Field (Visual) Classification of Soilscem.uaf.edu/media/187377/visual-soil-classification.pdf ·...
Transcript of Field (Visual) Classification of Soilscem.uaf.edu/media/187377/visual-soil-classification.pdf ·...
Field (Visual) Classification of Soils
by
Chuang LinDepartment of Civil and Environmental Engineering
University of Alaska Fairbanks
June 6, 2016
2016 Summer Transportation Institute, Fairbanks, AK
Background and Objectives
Soil types
Characterization of fine-grained soils
Characterization of coarse-grained soils
Outline
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Based on Unified Soil Classification System (USCS)
ASTM Standards
Lab classification: ASTM D2487
Field (visual) classification: ASTM D2488
Background
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Coarse0.2 in – 0.08 in
(4.75 mm – 2 mm)Medium
0.08 in – 0.02 in(2 mm – 0.4 mm)
Fine0.02 in – 0.004 in
(0.4 mm – 0.075 mm)
Sand
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AngularityShapeColorOdor
Moisture ConditionConsistencyCementation
Range of the Particle SizeMaximum Particle Size
Only for G or S
Only for G or S
Only for C or M
Description-Soil Properties
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Bulky – most cases
Soil particle shape Flaky – mostly clay mineralsNeedle shaped – less common
AngularBulky particles Subangular
SubroundedRounded
Soil Particle Shape
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MC - wet
MC - dry
What is soil consistency?A method of describing the degree of cohesion and
adhesion between soil particles as related to the resistance of the soil to deform or rupture
Consistency
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Two Broad Categories
Fine-grained: 50% or more finer than No. 200 sieve
M or C, H or L, portion of coarse-grained
Coarse-grained: less than 50% finer than No. 200 sieve
G or S, W or P, portion of fine-grained
Classification-Flow Charts
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Visual Determination
Remember that percent is by dry weight. Gravel Particles overpower gradation
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Textual Determination
When wetted thoroughly, coarse-grained soils have a distinct grittiness. Practice on known samples
Easiest method to differentiate fine-grained soil from fine sand
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Tests to Characterize Fine-Grained Soils
Liquid Limit Evaluation (M or C)
High - Greater than 50
Low - Less than 50
Plasticity Evaluation (H or L)
Plastic Index (PI) above or below A-line
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Dilatency Evaluation
When shaken, dilatent soil releases free water and becomes “livery” in appearance 28
Toughness Evaluation
Reaching plastic limit requires adding considerable dry soil.
Thread can be re-rolled several times after reaching plastic limit.
Thread requires pressure to form.
Thread can be rolled out to lengths > 2”
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Ribbon Evaluation
Length of ribbon Strength Related to clay content Ribbon more than 2 inches
long indicates clay content is >= 40%
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Dry Strength Evaluation
Very high: cannot be crushed even on hard surface
High: can be crushed only against hard surface
Medium: can be crushed with considerable pressure
Low: crumbles with some pressure
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Typical Behaviors-CH Soil
High liquid limit High toughness - easily roll out thread to 1/8 inch
diameter to length of several inches Zero dilatency Ribbons to more than 1-1/2 inch High dry strength High Shine
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Typical Behaviors-ML Soil
Low liquid limit Slight to no toughness - if a thread can be formed
to 1/8 inch diameter it can’t be reformed Medium to high dilatency Ribbons only slightly if any Low to very low dry strength No Shine (watch out for mica)
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Typical Behaviors-MH Soil
High liquid limit Medium toughness - if a thread can be formed to
1/8 inch diameter it has low strength Low dilatency Ribbons slightly to moderately Low to Medium dry strength Low Shine (watch out for mica)
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Typical Behaviors-CL Soil
Low liquid limit Medium to high toughness – a thread can be
formed to 1/8 inch diameter with medium toughness
Low to none dilatency Ribbons slightly to moderately Medium dry strength Low to Medium Shine (watch out for mica)
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Distinguishing between CL and MH Difficult
MH
CH
CL
MLPI is the same -
Only difference is in LL
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PI
LL40
Example Problem
A field sample has the following properties: Fine content 60%, gravel 18% and sand 22% Ribbon test < 1.5 in., high dilatency, water penetrates
into soil relatively fast and no heat generation, low dry strength
What is the Group Symbol and Group Name of this sample?Refer to the flow chart in the handout
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Tests to Characterize Coarse-Grained Soils
Visual Examination Determine whether G or S is predominant Determine if clean or dirty
If Clean Determine if poorly or well graded
If Dirty Determine if fines are plastic or not
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Clean or Dirty?
When wetted a clean coarse-grained soil will not leave a stain on your palm. A dirty soil will leave a stain
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Clean or Dirty?
If a soil sample is dropped in a beaker of water, a cloud remaining after about 30 seconds indicates more than 12 % fines
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Well Graded or Poorly Graded?
A well graded sample has a wide range of particle sizes that are about equally distributed by weight
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Well Graded or Poorly Graded?
A gap-graded sample has a range of particle sizes missing. This sample has coarse and fine but no medium size particles
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Example Problem
A field sample has the following properties: Gravel 65%, sand 18% and fines17% easily roll out threads into 1/8 in. dia. to several inches,
low dilatency, water can hardly penetrates into soil and heat generation, strong dry strength
What is the Group Symbol and Group Name of this sample?Refer to the flow chart in the handout
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