in drains maybe minimized by setting the pipe in filter
was provided beneath a slab-on-grade, the granular
fabric and pea gravel/sand.
material in the drain trench should be continuous with
the granular material beneath the floor slab. The per-
7-3. Stabilization techniques
forated pipe should be placed at least 12 inches deeper
Two effective and most commonly used soil stabi-
than the bottom of the granular layer. An impervious
lization techniques are controlled backfilling and
membrane should also be placed on the bottom and
continuous maintenance involving drainage control
sides of the drain trench but should not inhibit flow of
and limited watering of surface soil adjacent to the
moisture into the drain from beneath the floor slab.
structure during droughts. Other techniques, such as
Granular fills of high permeability should be avoided
moisture barriers and lime treatment, are not widely
where possible.
used in minimizing differential heave of single and
(c) Deep subsurface drains constructed to con-
multistory buildings. Presetting or pending for peri-
trol arising water table should be located at least 5
ods of a few months to a year prior to construction is
feet below a slab-on-grade. An impervious membrane
often effective but normally is not used because of
should not be placed in the drain trench. These drains
time requirements. Prewetting should not be used on
are only partially effective in controlling soil heave
fissured clay shales because swelling from water seep-
above the drain trench, and they are relatively expen-
ing into fissures may not appear until a much later
sive. A more economical solution may be to place a
date and delayed problems may result.
temporary (or easily removable slab-on-grade) with a
permanent slab after the groundwater table has
reached equilibrium.
of surface swelling soil and replacement with nonex-
(2) Outlets. Drains should be provided with out-
pansive, low permeable backfill will reduce heave at
lets or sumps to collect water and pumps to expel
the ground surface. Backfills adjacent to foundation
water if gravity drainage away from the foundation is
walls should also be nonswelling, low permeable mate-
not feasible. Sumps should be located well away from
rial. Nonswelling material minimizes the forces exert-
the structure. Drainage should be adequate to prevent
ed on walls, while low permeable backfill minimizes
any water from remaining in the drain (i.e., a slope of
infiltration of surface water through the backfill into
at least 1/8 inch per foot of drain or 1 percent should be
the foundation soil. If only pervious, nonexpansive
provided).
(granular) backfill is available, a subsurface drain at
(3) Drain trench material. The intrusion of fines
the bottom of the backfill is necessary to carry off in-
7-2
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