FCE 311 GEOTECHNICAL ENGINEERING

6. CONSISTENCY AND PLASTICITY

Department of Civil & Construction Engineering

University of Nairobi

6.1 State of Consistency

• The term consistency refers to the relative ease with which a soil mass can be deformed and is used to describe the degree of firmness of fine-grained soils for which consistency relates to a large extent to water content.

6.1 State of Consistency – cont’d• The four states of consistency suggested

by Atterberg are ;

18

Liquid Limit, LL

Liquid State

Plastic Limit, PL

Plastic State

Shrinkage Limit, SL

Semisolid State

Solid StateDry Soil

Fluid soil-water mixture

Incr

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ng w

ater

con

tent

6.1 State of Consistency – cont’d• The four states of consistency in a chart

form;

6.1 State of Consistency – cont’d• The water contents, which arbitrarily

define the boundary between the four states of consistency, are referred to as consistency limits or Atterbag limits.

• The three consistency limits are Liquid Limit, Plastic Limit and Shrinkage Limit.

• Liquid limit is denoted by WL and is the boundary between plastic and liquid states of consistency. It is the minimum water content at which the soil mass still flows like a liquid.

6.1 State of Consistency – cont’d• Liquid limit is defined as the water

context at which a groove, cut with a standard grooving tool, in soil pat taken in the cup of a standard liquid limit device closes for a distance of 13mm when the cup is imparted 25 blows.

6.1 State of Consistency – cont’d• The standard liquid limit device

6.1 State of Consistency – cont’d• Plastic limit is denoted by WP and is the

boundary between semi-solid and plastic states of consistency. It is the minimum water content at which the soil mass can still be deformed without cracking.

• Plastic limit is defined as the water content at which the soil mass can be rolled into a threat of 3mm diameter and the thread first shows signs of cracking.

6.1 State of Consistency – cont’d• Shrinkage limit is denoted by WS and is

the boundary between solid and semi-solid state of consistency. It is defined as the maximum water content at which there is no reduction in volume of soil mass accompanying reduction in water content.

• The laboratory determination of these states is discussed in Chapter 7.