Page 222 - Masonry and Concrete

Footings, Foundation Walls, Basements, and Slabs
221
FOOTINGS, FOUNDATION WALLS, BASEMENTS, AND SLABS

soil test is not available, excavations at the site can provide some con-
temporaneous information. Water which seeps into an excavation will
rise to the level of the current water table. If the excavations are made
during the dry season, the water table is probably lower than normal.
If the excavations are made during the rainy season, the water table
may be at its highest. Planning for drainage and waterproofing a base-
ment space should be based on worst-case scenario because it is very
difficult and very expensive to correct below-grade moisture problems
after the backfill is in place. The cost of adding an extra measure of
protection up front is minimal and can reduce or eliminate the call-
backs required to deal with leaky or damp basements.
Hydrostatic pressure is the pressure exerted by the weight of a fluid
such as water. The hydrostatic pressure exerted by groundwater at any
point against a basement wall is equal to the depth of that point below
the water table times the unit weight of water (which is 62.4 pcf). If the
bottom of the wall is 8 ft. below the water table, the hydrostatic pres-
sure at that point is 8 62.4 499.2 lbs. per square foot of wall area.
The lateral pressure of the soil itself is slightly reduced because of the
buoyancy of the water it contains, but the added hydrostatic pressure
significantly increases the structural load on the wall. The hydrostatic
uplift pressure on the bottom of a basement slab is calculated in the
same way. Both structures and waterproofing membranes must be able
to withstand the lateral and uplift loads created by hydrostatic pres-
sure. As an alternative to resisting the full force of the hydrostatic load,
groundwater can be diverted away from a basement by installing sub-
surface drains to lower the water table. Draining water away from a
building reduces structural loads on walls, footings, and slabs as well
as hydrostatic pressure on waterproofing membranes.
In addition to lateral gravity flow, water can move upward through
soil from the water table by capillary action. The rate at which this
capillary rise occurs depends on particle size and distribution and the
resulting size of voids or pores between soil particles. Clay soils have
the finest pore structure and can draw capillary moisture upward
from a water table many feet below. Coarse, sandy soils generally have
a pore structure so large that capillary rise is minimal. The capillary
moisture content of soil varies in direct proportion to the fineness of
the soil. Capillary moisture cannot be drained out of soil because the
surface tension within the pore structure of the soil holds the water

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