Consolidation (geology)

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Consolidation is a process by which soils decrease in volume. It occurs when stress is applied to a soil that causes the soil particles to pack together more tightly, therefore reducing volume. When this occurs in a soil that is saturated with water, water will be squeezed out of the soil. The magnitude of consolidation can be predicted by many different methods. In the Classical Method, developed by Karl Terzaghi, soils are tested with an oedometer test to determine their compression index. This can be used to predict the amount of consolidation.

When stress is removed from a consolidated soil, the soil will rebound, regaining some of the volume it had lost in the consolidation process. If the stress is reapplied, the soil will consolidate again along a recompression curve, defined by the recompression index. The soil which had its load removed is considered to be overconsolidated. This is the case for soils which have previously had glaciers on them. The highest stress that it has been subjected to is termed the preconsolidation stress. A soil which is currently experiencing its highest stress is said to be normally consolidated.

The process of consolidation is different from secondary compression, compaction, and other processes of volume reduction.

Contents

[edit] Consolidation analysis

[edit] Classical Method

This methods assumes consolidation occurs in only one-dimension. Laboratory data is used to construct a plot of strain or void ratio verses effective stress where the effective stress axis is on a logarithmic scale. The plot's slope is the compression index or recompession index. The equation for consolidation settlement of a normally consolidated soil can then be determined to be:

\delta_c = \frac{ C_c }{ 1 + e_0 } H \log \left( \frac{ \sigma_{zf}' }{ \sigma_{z0}' } \right) \

where

δc is the settlement due to consolidation.
Cc is the compression index.
e0 is the initial void ratio.
H is the height of the soil.
σzf is the final vertical stress.
σz0 is the initial vertical stress.

Cc can be replaced by Cr (the recompession index) for use in overconsolidated soils where the final effective stress is less than the preconsolidation stress. When the final effective stress is greater than the preconsolidation stress, the two equations must be used in combination to model both the recompession portion and the virgin compression portion of the consolidation process.

[edit] Time dependency

The time for consolidation to occur can be predicted. Sometimes consolidation can take years. This is especially true in saturated clays because their hydraulic conductivity is extremely low, and this causes the water to take an exceptionally long time to drain out of the soil. While drainage is occurring, the pore water pressure is greater than normal because it is carrying part of the applied stress (as opposed to the soil particles).

[edit] See also

[edit] References

Coduto, Donald (2001), Foundation Design, Prentice-Hall, ISBN 0-13-589706-8

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