Foundation, Concrete and Earthquake Engineering

### Electric Double Layer Theory and Dry Density of Soil

In our last post we have learnt how dry density of soil is related to water content in compaction process. Here we will discuss this phenomenon in light of electric double layer theory and soil structure. Lambe in 1958 provided this relation as discussed below.

At first we will discuss about adsorbed water as it is related to this discussion. We know in some soil surface, there have electro-chemical charges which can hold water. This electro-chemically bond water is adsorbed water. As electrical force act on this water, the behavior of adsorbed water is quite different from normal water.

Plasticity of cohesive soil is due to this adsorbed water. Dear reader in our upcoming post we will learn in details about adsorbed water. Here we want to co-relate its influence on changes in dry density.

In this part we simply explain the phenomena like – the attraction forces in layer of adsorbed water are large when water content is low and produce more resistance against movement of soil particles thus wasting compacting effort.

When water content in soil mass in increased, the repulsive forces remain in particles are increased to allow sliding over one another and finally lead to closely packed condition.

We think you are not satisfied with this explanation, to know how do actually repulsive force developed please read next part, will be published consecutively.
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.

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.