SOIL PHASE RELATIONSHIPS, INDEX PROPERTIES AND CLASSIFICATION
Case 4: When the soil is submerged
3.13 DISCUSSIO N ON LIMITS AN D INDICES
Glass plate
Figure 3.16 Determinatio n o f dr y volum e b y mercury displacemen t metho d
Therefore, v v = •
M -xlOO = xlOO (3.42)
or w = -^ --- - xlO O (3.43)
where, p = 1 for all practical purposes .
Soil Phas e Relationships , Inde x Propertie s an d Soil Classificatio n 53
4 6 1 0 2 0 2 5 4 0 6 0 10 0
Log number of blows N
Figure 3.17 Tw o sample s of soils with different flo w indice s
Plasticity Inde x lp
Plasticity index / indicate s th e degree o f plasticity of a soil. The greater th e difference betwee n liquid an d plasti c limits , th e greate r i s th e plasticit y o f th e soil . A cohesionles s soi l ha s zer o plasticity index. Such soils are termed non-plastic . Fat clays are highly plastic an d possess a high plasticity index. Soils possessing larg e values of w, and / ar e said to be highly plastic or fat. Those with low values are described a s slightly plastic or lean. Atterberg classifies the soils according to their plasticity indices a s in Table 3.9 .
A liqui d limi t greate r tha n 10 0 is uncommo n fo r inorgani c clay s o f non-volcani c origin . However, fo r clay s containin g considerabl e quantitie s o f organi c matte r an d clay s o f volcani c origin, the liquid limit may considerably exceed 100 . Bentonite, a material consisting of chemically disintegrated volcanic ash, has a liquid limit ranging from 400 to 600. It contains approximately 70 percent o f scale-like particle s o f colloidal siz e a s compared wit h abou t 30 per cen t for ordinary highly plastic clays. Kaolin and mica powder consist partially or entirely of scale like particles of relatively coars e siz e i n compariso n wit h highl y colloida l particle s i n plasti c clays . The y therefore posses s les s plasticit y tha n ordinary clays. Organi c clay s posses s liqui d limit s greate r than 50 . The plasti c limit s of such soils ar e equally higher. Therefore soil s wit h organic conten t have low plasticity indices correspondin g t o comparatively high liqui d limits.
Table 3.9 Soi l classification s accordin g t o Plasticit y Inde x
Plasticity inde x Plasticity
0
<7 7-17
Non-plastic Low plastic Medium plastic Highly plastic
Toughness Index , lt
The shearing strength of a clay at the plastic limi t is a measure of its toughness. Tw o clays havin g the same plasticity index possess toughness which is inversely proportional t o the flow indices. An approximate numerica l valu e for the toughness can be derived as follows.
Let s l = shearin g strength corresponding t o the liquid limit, wf, whic h is assumed t o be constant for all plastic clays.
s = shearin g strengt h a t th e plasti c limit , whic h ca n b e use d a s a measur e o f toughness of a clay.
Now Wj = -lf logAf , + C, wp = -If logN p + C
where N( an d N ar e the number of blows at the liquid and plastic limits respectively. The flow curve is assumed to be a straight line extending into the plastic range as shown in Fig. 3.17.
Let, N{ = msr N } = ms , wher e m is a constant.
We can write
wl = -I, \ogms [ + C, w - -I,\ogms + C
Therefore l p = wi~wp = If(logmsp-\ogmSl)= I f\og-?- s i
or t= T= g ~ ( 3-44>
Since w e ar e intereste d onl y i n a relativ e measur e o f toughness , lt ca n b e obtaine d fro m Eq. (3.44 ) a s the rati o o f plasticity index an d flo w index . The valu e of I( generall y fall s betwee n 0 and 3 for most clay soils. When It is less than one, the soil is friable at the plastic limit. It is quite a useful inde x to distinguish soils of different physica l properties .
Liquidity Inde x /,
The Atterberg limits are found for remolded soi l samples. These limits as such do not indicate the consistenc y o f undisturbe d soils . Th e inde x tha t i s use d t o indicat e th e consistenc y o f undisturbed soils is called the liquidity index. The liquidity index is expressed a s
7/=^—~ (3.45 )
where, wn is the natural moisture content of the soil in the undisturbed state. The liquidity index of undisturbed soil can vary from les s tha n zero to greater tha n 1 . The valu e of I{ varie s according t o the consistency o f the soil as in Table 3.10.
The liquidity index indicates the state of the soil in the field. If the natural moisture content of the soil is closer to the liquid limit the soil can be considered a s soft, and the soil is stiff if the natural moisture content is closer to the plastic limit. There are some soils whose natural moisture content s are highe r tha n th e liqui d limits . Such soil s generall y belon g t o th e montmorillonit e grou p an d possess a brittle structure. A soil of this type when disturbed by vibration flows like a liquid. The liquidity inde x value s o f suc h soil s ar e greate r tha n unity. On e ha s t o b e cautiou s i n usin g suc h soils fo r foundations of structures.
Soil Phas e Relationships , Inde x Propertie s and Soil Classification 5 5
Table 3.10 Value s of / / and lc accordin g t o consistenc y o f soi l
Consistency / / l c
Semisolid or solid stat e Negativ e > 1 Very stif f stat e (wn = wp) 0 1
Very sof t stat e (wn = wl) 1 0
Liquid stat e (whe n disturbed ) > 1 Negativ e
Consistency Index , /C
The consistency index may be defined as
/ (3.46p )
The index lc reflects the state of the clay soil condition in the field in an undisturbed state just in the same wa y as It describe d earlier . Th e values of / fo r different state s o f consistency ar e given in Table 3.10 along with the values Ir I t may be seen that values of 7, and Ic are opposite to each other for the same consistency of soil.
From Eqs (3.45) and (3.46) we have wl — w
Ii+Ic= j Pp =l (3.47 )
Effect o f Dryin g o n Plasticit y
Drying produces a n invariable change in the colloidal characteristics of the organic matter in a soil.
The distinction between organi c and inorganic soils ca n be made by performing tw o liquid limit tests on the same material . On e test is made on an air-dried sample an d the other on an oven-drie d one. If the liquid limit of the oven-dried sample is less than about 0.75 times that for the air-drie d sample, the soils may be classed a s organic. Oven-drying also lowers the plastic limit s of organic soils, but the drop in plastic limit is less than that for the liquid limit.
Shrinking an d Swellin g o f Soil s
If a mois t cohesiv e soi l i s subjecte d t o drying , i t lose s moistur e an d shrinks . Th e degree o f shrinkage, S , is expressed a s
. , = - x (3.48a )
o
where,
Vo = original volume of a soil sample at saturated state Vd = final volume of the sample at shrinkage limit
On the basis of the degree o f shrinkage, Scheidig (1934) classified soils as in Table 3.11.
Shrinkage Rati o SR
Shrinkage rati o i s define d a s th e rati o o f a volum e chang e expresse d a s a percentag e o f dr y volume to the corresponding change in water content above the shrinkage limit .
Table 3.1 1 Soi l classificatio n accordin g t o degre e o f shrinkag e Sr
Sr% Qualit y o f soi l
< 5 Goo d
5-10 Mediu m goo d
10-15 Poo r
> 1 5 Ver y poo r
(V -V,)/V,
SR=' ° d)l d xlO O (3-48b )
W0~WS
where
Vo = initial volume of a saturated soi l sample at water content wo Vd = the final volum e of the soil sample at shrinkage limit ws
(wo-ws) = change in the water content
Md = mass of dry volume, Vd, of the sample
Substituting for (wo-ws) i n Eq (3.48b) and simplifying, we have
• ; - • - •
Thus the shrinkage ratio of a soil mass i s equal to the mass specific gravity of the soil in its dry state .
Volumetric Shrinkag e Sv
The volumetric shrinkage or volumetric change is defined as the decrease i n volume of a soil mass, expressed as. a percentage of the dry volume of the soil mas s whe n the water conten t i s reduce d from th e initial wo to the final ws at the shrinkage limit.
d
(3.49)
Linear shrinkage ca n be computed fro m the volumetric change b y the following equatio n
1/3
5.. +1.0
LS= l~ c 1 m Xl °° percen t (3-50 )
The volumetri c shrinkag e Sv i s use d a s a decima l quantit y in Eq . (3.50) . Thi s equatio n assumes that the reduction in volume is both linear and uniform in all directions.
Linear shrinkage can be directly determined by a test [this test has not yet been standardized in the United States (Bowles, 1992)]. Th e British Standard BS 1377 used a half-cylinder of mold of diameter 12. 5 mm an d length Lo = 140 mm. The wet sample filled int o the mold is dried an d the final lengt h L,is obtained. Fro m this , the linear shrinkage LS is computed a s
Soil Phas e Relationships, Inde x Propertie s an d Soil Classification 57
LS = L-L.
(3.51) Activity
Skempton (1953) considers that the significant change in the volume of a clay soil during shrinking or swelling is a function o f plasticity index and the quantity of colloidal cla y particles presen t in soil. The clay soil can be classified inactive, normal or active (after Skempton, 1953) . The activity of clay is expressed a s
Activity A = Plasticity index, /
Percent fine r tha n 2 micro n (3.52)
Table 3.12 give s the typ e o f soi l accordin g t o th e valu e of A. The cla y soi l whic h has a n activity value greater than 1.4 can be considered as belonging to the swelling type. The relationship between plasticity index and clay fraction i s shown in Fig. 3.18(a).
Figure 3.18(b ) show s result s o f som e test s obtaine d o n prepare d mixture s o f variou s percentage o f particle s les s than and greate r tha n 2 /^. Severa l natura l soils wer e separate d int o fractions greater and less than 2 /z and then the two fractions were combined as desired. Fig 3.18(c) shows the results obtained on clay minerals mixed with quartz sand.
Table 3.12 Soi l classification accordin g to activit y
A Soi l type
<0.75 Inactiv e
0.75-1.40 Norma l
>1.40 Activ e
Plasticity index, Ip •— K> U> -£>. Lf> O\ D O O O O O O Acti
/
//
VQ soil
// /
/ 1
{ s
A /
Nformal s o
/*
Inactiv
/
11 jr
^/l = (
e soil
•0
.75
10 2 0 3 0 4 0 5 0
Percent finer than 2 micron 60 Figure 3.18(a) Classificatio n o f soi l accordin g t o activit y
58 Chapter 3
1UU
80
X
1
6 0'o1 40 a,
20
° (
/
»&
^o> y/ / y
<*/ y ( 1ShellLoneWeald cla y(().95)haven33)Ion clay
^ (0-95 ) Horten
(0.95)
) 2 0 4 0 6 0 8 0 1 G
500 400 300
200
100
mon
f,
/
Sodium tmorillo
(1.33)
/ /
^ „ — — — ""
aite /
/
.---"KaoHnite
T (A=0.9_)
Clay fraction (< 2//) (%) (b)
20 4 0 6 0 8 0
Clay fractio n ( < 2//)(%) (c)
100
Figure 3.18(b, c) Relatio n between plasticit y inde x an d clay fraction. Figure s in parentheses ar e the activities o f th e clay s (afte r Skempton, 1953 )
Consistency o f Soil s a s per the Unconfine d Compressiv e Strengt h
The consistency of a natural soil is different fro m that of a remolded soi l at the same water content.
Remolding destroys th e structure of the soil and the particle orientation. The liquidity index value which is an indirect measure of consistency is only qualitative. The consistency of undisturbed soil varies quantitativel y o n th e basi s o f it s unconfine d compressiv e strength . Th e unconfme d compressive strength , qu, i s define d a s th e ultimat e loa d pe r uni t cros s sectiona l are a tha t a cylindrical specime n o f soil (with height to diameter ratio of 2 to 2.5) can take under compressio n without an y latera l pressure. Water conten t of the soi l i s assumed t o remain constan t during the duration of the test which generally takes only a few minutes. Table 3.13 indicates the relationship between consistency and qu.
As explaine d earlier , remoldin g o f a n undisturbe d sampl e o f cla y a t th e sam e wate r content alters its consistency, because o f the destruction of its original structure. The degree of disturbance of undisturbed clay sample due to remolding can be expressed a s
Table 3.13 Relationshi p betwee n consistenc y o f clay s an d qu Consistency
Very sof t Soft Medium
qu, kN/m2
<25 25-50 50-100
Consistency Stiff
Very stif f Hard
qu, kN/m2
100-200 200-400
>400
Table 3.14 Soi l classification on the basis of sensitivity (afte r Skempton an d Northey, 1954 )
st
1
1-2 2-4
Nature o f cla y
Insensitive clays Low-sensitive clay s Medium sensitiv e clays
St 4-8 8-16
Nature o f cla y
Sensitive clays Extra-sensitive clay s Quick clay s
Soil Phas e Relationships, Inde x Propertie s an d Soi l Classificatio n 59
Sensitivity, Sr = qu, undisturbed
q'u, remolde d (3.53)
where q'u is the unconfmed compressive strength of remolded cla y at the same water content as that of the undisturbed clay.
When q'u is very low as compared to qu the clay is highly sensitive. When qu = q'u the clay is said to be insensitive to remolding. On the basis of the values of St clays can be classified as in Table 3.14.
The clay s that have sensitivit y greater tha n 8 should be treated wit h care durin g constructio n operations because disturbance tends to transform them, at least temporarily, into viscous fluids. Such clays belong to the montmorillonite group and possess flocculent structure .
Thixotropy
If a remolde d cla y sampl e wit h sensitivity greate r tha n on e i s allowe d t o stan d withou t furthe r disturbance an d chang e i n wate r content , i t may regai n a t least par t o f its origina l strengt h an d stiffness. This increase in strength is due to the gradual reorientation o f the absorbed molecule s of water, and is known as thixotropy (fro m th e Greek thix, meaning 'touch' and tropein, meanin g 'to change'). Th e regainin g o f a par t o f th e strengt h afte r remoldin g ha s importan t application s i n connection wit h pile-driving operations , an d other type s of construction i n which disturbance of natural clay formations is inevitable.