Soils Investigation
Why Test the Soil?
• Buildings are supported by soil
• Engineering properties of soil are highly variable
• Engineers need reliable soil information for
structural and site design
The knowledge gained from soil
investigation allows the engineer to make
estimates for:
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

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Bearing Capacity of the soil
Settlement of the Foundation (amount and rate)
Earth Pressure – both lateral and vertical
Drainage characteristics
Wastewater Disposal limitations
What Is Soil?
Organics
5%
Water
25%
Air
25%
Mineral
45%
Minerals
GRAVEL
Clay
SAND
Silt
Soil Information
• Preliminary Information: USDA Web Soil Survey
http://websoilsurvey.nrcs.usda.gov/
• Soil Testing/Analysis
– Previous soils investigations on or near the site
– Site inspection and simple soil testing
– Soil borings taken at proposed foundation
locations
• Local Building Department or Other Codes
and Regulations
USDA Web Soil Survey
http://websoilsurvey.nrcs.usda.gov/
Soil Boring Log
Soil Testing
Unified Soils Classification System
• (USCS) is a method for identifying
and grouping soils
• First developed by Casagrande for
military construction of airfields
Unified Soil Classification System
− Coarse-Grained
− Fine-Grained
− Highly Organic
Only particles sizes smaller than 3 inches are
considered in the USC System.
Soil Color
• Can vary with moisture content
• May indicate the presence of certain
chemicals or impurities
– Dark brown /black may indicate organic material
– Gray, olive green indicates inorganic soils
– Red or yellow may indicate iron oxides
– Gray-blue or gray-yellow indicates poor drainage
– White to pink may indicate silica, calcium
carbonate, or aluminum compounds
Coarse vs. Fine-Grained Soils
• Coarse-Grained Soils described by
grain size
• Fine-Grained Soils described on the
basis of their plasticity
Sieve Analysis
Gravels range from 3 inches
down to the size of peas
Silt and clay can pass
through the #200 sieve
13
Sieves
#4 sieve (¼ in. squares, similar to hardware cloth)
#40 sieve (similar to window screen)
Baseball sized
Grain Size
(Gradation)
Pea sized
Rock salt sized
Gravel
Boulder
Sand
Cobbles
Silt &
Coarse
>12”
Sugar sized
12”
3”
Fine
¾”
Coarse
#4
Medium
#10
#40
Fine
Clay
#200 <#200
NOTE: Particles finer than fine sand (#200 sieve) can not be seen by
the naked eye at a distance of 8 inches.
Gravel and Sand
Graded Soils
Coarse-Grained Soils can be defined by how the
particle sizes are distributed within the soil
sample.
• Well-Graded Soils provide a good representation
of all particle sizes from the largest to smallest.
Gravel and Sand
Graded Soils
Coarse-Grained Soils can be defined by how the
particle sizes are distributed within the soil
sample.
• Poorly-Graded Soils
 Uniformly Graded – Soil particles are nearly the same
size.
 Gap Graded – Contains both large and small particles,
but the graduation continuity is broken by the
absence of some particle sizes
Gravel and Sand
Clean and Dirty
• Clean soil contains little or no fines
(<5%).
• Dirty soil contains an appreciable
amount of fines (>12%).
• Silty
• Clayey
Clay and Silt
Plasticity
Fine-grained soils are classified by the
plasticity of the soil.
Plasticity refers to the consistency of
fine-grained soils as the water content
varies.
Soils Groups
Soil Type
Gravel – G
Sand – S
Silt – M
Clay – C
Organic – O
Peat – Pt
Gradation
Well Graded – W
Poorly Graded – P
Plasticity
High Plasticity – H
Low Plasticity – L
These letters are used in combination to indicate soil
classifications.
Sieve Analysis Results
USCS Classification for Coarse-Grained Soils
• Coarse-grained soil (granular soil) has
more than half of the soil grains visible to
the naked eye
– If the percentage of GRAVEL and SAND is greater
than 50% of sample, then the soil is a course-grained
or granular soil.
SAND if more than half of the coarse grains are
smaller than #4 sieve
GRAVEL if more than half of the coarse grains are
larger than #4 sieve
Sieve Analysis Results
USCS Classification for Fine-Grained Soils
Fine-grained soils (cohesive soils) contain
greater than 50% SILT and CLAY
particles.
• In addition to the sieve analysis, the following tests
will be performed to determine the plasticity
characteristics of the fine-grained soils (in lieu of
LL and PL).
Dry Strength – crushing characteristics
Dilatancy – reaction to shaking
Toughness – consistency near plastic limit
Soils Test Results for Fine-Grained Soils
FINE
GRAINED
SOILS
Highly
Organic
Soils
Dry Crushing
Strength
Dilatancy
Toughness
Soils Type
None to slight
Rapid
Low
ML
Medium to high
None to very slow Medium to high
CL
Slight to medium
Slow to none
Medium
MH
High to very high
None
High
CH
Identifiable by color, odor, spongy feel, and/or fibrous
texture
OL, OH,
and Pt
Clay and Silt
Plasticity
In lieu of dry strength, dilatancy, and toughness,
ATTERBERG LIMITS can be used to classify finegrained soils.
– Plastic Limit (PL) – lowest moisture content at which soils
can be rolled into 1/8 in. dia. thread without breaking
– Liquid Limit (LL) – minimum moisture content at which
soil will flow when a small shear or cutting force is applied
– Plastic Index (PI) – difference between the LL and PL
PI = LL - PL
USCS Soil Classification Chart
50% or more retained
on the No. 200 sieve
More than 50% passes
the No. 200 sieve
Plasticity Chart
Soil Classifications for Foundations
• Sand and gravel – Best
• Medium to hard clays – Good
• Soft clay and silt – Poor
• Organic silts and clays – Undesirable
• Peat – No Good/Avoid
Estimated Soil Bearing Capacities
Soil Type
BEDROCK
GRAVELS
GRAVELS w/ FINES
SAND
SAND W/ FINES
SILT
CLAYS
ORGANICS
Allowable Bearing
(lb/ft2)
4,000 to 12,000
3,000
3,000
2,000
2,000
1,500
1,500
0 to 400
Drainage
Poor
Good
Good
Good
Good
Medium
Medium
Poor
Resources
Sowers, G.F. (1979). Introductory soil mechanics and
foundations: geotechnical engineering. New York,
NY: Macmillan Publishing.
Lindeburg, M. R. (1994). Civil engineering reference
manual (9th ed.). Belmont, CA: Professional
Publications, Inc.
Guthrie, P. (2003). Architects portable handbook: First
step rules of thumb for building design. NY:
McGraw-Hill.
Unified soil classification. Retrieved August 5, 2004,from
http://www.hydro.unr.edu/homepages/benson/
classes/hydro/uscs.html
Resources
U.S. Army Corps of Engineers. (1953). The unified soil
classification system. U.S. Army Technical
Memorandum, No.3-357.
US Army unified soil classification system. Retrieved
August 5, 2004, from
http://www.adtdl.army.mil/cgi- bin/atdl.dll/fm/5472/apb.pdf
Image Sources
Fox, A. (photographer). University of South Carolina
U.S. Army Corp of Engineers. Technical Manual FM5-410
Soils Engineering