Consolidation (soil)

For compaction near the surface, see Soil compaction; for natural compaction on a geologic scale, see Compaction (geology). Soil compaction occurs when weight of Livestock or heavy machinery compresses Soil, causing it to lose Pore space Compaction (geology refers to the process by which a Sediment progressively loses its Porosity due to the effects of loading

Consolidation is a process by which soils decrease in volume. Soil, often typeset as SOiL, is a four piece rock band from Chicago Illinois United States founded by Shaun Glass Tom Schofield Tim King and Adam Zadel It occurs when stress is applied to a soil that causes the soil particles to pack together more tightly, therefore reducing its bulk volume. Stress is a measure of the average amount of Force exerted per unit Area. 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 von Terzaghi, soils are tested with an oedometer test to determine their compression index. Karl von Terzaghi ( October 2, 1883 &ndash October 25, 1963) was an Austrian Civil engineer and Geologist Geotechnical investigations are performed by geotechnical engineers or engineering geologists to obtain information on the physical properties of soil and rock around a site to design 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. "Glacial" and "Glaciation" redirect here For the geological periods see Glacial period. The highest stress that it has been subjected to is termed the preconsolidation stress. The over consolidation ratio or OCR is defined as the highest stress experienced divided by the current stress. A soil which is currently experiencing its highest stress is said to be normally consolidated and to have an OCR of one. A soil could be considered underconsolidated immediately after a new load is applied but before the excess pore water pressure has had time to dissipate. Pore water pressure refers to the pressure of Groundwater held within a Soil or rock, in gaps between particles ( Pores)

1 Consolidation analysis
2 Time dependency
3 See also
4 References

Consolidation analysis

Spring analogy

The process of consolidation is often explained with an idealized system composed of a spring, a container with a hole in its cover, and water. A spring is a flexible elastic object used to store mechanical Energy. In this system, the spring represents the compressibility or the structure itself of the soil, and the water which fills the container represents the pore water in the soil.

The container is completely filled with water, and the hole is closed. (Fully saturated soil)
A load is applied onto the cover, while the hole is still unopened. At this stage, only the water resists the applied load. (Development of excessive pore water pressure)
As soon as the hole is opened, water starts to drain out through the hole and the spring shortens. (Drainage of excessive pore water)
After some time, the drainage of water no longer occurs. Now, the spring alone resists the applied load. (Full dissipation of excessive pore water pressure. End of consolidation)

Primary Consolidation

This method 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. Karl von Terzaghi first proposed the relationship for effective stress in 1936 Definition and base Logarithmic scales are either defined for ratios of the underlying quantity or one has to agree to measure 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

δ_c is the settlement due to consolidation.

C_c is the compression index.

e₀ is the initial void ratio.

H is the height of the soil.

σ_zf is the final vertical stress.

σ_z0 is the initial vertical stress.

C_c can be replaced by C_r (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 recompression portion and the virgin compression portion of the consolidation process, as follows:

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

where σ_zc is the preconsolidation stress of the soil.

Secondary consolidation

Secondary consolidation is the compression of soil that takes place after primary consolidation. Secondary consolidation is caused by creep, viscous behavior of the clay-water system, compression of organic matter, and other processes. Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses It occurs as a result of long term exposure to levels of In sand, settlement caused by secondary compression is negligible, but in peat, it is very significant. Sand is a naturally occurring Granular material composed of finely divided rock and Mineral particles Peat is an accumulation of partially Decayed Vegetation matter.

Secondary consolidation is given by the formula

$S_s=\frac{H_0}{1+e_0} C_{a} \log \left( \frac {t} {t_{90} } \right) \$

Where H₀ is the height of the consolidating medium
e₀ is the initial void ratio
C_a is the secondary compression index

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. Hydraulic conductivity, symbolically represented as K is a property of vascular plants soil or rock that describes the ease with which water can move through pore spaces 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). Pore water pressure refers to the pressure of Groundwater held within a Soil or rock, in gaps between particles ( Pores)

References

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

(Korean) Kim, Myung-mo (2000), written at Seoul, Soil Mechanics (4 ed. Soil mechanics is a discipline that applies principles of Engineering mechanics, e Seoul ( soʊl is the Capital and largest City of South Korea. ), Munundang, ISBN 89-7393-053-2

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