SOIL PHASE RELATIONSHIPS, INDEX PROPERTIES AND CLASSIFICATION
Case 4: When the soil is submerged
3.20 UNIFIE D SOI L CLASSIFICATION SYSTEM (USCS)
Soil Phas e Relationships, Inde x Propertie s an d Soil Classification 73 70
60
50
30
20
10
0
Note: A 3;-2 so
5%fi
t ilsco nertntain
lan > less Jo.2(than
)0sk
A-6 and A-2-6
^-4 a id A-2-4
/ A-
/
1
7-6
/
\-5i /
•>
/
/ j /
A-7-5 and A-2-7
ndA-2-5
/ /
/ /
/
0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 10 0
Liquid limit w,
Figure 3.21 Char t fo r us e in AASHTO soil classification syste m
Table 3.17 Th e Unified Soi l Classificatio n Syste m (Source : Bowles, 1992 )
Major divisions
Coarse-grained soils (more than half of material is larger than No. 200)Fine-grained soils f of material is smaller than No. 200)(more than ha
s ;oars
e fraction 4 sieve size)Grave (more than half of is larger than No.Sands an half of coarse fraction er than No. 4 sieve size)(more th is smal Gravels with fines Clean gravels (appreciable (little or no amount of fines) fines)
Sands with fines Clean sands (appreciable (little or no amount of fines) fines)
Silts and clays (liquid limit < 50)ills and clays
c/o i/iA
6
T33 2"
£ o
Group symbol GW GP
GM
GC du
SW SP
SM
SC u
ML
CL
OL
MH
CH OH Pt
Typical name s Well-graded gravels, gravel-san d
mixtures, little or no fines Poorly grade d gravels , gravel- sand mixtures, little or no fines Silty gravels, gravel-sand-silt
mixture
Clayey gravels, gravel-sand-clay mixture
Well-graded sands , gravelly sands, littl e or no fines Poorly graded sands , gravelly
sands, littl e or no fines Silty sands, sand-silt mixtur e
Clayey sands, sand-sil t mixture
Inorganic silt s and very fine sands, rock flour, silty or clayey fine sands, o r clayey
silts with slight plasticity Inorganic clays of very low to medium plasticity, gravelly clays, sandy clays, silty clays,
lean clay s Organic silts and organic silty
clays of low plasticity Inorganic silts, micaceous or di-
atomaceous fine sandy or silty soils, elastic silts Inorganic clays or high plasticity,
fat clays
Organic clays of medium to high plasticity, organic silts Peat and other highly organic soil s
Classification criteri a for coarse-grained soil s Cu> 4
1 < Cc < 3
Not meeting all gradation requirements for GW (Cu < 4 or 1 > C, > 3) Atterberg limits Abov e A line with
below A line or 4 < / < 7 are IP < ^ borderlin e cases Atterberg limits symbol s above A line with
/„>?
Cu> 6 1 < Cc < 3
Not meeting all gradation requirements for S W (Cu < 6 or 1 > Cc > 3)
Atterberg limits Abov e A iine with
below A line or 4 < / < 7 are
'p < borderlin e cases
Atterberg limits symbol s above A line with
1 . Determin e percentages o f sand an d gravel from grain-size curve . 2. Dependin g on percentages o f fines
(fraction smalle r than 200 sieve size), coarse-grained soil s are classified as follows:
Less than 5%-GW, GP, SW, SP More tha n 12%-GM , GC, SM, SC 5 to 12%-Borderline cases requiring dual symbol s
c
-=ft
Gravels and sands are GM, GC, SM, or SC
if mor e tha n 1 2 percent passe s th e No . 20 0 sieve ; M = silt; C = clay. Th e sil t o r cla y designation is determined by performing the liquid and plastic limit tests on the (-) No. 40 fraction an d usin g th e plasticit y char t o f Fig . 3.22 . Thi s char t i s als o a Casagrand e contribution t o the USC system , and the A line show n on this chart i s sometimes calle d Casagrande's A line.
Soil Phas e Relationships , Inde x Propertie s and Soil Classificatio n 60
75
3.
4.
50
- 30
20 30 4 0 5 0
Liquid limit w, percent
60 70 80
Figure 3.22 Plasticit y char t fo r fine-graine d soil s
The chart as presented her e has been slightl y modified based o n the Corps o f Engineers findings that no soil has so far been found with coordinates that lie above the "upper limit "
or U line shown. This chart and lines are part of the ASTM D 2487 standard . Gravels and sands are (note using dual symbols)
GW-GC SW-S C GP-G C SP-SC , or GW-GM SW-S M GP-G M SP-S M
if between 5 and 1 2 percent of the material passes the No. 200 sieve. It may be noted that the M o r C designatio n i s derive d fro m performin g plasti c limi t test s an d usin g Casagrande's plasticit y chart .
Fine-grained soil s (more than 50 percent passes th e No. 200 sieve) are:
ML, OL, or CL
if the liquid limits are < 50 percent; M = silt; O = organic soils; C = clay. L = Less than 50 percent for \vt
Fine grained soils are MH, OH, or CH
if the liquid limits are > 50 percent; H = Higher than 50 percent. Whether a soil is a Clay (C), Silt (M), or Organic (O) depends on whether the soil coordinates plo t above or below the A line on Fig. 3.22 .
The organi c (O ) designatio n als o depend s o n visua l appearanc e an d odo r i n th e US C method. In the ASTM metho d th e O designation is more specificall y defined by using a comparison o f the air-dry liquid limit vv/ and the oven-dried w'r If the oven dried value is
0.75w
and the appearance and odor indicates "organic" then classify th e soil as O.
Table 3.1 8 Unifie d Soi l Classificatio n Syste m —fine-grained soil s (mor e than hal f of materia l i s larger than No . 20 0 siev e size )
Soil
andSilt clays
Highly organic soils
Major divisions
Liquid limit less than 50
Liquid limit mor e than 50
Group symbols
ML CL OL MH CH OH
Identification procedures on fraction smalle r tha n No. 40 sieve siz e
Drystrength
None to slight Medium to high Slight to medium Slight to medium High to very hig h Medium to high
Dilatancy
Quick to slow None t o very slo w Slow Slow t o none None None to very slo w
Toughness
None Medium Slight Slight to medium High Slight t o medium Pt Readil y identifie d by color, odor,
spongy fee l and frequently by fibrous textur e
The liquid and plastic limits are performed o n the (-) No . 40 sieve fraction of all of the soils, including gravels, sands , and the fine-grained soils. Plasticity limit tests ar e not required fo r soil s where th e percen t passin g th e No . 20 0 siev e < 5 percent . Th e identificatio n procedure o f fin e grained soil s ar e given in Table 3.18 .
A visua l descriptio n o f th e soi l shoul d accompan y th e lette r classification . Th e AST M standard includes some descriptio n in terms of sandy or gravelly, but color i s also ver y important . Certain area s ar e underlain wit h soi l deposit s having a distinctive colo r (e.g. , Boston blu e clay , Chicago blue clay) which may be red, green , blue , grey, black, and so on. Geotechnical engineer s should becom e familia r wit h th e characteristic s o f thi s materia l s o th e colo r identificatio n is of considerable ai d in augmenting the data base o n the soil.
3.21 COMMENT S O N THE SYSTEMS O F SOIL CLASSIFICATIO N