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Engineering Classification of Soils
I. Overview
A. Two Systems of Classification
1. Pedological Classifications
(soil weathering, texture, chemistry, profile thickness, etc.)
2. Engineering Classifications– soil texture – degree of plasticity (Atterberg Limits)
WentworthScale
“Gravel”75-2 mm
“Sand”2-0.075 mm
“Silt and Clay”<0.075 mm
RemoveCobbles andBoulders from Analysis (>75mm)
Soil Texture
B. Overview of Mechanics
As water content increases,the shear strength decreases
It’s all about shear strength
C. Liquid Limit
•Soil is practically a liquid•Shows minimal shear strength•Defined as the moisture content required
to close a distance of 0.5 inch alongthe bottom of a groove after 25 blowsof the liquid limit device.
animation
C. Liquid Limit
D. Plastic Limit•Water content at which the soil is a plastic•Less water content than liquid limit•Wide range of shear strengths at plastic limit•Defined as the moisture content % at which the soil begins to crumble when rolled into 1/8” diameter threads
animation
C. Liquid Limit
D. Plastic Limit
E. Plasticity Index (PI)• Difference between Liquid Limit and Plastic Limit• Important measure of plastic behavior
C. Liquid Limit
D. Plastic Limit
E. Plasticity Index (PI)• Difference between Liquid Limit and Plastic Limit• Important measure of plastic behavior
In general….
PI Degree of Plasticity0 Nonplastic1-5 Slightly plastic5-10 Low plasticity10-20 Medium plasticity20-40 High plasticity40+ Very high plasticity(from Burmister, 1949)
Procedure for AASHTO Classification
(American Association of StateHighway and Transportation
Officials)
Developed in 1929 as the Public Road Administration Classification SystemModified by the Highway Research Board (1945)
Procedure for AASHTO Classification
• Determine the percentage of soil passing the #200 sieve
• Determine the subgroups– For coarse-grained soils (gravel and sand),
determine the percent passing the #10, 40, and 200 sieves, AND
– Determine the liquid limit and plasticity index– THEN, determine soil group or subgroup from
Table 9.1
– For coarse-grained soils (gravel and sand), determine the percent passing the #10, 40, and 200 sieves. Cobble
GravelVery Coarse to Med SandFine/Very Fine SandSilt/Clay
3 “#10#40
#200
#10
#40
#200
Procedure for AASHTO Classification
• Determine the percentage of soil passing the #200 sieve
• Determine the subgroups– For fine-grained soils (silt & clay), determine
the liquid limit and plasticity index– Determine soil group or subgroup from Table
9.2
AASHTO Classification for Soils
• Determine the Group Index (usually reflects the relative strength of the material, where low values have the greatest shear strength)
• Determine the group index
• Determine the group index
fine
{e.g.: A-7-5(9)}
Example Problem
A-7-6(10)
II. Unified System
A. Overview
A. Arthur Casagrande (USAF) proposed for the construction of Airfields
B. Basis
-Over half of material retained on #200
sieve, use textural characteristics
-Over half of material passes the #200 sieve, use plasticity-compressibility characteristics
II. Unified System
B. The classification scheme
II. Unified System
C. The procedure
1. Determine the percent passing through the #200 sieve (boundary between sand and silt/clay).
1. If less than 50% passes, then it is a coarse grained soil (gravel and sand)
2. If greater than 50% passes, then it is a fine grained soil (silt and clay)
Uniformity Coefficient = D60/D10, where we use the % finer by weight (% passing through) for the values
“clayey materials”
“silty materials”
Sieve Analysis -- % finer than
Soil Sample #10 #40 #200 Liquid Lmt
Plastic Lmt
A 95 79 53 36 21B 100 95 78 65 26C 100 80 62 35 20D 90 55 45 28 20E 90 71 60 40 26
HOMEWORK:Classify the following soils by both the AASHTO and Unified Systems,and give the group index for the AASHTO system.
Alternate method for classifying soils using Unified Method…..(bonus information!)
For fine grained soils:
Where R = ‘retained’ F = ‘falling through’
For fine grained soils:
For fine grained soils:
• For coarse-grained soils:
See next slide
Uniformity Coefficient = D60/D10, where we use the % finer by weight (% passing through) for the values
Fine grained soils
Fine grained soils
Gravelly soils
sandy soils
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