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Plumbing 101 - Underground Drainage

Most buildings need some kind of drainage. Without it, water from rain, sleet and snow can sink into soil, which will become waterlogged over time and may threaten the stability of building foundations and damage garden plants, or flood across impermeable surfaces, creating hazardous conditions. One option is surface drainage which routes water into storm drains on the streets. However, a less unsightly option is an underground drainage system.


Underground drainage is, essentially, the placement of plumbing below ground level. Water is diverted from above ground into these systems by which it is carried away from one location and deposited in another that is downhill from the first.


In the past, the guttering on the side of a house will have channelled water to a drainpipe that either expelled the water directly onto the ground or deposited it in a storage barrel. These days it is more common for the drainage plumbing to be routed below the surface and linked to the underground systems.

French Drain:

One common form of such drainage system is known as a French drain. This is a ditch - orientated on a slope so that water drains downhill - into which a perforated clay pipe is laid. The ditch is filled with gravel over which the topsoil is laid. This system allows excess groundwater to percolate through the gravel and enter the pipe to be conveyed away from the property. It slows the percolation of water through the soil, meaning that plants are able to access all they need from the moisture in the soil.


One primary consideration when considering underground drains is the ability of the pipes utilised in the system to repel plant and tree roots. Reinforced plastic is probably the most common material used for today's drainage systems as it is both strong enough but also slightly flexible to enable it to accommodate nearby root growth. Homeowners should also consult local building codes before installing underground pipes and tanks.


Where the water conveyed by an underground system is deposited can vary. In some cases it flows into municipal storm drains. It may also be routed into storage tanks or to dry wells from where it can be reintegrated into the potable water system. A third option is what is referred to as a rain garden, which is a location filled with wetland plants which absorb the excess moisture and release it back into the atmosphere from where, eventually, it will become rain and repeat the cycle.


Municipal authorities have long used underground systems to facilitate the extraction of water from city streets and community buildings. These systems - networks of underground pipes linked to the storm drains in the streets - typically drain into to treatment plants from where the water can be returned into the system for consumption. Alternatively, they route the water into rivers or the sea, as both of these features generally lie at a lower level than urban settlements.

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