Page 228 - Masonry and Concrete

Footings, Foundation Walls, Basements, and Slabs
227
FOOTINGS, FOUNDATION WALLS, BASEMENTS, AND SLABS

gravel bed must be protected from soil clogging with a filtering cover
made from landscape fabric. This will allow water to flow toward the
drain but keep soil from clogging the voids between gravel particles.
For clay soils, which have poor drainage and only limited amounts of
groundwater flow, a 4-in. drain is usually adequate. For sandy soils
with better drainage and more groundwater flow, a 6-in. drain is
needed. For gravely soils with good drainage and large ground water
flow, a drain as large as 8 in. may be necessary.
Subsurface drainage can also be used to relieve hydrostatic pres-
sure against the full height of a basement wall. A free-draining gravel
backfill that extends the height of the wall allows groundwater to flow
by gravity down to the level of the drain (Figure 6-36 top). The gravel
should be carried up the wall to within a few inches of the ground sur-
face with only a covering of topsoil for landscaping purposes. Propri-
etary insulation board with vertical drainage channels can be used
instead of the gravel backfill (Figure 6-36 bottom). These drainage
mats are generally easy to install and help to insulate the basement as
well. The insulation is a polystyrene board which is impervious to
moisture damage.

NATURAL
GROUND
SLOPE
NATURAL
WATER
TABLE

WATER TABLE LOWERED WATER TABLE HIGHER
BY FOUNDATION DRAINS AT CENTER OF BUILDING
FIGURE 6-35
Drains lower water table. (from Callendar, John H., Timesaver Standards for Architectural Design Data, McGraw-
Hill, New York).

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