Geocomposite Drainage
Geocomposite drainage is used in civil engineering, construction and infrastructure works to carry subsurface water away and relieve hydrostatic pressure. These thin geosynthetic layers are installed behind retaining walls, beneath highways, around foundations and in landfill engineering, where they take water away from structures and keep the surrounding ground stable. HERMEQ supplies three formats: cuspated drainage board, drainage net and fin drain, each made for permanent installation under backfill and load. The range is manufactured in the UK, with nationwide delivery and the option to buy online.
Drainage geocomposite types: cuspated board, net and fin drain
Each format pairs a high-density polyethylene (HDPE) core, which holds open a path for water, with a geotextile filter that lets water in and keeps soil out, and the choice between them follows the shape of the area to be drained, with cores from 4.5mm on the thinnest net up to 25mm on cuspated board.
Cuspated drainage board
Cuspated board has a stiff core moulded into rows of cusps, the studs that hold the filter off the structure and leave open channels for water to run down. It is the format to choose for walls, foundations and embankments, where water is collected over a tall face and carried down to a drain at the base. The trade also calls it drainage board or cuspated drainage membrane, and HERMEQ supplies it as Techdrain cuspated drainage board.
Geocomposite drainage net
A drainage net, also known as a geonet composite, is a thin extruded HDPE mesh with a non-woven polypropylene geotextile bonded to it. It lies flat, follows the ground and comes in wide rolls, so it suits large horizontal and sloped areas such as landfill cells and capping layers. Choose the geocomposite drainage net where the drainage layer sits between a liner and fill, or between two layers of soil.
Fin drain and ventformer
A fin drain is a narrow strip of cuspated core fully wrapped in geotextile, stood on edge in a slim trench to intercept groundwater along its length. The Techdrain FIN25 fin drain is used as a Type 5 fin drain in highway drainage, and it is also a ventformer, a passive venting layer for gases and liquids. Choose it for linear runs, such as highway verges, where a full-width drainage layer is not needed.
Single or double sided geotextile filter
The filter goes wherever soil touches the core, because the filter is what stops fines entering the flow channels. Where the drain sits against a retaining wall, basement wall, slab or waterproofing membrane, with soil on the other face only, single sided board is the right choice: the plain face of the core bears on the structure and the filter faces the backfill. Where soil or fill lies against both faces, as in a drainage layer within an embankment or between two fills, the core needs a filter on both sides. Techdrain is made in both versions, so the choice is made when the order is placed. The geocomposite drainage net and the FIN25 fin drain are filtered on both faces as standard.
Geocomposite drainage layer or stone drainage
A geocomposite drainage layer replaces a granular drain where the job is to collect water from a wall face or a formation and carry it to an outlet. The trench or working space only has to be wide enough to place the sheet, so excavation is smaller, no stone is imported, placed and compacted, and the rolls are light enough to handle by hand behind walls and in narrow trenches. A stone drain also needs a geotextile wrap to keep fines out of the stone, which is a second material and a second operation on site, while a geocomposite arrives with its filter already in place. Stone remains the usual choice for drains that take surface water in through their top, such as filter drains at a carriageway edge, and for the bed and surround of a carrier pipe. Where a wrapped stone drain is the right design, HERMEQ supplies non-woven geotextiles for the wrap.
Geocomposite drains for walls, highways, landfill and tunnels
Retaining walls and basements
Behind a retaining wall or basement wall, the drain relieves the water pressure that would otherwise build up in the backfill against the structure. Single sided cuspated board is the usual choice, fixed to the wall and run down to the drain at the footing.
Foundations and below-ground structures
Around foundations, pile caps, lift pits and other below-ground structures, a drainage layer keeps groundwater moving away from the concrete and any tanking. On housing work the same board is used against foundation walls, and the double sided version suits drains laid within the backfill itself.
Highway embankments and verges
On highway schemes, cuspated board is laid within embankments and the FIN25 strip is installed in verges as a Type 5 fin drain, intercepting groundwater before it reaches the pavement foundation.
Landfill engineering
In landfill cells and capping systems, the drainage net is laid over geomembrane liners to carry leachate or surface water to the collection drains.
Tunnels
In cut and cover tunnels, underpasses and culverts, the drainage layer runs over the roof and down the walls of the structure before backfill, so water reaching the box drains to collectors at its base and does not stand against the waterproofing.
Gas venting beneath structures
On brownfield sites, ground gas has to be vented before it can collect beneath a building. Laid in the venting layer under a floor slab and run to outlets at the perimeter, the FIN25 ventformer is a passive venting path for gases and liquids, used beneath a gas-resistant membrane as part of the gas protection design.
Core thickness, coverage and installation
Backfill and traffic load
The core has to keep its channels open under the weight of backfill and anything that runs over it, so deeper fill, heavy compaction plant and trafficked areas above the drain call for a stiffer cuspated core. Where loads are light, such as landscaped areas or a shallow garden wall, a thinner core does the job.
Roll widths and coverage
Planning the layout around the roll width reduces the number of joints, and every joint is a point where soil can enter if it is left uncovered. On slopes, rolls are run down the fall so that water flows along the core and not across a lap.
Checks before installation
Three things are checked before any backfill is placed against the drain.
- The filtered face is against the soil along the whole run, including at corners, laps and cut edges.
- The base of the drain discharges into a collector pipe or an outlet, so the water it gathers has somewhere to go.
- Waterproofing behind the drain is complete and protected, and the top edge of the board is closed off so fill cannot fall into the core.
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Frequently Asked Questions
What is a geocomposite?
A geocomposite is a product made from two or more geosynthetic materials combined into one, such as a plastic drainage core bonded to a geotextile filter. Other geocomposites combine geotextiles with geogrids for reinforcement or with bentonite clay for lining. The word describes how the material is made, while geocomposite drainage describes the job it does.
Is a geocomposite drain the same as a drainage membrane?
The terms overlap, and on site drainage membrane is often used for any dimpled HDPE sheet laid against a wall. A geocomposite drain carries a geotextile filter on its core, so it can be placed directly against soil. A dimpled membrane without a filter needs a separate geotextile between it and the soil.
Can geocomposite drainage be cut to size on site?
Yes. Boards and rolls are cut with a sharp knife or a fine-toothed saw to fit around corners, pipe penetrations and changes of level. Cut edges are then covered with geotextile, either by folding the filter back over the edge or by taping a strip over it, so soil cannot enter the exposed core.
How are geocomposite drainage rolls joined?
Adjacent rolls are butted or overlapped so that the flow channels line up, and the geotextile is lapped over the joint so no part of the core is left open to the soil. Where there is no filter flap to lap, a strip of geotextile is laid over the joint and taped. On slopes, the upper roll laps over the lower one in the direction of flow.
How should geocomposite drainage be stored on site?
Rolls are stored flat on clean, level ground or on pallets, away from wet and muddy surfaces that would soil the filter. Geotextile filters are not made for long exposure to sunlight, so rolls stay in their wrapping or under cover until they are laid. Once installed, the drain is backfilled as soon as the work allows.