High-Performance Drainage Blankets

In the same way that waterproofing ground with puddle clay or concrete has been superseded over the years by the use of plastic geomembrane linings, drainage geocomposites are rapidly eliminating the need for sand and gravel in traditional land drainage blankets. Applications include:

• road embankments
• water table drainage below developments (such as car parks)
• landfill cells and caps (water/leachate/gas)
• leak monitors below liners
• sports pitches

Drainage geocomposites come in a variety of types, foremost of which for reliability and performance is Aztex Draintube/Somtube. This drainage geocomposite is the only geocomposite with ‘built-in’ perforated mini-pipes to ensure the highest drainage efficiency and provide absolute flow direction control. Since Roman times and before, pipes have been and will remain the most efficient means of conveying fluids. Water in a homogenous drainage layer – such as sand, gravel, or geocomposites using nets or cuspates – is impeded in its flow by the homogenous medium concerned, which generates a pressure within the drainage layer and lead to saturation of adjacent ground. In contrast, with the Draintube/Somtube, water entering at any point in the mat has minimal distance to travel before entering a perforated mini-pipe through which it can flow efficiently.

Cost savings can be considerable, particularly through the elimination of multi-trucking of granular material either to or on site. This is because a single trailer-load of geocomposite can cover an area otherwise requiring more than 150 truck-loads of granular material. To this must be added the savings through speed of installation. (A small work crew can easily install 4000m2 of geocomposite per day.) In addition, there are savings in administration and programming.

The Draintube/Somtube geocomposite comprises a coarse geotextile draining web needle-punched to fine filter geotextile on one side or on both sides, with the perforated mini-pipes sandwiched between. The mini-pipes run through the entire length of the geocomposite mat, typically 4m wide × 100m long, and are set at spacings across the width of the mat ranging from 250mm to 2000mm depending upon the drainage efficiency required. Water can be drained from the ground either side of the geocomposite, or from one side only where used in conjunction with an impermeable geomembrane.

Drain and Somtubes were developed by Geosynthetic Technology Limited in collaboration with a university geotechnical faculty together with an independent laboratory. A large-scale drainage cell was constructed to determine key data such as water pressure levels in the geocomposite under various operating conditions. The data was then adapted to produce design-aid software called Lymphea. Using design input data for a project, Lymphea will determine the appropriate grade of Aztex Draintube/Somtube to be used and calculate the thickness of gravel or sand necessary to provide an equivalent transmissivity.

The extra efficiency gained with the perforated mini-pipes is particularly apparent in applications where long drain distances are involved, such as landfill cappings, car parks, and sport pitches. Draintubes can drain water much more efficiently over long distances than other types of geocomposites, requiring fewer main collector drains, and ensuring that the capping soil does not become saturated. Somtubes (filter both sides) have been used extensively in road embankment construction to relieve pore pressure in the fill, and/or as basal foundation drainage where it is laid in contact with wick drains. Draintubes (filter one side) – which also provide puncture protection to the geomembrane – is ideal for capping landfills or contaminated ground where the geocomposite is laid above (water drainage) or below (gas venting) an impermeable geomembrane.