C1 Hydrostatic Head

The force of water in the earth below an area's local water table can turn a building's basement into an empty boat and pop it out of the ground —a phenomenon well-known to swimming pool contractors. Where moist subsoil conditions could rise above a basement's floor level during any part of the year, the basement should have foundation drains laid no more than 16 ft apart beneath its floors as well as around them, and each drain should slope down at least 1/16 in/— to an outfall well away from the building.

A 20 x 40 ft concrete basement floor in a residence lies below the surrounding water table during part of the year. What is the total pressure potential beneath this floor if no sub-slab drains are laid?

P = total pressure against underside of construction, ? lb

A = area of underside of construction, sf. A of basement floor, 24 x 40 = 960 sf.

H = hydrostatic head, psf. Water-saturated soil tends to act as a static fluid in which an immersed object obeys the lays of buoyancy; thus such soil can exert an upward force that equals its unit weight which may be as much as 140 pcf, or 140 psf against a surface.


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