What is Consolidation of Soil?

Consolidation of Soil

Consolidation is the process by which soil particles are packed more closely together under the action of static load. it is a natural process and its take a long time. It is a two-stage process.

Types of Consolidation

1. Primary Consolidation
2. Secondary Consolidation

1. Primary Consolidation - it is an instantaneous process of reducing the volume of voids due to the expulsion of air voids when the degree of saturation is S <1.

2. Secondary Consolidation - It is a time-dependent process of reducing the volume f voids due to plastic readjustment of soil solids.

To understand the process of consolidation let us assume we have a ground which is fully saturated that means water is present at the top surface, middle and bottom. Now we build a building over the ground.

1. When t = 0

Now there are additional stresses on the soil. Before building the structure there is only effective stress in the soil. Due to these additional stresses, there is the development of pore water pressure which is equal to additional stresses. The total building load is taken by only water, soil particle will not take any load. At this condition, effective stress is zero.

effective stress = total stress - pore water pressure

Where

Δσ = stress
Δu = pore water pressure
Δσ̅ = effective stress
t  =  time

2.When 0 > t < ∞

As time passes the water try to drain out, very first it will drain out from top surface then bottom and at last from the middle. Now there is no water to generate pore water pressure. As the water draining out pore water pressure get reduce. As shown in the graph at the top and bottom pore water pressure is zero but in the middle, there is some pore water pressure. At this condition, effective stress at top and bottom is more but in the middle, it is less as shown in the graph.

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3.When t > ∞

In actual noting is greater than infinite but it is just only to show. At this condition water present in the middle layer completely drained out, now the primary consolidation is complete. Now soil particles start taking the load. At this condition pore water pressure is zero because there is no water at the top, bottom and in the middle layer. At this condition, effective stress is equal to total stress that means the total load is taken by only soil particles. Now the process of secondary consolidation starts.