UFC 3-220-01N
15 AUGUST 2005
8-7.6.3
Strips and Membranes. Low-cost, durable waterproof membranes, such
as polyethylene, polypropylene asphalt, and polyester fabric asphalt, have had
application as moisture barriers. At the same time, these materials have sufficient
tensile strength that when used in envelope construction, such as surrounding a
well-compacted, fine-grained soil, the composite structure has a greater resistance to
applied loads than conventional construction with granular materials. The reason is that
any deformation of the enveloped soil layer causes tension in the membrane, which in
turn produces additional confinement on the soil and thus increases its resistance to
further deformation.
In the case of a granular soil where moisture infiltration is not likely to be
detrimental to strength, horizontally bedded thin, flat metal or plastic strips can act as
tensile reinforcing elements. Reinforced earth has been used mainly for earth retaining
structures; however, the feasibility of using reinforce earth slabs to improve the bearing
capacity of granular soil has been demonstrated.
Model tests have shown that the ultimate bearing capacity can be
increased by a factor of 2 to 4 for the same soil unreinforced. For these tests, the
spacing between reinforcing layers was 0.3 times the footing width. Aggregate strip
width was 42 percent of the length of strip footing.
8-7.6.4
Thermal Methods. Thermal methods of foundation soil stabilization,
freezing or heating, are complex and their costs are high.
8-7.6.4.1
Artificial Ground Freezing. Frozen soil is far stronger and less pervious
than unfrozen ground. Hence, artificial ground freezing has had application for
temporary underpinning and excavation stabilization. More recent applications have
been made to back-freezing soil around pile foundations in permafrost and maintenance
of frozen soil under heated buildings on permafrost. Design involves two classes of
problems; namely, the structural properties of the frozen ground to include the strength
and the stress-strain-time behavior, and thermal considerations to include heat flow,
transfer of water to ice, and design of the refrigeration system.
8-7.6.4.2
Heating. Heating fine-grained soils to moderate temperatures, e.g.,
100C +, can cause drying and accompanying strength increase if subsequent rewetting
is prevented. Heating to higher temperatures can result in significant permanent
property improvements, including decreases in water sensitivity, swelling, and
compressibility; and increases in strength. Burning of liquid or gas fuels in boreholes or
injection of hot air into 15 to 23 mm (6 to 9 in) diameter boreholes can produce 1.22
meters to 2.13 ms (4 to 7 ft) diameter strengthened zones, after continuous treatment
for about 10 days. Dry or partly saturated weak clayey soils and loess are well suited
for this type of treatment, which is presently regarded as experimental.
8-7.7
Miscellaneous Methods.
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