TM 5-818-7
CHAPTER 7
MINIMIZATION OF FOUNDATION MOVEMENT
7-1. Preparation for construction
a. Grading. The most commonly used technique is
The foundation should always be provided with ade-
grading of a positive slope away from the structure.
The slope should be adequate to promote rapid runoff
quate drainage, and the soil properly prepared to mini-
and to avoid collecting, near the structure, ponded
mize changes in soil moisture and differential move-
water, which could migrate down the foundation/soil
ment.
interface. These slopes should be, greater than 1 per-
a. Removal of vegetation. Existing trees and other
cent and preferably 5 percent within 10 feet of the
heavy vegetation should be removed. New plantings of
foundation,
like items installed during postconstruction landscap-
(1) Depressions or water catch basin areas should
ing should not be located within a distance away from
be filled with compacted soil (para 7-3a) to have a
the structure ranging from 1 to 1.5 times the height of
positive slope from the structure, or drains should be
the mature tree.
provided to promote runoff from the water catch basin
b. Leveling of site. Natural soil fills compacted at
areas. Six to twelve inches of compacted, impervious,
the natural water content and the natural density of
nonswelling soil placed on the site prior to construc-
the in situ adjacent soil minimize differential move-
tion of the foundation can ensure the necessary grade
ment between cut and fill areas of sloping ground,
and contribute additional uniform surcharge pressure
trenches, or holes caused by removal of vegetation.
to reduce uneven swelling of underlying expansive
The volume of cut portions should be kept to a mini-
soil.
mum. Cut areas reduce the overburden pressure on
(2) Grading and drainage should be provided for
underlying swelling soil and lead to time-dependent
structures constructed on slopes, particularly for
heave.
slopes greater than 9 percent, to rapidly drain off
water from the cut areas and to avoid pending of water
c. Excavation.
in cuts or on the uphill side of the structure. This
(1) Construction in new excavations (within a few
drainage will also minimize seepage through backfills
years of excavating) without replacement of a sur-
into adjacent basement walls.
charge pressure equal to the original soil overburden
pressure should be avoided where possible because the
b. Subsurface drains. Subsurface drains (fig. 7-1)
may be used to control a rising water table, ground-
reduction in effective stress leads to an instantaneous
water and underground streams, and surface water
elastic rebound plus a time-dependent heave. The re-
penetrating through pervious or fissured and highly
duction in overburden pressure results in a reduction
permeable soil. Drains can help control the water table
of the pore water pressure in soil beneath the excava-
before it rises but may not be successful in lowering
tion. These pore pressures tend to increase with time
the water table in expansive soil. Furthermore, since
toward the original or equilibrium pore pressure pro-
drains cannot stop the migration of moisture through
file consistent with that of the surrounding soil and
expansive soil beneath foundations, they will not pre-
can cause heave.
vent all of the long-term swelling.
(2) Ground surfaces of new excavations, such as
(1) Location of subsurface drains, These drains
for basements and thick mat foundations, should be
immediately coated with sprayed asphalt or other seal-
are usually 4- to 6-inch-diameter perforated pipes
ing compounds to prevent drying of or the seepage of
placed adjacent to and slightly below the baseline of
the external wall to catch free water (fig. 7-1).
ponded water into the foundation soil during construc-
tion (fig. 7-1). Rapid-cure RC 70 or medium-cure MC
(a) An impervious membrane should be placed
30 cutback asphalts are often used as sealing com-
beneath the drain in the trench to prevent migration
of surface moisture into deeper soil. The membrane ad-
pounds, which penetrate into the soil following com-
paction of the surface soil and cure relatively quickly.
jacent to the foundation wall should be cemented to
the wall with a compatible joint sealant to prevent
7-2. Drainage techniques
seepage through the joint between the membrane and
the foundation.
Drainage is provided by surface grading and subsur-
(b) If a 6- to 12-inch layer of granular material
face drains.
Previous Page
