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03- Classification of Soils
*01. Determine the percentage of each of the four grain sizes.
*02. Coefficients of uniformity and curvature of granular soils.
*03. Classify two soils using the USCS.
*04. Manufacturing a “new” soil from two unacceptable soils.
*05. Use Casagrande A-chart to classify a soil via USCS.
*06. Identify a very highly plastic soil.
*07. Find the Activity number of a clay.
*08. Classify a soil comparing USCS with AASHTO.
*09. Classify a soil comparing USCS with AASHTO.
*10. Classify five soils using both USCS and AASHTO.
*11. Find the fine’s portion particle size with a hydrometer.
*12. Using a hydrometer to classify the fines portion of a soil.
*13. Compare the relative density versus the relative compaction.
**14. Find the range of relative densities Dr for a granular soil.
**15. Relative densities of quartzitic versus calcareous sands.
**16. The theoretical minimum voids ratio of uniform sand.
Symbols for the Classification of Soils
Cc → Coefficient of curvature (also known as coefficient of gradation). Cu → Coefficient of uniformity. RC→ Relative compaction. Dx → Diameter of the grains (at % finer by weight). Dr → Relative density of a granular soil. e→ Voids ratio. emin → Minimum voids ratio. emax → Maximum voids ratio. IP → Index of plasticity (also referred to as PI, plasticity index). K→ Constant of the yield value. LL→ Liquid limit. PL→ Plastic limit. SL→ Shrinkage limit. V→ Volume of the soil sample. W→ Weight of the soil sample. γd(min) → Dry unit weight in loosest condition (voids ratio emax ). γd → In-situ dry unit weight (voids ratio e ). γd(max) → Dry unit in densest condition (voids ratio emin )
The Unified Soils Classification System:
GROUP SYMBOLS TYPICAL NAME 1 3 4 GW
Well-graded gravels & gravel- sand mixtures, little or no fines GP
Poorly-graded gravels & gravel- sand mixtures, little or no fines GM Sitly gravels, gravel-sand-siltmixtures
GC Clayey gravels, gravel-sand-claymixtures
SW
Well-graded sands & gravelly sands, little or no fines SP
Poorly-graded sands & gravelly sands, little or no fines SM Sitly sand, sand-silt mixutres
SC Clayey sands, sand-clay mixtures
GROUP SYMBOLS TYPICAL NAME 1 3 4
Dry strength (crushing characteristic s)
Dilatancy (reaction to shaking)
Toughness (consistency near PL) ML
Inorganic silts, very fine sands, rock flour, silty or clayey fine sands
None to slight
Quick to Slow None
CL
Inorganic clays of low to medium plasticity gavely clays, sandy clays, silty clays, lean clays
Medium to high
None to very slow Medium
OL Organic silts & organic silty claysof low plasticity Slight tomedium Slow Slight
MH
Inorganic silts, micaceous or diatomaceous fine sands or silts, elastic silys
Slight to medium
Slow to none
Slight to medium
CH Inorganic clays of high plasticity,fat clays very highHigh to None High
OH Organic clays of medium to highplasticity, organic silts Medium tohigh very slowNone to Slight tomedium
PT Peat & other highly organic soils
MAJOR DIVISIONS
FIELD IDENTIFICATION PROCEDURES (excluding particles larger than 75mm & basing fractions on estimated weights)
Coarse
Grained
Soils
M ore
than
half of material is
larger than
No. 200
(
μm) sieve
size
SANDS
More
than
half of Coarse
Fraction
is
smaller than
No. 4
Sieve
Size
(4.75mm)
2 5
Wide range in grain sizes & substantial amounts of all intermediate particle sizes Predominantly one size or a range of sizes with some intermediate sizes missing Nonplastic fines or fines with low plasticity (for identification procedures see ML below) Plastic fines (for indentification procedures see CL below) Wide range in grain sizes & substantial amounts of all intermediate particle sizes Predominantly one size or a range of sizes with some intermediate sizes missing
Sands
with
fines
(appreciableamount of fines)
Clean
sands
(little
or no
fines)
5
FIELD IDENTIFICATION PROCEDURES (excluding particles larger than 75mm & basing fractions on estimated weights)
Identification Procedures on fractions smaller than No. 40 sieve size
Readily identified by color, odor, spongy feel, & frequently by fibrous texture
Nonplastic fines or fines with low plasticity (for identification procedures see ML below) Plastic fines (for indentification procedures see CL below)
Silt and
Clays
Liquid
Limit Less
than
50
Silt and
Clays
Liquid
limit greater than
50
Highly Organic Soils
Fine
Grained
Soils
M ore
than
half of material is
smaller than
No. 200
(
μm) sieve
size
MAJOR DIVISIONS
2
Clean
Gravels
(little
or no
fines)
Gravels with
fines
(appreciableamount of fines)
GRAVELS
More
than
half of gravel fraction
is
larger than
No. 4
Sieve
Size
(4.75mm)
For visual classifications, 5mmmay
be
used
as
equivalent to
the
No. 4
sieve
size
The AASHTO Classification table :
General Highly Classification Organic
A- Group Classiification A-1-a^ A-1-b^ A-3^ A-2-4^ A-2-5^ A-2-6^ A-2-7^ A-4^ A-5^ A-6^ A-7-5^ A- A-7- Sieve Analysis percent passing: #10 <^50 #40 < 30 < 50 < 51 #200 < 15 < 25 < 10 < 35 < 35 < 35 < 35 > 36 > 36 > 36 > 36
Characteristics of fraction passing #40: b Liquid limit < 40 > 41 < 40 > 41 < 40 > 41 < 40 > 41 Plasticity index (^) NP a^ < 10 < 10 > 11 > 11 < 10 < 10 > 11 > 11
Usual types of Fine Peat or significant sand muck constituent materials
General as Unstable subgrade
passing No. 200 sieve)
sand
Silty or clayey gravel and sand
Granular Materials (35 % or less passing No. 200 sieve) c
A-1 A-
< 6
Silty soils Clayey soils
Fair to Poor
Stone fragments; gravel and
Excellent to good
Silty-Clay Materials (more than 35 %
Types of grain-size distribution curves:
Correlation between the symbols of USCS and AASHTO
USCS AASHTO
Most Probable Possible
GW A-1-a
GP A-1-a A-1-b
GM A-1-b, A-2-
A-2-5, A-2-
A-2-
GC A-2-6, A-2-7 A-2-4, A-
SW A-1-b A-1-a
SP A-3, A-1-b A-1-a
SM A-1-b, A-2-
A-2-5, A-2-
A-2-6, A-4, A-
SC A-2-6, A-2-7 A-2-4, A-
A-4, A-7-
ML A-4, A-5 A-6, A-7-
CL A-6, A-7-6 A-
OL A-4, A-5 A-6, A-7-5, A-7-
MH A-7-5, A-
CH A-7-6 A-7-
OH A-7-5, A-
Pt A-
*Classify–02: Coefficients of uniformity and curvature of granular soils.
(Revision: Sept.-08) Determine the uniformity coefficient Cu and the coefficient of gradation Cc for soil A.
Solution: From the grain distribution curve, D 60 = 1.4 mm, D 30 = 0.95 mm and D 10 = 0.50 mm, therefore the coefficients are,
( ) ( )( )
2 2 60 30 10 60 10
U C
D mm D
C and C
D mm D D
A uniform soil has a coefficient of uniformity Cu less than 4, whereas a well-graded soil has a uniformity coefficient greater than 4 for gravels and greater than 6 for sands. Since soil A has a low value of 2.8, and it is sand, this corresponds to a poorly-graded sand (SP). Steep curves are uniform soils (low Cu ) whereas diagonal curves are well-graded soils (high Cu ). Smooth curved soils have coefficients of curvature Cc between 1 and 3, whereas irregular curves have higher or lower values. Soils that are missing a type of soil (a gap) are called gap-graded ( Cc will be less than 1 or greater than 3 for gap-graded soils). Therefore, this soil is classified as poorly-graded sand (or SP).
*Classify-03: Classify two soils using the USCS.
(Revision: Aug.-09) Use the grain-size distribution curve shown below to classify soils A and B using the USCS. Soil B’s Atterberg limits are LL = 48% and PL = 44%?
Solution:
Classify Soil A: For soil A, the distribution is G = 2%, S = 98%, M = 0% and C = 0%.
60 10
U
D mm C D mm
= = = , therefore, soil A is a poorly graded sand (SP).
Classify Soil B: For soil B, the distribution is G = 0%, S = 61%, M = 35% and C = 4%.
60 10
U 0.
D mm C D mm
= = = , therefore, soil A is very well graded silty sand (SM).
A third method to classify soil B is the combination of the plasticity chart and the flow chart, as follows:
*Classify-04: Manufacturing a “new” soil from two unacceptable soils.
(Revision: Aug.-09)
Sieve No. Grain^ Percent Finer Size (^) Soil A Soil B 4 4.750 98 100 10 2.000 92 100 20 0.850 24 100 40 0.425 7 99 60 0.250 1 94 100 0.150 0 85 200 0.075 0 67 0.030 0 48 0.020 0 41 0.010 0 27 0.006 0 19 0.002 0 12
Hydrometer Analysis
A site has an unsuitable in-situ soil A that does not
compact properly. In lieu of removing that soil A, you
have decided to improve it by mixing it with a borrow pit
soil B to produce an improved new soil C that will
compact better.
You desire a coefficient of uniformity Cu of about 100 for
the new soil C. Determine the relative percentages of
these two uniform soils A and B so that they will result in
better graded soil C. Plot your results.
The plots of soils A and B are as shown below,
*Classify–05: Use Casagrande A-chart to classify a soil via USCS.
(Revision: Aug.-09) Laboratory tests were performed on a light-brown sandy soil; the fines had a plastic limit PL = 27 % and the liquid limit LL = 35%. Classify this soil using USCS.
Sieve No. % Passing 4 98. 40 46. 200 30.
Solution: Note that the 30.0% that passed the #200 sieve are fine-grained and the remaining 70.0% are sands and gravels, but mostly sand since there is only 2% of gravels. Since IP = PI = LL - PL = 35%–27% = 8%, the A -chart indicates that the fines portion of the soil is an inorganic silt of medium plasticity. This soil could be classified as medium plasticity silty sand (SM).
*Classify–06: Identify a very highly plastic soil.
(Revision: Aug.-09)
What type of soil would have a LL = 150% and a PI = 80%.
Solution:
Determine the value of the plasticity index PI from the A-line equation, using LL = 150%, PI = 0.73( LL – 20) = (0.73) (150 – 20) ~ 95. Checking, the plasticity index PI from the U-line equation, using LL = 150%, PI = 0.90( LL – 8) = (0.90) (150 – 8) ~ 128.
Obviously, a PI = 80% and a LL = 150% falls below the A-line, and therefore the soil is a very highly plastic silt (MH). Some research indicates that this soil could also be an organic clay.
*Classify–08: Classify a soil comparing USCS with AASHTO.
(Revision: Sept.-08) Given the grain-size chart shown below, classify this soil using both USCS and AASHTO, if LL for soil = 23.2% and PL=15.7%.
Solution:
Determining the coefficients of uniformity and curvature for this soil.
Coefficient of uniformity Æ = = = 17.
Coefficient of curvature Æ = = 0.
Based on these values the soil appears to be a well graded sandy-silt, since the coefficient of
uniformity is very high (well above 6 for sands), and also, the coefficient of curvature is
almost 1 (between 1 and 3, the soil is well graded). By method of coefficients of curvature
and uniformity of the soil, it appears to be an SW as per USCS.
Classify this soil using both USCS and AASHTO.
Soil Composition is as follows: Gravel 0.0% Sand 56.8% Silt 36.9% Clay 6.3%
Using the ‘Unified Soil Classification System’ developed by Arthur Casagrande in 1948:
1. Since less than 50% pass through the #200 Sieve, the soil is ‘Coarse Grained’: **= 100 - = 100 – 43.2 = 56.8 50
- The soil is not a peat.
- = 100 - = 100 – 100 = 0 --** Æ = 0 0.5 (We now refer to Table 3.3, lecture 12
for the soil symbol).
4. Per USCS, the percentage passing the #200 sieve is more than 12 percent, and the and Atterberg limits fall in the hatched area marked CL-ML on in Figure 3.3. So, per USCS: (Well graded) SM****.
For the AASHTO soil classification we proceed as follows:
Soil is coarse grained: Soil passing #10 Sieve: 80.0 percent Soil passing #40 Sieve: 75.0 % Soil passing #200 Sieve: 43.2 %
Using the AASHTO Formula, since more than 35% passes #200 sieve:
From where GI = 0.
Flow chart we determine that the soil is an A-4 (0.359).
For the clay in the soil from the Activity formula we obtain: = 0.079/0.063 = 1.
Based on the Plasticity index: LL - PL = 0.232 – 0.157 = 0.
The clay appears to be an Illite or Horton clay.
*Classify–10: Classify five soils using both USCS and AASHTO.
(Revision: Aug.-09) Using the grain size distribution curves shown on the next page and the LL+PI shown below, classify each soil.
Solution:
Soil # 1: LL = 19, PI = 5; the curve shows G = 0%, S = 45%, M = 40% and C = 15%. USCS Classification: sandy low plasticity silt (ML). Soil passing #10 sieve = 93%; #40 sieve = 86%; #200 sieve = 55%; AASHTO Classification: A-4 (non-plastic silty soil).
Soil # 2: LL = 24, PI = 0; the curve shows G = 4%, S = 83%, M and C = 13%. USCS Classification: well graded sand (SW). Soil passing #10 sieve = 88%; #40 sieve = 38%; #200 sieve = 13%; AASHTO Classification: A-1-b (coarse sand).
Soil # 3: LL = 30, PI = 13; the curve shows G = 0%, S = 4%, M = 22% and C = 74%. USCS Classification: low plasticity clay (CL). Soil passing #10 sieve = 100%; #40 sieve = 100%; #200 sieve = 96%; AASHTO Classification: A-6 (clayey soil).
Soil # 4: LL = 55, PI = 25; the curve shows G = 0%, S = 33%, M = 45% and C = 22%. Activity A = PI / % of clay = 25/22 = 1.13 placed almost on the A-line; USCS Classification: borderline high plasticity clay (CH) (Illite). Soil passing #10 sieve = 97%; #40 sieve = 90%; #200 sieve = 67%; AASHTO Classification: A-7-6 (high plasticity clayey soil).
Soil # 5: LL = 17, PI = 0; the curve shows G = 69%, S = 23%, M and C = 8%. Cu = D 60 / D 10 = 16 mm / 0.15 mm = 107 very well graded; Cc = ( D 30 )^2 / ( D 10 )( D 60 ) = (4.3 mm)^2 / (0.15)(16) = 8 USCS Classification: well and gap-graded sandy gravel (GW). Soil passing #10 sieve = 26%; #40 sieve = 16%; #200 sieve = 8%; AASHTO Classification: A-1-a (stone fragments and gravelly soil).
Notes: The AASHTO soil classification group index GI formula is,
GI = (^) ( F 200 (^) − (^35) ) ⎡⎣ 0.2 + 0.005 (^) ( LL − (^40) ) ⎤⎦+ 0.01( F 200 − (^15) ) (^) ( PI − (^10) )
The GI is reported to the nearest whole integer. For granular soils GI = 0. If GI < 0, set to zero.
For groups A-2-6 and A-2-7 use the partial group index PGI^ =^ 0.01(^ F 200^ −^15 )^ ( PI −^10 )
The higher the PGI integer, the lower the quality of the soil as a subgrade material; the PGI has to be less than 20 for any of the groups between A-4 and A-7 (A-8 = peat).
