Consolidation (soil mechanics)

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Consolidation (also: consolidation ) describes in soil mechanics the delayed compression of the soil as a result of an increase in load . The cause is the hindered flow movement of the pore water squeezed out in the course of compression .

Process of consolidation

Soils are essentially compacted by compressing the pores . If the pores are filled with water, due to the incompressibility of the water, compaction can only be achieved by pressing out pore water. If water movement is hindered due to poor permeability and long drainage paths, the increase in load is initially only absorbed by the pore water - it comes under overpressure. As the pore water drains away, this overpressure is gradually transferred to the grain structure , which then compacts until it can just absorb the increase in load.

This process can be illustrated with a spring system:

Consolidation spring analogy colored.jpg
  1. The container is filled with water, the only opening is closed, the plate is unloaded and the spring is thus relaxed. This corresponds to an unpolluted soil whose pores are filled with water.
  2. A load is applied to the plate, but the opening is still closed. This corresponds to polluted soil, the drainage of which is hindered. The pore water is under excess pressure and takes the load.
  3. The opening is opened so that the water can be slowly pressed out of the container. The load is taken more and more by the spring. This process is known as consolidation . This slow drainage also takes place in the soil and the grain structure begins to take up the load.
  4. After a certain time, so much water was squeezed out of the container that the spring barely bears the load on its own. Analogously, this is the end of the consolidation of a soil. The grain structure bears the load and the pore water is relaxed.

When the load is lowered, the reverse process is carried out. The pore water comes under negative pressure, which means that the surrounding water is sucked into the sample with a delay. This gradually reduces the negative pressure, the grain structure is relieved and loosens up somewhat. This process is also known as negative consolidation .

The duration of the pore water pressure equalization is theoretically infinite, since its rate tends asymptotically to zero. In practice, the period of time after which 98% of the pore water pressure is equalized is defined as the consolidation time. The consolidation coefficient results from the square of the drainage length related to the consolidation time and is a soil constant. According to these specifications, the level of the pore water pressure therefore has no influence on the consolidation time.

Under-, normal- and over-consolidated floors

Since only fine-grained soils are noticeably impeded to flow, the term consolidation can only be used in cohesive soils . A distinction is made between under-, normal- and over-consolidated. In the under-consolidated (UC) case, the grain structure has not yet compacted as much as is necessary to accommodate the increase in load, i.e. This means that more pore water has to be squeezed out. In the normally consolidated (NC) case, the grain structure is compressed to the extent necessary to accommodate the increase in load. In the overconsolidated (OC) case, the grain structure is denser than required for the current load due to a previously greater load. In the last two cases mentioned NC and OC there is no movement of water. On the other hand, the negative consolidation takes place during the transition from OC to NC.

In under- and normally consolidated soils, the shear strength is proportional to the grain structure pressure (and thus to the density ), i.e. H. the angle of friction is constant and the soil is cohesive . Overconsolidated soils initially have a higher shear strength with the same grain structure pressure, which is based on the tendency to loosen up dense grain structures during shear ( dilatancy ). Any pore water that is still present delays this process, as the increase in pore volume associated with the loosening causes it to come under negative pressure, which stabilizes the grain structure against shear and tensile stresses. Common shear laws describe this situation either by applying the (fictitious) cohesion and a (fictitious and compared to the NC case smaller) angle of friction or by applying a (also fictitious and compared to the NC case larger) angle of friction. With increasing shear, the tendency to loosening is lost and the OC shear strength returns to the value at NC.

For normal consolidated soil, the maximum stress is equal to the current stress. An overconsolidated floor has experienced greater tensions in the past than it currently does. B. the load from melted inland ice. OCR (over consolidation ratio) represents the extent of over consolidation . It is the ratio between the maximum vertical stress in the past and the current vertical stress. For consolidated soils, OCR = 1. Often the maximum stress is unknown in the past; But there are laboratory tests by Casagrande that compare the compression behavior of normal and over-consolidated soils. Overconsolidation has a major impact on the shear strength of cohesive soils. Due to the pre-load, non-cohesive soils acquire cohesion in addition to the frictional forces. In nature, overconsolidation often creates a hard crust in which the shear strength is greater than in the normally consolidated soil below the crust. Overconsolidated soils are usually also stiffer and less settled . The earth pressure at rest, i.e. the ratio of horizontal stresses to vertical stresses, is also dependent on OCR. This has far-reaching consequences for the determination of earth pressure and the dimensioning of structures.

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Wiktionary: Consolidation  - explanations of meanings, word origins, synonyms, translations