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.
Theliquid limitof cohesive soils represents a near-liquid state, that is, an
undrained shear strength about 0.01 lb/ft^2 (0.0488 kg/m^2 ). 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 Ip, the liquidity index Il, the shrinkage
indexIs, and the activity Ac. These parameters are defined in Table 8.1.
Relative density Drof cohesionless soils may be expressed in terms of void
ratioeor unit dry weight d:

(8.4)

(8.5)

Drprovides cohesionless soil property and parameter correlations, including
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 ofwater contained in
the soil. Three unit weights are in general use: the saturated unit weight sat, the
dry unit weight dry, and the buoyant unit weight b:

Dr

1/min1/d

1/min1/max

Dr

emaxe 0

emaxemin

TABLE 8.1 Soil Indices

Index Definition* Correlation

Plasticity IpWlWp Strength, compressibility, compactibility, and so forth

Liquidity Compressibility and stress rate

Shrinkage IsWpWs Shrinkage potential

Activity Swell potential, and so forth

*Wlliquid limit; Wpplastic limit; Wnmoisture content, %; Wsshrinkage limit; percent
of soil finer than 0.002 mm (clay size).

Ac

Ip



Il

WnWp

Ip