Consolidation term is generally used in case of cohesive soil. The settlement that occur due to consolidation beneath foundation may continue several years depending on loading intensity, permeability of respective soil and drainage path available for this process.
Definition of consolidation:
when a soil layer having low permeability having certain degree of saturation is loaded to increase stress in soil mass to generate excess pore water pressure within clay layer, the soil being less permeable takes time to dissipate this pressure. Obviously this dissipation will be associated with certain volume change which leads to certain degree of settlement of soil mass. This phenomenon is termed as consolidation and respective settlement is consolidation settlement.
Depending on direction, nature of drainage and deformation, consolidation can be classified as:
a. One dimensional consolidation
b. Two dimensional consolidation
c. Three dimensional consolidation
a. One dimensional consolidation:
In this condition, only vertical drainage and subsequent strain occurred. This consolidation is generally happened below a load of wide extent i.e. there is no way but to drainage in vertical direction as drainage in other two horizontal directions is impossible due to
-long travel distance
-impermeability of clay deposit
Consolidation phenomenon can be happened in other than wide loading area where thickness of layer is small relative to its width.
General examples are:
• Under large fill
• Beneath center of circular or square footing
• Alteration of ground water table i.e. lowering
• Beneath center of mat/raft foundation
Two dimensional consolidations:
Terzaghi provided solution for one dimensional consolidation which is extended next for two dimensional and three dimensional consolidations. Normally two dimensional consideration taken where drainage can be happened in one of the two horizontal direction with as usual in vertical. In natural condition, without providing any external consolidation drainage path, this type of consolidation is observed when strip load i.e. any narrow loading beneath which with vertical drainage, in short direction of loading some horizontal drainage is occurred.
This drainage generally happen when drainage and strain are both in vertical and any of horizontal direction. The strain may be vertical but drainage occur in both directions.
Examples are:
• Strip footing
• Embankment
• dam
Three dimensional consolidation
In this condition of consolidation drainage and consolidation are happened in three directions i.e. vertical and two horizontal directions. The strain may be in vertical but resulting drainage in three directions.
When vertical sand drains or PVD drains are used, radial drainage can be obtained. Simultaneously there will also vertical drainage. Radial drainage associated with vertical drainage reduces consolidation time abruptly. Design of sand drain will be discussed in our upcoming posts.
Effects of two dimensional and three dimensional consolidation:
The effects of 2D and 3 D consolidations are decrease in time which will help engineers to implement construction stage in short time. Now we will learn how time is reduced:
• Footing area is deceased as new drainage path introduced
• Thickness of soil layers is decreased for same cause of reducing drainage path
• Horizontal permeability of soil increased
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