Sand and Wick Drains
Drains are mainly used in the consolidation of extensive areas of loading such as large storage areas, airport runways, road embankments and reservoirs. Another notable application has been in conjunction with the driving of piles in soft clayey soils.
The drain dissipates the great increase in interstitial pressure generated during driving, so eliminating any danger to piles already placed.
These involve the excavation of a vertical shaft filled with appropriately graded sand to act as a filter. The techniques currently in use and chosen by Trevi on the basis of the most satisfactory results include the technique which uses the dynamic energy of water under pressure for drilling and displacement of the sol/. The excavated material is carried to the surlace in suspension by the rising flow of water, which at the same time serves to keep the shaft open until it is filled with sand.
For drains made with the continuous auger technique, the method consists in introducing a continuous auger into the soil to the desired depth and then introducing the sand through the hollow shaft of the auger at the same time, as the auger itself is withdrawn with a rotary movement.
Average sand drain diameters vary from 25 to 40 cm and depths of 40 to 45 m can be reached. Average production can be from 70 to 100 linear m per hour, depending on soil characteristics.
These are prefabricated drains placed in the soil by means of appropriate equipment. Many types of prefabricated drains with practically identical characteristics are currently available on the market. However, it is important, before installing a drain, to know the permeability and resistance characteristics of its filter, and of the capacity of the drain to tolerate movements of the soil.
Trevi uses a driving technique employing hydraulic vibrators. The vibrating element is situated at the head of the driving tube. In special cases a vibrating chuck can be employed with the vibration being applied at various tube depths. Vibration driving allows rapid crossing of any compacted layers above the layer to be drained.
Average production can be between 200 and 500 linear metres (m) per hour - in fact, depending on soil characteristics, driving speeds of up to 1.5 m/sec. can be obtained. Average lengths are around 25 to 30 m, with the possibility of reaching depths of the order of 40 m.