TM 5-018-7
Uniform limited watering around the struc-
filtrated water (fig. 7-1) and to minimize seepage of
(6)
ture during droughts to replace lost moisture.
water into deeper desiccated foundation expansive
soils.
c. Moisture barriers. The purpose of moisture bar-
riers is to promote uniform soil moisture beneath the
soil and compaction control has been satisfactory in
foundation by minimizing the loss or gain of moisture
some cases if nonswelling backfill is not available.
through the membrane and thus reducing cyclic edge
However, this use of backfill should be a last resort,
movement, Moisture may still increase beneath or
(a) In general, the natural soil should be com-
within areas surrounded by the moisture barriers lead-
pacted to 90 percent of standard maximum density
ing to a steady but uniform heave of the foundation or
and should be wet of optimum water content. Founda-
slab-on-grade.
tion loads on fills should be consistent with the allow-
(1) Types of barriers. These barriers consist of
able bearing capacity of the fill. Overcompaction
horizontal and vertical plastic and asphalt membranes
should be avoided to prevent aggravating potentially
and granular materials. Concrete is an ineffective
swelling soil problems such as differential heave of the
moisture barrier. Longlasting membranes include
fill. Compaction control of naturally swelling soil is us-
chlorinated polyethylene sheets, preferably placed
ually difficult to accomplish in practice. Some soils be-
over a layer of catalytically blown or sprayed asphalt.
come more susceptible to expansion following remold-
All joints, seams, and punctures should be sealed by
ing, and addition of water to achieve water contents
plastic cements or concrete/asphalt joint sealants.
necessary to control further swell may cause the soil to
ASTM D 2521 (Part 15) describes use of asphalt in
be too wet to work in the field.
canal, ditch, and pond linings (app A).
(b) As an alternative, backfills of lime-treated
(2) Horizontal.
natural soil compacted to 95 percent standard maxi-
(a) An impervious membrane on the ground sur-
mum density at optimum water content may be satis-
face in a crawl space where rainfall does not enter may
factory if the soil is sufficiently reactive to the lime (d
help reduce shrinkage in clayey foundation soils with
below), Lime treatment may also increase soil strength
deep groundwater levels by minimizing evaporation
and trafficability on the construction site.
from the soil. A vapor barrier should not be placed in
(2) Backfill adjacent to walls. A IV on lH slope
ventilated crawl spaces if there is a shallow water ta-
cut into the natural soil should dissipate lateral swell
ble or if site drainage is poor because heave maybe ag-
pressures against basement or retaining walls exerted
gravated in these cases. Figure 7-2 illustrates a useful
by the natural swelling material. The nonswelling
application of horizontal membranes,
backfill should be a weak material (sand fill with fric-
(b) Other applications include the use of hori-
tion angle of 30 degrees or lessor cohesive fill with co-
zontal moisture barriers around the perimeter of struc-
hesion less than about 0.5 tons per square foot) to al-
tures to reduce lateral variations in moisture changes
low the fill to move upward when the expansive natu-
and differential heave in the foundation soil. Plastic or
ral soil swells laterally. Restraining loads should not
other thin membranes around the perimeter should be
be placed on the surface of the fill. A friction reducing
protected from the environment by a 6- to 12-inch-
medium may be applied on the wall to minimize fric-
thick layer of earth.
tion between the wall and the backfill, TM 5-818-4
(c) A disadvantage of these barriers is that they
discusses details on optimum slopes of the excavation
are not necessarily reliable and may be detrimental in
and other design criteria.
some cases. For example, most fabrics and plastic
membranes tend to deteriorate with time. Undetected
b. Maintenance. Maintenance programs are di-
(and hence unrepaired) punctures that allow water to
rected toward promoting uniform soil moisture be-
get in, but not to get out, commonly occur in handling
neath the foundation. A good program consists of the
on placement. Punctures may also occur during plant-
following:
ing of vegetation. If the barrier is a concrete slab, the
(1) Maintenance of a positive slope of about 5 per-
concrete may act as a wick and pull water out of the
cent around the structure for drainage and elimination
soil.
of water catch areas.
(3) Vertical.
(2) Maintenance of original drainage channels and
(a) Plumbing or utility trenches passing
installation of new channels as necessary.
through the barrier may contribute to soil moisture be-
(3) Maintenance of gutters around the roof and di-
neath the foundation.
version of runoff away from the structure.
(b) The vertical barrier (fig. 7-3) should extend
(4) Avoidance of curbs or other water traps
to the depth of the active zone and should be placed a
around flower beds.
(5) Elimination of heavy vegetation within 10 to
minimum of 3 feet from the foundation to simplify
construction and to avoid disturbance of the founda-
15 feet of the foundation or 1 to 1.5 times the height
tion soil. The barrier may not be practical in prevent-
of mature trees.
1-3
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