WICKDRAINS

The Problem
Historically, the design of structures on soft compressible soils (clays) has created problems for civil engineers. Construction without some sort of soil treatment is usually impractical due to unpredictable long-term settlement. Simple surcharging as a soil consolidation method can take many years. Although surcharging increases water pore pressure, settlement can take considerable time, as the water lacks easy path to leave the soil.

The Solution
Soil consolidation using prefabricated vertical drains (also commonly called wick drains or band drains) can reduce settlement times from years to months. Most settlement can occur during construction, thus keeping post-construction settlements to a minimum.
Consolidation of water-saturated, fine-grained soil occurs very slowly because the low permeability of these soils impedes the escape of pore water from the soil voids. Even under large temporary surcharge loads, settlements can take years because of this slow water movement and the great distance the water must move to exit the soil. The installation of prefabricated vertical drains greatly reduces the distance the water must move to reach a free drainage path, and therefore greatly increases settlement rate (Figure 1). Drain spacing may be adjusted to match the required settlement time.
Prefabricated drains are installed vertically to depths as great as 50 meters (164 feet). The water, under pressure in excess of hydrostatic, flows through the filter and into the channels where it is led vertically out of the soil. This may be either up or down to intersecting natural sand layers or to the surface where a sand drainage blanket or prefabricated strip drains are provided. The water in the soil only has to travel the distance to the nearest drain to reach a free drainage path. The drains are usually placed in a triangular configuration of 1 to 10 meters (3 to 33 feet) – depending on the desired consolidation time. As a result of this method of accelerating the consolidation process, uneven post-construction settlements can be virtually eliminated.

Background
Prior to the 1980s, most consolidation of soft compressible soils (clays) was accomplished using sand drains with a horizontal sand blanket for lateral drainage .This technique was reasonably effective, but sand drain installation was slow and expensive. In addition, sand drains were subject to clogging from silt infiltration.
In the early 1980s, prefabricated plastic vertical drains began to replace sand drains in the United States. The plastic drains could be installed much more quickly and economically and provided more reliable drainage. By the end of the 1980s, prefabricated drains had completely replaced sand drains. However, the sand blanket was still used for horizontal drainage.
In the late 1980s, larger prefabricated drains called “strip drains” were developed. These new drains were similar to vertical drains but had higher flow capabilities and higher compressive strengths. In 1987, strip drains were used in conjunction with a sand blanket for horizontal drainage on a vertical soil drain in Jacksonville, Florida. More recently, strip drains were used with a sand blanket on a job-site in Massachusetts. However, more commonly, strip drains have been used alone (without any sand blanket) in vertical drain installation. 
Strip drains offer three advantages over a sand blanket in a vertical drain installation. First, strip drains are less expensive. Both material, freight and installation costs are usually lower for the prefabricated drains. Second, strip drains may be installed more quickly and with less manpower and equipment. And third, strip drains provide better drainage as their flow capacity is more predictable, less subject to clogging and not reduced by the surcharge.

Vertical and strip drains
Prefabricated vertical drains consist of a formed polymeric core surrounded by a geotextile filter fabric. The fabric allows water to pass into the core while restraining soil particles which might clog the core while restraining soil particles which might clog the core. The core provides a conduit for water flow upward (or downward) out of the soil. Typical flow capacity of a vertical drain is 1 gallon per minute (100x10-6m3/sec) under a soil loading of 50 psi (350 kPa). Vertical drains are usually 4” (100 mm) wide, 1/8” (3 mm) thick and come in 1000’ (305 meter) rolls.
Prefabricated strip drains such as AKWADRAINTM are similar in construction to vertical drains, except that flow capacities are greater-in the range of 20 gpm (75 1pm) per foot of width, and compressive strengths are greater-up to 12,000 psf (575 kN/m2). Strip drains are typically available in 6, 12 and 18” (150, 300 & 450 mm) widths. Thickness if usually 1” (25 mm) and roll lengths vary from 100 to 500’ (30 to 150 meters). Standard fittings are available to connect sections of strip drain to each other and to outlet piping.

TYPICAL APPLICATIONS FOR WICKDRAIN

Embankment construction
Vertical drains may be used to accelerate settlement of embankments for roadways, railroad tracks, runways, or bridge approaches which must be put into operation very soon after construction is completed. Presettlement can greatly reduce long-term maintenance costs that would result from extended periods of settlement during the life of the project.

Underwater consolidation
Vertical drains may be used to accelerate settlement of soil below water level. The differential pressure created by the surcharge is as effective under water as on land. This technique can be used in preparation for placing tunnel sections in a river bed, for example.

Tank farm foundations & material storage areas
Because of high unit loadings, liquid storage tanks are subject to settlement in soft soils. Vertical drains used in conjunction with a horizontal strip drainage can provide rapid soil consolidation prior to construction. Storage sites for solid materials – coal, ore paper – also can benefit from vertical drainage prior to use.

Landfill Areas
Fill is often placed behind sheet piling walls or cofferdams for use as docks or industrial sites. Vertical drainage is an effective method to accelerate settlement thereby making the site available for use in the shortest possible time.

HOW THEY WORK

WICKDRAIN prefabricated vertical drain has two components – a core which serves as a water conduit and a geotextile filter fabric which allows water to pass into the core while restricting the movement of soil particles which might clog the core.

The fabric
The WICKDRAIN filter material uses a strong, tough, permeable nonwoven filter fabric of 100% polypropylene specifically designed for drainage. Its continuous filaments are arranged preferentially in the length and width directions of the sheet and thermally bonded. WICKDRAIN filter fabric resists mildew, rotting, insects and chemicals normally encountered in a subsurface drainage system. It is dimensionally stable, wet or dry, has good tear and puncture resistance and will not shrink, grow or unravel.
WICKDRAIN fabric has a unique structure that enhances its function as a filter fabric. It has a large number of openings with a range of opening sizes throughout its structure instead of a few openings of fixed size as in woven fabrics. Its bonded fabrics create a tortuous pathway resembling that of a well graded aggregate filter rather than a simple, straight line exit for soil particles. Because of its unique structure, the filter fabric has both high permeability and the ability to restrict the movement of most soil particles, while allowing the very fine silts to flow into and out of the drain. The initial removal of very fine silts from the soil is beneficial because this leaves the larger particles to form a highly permeable soil network against the fabric. The soil network restricts the further movement of fine soil particles and helps to develop a graded soil filter. This soil filter effectively stops piping of soil and prevents other fine particles from entering the drain. The fabric filter, being more permeable than the soil filter and the natural soil, does not restrict the flow of water into the drain.

The effectiveness of the filter fabric has been proven in government and commercial projects in a wide range of soil types. Its effectiveness has also been confirmed by extensive laboratory tests at the Colorado State University Engineering Research Center. Tests were run with soil mixtures of fine sand, silts and clays which simulated actual systems. Under these conditions, the tests confirmed that, after the initial passage of fine silts through the filter there was:
a)No measurable migration of soil fines within the soil filter or into the drain, and
b)No measurable pressure drop across the filter indicating no reduction of water flow through the fabric. Scanning electron microscopy confirmed that there was no clogging of the fabric.

The core
The WICKDRAIN core is a strong, tough structural member of extruded from 100% polypropylene specifically designed for vertical drain systems. A total of 38 longitudinal grooves distributed on both sides of the core provide discharge passages for water flowing to the surface. The core is dimensionally stable when wet, has good puncture and collapse resistance and will not shrink or rot

INSTALLING WICKDRAINS

Installing AMERDRAIN vertical drains

AMERDRAIN vertical drains may be installed employing either vibratory of static crowd methods. In either case the drain is enclosed in a tubular steel mandrel of small cross-sectional area (usually 2 x 5 inches). A small steel anchor plate is attached to the wick at the bottom of the mandrel. The mandrel is then driven into the soil either with a static crowd or vibratory rig. When the design depth is reached, the mandrel is extracted. The anchor plate retains the drain in the soil. When the mandrel is fully extracted, the drain is cut off, a new anchor plate is installed and the process begins again.

INTRODUCING E-QUAKE DRAINS

A new method for treating liquefiable soils uses prefabricated vertical drains in conjunction with a reservoir to maintain pore water pressures within the soil at safe levels.
The efficiency of vertical prefabricated wick drains for acceleration of consolidation settlement is well established. This method has virtually replaced the older sand drain method. Wick drains provide a shortened drainage path for dissipation of excess pore water pressures within the soil and provide a clear drainage path to conduct expelled water to the surface. This process is very efficient and cost-effective for acceleration of the consolidation process which takes place over a period of time, often several months.

In an earthquake, efficient drainage is required within seconds. Head losses from water flow within the drain itself become significant. E-QUAKE DRAIN (Patent Pending) makes use of a prefabricated drain with a large cross-sectional area, with flow capacity up to 10,000 times that of the usual wick drain. Head losses within this drain are only a fraction of that in a three-foot diameter stone column with equivalent flow. For further efficiency, a reservoir is provided to accommodate the expelled water without requiring its expulsion to the surface. This reservoir may occur naturally, such as a sand or gravel layer above the water table, or may be installed as an integral part of each drain.

E-QUAKE DRAINS are installed with specialized equipment, capable of penetrating hard intervening layers. The specially-shaped installation mandrel is vibrated during installation as an facilitate penetration, and to densify the liquefiable soil.
These factors combine to provide an extremely cost-effective and efficient system for liquefaction control.