or basements decrease the vertical confining pressure
board, or other retaining forms that deteriorate and
collapse (para 6-4d).
and pore water pressure, which can cause the underly-
ing expansive foundation soil to heave from adjust-
(3) Joints. Joints should be provided in interior
walls and other interior construction if slab-on-ground
ment of the moisture regime back to the natural pore
is used (fig. 6-3). The slab should be isolated from the
water pressures.
walls with a flexible impervious compound.
c. Basements. Basements and long continuous foot-
(4) Lateral earth pressure on wall. The coefficient
ings constructed in excavations are subject to swell
of lateral earth pressure can exceed one if the backfill
pressures from underlying and adjacent expansive soil.
is heavily compacted and expansive, or the natural soil
(1) Walls. Basement walls of reinforced concrete
adjacent to the wall is expansive. Controlled backfills
can be constructed directly on the foundation soil
are recommended to minimize lateral pressures and in-
without footings provided foundation pressures are
crease the economy of the foundation (para 7-3a).
less than the allowable bearing capacity (fig. 6-4a).
Steel reinforcement can provide the necessary re-
However, placing heavy loads on shallow footings may
straint to horizontal earth pressures, Unreinforced
not be effective in countering high swell pressures be-
masonry brick and concrete blocks should not be used
cause of the relative small width of the footings. The
to construct basement walls.
stress imposed on the soil is very low below a depth of
about twice the width of the footing and contributes
d. Design. Standard design procedures for founda-
little to counter the swell pressure unless the expan-
tions of buildings and other structures are presented in
TM5-818-1.
sive soil layer is thin.
(2) Voids. Voids can also be spaced at intervals be-
6-3. Reinforced slab-on-grade founda-
neath the walls to increase loading pressures on the
tions
foundation soil and to minimize flexing or bowing of
the walls (fig. 6-4b). The voids may be made with re-
a. Application. The reinforced mat is often suitable
movable - wood forms, commercially available card-
for small and lightly loaded structures, particularly if
INTERIOR
Previous Page
