foundation system of burj al arab

Electric Double Layer Theory and Dry Density of Soil (Part-2)

Dear reader in last post we have explained water content-dry density relation of compacted soil with lubrication phenomenon. In this post we are discussing this according to Lambe. He tried to co-relate this relation in the light of to elastic double layer theory.


Dear reader let consider a cohesive soil deposit to be compacted. In cohesive soil attraction force between adjacent soil particles is defined as van der waals’ force. Dear reader we have already learnt about adsorbed water which produces repulsive forces.

Double layer of liquid in contact with a (–)ve charged solid

When double layers of this water come close to each other the repulsive force is generated. Repulsive force between two layers has direct relation with size of double layers; but attractive force remains identical at the same magnitude.


If the resultant of these two forces is attractive, a flocculated structure is found. But if resultant is repulsive, our purpose of reducing resistance of movement is achieved i.e. particles will have tendency to move away i.e. dispersed.


When water content is low, double layer producing repulsive force do not developed completely; thus attraction force governs over repulsion. Thus more compaction effort is required to reach a certain degree of compaction and the consequence is low dry density of compacted soil.


With the increase in water content, double layer is expanded to increase repulsive forces. Now the particles can easily slide past one another reducing compacting effort and becomes packed easily and more closely. The consequence is high dry density.


The double layer’s expansion is not infinite. At optimum water content (optimum moisture content, OMC), this expansion process is completed and we can notice from compaction curve that maximum dry density reached at this water content. Beyond OMC, water fills the space between soil grains increasing void in soil mass thus reducing dry density.

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