Page 226 - Masonry and Concrete

Footings, Foundation Walls, Basements, and Slabs
225
FOOTINGS, FOUNDATION WALLS, BASEMENTS, AND SLABS

from a building at the ground surface cannot enter the soil and con-
tribute to below-grade moisture problems. Roof overhangs, gutters,
and downspouts provide effective control for sloped roofs by diverting
the runoff away from the building (Figure 6-34 bottom). Site selection,
building orientation, and grading should provide slopes away from the
building, and ground swales and troughs can also be used to redirect
surface runoff.
Backfill adjacent to a building should be compacted sufficiently to
prevent settlement and the possibility of ponding water, which might
drain toward the foundation wall. Backfill materials that contain a
high percentage of fines may absorb and hold surface water and rain
water, concentrating the moisture immediately adjacent to the build-
ing. A low-permeance cap of compacted clay soil can be installed
under grassy areas. Planting beds located next to the building walls
should always be well drained to avoid concentrating moisture along
the foundation line. Sidewalks located adjacent to a building can pre-
vent groundwater absorption but may cause backsplash and soiling on
the walls. Sidewalks should always be sloped away from the building
1
a minimum of 2 in. per foot. The joint between the sidewalk and the
building should be sealed with a traffic-grade silicone or urethane
sealant if substantial rainfall, accumulated snow drifts, or exposure to
roof or site runoff is expected.
Subsurface drainage systems can collect and divert groundwater
away from the walls and floor of a basement and relieve hydrostatic
pressure. The most common method of keeping groundwater away
from basement structures is to provide a perimeter drain or footing
drain in the form of perforated, porous, or open-jointed pipe at the
level of the footings. Perforated drains are generally preferable to the
porous pipe and open-jointed systems. When perforated drains are
used, they should be installed with the perforations on the bottom so
that water rises into the pipe. Perimeter drains artificially lower the
water table below the elevation of the floor and eliminate hydrostatic
pressure against the walls and the bottom of the slab (Figure 6-35).
Perimeter drains must be placed below the floor level but above the
bottom of the footing. As a rule of thumb, the bottom of the footing
should be at least 4 in. below the bottom of the drain to prevent under-
mining the footing stability. Crushed stone or gravel should always be
placed above and below perimeter drains to facilitate water flow. The

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