DRAINAGE FOR SUBGRADE STRUCTURES BASICS AND TUTORIALS

DRAINAGE FOR SUBGRADE STRUCTURES BASIC INFORMATION
How To Design Drainage Subgrade Structures?


Subgrade structures located above groundwater level in drained soil may be in contact with water and wet soil for periods of indefinite duration after long continued rains and spring thaws.

Drainage of surface and subsurface water, however, may greatly reduce the time during which the walls and floor of a structure are subjected to water, may prevent leakage through openings resulting from poor workmanship and reduce the capillary penetration of water into the structure.

If subsurface water cannot be removed by drainage, the structure must be made waterproof or highly water-resistant.

Surface water may be diverted by grading the ground surface away from the walls and by carrying the runoff from roofs away from the building. The slope of the ground surface should be at least 1⁄4 in / ft for a minimum distance of 10 ft from the walls.

Runoff from high ground adjacent to the structure should also be diverted. Proper subsurface drainage of ground water away from basement walls and floors requires a drain of adequate size, sloped continuously, and, where necessary, carried around corners of the building without breaking continuity.

The drain should lead to a storm sewer or to a lower elevation that will not be flooded and permit water to back up in the drain.

Drain tile should have a minimum diameter of 6 in and should be laid in gravel or other kind of porous bed at least 6 in below the basement floor. The open joints between the tile should be covered with a wire screen or building paper to prevent clogging of the drain with fine material.

Gravel should be laid above the tile, filling the excavation to an elevation well above the top of the footing. Where considerable water may be expected in heavy soil, the gravel fill should be carried up nearly to the ground surface and should extend from the wall a distance of at least 12 in (Fig. 3.7).


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