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Drainage Around Structures

Drainage Around Structures

Site Drainage

Drainage is key

All surface areas should be provided sufficient surface slopes & proper sub “underground” drainage and/or preferably both. This is done so water will not stand anywhere and the ground will rapidly dry and soil will firm. Special care may be required to sub-drain any soil touching basement walls, retaining walls and floors.

When soil is porous “pervious” sand or gravel in nature and the water table is low, drainage is usually automatic and mistakes made in grading will show only briefly during rains. Impervious soil on the other hand is a different story and will demand more care in shaping so that they will drain completely. If this is done incorrectly big problems may arise.

If pervious fill is placed on top of impervious native soil the surface of the impervious native soil should be graded to drain before pervious fill is placed to avoid underground trapping of water forming pockets. If the native soil to be covered is pervious on the other hand, it does not need to be graded no matter the fill being placed over top.

Where areas are large, rainwater flowing on the surface may cause serious nuisance, “even if it does not erode the ground” some times depending on location, the cost may be justified to install catch basins and underground piping to remove water from the area.

Erosion can be another big problem in drainage systems. Water at times must be slowed down to reduce friction eroding the soils and surrounding areas. This can be achieved by changing grades from +5% to more subtle grades in the 2% to 3% range.

Drainage pipe diameter is also a big factor in drainage. If the pressure behind water flow in a pipe is kept constant, the size of the pipe will directly affect the flow rate. If the diameter of the pipe is increased, the flow rate will decrease. Also, If the length of the pipe is increased, the flow rate will decrease due to friction.

For more information on flow rates check out Pipe flow Calculations.


Owner and operator of Willhite Grading

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