Subsurface Drains

Subsurface drains involve excavating a trench and placing a perforated pipe and coarse material such as gravel in the trench. The drain usually drains by gravity to a sump where the water is pumped to the surface for treatment. Subsurface drains essentially function like an infinite line of extraction wells, creating a continuous zone of depression in which groundwater flows towards the drain. Two types of subsurface systems are relief drains and interceptor drains. The major difference between these drains is that the drawdown created by an interceptor drain is proportional to the hydraulic gradient, whereas the drawdown created by a relief drain is a function of the hydraulic conductivity and depth to the impermeable layer below the drain.

Environmental engineers use relief drains primarily to lower the water table and prevent its contact with waste material or to contain a plume in place and prevent contamination from reaching a deeper aquifer. Relief drains can be installed in parallel on either side of a waste site or completely around the perimeter of the waste site as shown in Figure 9.17.4. The areas of influence of relief drains should overlap to prevent the contaminated groundwater from escaping between the drain lines.

Engineers use interceptor drains to intercept a plume hydraulically downgradient from its source and prevent the contamination from reaching wells and surface water located downgradient from the site. Interceptor drains are installed perpendicular to groundwater flow and down-gradient of the plume of contamination as shown in Figure 9.17.5. In some cases, engineers use interceptor drains in conjunction with a barrier wall to prevent infiltration of clean water from downgradient of the drain thereby reducing treatment costs (see Figure 9.17.6). A series of interceptor drains or collector pipes (laterals) can be connected to a main pipe (header) as shown in Figure 9.17.7.

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