Let Nilex's experience work for you to design a more efficient drainage system, avoid potential problems, and save time and money. Nilex provides the following recommendations to help the engineer unfamiliar with wick drain system. These guidelines are by no means comprehensive; contact Nilex for additional information or design assistance.

Developing the Design: Investigation

A sound soil investigation can save the owner money, time, and provide a better product. For a wick drain design, penetration resistance and rate of soil consolidation are the key elements to optimize the wick drain system.

• Soil borings

• Cone Penetration Tests (CPTs) and Standard Penetration Tests (SPTs): An initial grid of the site with a CPT rig, allows the contractors and soils testing agency to see the entire (continuous) soil profile to determine penetration resistance, and the compressible layer(s). After analyzing the CPT results, a second set of testing with SPT's provides an opportunity to get samples from the problem areas/layers. By using the Seismic and Pore pressure options (called a SCPTu) on the CPT rig, and allowing the pore pressure to dissipate to T₅₀ , additional information and correlation of consolidometer testing can be obtained.
• Penetration resistance: Typical installation suffers, and becomes more expensive for "N" values greater than 20.
• Sampling: Based upon CPT values, take samples from soft/compressible layer(s).
• Testing: Consolidometer tests to determine Ch values and expected settlement. If the SCPTu with porewater dissipation test was performed on site, a direct shear test in the lab can provide information to corroborate C_h values.

• Site History

• Utility Locations

Developing the Design: Design Parameters

Here are some questions you'll want to answer to obtain design parameters for your wick drain system:

• What consolidation percentage do you wish to obtain?
• How much surcharge can you place?
• How much residual primary settlement can you tolerate?
• How long can you wait?
• What is your horizontal coefficient of consolidation (Ch)?
• Are there any complicating issues, including any of the following:
• Multiple soil conditions.
• Unusual loading conditions.
• Very deep drains.
• pore water discharge/outfall issues

By using the 3 basic inputs above, a spacing can be determined. The percentage of consolidation, being a function of surcharge and settlement, can be adjusted depending on the project. For example, if a 10 ft. embankment is expected to induce 10" of settlement, and 1" of remaining settlement is tolerable, a U (percentage of consolidation) of 90% can be utilized without additional surcharge material. In order to meet the project schedule, this may require a 3 ft. isosocles triangular spacing. For this example, this may make the wick drain quantity 1,000,000 linear feet, which at $0.50/l.f., would cost the project $500,000. If the project had a stockpile of surcharge available, and it is determined that 10 ft. of surcharge above the embankment would induce a total of 16 inches of primary settlement, then the percentage of consolidation required would be 56% (9/16). Analysis may determine that a 6 ft. spacing would achieve this consolidation in the time allowed, and may reduce the wick drain quantity to 250,000 linear feet, which could increase the unit cost to $0.58/l.f. for a wick drain cost of $145,000. The cost of acquiring/moving/placing/removing the surcharge material may make this a more feasible option.

Implementing the Design: Specifications

When you're putting together your wick drain system design, you'll want to include the following specifications:

• Materials
• Wick Drain
• Pore water discharge
• Horizontal Strip Drains
• Gravel/Sand Blanket
• Installation
• Equipment Requirements
• Data Logging
• Payment
• Monitoring
• Settlement
• Pore Pressure