About
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.
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