Wednesday, December 12, 2012

Structures: Ground treatment (geotechnical processes)

Soil properties can change under the action of superstructure loading. It compacts, consolidates and drains, and so becomes denser, stronger and less prone to settlement.

These improvements can also be induced by a variety of geotechnical processes  before construction. The ground can be temporarily loaded before construction (preconsolidated), hammered by heavy weights to compact it (dynamic consolidation), vibrated to shake down and reduce the voids ratio (vibro stabilization), the soil moisture drained off (dewatering, sand wicks), the voids filled with cementitious material (grouting, chemical injection), and similar techniques.

Imported material (usually sandy gravel) can be laid over weak ground and compacted so that the pressure from column pad foundations can be spread over a greater area.

Imported material can also be used to  seal contaminated sites. Imported soils can also be laid and compacted in thin (say 150 mm) layers with polymer nets placed between each layer. The composite material, known as reinforced soil, has been widely used in retaining walls and embankments.

These techniques are discussed in detail in later. The development of these techniques has made it possible to build economically on sites which, until recently, were too difficult and expensive to be considered as building land.

Temporary geotechnical processes can be used to ease excavation. Typical cases are:

(1) Temporary dewatering to allow the excavation to be carried out in the dry,

(2) Chemical injection, freezing, grouting and the like to
maintain sides of excavations, etc.

Permanent processes are employed to improve the ground properties by:

(1) Compaction (making the soil denser and thus stronger), and

(2) Consolidation and drainage processes to reduce the magnitude of settlement. (Such measures are discussed in detail later.)

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