Test: Secondary Consolidation & Three Dimensional Consolidation - Civil Engineering (CE) MCQ

The secondary consolidation occurs at a very slow rate. Even after the excess pore pressure due to consolidation is dissipated, change in void ratio continues and this phenomenon is known as secondary consolidation.

During the secondary consolidation, there is relative plastic readjustment of soil particles and of the absorbed water due to the continued stress and progressive fracture of some soil particles.

Since the dissipation of the excess hydrostatic pressure does not govern the secondary consolidation, the Terzaghi’s theory of consolidation is in valid as it cannot be used to find the rate of secondary consolidation.

It is often assumed that the secondary compression proceeds linearly with the logarithm of time. It appears as a straight line in the graph in the plot of voids ratio as a function of logarithm of time.

The secondary consolidation is represented by the equation of,

Where α = coefficient representing the rate of secondary compression
t₁ and t₂ = total elapsed time since load was applied to the soil.

Secondary consolidation is also known as secondary compression. During the secondary compression, some of the highly viscous fluids between the point of contact, is forced out from between the grains.

In many soil deposits, the secondary consolidation is so small and insignificant that it can be neglected. Also for the secondary consolidation to occur, it takes a lot of time.

Secondary consolidation starts in the range of primary consolidation itself, although ots magnitude is very small. This is one of the reason why the experimental curve is in agreement with the Terzaghi’s theory up to U = 60%.

For highly organic soils, highly micaceous soils and some loosely deposited clays, secondary consolidation may constitute a substantial percentage of total settlement.

For a soil at a given void ratio, the coefficient of compressibility increases with increase in magnitude of consolidating pressure, and hence it should be determined in the laboratory consolidation test for the range of pressure that would be encountered in practice.