Transcript Engineering Classification of Soils

Engineering Classification of
Soils
I.
A.
Overview
Two Systems of Classification
1. Pedological Classifications
(soil weathering, texture, chemistry,
profile thickness, etc.)
2. Engineering Classifications
–
–
soil texture
degree of plasticity (Atterberg Limits)
Soil Texture
Remove
Cobbles and
Boulders from
Analysis (>75mm)
“Gravel”
75-2 mm
“Sand”
2-0.075 mm
“Silt and Clay”
<0.075 mm
Wentworth
Scale
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 along
the bottom of a groove after 25 blows
of 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
In general….
D. Plastic Limit
PI
Degree of Plasticity
0
Nonplastic
1-5
Slightly plastic
5-10
Low plasticity
10-20 Medium plasticity
20-40 High plasticity
40+
Very high plasticity
(from Burmister, 1949)
E. Plasticity Index (PI)
• Difference between Liquid Limit and Plastic Limit
• Important measure of plastic behavior
Procedure for AASHTO
Classification
(American Association of State
Highway and Transportation
Officials)
Developed in 1929 as the Public Road Administration Classification System
Modified 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,
Cobble
and 200 sieves.
3“
Gravel
#10
#10
Very Coarse to Med Sand
#40
Fine/Very Fine Sand
#200
Silt/Clay
#200
#40
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.
2.
If less than 50% passes, then it is a coarse
grained soil (gravel and sand)
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”
HOMEWORK:
Classify the following soils by both the AASHTO and Unified Systems,
and give the group index for the AASHTO system.
Sieve
Analysis
-- % finer
than
Soil
Sample
A
B
C
D
E
#10
95
100
100
90
90
#40
79
95
80
55
71
#200 Liquid Lmt
53
36
78
65
62
35
45
28
60
40
Plastic
Lmt
21
26
20
20
26
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