Soil Testing
Purpose
Almost any structure we build rests on the ground. Before we can design the structure or
determine a site grading plan, we must understand the soil conditions. Soil is made up of many
different size grains of broken-down rock (sand, silt, and clay). How much water a soil will hold,
how easily water passes through the soil, and what happens to the soil as it dries depend on the
combination of these materials in your particular soil. The regions climate and weather
conditions and their affect on the soil must be considered. Before the design of a major project,
such as a highway, building, bridge or dam, can begin, the ground underneath the proposed
construction site must be tested to determine composition and other factors affecting its loadbearing capacity. On large projects, core samples are used. Engineers use a drill to remove
samples of earth at prescribed intervals. Sometimes core samples are taken from a depth of
one hundred feet or more. The samples are then brought to the surface for study. A
geotechnical engineer, which is a type of civil engineer, collaborates with a surveyor to map
grades, slopes and any modifications that may have been made to the site. The engineers will
determine appropriate sites and depths for taking soil samples. Boring sites will include areas
where filling and cutting may occur during construction. In addition, engineers will develop
preliminary concepts for drainage structures that may be needed to direct water away. Not every
project needs test borings. However, you must know the basic soil type before you can proceed.
In this activity, you will be using some tests to quickly determine a soil type. By identifying the
soil type and properties using a chart, you will be able to make some preliminary decisions
regarding grading and load bearing characteristics.
SOIL CATEGORIES
Soil rarely exists in nature as sand alone, gravel alone, or any other single component. Usually,
soil occurs as mixtures with varying proportions of particles of different sizes. The Unified Soil
Classification System is based on the characteristics of the soil that indicate how it will behave
as a construction material. There are many indicators and descriptors for soil.
In the USCS, all soils are placed into one of three major categories. They are
 Coarse-grained.
 Fine-grained.
 Highly organic.
The USCS further divides soils that have been classified into the major soil categories by letter
symbols, such as
 S for sand.
 G for gravel.
 M for silt.
 C for clay.
A soil that meets the criteria for a sandy clay would be designated (SC). There are cases of
borderline soils that cannot be classified by a single dual symbol, such as GM for silty gravel.
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What to look for
Granular soils, fine sands Plastic (cohesive) soils,
and silts
clay
Visual appearance and feel
Coarse grains can be seen;
feels gritty when rubbed
between fingers
Movement of water in the
spaces
Plasticity when moist
Cohesion in dry state
Grains cannot be seen by
the naked eye; feels smooth
and greasy when rubbed
between fingers
When a small quantity is
When a small quantity is
shaken in the palm of the
shaken in the palm of the
hand, water will appear on the hand, it shows no sign of
surface of the sample. When
water moving out of the
shaking is stopped, water will voids.
gradually disappear.
Very little or no plasticity
Plastic and sticky, sample
can be rolled
Little or no cohesive strength High dried strength,
in dry state; will crumble
crumbles with difficulty and
readily
disintegrates slowly in water
Particle size
Gravels range from about 3” down to the size of peas. Sands start just below this size and
decrease until the individual grains are just distinguishable by the naked eye. The eye can
normally see individual grains about 0.07 millimeter in size, or about the size of the No. 200
sieve. Silt and clay particles, which are smaller than sands, are indistinguishable as
individual particles.
Size Group
Cobbles
Gravels
Sands
Fines (clay or silt)
Sieve Size
Passing
Retained On
No maximum size
3 inches
No. 4
No. 200
3 inches
No. 4
No. 200
No minimum size
Procedure
These test methods provide information about the size of the soil particles, the amounts of the
various sizes, and the characteristics of the very fine grains. See Table 1 for a summary of the
test results and the USCS soil types.
Simple Grain Size Analysis
1. Divide the soil sample into approximately two equal portions. Set aside one portion for
later testing.
2. Dry one soil sample in an oven. Check soil occasionally and stir.
3. While soil is drying, weigh glass containers, sieves, and bottom sieve pan.
4. Remove soil from oven and place in mortar and pestle and pulverize the soil.
5. Stack sieves with No 4 on top, then the No 40 sieve below that, and then the bottom pan.
6. Place soil in No 4 sieve, place cover on top and shake vigorously.
7. Pull the cover off of the No 4 sieve and inspect contents to be sure that small dirt clumps
do NOT exist on the No. 4 sieve. Manually break up small dirt clumps and sift again.
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8. Remove the No 4 sieve from the stack with its soil contents and weigh them.
9. Record and subtract the weight of the No 4 sieve only (i.e., without the soil sample). The
difference is the GRAVEL fraction in the soil sample.
10. Repeat steps 7 and 8 for the No 40 sieve. The difference is the coarse and medium
grained SAND fraction in the soil sample.
11. Weigh the bottom pan and its contents. Record and subtract the weight of the empty pan.
The difference is the fine SAND, SILT, and CLAY fraction of the soil sample.
12. Pour the contents of the bottom pan into the Mason jar. Weigh and record this weight.
13. Add water to the jar so that ¾ of the jar is filled.
14. Shake the Mason jar, allow the fine sand to settle for 5 to 10 seconds, remove the top,
and pour off the dirty water into a 5-gallon pail. Be careful not to spill any of the soil that
has settled to the bottom of the jar into the 5 gallon pail.
15. Repeat until water is clear. The material remaining in the Mason jar is the fine grained
SAND. SILT, and CLAY that has been removed from the soil sample.
16. Place the Mason jar, without top, and contents into the oven and dry.
17. Remove the jar, being careful, as the glass jar will be hot.
18. Weigh the jar and contents. Record the weight and subtract the weight of the empty jar.
The difference is the fine grained SAND fraction in the soil sample.
19. Determine the weight of SILT and CLAY in the soil sample. Record and present the
results in a table similar to the sample table provided.
Item
No 4
sieve
No 40
sieve
Bottom
Pan
Mason
Jar
Item
Mason jar
and soil
Soil in
mason jar
Mason jar
and fine
SAND
Fine SAND
in mason
jar
SILT and
CLAY in
mason jar
Fine SAND
in pan
SILT and
CLAY in
pan
Weight,
grams
Weight,
grams
Item
No 4 sieve and
retained soil
GRAVEL
Fraction in
mason jar
Soil Sample
wgt, grams
Weight,
grams
Soil Sample
wgt, grams
No 40 sieve and
retained soil
Med. and coarse
SAND
Bottom pan and
soil
Fine SAND,
SILT and CLAY
Presentation of results
Item
GRAVEL
Med. and
coarse
SAND
Fine SAND
Soil Sample
wgt, grams
Percent in soil
sample
Total
SAND,
%
SILT and
CLAY
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USCS Class. Technique for Coarse-Grained Soils & Fine-Grained Soils
COARSEGRAINED
SOILS
(GRANULAR)
More than 50%
GRAVEL and
SAND
Gravelly Soils
More than 50%
GRAVEL
Sandy soils
More than 50%
coarse,
medium, and
fine grained
SAND
FINEGRAINED
SOILS
(COHESIVE)
More than 50%
of the soil is
SILT and CLAY
Highly Organic
Soils
Dry Crushing
Strength
None to slight
Medium to high
Table 1
GW. Well-graded GRAVEL.
Clean Gravels
will not leave a Various sizes of gravel in sample.
stain on a wet
GP. Poorly graded GRAVEL.
palm.
One size of gravel in sample.
Uniform.
GM. Silty GRAVEL. Non-plastic
Dirty Gravels
will leave a
fines. See ML below to identify.
stain on a wet
GC. Clayey GRAVEL. Plastic
palm.
fines. See CL below to identify.
SW. Well-graded SAND. Various
Clean Sands
will not leave a sizes of sand in sample.
stain on a wet
SP. Poorly graded SAND. One
palm.
size of sand in sample. Uniform.
Dirty Sands will SM. Silty SAND. Non-plastic
leave a stain on fines. See ML below to identify.
a wet palm.
SC. Clayey SAND. Plastic fines.
See CL below to identify.
Dilatancy
Toughness
Rapid
Low
ML
None to very
slow
Slow to none
Medium to high
CL
Slight to
Medium
medium
High to very
none
High
high
Readily identifiable by color, odor, spongy feel, and
frequently by fibrous texture
MH
CH
OL, OH, and Pt
Well-Graded Soils. A well-graded soil is defined as having a good representation of all particle
sizes from the largest to the smallest.
Poorly-Graded Soils. There are two types of poorly-graded soils.
 A uniformly graded soil consists primarily of particles of nearly the same size.
 A gap-graded soil contains both large and small particles, but the gradation
continuity is broken by the absence of some particle sizes.
USCS Classification Technique for Fine-Grained Soils
1) Use half of the original soil sample that was set aside to complete:
a) Dry sample in oven
b) Sift soil though No 40 sieve
c) Use the soil retained in the bottom pan for the 3 tests described below.
2) Dry Strength (Crushing characteristics)
3) Dilatancy (Reaction to Shaking)
4) Toughness
5) Smear Test
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Soil Types and Properties
Division
Letter
code
Color
Gravel and
Gravelly
Soils
GW
Red
GP
GM
Sand and
Sandy Soils
Soil Description
Well-Graded Gravel or
gravel sand mixture,
little or no fines
Red
Poorly-graded gravel
or gravel sand mixture,
little or no fines
Yellow Silty gravels, gravel
sand silt mixture
GC
Yellow Clayey gravels; gravel
clay sand mixture
SW
Red
SP
SM
SC
Well-graded sands or
gravelly sands, little or
no fines
Red
Poorly-graded sands or
gravelly sands, little or
no fines
Yellow Silty sands, sand silt
mixtures
Yellow Clayey sands, sand
clay mixture
Value for Embankments
Very stable, pervious
shells of dikes and dams
Reasonably stable,
pervious shells of dikes
and dams
Reasonably stable, not
particularly suited to
shells but may be used
for impervious cores or
blankets
Fairly stable, may be
used for impervious
cores
Very stable, pervious
sections, slope protection
required
Reasonably stable, may
be used in dike section
with flat slopes
Fairly stable, not suited
to shells but may be used
for impervious cores or
dikes
Fairly stable, may be
used for impervious core
for flood control
structures
Value as a
Foundation
Material
Excellent
Frost
Action
Drainage
Maximum
Gradient
None
Excellent
2.5:1
Good
None
Excellent
2.5:1
Good
Slight
Poor
2:1
Good
Slight
Poor
1.75:1
Good
None
Excellent
3:1
Fair
None
Excellent
3:1
Fair
Slight
Fair
2.5:1
Fair
Medium
Poor
2.5:1
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Division
Letter
code
Color
Soil Description
Value for Embankments
Silts and
Clays
Liquid
Limit
<50%
(plasticity
level)
ML
Green
Poor stability, may be
used for embankments
with proper control
CL
Green
OL
Green
MH
Blue
CH
Blue
Inorganic silts, rock
flour, silty or clayey
fine sands or clayey
silts with slight
plasticity
Inorganic clays of low
to medium plasticity,
gravelly clays, silty
clays, lean clays
Organic silt clays of
low plasticity
Inorganic silts,
micaceous or
diatomaceous fine
sandy or silty soils,
elastic silts
Inorganic clays of high
plasticity, fat clays
OH
Blue
Silts and
Clays
Liquid
Limit
>50%
Highly
Organic
soils
Pt
Organic clays of
medium to high
plasticity, organic silts
Orange Peat and other highly
organic soils
Value as a
Foundation
Material
Fair
Frost
Action
Drainage
Maximum
Gradient
Very
High
Poor
1.25:1
Stable impervious cores
and blankets
Fair
Medium
Impervious 1.25:1
Not suitable for
embankments
Poor stability, core of
hydraulic fill dam, not
desirable in rolled fill
construction
Poor
High
Impervious 1.5:1
Poor
Very
High
Poor
Fair stability with flat
slopes, thin cores,
blankets and dike
sections
Not suitable for
embankments
Very Poor
Medium
Impervious 5:1
Very Poor
Medium
Impervious 5:1
Not used
Not suitable
Slight
Poor
5:1
2.5:1
Source: U.S. Army Corps of Engineers. (1953). The unified soil classification system. U.S. Army Technical Memorandum, No.3-357.
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