Consolidation (soil)

Consolidation (also: consolidation) referred to in soil mechanics, the time-delayed compression of the soil due to a load increase. Cause is disabled flowage of the squeezed during the compression pore water.

Process of consolidation

The mechanical compaction is performed by essentially crushing the pores. If the pores are filled with water, compaction can be achieved only by Extraction of pore water due to the incompressibility of water. The water movement impeded due to low permeability and long drainage paths, the load increase is initially absorbed by the pore water only - it comes under pressure. With drainage of pore water this pressure is gradually transferred to the soil skeleton, which then as far thickens until it can absorb the load increase straight.

This process can be illustrated with a spring system:

For load reduction of the reverse process is run. The pore water gets under vacuum through which surrounding water is delayed sucked into the sample. Thus, the negative pressure gradually goes back, the grain structure is relieved and loosens something up. This process is also referred to as a negative consolidation.

The duration of the pore water pressure equalization is theoretically infinite because its rate asymptotically approaches zero. Practically you put as the consolidation firm that period of time, are balanced after just 98 % of the pore water pressure. The Konsolidierungsbeiwert arises from the square of the drainage length based on the consolidation time and soil is a constant. According to these definitions, the amount of pore water pressure therefore has no influence on the consolidation time.

Under-, normal - and over- consolidated soils

Since only in fine-grained soils a remarkable flow obstruction of the pore water occurs, the concept of consolidation is applicable only in cohesive soils. These are divided into under -, normal - and consolidated. In sub-consolidated (UC ) case, the granular structure has not so far compressed as needed to accommodate the increase in load, that is, it still needs further pore water are pressed. Normally consolidated (NC) case, the granular structure is compressed just as much as to accommodate the load increase required. In überkonsoliderten (OC ) case, the granular structure is denser by a formerly greater burden than for the actual load required. In the latter two cases, the NC and OC so no water movement takes place. In the transition from OC after NC, however, the negative consolidation expires.

Less than normal and the shear strength of consolidated soil is proportional to the grain structure pressure (and thus density ), i.e., the angle of friction is constant and the bottom cohesionless. About Consolidated soils with the same soil skeleton pressure initially a higher shear strength, which is based on the tendency for loosening tight grain skeletons in shear ( dilatancy ). Still present pore water slows down this process, as it comes through the loosening associated with the enlargement of the pore volume under reduced pressure, stabilizes the soil skeleton to shear and tensile stresses. Common shear laws describe it either approach the ( fictional) cohesion and a (fictional and compared to the NC case smaller ) angle of friction or by listing an ( equally fictitious and compared to the NC case larger ) friction angle. With increasing shear, the tendency to loosen up is lost and the OC- shear strength to the value at NC back.

In a normal floor konsoliderten the maximum voltage equal to the present voltage. An overconsolidated soil has experienced in the past, higher voltages than currently available, such as the applied load from abgeschmolzenem ice. OCR (over- consolidation ratio) represents the extent of the consolidation dar. 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 in the past is unknown; there are laboratory tests of Casa Grande, compare the compression behavior normal and overconsolidated soils. The Überkonsolidation has great influence on the shear strength of cohesive soils. Due to pre obtain non-cohesive soils, in addition to the frictional forces cohesion. In nature produced by the Überkonsolidation often a hard crust in which the shear strength is greater than in the normally consolidated ground beneath the crust. About Consolidated soils are also stiffer and implementation poorer in general. The Erdruhedruckbeiwert, ie the ratio of vertical stress on horizontal stresses is, moreover, also depends on OCR. This has far -reaching consequences for the earth pressure and the design of structures.