What is the difference between normally-consolidated and over-consolidated soils? What are the effects of over consolidation on the physical properties of clay? What are the causes of over consolidation?

Normally consolidated clays are these that are currently experiencing the maximum vertical overburden effective pressure they have ever experienced in their history.

Over consolidated clays have experienced a higher overburden stress in the past.

𝑂𝐶𝑅 =𝑚𝑎𝑥𝑖𝑚𝑢𝑚 𝑜𝑣𝑒𝑟𝑏𝑢𝑟𝑑𝑒𝑛 𝑒𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑠𝑡𝑟𝑒𝑠𝑠

𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑜𝑣𝑒𝑟𝑏𝑢𝑟𝑑𝑒𝑛 𝑒𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒 𝑠𝑡𝑟𝑒𝑠𝑠

Over consolidation causes an increase in strength, reduction in

permeability and a reduction in settlement. Several causes of over consolidation are: 1- Excavation/corrosion. 2- Glaciers. 3- desiccation of soils 4- Lowering of ground water table.

Consolidation

The decrease in soil volume by the squeezing out of the pore water on account of gradual dissipation of excess hydrostatic pressure induced by an imposed total stress.

Degree of consolidation

The ratio of compression after an elapsed time to the final compression

Theoretical and experimental consolidation curve

There is a fair degree of convergence up to a degree of consolidation of about 80%, but beyond that, the experimental curve indicates continued compression at a slow rate.

Theoretical curve tends to become asymptotic.

This delayed compression is ascribed to secondary compression, which occurs after almost all excess pore water pressure has dissipated.

Assumptions

Compression and flow are one-dimensional (vertical).

Darcy’s law is valid.

The soil is homogeneous.

The soil is completely saturated.

The soil grains and water are both incompressible.

Strains are small; that is, the applied load increment produces

virtually no change in thickness, and 𝑘 and 𝑎𝑣 remain constant.

There is a unique relationship, independent of time, between void

ratio and effective stress, that is,

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Example: A 6-m-thick layer of brown normally consolidated clay is saturated between a layer of rock beneath it and a layer of loose sand above it. The layer undergoes a total settlement of 2.1 cm under an applied uniform surcharge load of 24 kN/m2. What is the coefficient of consolidation, Cv, of the brown clay if 120 days were required for 1.9cm.

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