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Consolidation of soft soils in conjunction with preloading
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Pressure relief wells to reduce pore pressures caused by seepage
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Draining
Any site that requires consolidation can benefit from wick drains.
The following subsurface criteria must be met in order for wick drains to accelerate consolidation and settlement:
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Moderate to highly compressible
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Fully saturated
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Final loads generally exceed maximum past pressure
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Secondary consolidation is acceptable
Some site and soil conditions that can affect the economic and technical feasibility of wick drain systems include:
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Uneven or unstable working surface
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Limited head room
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Obstructions above the compressible layer (may be addressed with predrilling)
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Stiff to very stiff layers (addressed with predrilling)
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Extremely soft anchoring layer
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Poor site accessibility
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Overhead or subsurface utilities
Excess pore water removed by wick drains will need to be collected and treated if the in situ soils are contaminated. For contaminated sites, if a highly permeable layer exists below the treatment zone, caution must be taken to ensure that the drain does not penetrate into an uncontaminated aquiclude.
Before construction can begin, the working surface must be graded to ensure a stable working platform. Since wick drain sites are typically soft, a sand or stone blanket may be placed to provide support for the equipment. The sand and stone will also act as a drainage blanket to direct water off-site.
The installation equipment consists of the drain spools, mandrel, and is mast-mounted on either a track-mounted excavator or crane. Drains can be installed up to 130 feet from a track-mounted excavator. Drains greater than 130 feet often require the mast to be mounted on a crane unit for stability.
The installation force is typically provided by vibratory hammers, static methods, or a combination of these methods depending on the soil conditions. Water may be utilized to lubricate the mandrel during installation to reduce the friction on the mandrel. The wick drain is threaded through the mandrel which protects it from damage as it is driven through the soil. After target depth is reached, the drain is anchored to keep it in place while the mandrel is withdrawn. The drain is then cut approximately 6 to 8 inches above ground, and a new anchor fastened to the wick at the bottom of the mandrel in preparation for the next installation point.
Layout usually consists of triangular or square grid patterns.
Soil targeted for consolidation is usually highly compressible with low permeability and fully saturated. Obstructions in and above the treatment zone can slow down installation. Predrilling can be performed to address obstructions.
Wick drains are most commonly used in silts, clays, organic silts, organic clays, muck, peat, swamps, muskeg and sludges.
Wick drains may not be applicable in soils where significant secondary settlement will occur (such as organic materials) if the anticipated secondary settlement is excessive for the planned construction.
- Reduces construction time
- Minimal post-construction settlement
- Low cost
- Increases the rate of strength gain due to consolidation of soft soils when stability is a concern
