TM 5-818-7
until all parts of the foundation are at the same eleva-
make important changes in soil moisture and can lead
tion. If a heaved area is lowered to the same elevation
to differential heave.
as the rest of the foundation, such as to repair a mush-
9-3. Stiffened slab foundations
roomed or dome-shaped heave pattern, the slab is first
supported before digging out the soil to prevent the
Most slab foundations that experience some distress
slab from creeping down on the work crew during the
are not damaged sufficiently to warrant repairs. Dam-
digging. Attempts at leveling dome-shaped distortion
age is often localized by settlement or heave of one side
by raising the perimeter may be unsuccessful because
of the slab. The cause of the soil movement, whether
mudjacking usually causes the entire slab to rise.
settlement or heave, should first be determined and
then corrected.
9-4. Drilled shaft foundations
a. Stabilization of soil moisture. Drainage improve-
Most damage to structures with shaft foundations con-
ments and a program to control soil moisture at the
sists of fractured slabs-on-grade. The shaft may con-
perimeter of the slab are recommended (chap 7) for all
tribute to the damage caused by migration of moisture
damaged slab foundations.
down the shaft/soil interface into swelling soil beneath
b. Remedial procedures. Remedial work on slabs de-
the shaft footing. The fracture pattern of open cracks
pends on the type of movement, Repair of a settled
in the floor slab parallel to and several feet from the
area requires raising of that area, while repair of a
wall often shows that the slab had not been free to
heaved area often requires raising the entire unheaved
move near the walls. Damage to drilled shafts is often
portion of the slab up to the level of the heaved por-
caused by upward movement of the shaft from swell-
tion. Repair costs are consequently usually greater for
ing soil beneath its base and by uplift forces on the
heaving than settling cases.
shaft perimeter from adjacent swelling soil.
(1) Repair of a damaged slab consists of a combi-
nation of underpinning and mudjacking using a
ments and a program to control soil moisture around
cement grout. Mudjacking using a cement grout is re-
the perimeter of the foundation are recommended
quired simultaneously with underpinning to fill voids
(chap 7).
during leveling of the slab. Fractured slabs are usually
easier to repair than unfractured slabs that have been
removal of the slab and underlying wet soil, replace-
distorted by differential movement because usually
ment with nonswelling soil, and placement of a new
only the fractured portion of the slab requires treat-
ment. The distortion of unfractured slabs can also
slab isolated from the perimeter walls. Repair of
cause considerable damage to the superstructure and
drilled shafts consists of cutting down the top of the
inconvenience to the occupants.
shaft and releveling the foundation. The tops of the
drilled shafts are cut to the elevation of the top of the
(2) Underpinning and mudjacking are applied si-
lowest shaft where possible.
multaneously and usually clockwise around the slab
Table 9-1. Remedial Measures
Measure
Description
Drainage
Slope ground surface (positive drainage) from structure; add drains
for downspouts and outdoor faucets in areas of poor drainage,
and discharge away from foundation soil; provide subdrains if
perched water tables or free flow of subsurface water are prob-
lems; provide flexible, watertight utility connections.
Moisture stabilization
Remove natural swelling soil and recompact with impervious, non-
(maintenance of
swelling backfill; install vertical and/or horizontal membranes
constant moisture
around the perimeter; locate deep-rooted vegetation outside of
whether at high or
moisture barriers; avoid automatic sprinkling systems in areas
low levels)
protected with moisture barriers; provide a constant source of
moisture if a combination of swelling/shrinking soils is oc-
curring; thoroughly mix 4 to 8 percent lime into soil to reduce
potential for swell or pressure-inject line slurry around
the perimeter of the structure.
Superstructure
Free slabs from foundation by cutting along foundation walls; pro-
adjustments
vide slip joints in interior walls and door frames; reinforce ma-
sonry and concrete block walls with horizontal and vertical tie
bars or reinforced concrete beams; provide fanlights over doors
extended to the ceiling.
9-2
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