Liquid Limit
> 50
Liquid Limit
< 50
If Between 5% & 12% If More than 12%
of Total Sample
of Total Sample
Plot Atterberg
Test Data
CL-ML ML or MH CL or CH ML or MH CL or CH CL-ML
Fines
Fines
Fines
Fines
Fines
Fines
Classify
as
If Less than 5%
of Total Sample
Classify
as
SW-SM
Classify
as
SW-SC
Classify
as
SP-SM
Classify
as
SP-SC
Classify
as
SP-SM-SC
PLASTICITY INDEX (PI)
If Less than 5% If Between 5% & 12%If More than 12%
of Total Sample
of Total Sample
of Total Sample
Classify with
Dual Symbol
Plot Atterberg
Test Data
It is a Sand
Determine %
Passing #200
Determine
Cu & Cc
If 50 % or More of Total Sample
It is a Fine-grained Soil
50
CH OR OH
40
30
20
CL OR OL
10
7 CL - ML
4
0
10 1620
MH OR OH
ML OR OL
30
40 50 60 70
LIQUID LIMIT (LL)
80
PLASTICITY CHART
90
Any Questions?