Vertical Drains/ Wick Drains
FS is engaged in the design and formation of all types of prefabricated and sand drains including soil consolidation by drain and surcharge.
A relatively inexpensive, effective method to improve poor foundation soil in advance of construction of permanent facilities is to use preloading. The preload may consist of soil or sometimes rock, and in the case of oil or water tanks, gradual filling of the tanks may obtain preload goals.
Precompression is the two major goals:
1. Temporary surcharge loads are used to eliminate settlements that would otherwise occur after the structure is completed.
2. Improve the shear strength of the subsoil by increasing the density, reducing the void ratio, and decreasing the water content.
Preloading is most effective on normal to lightly overconsolidated silts, clays and organic deposits. If the deposits are thick and do not have alternating sand seams the preloading may necessitate using sand drains to reduce the time necessary to effect consolidation.
Drainage Using Vertical Drains
The concept of sand blankets, sand drains and paper drains is to provide shorter drainage paths for excess soil pore pressure once the preload is placed. For cohesive soils, consolidation takes place as water is expelled out from the voids in between soil particles. Since silts and clays are relatively impermiable compared to sands, sand drains were devised to accelerate water movement in cohesive soils.Thus consolidation takes place with in months instead of years.
Vertical sand drains are installed by several procedures in diameters ranging from 150 to 750 mm. The particular advantage of dense sand drains is the strengthening effect on the soil - particularly if the site is soft clay.
Sand blanket is placed over the area to be preloaded before placing the surcharging. This provides the horizontal path for water to drain. Any water squeezed out from the underlying soil will enter the sand and flow laterally. Thus speeding up the whole daining process.
Wick drains are now being widely used in lieu of sand drains. A wick drain is a geotextile or filter paper sleeve with a grooved plastic core to produce a "wick" with size ranging from about 100 to 300 mm wide and 4 to 6 mm thick as long as needed. it usually comes in rolls of 100 or more meters.. The membrane cover provides a permeable soil barrier to reduce core clogging. The core provides a ready conduit to the surface into a sand or textile filter or into horizontal trench drains.
The particular attraction of paper drains is economy since per meter installation costs are typically one-quarter that of sand drains. The wick drains can be installed as deep as needed using a conventional vibratory hammer or hydraulic jacks and a mandrel of proper design.