Page 252 - Civil Engineering Formulas

186 CHAPTER EIGHT

INDEX PARAMETERS FOR SOILS

Index parameters of cohesive soils include liquid limit, plastic limit, shrinkage
limit, and activity. Such parameters are useful for classifying cohesive soils and
providing correlations with engineering soil properties.
The liquid limit of cohesive soils represents a near-liquid state, that is, an
2
2
undrained shear strength about 0.01 lb/ft (0.0488 kg/m ). The water content at
which the soil ceases to exhibit plastic behavior is termed the plastic limit. The
shrinkage limit represents the water content at which no further volume change
occurs with a reduction in water content. The most useful classification and cor-
relation parameters are the plasticity index I , the liquidity index I , the shrinkage
l
p
index I , and the activity A . These parameters are defined in Table 8.1.
c
s
Relative density D of cohesionless soils may be expressed in terms of void
r
ratio e or unit dry weight :
d
D r e max e 0 (8.4)
e max e min
D r 1/ min 1/ d (8.5)
1/ min 1/ max
D provides cohesionless soil property and parameter correlations, including
r
friction angle, permeability, compressibility, small-strain shear modulus, cyclic
shear strength, and so on.
RELATIONSHIP OF WEIGHTS AND
VOLUMES IN SOILS

The unit weight of soil varies, depending on the amount of water contained in
the soil. Three unit weights are in general use: the saturated unit weight , the
sat
dry unit weight , and the buoyant unit weight :
dry
b
TABLE 8.1 Soil Indices
Index Definition* Correlation
Plasticity I p W W p Strength, compressibility, compactibility, and so forth
l
W n Wp
Liquidity I l Compressibility and stress rate
I p
Shrinkage I s W W s Shrinkage potential
p
Ip
Activity A c Swell potential, and so forth

*W l liquid limit; W p plastic limit; W n moisture content, %; W s shrinkage limit; percent
of soil finer than 0.002 mm (clay size).