presentation on soil and foundation engg.

soil mech.

1. Consolidation of Soil: When soil is loaded because of the construction of a structure, the volume of the soil will decrease due to the rearrangement of soil particles. If the soil is in a completely saturated state, volume reduction takes place due to the expulsion of pore water from voids. This process is called the consolidation of soil. It is a time-dependent phenomenon. The terms “consolidation” and “compaction” are frequently interchanged. Compaction reduces the volume of air in the voids, increasing the density of unsaturated soil. Consolidation, on the other hand, is a time-consuming procedure for raising the density of saturated soil by draining some of the water from the spaces. To assure the serviceability of structures built on a compressible soil layer, consolidation theory is essential for the prediction of both the magnitude and rate of consolidation settlements.

2. Type of Consolidation of Soil: Total consolidation of soil can be divided into two types. Primary consolidation Secondary consolidation   Primary Consolidation: This process is started when the soil is fully saturated. When effective stress over-saturated soil mass increases, then the pore water pressure increases. If a drainage facility is provided, the expulsion of pore water will occur. Primary consolidation is completed when pore water expulsion stops. During this process of consolidation, the soil remains in saturated condition and the flow of water is under laminar condition i.e <1. The volume of expelled water is equal to the change in volume of soil.

3. The soil mass is considered semi-infinite. So the change in volume is equal to the change in depth. Due to primary consolidation, settlements occur which is time-dependent. The time required for settlement depends upon the following factors:  Rate of application of load. Coefficient of permeability of the soil. Availability of drainage facility (one way or two way). Length of the drainage path. Secondary Consolidation: After completion of primary consolidation, when the expulsion of pore water is stopped and load continues to act, then at a very slow rate volume changes may be recorded. This is due to plastic readjustment of soil. This process is called secondary consolidation. It is a time-dependent process and much slower than the primary consolidation process.

4. Secondary consolidation is more seen in plastic soil and in highly plastic clays. It is nearly 10-20% of total volume change. In coarse-grained soil like gravels and sands, secondary consolidation is negligible. Classification Of Soil Based On Consolidation a) Normally Consolidated Soil: These are the soils that are loaded for the first time to the present applied effective stress. It also means that the past applied effective stress was lower than the present applied effective stress. These are more compressible soils. b) Over Consolidated Soil: Over consolidated soils are those which have been subjected to effective stress in the past greater than the present applied effective stress. It is also called pre-consolidated or pre-compressed soil.

5. Soil Consolidation In Construction: Understanding the potential for soil consolidation at a site while the project is still in the design phase can help to mitigate potentially expensive future issues associated with the foundation settling caused by consolidation. This may later manifest as cracking in the façade, an uneven slab, damage to utilities entering the structure, or a door sticking in the frame, among other issues. By measuring the potential for consolidation of the land on which a building is being constructed, it can be designed in a manner that will keep any settlement at a tolerable limit that does not put the structure in danger. However, consolidation isn’t the only process that soil undergoes in construction. Soil compaction is routinely carried out prior to the construction process to give the foundation a strong working platform. This helps to reduce the chances that settlement will occur and damage the building or road in the future.

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