TM 5-818-1 / AFM 88-3, Chap. 7
(3) Model tests have shown that the
16-6. Reinforcement. The supporting capacity of
ultimate bearing capacity can be increased by a factor of
soft, compressible ground may be increased and
2 to 4 for the same soil unreinforced. For these tests,
settlement reduced through use of compression
the spacing between reinforcing layers was 0.3 times the
reinforcement in the direction parallel to the applied
footing width. Aggregate strip width was 42 percent of
stress or tensile reinforcement in planes normal to the
the length of strip footing.
direction of applied stress.
Commonly used
d. Thermal methods. Thermal methods of
compression reinforcement elements include mix-in-
founda- tion soil stabilization, freezing or heating, are
place piles and walls. Strips and membranes are used
complex and their costs are high.
for tensile reinforcement, with the latter sometimes used
(1) Artificial ground freezing. Frozen soil
to form a moisture barrier as well.
is far stronger and less pervious than unfrozen ground.
a. Mix-in-place piles and walls.
Several
Hence, artificial ground freezing has had application for
procedures are available, most of them patented or
temporary underpinning and excavation stabilization.
proprietary, which enable construction of soil-cement or
More recent applications have been made to back-
soil-lime in situ. A special hollow rod with rotating vanes
freezing soil around pile foundations in permafrost and
is augered into the ground to the desired depth.
maintenance of frozen soil under heated buildings on
Simultaneously, the stabilizing admixture is introduced.
permafrost. Design involves two classes of problems;
The result is a pile of up to 2 feet in diameter. Cement,
namely, the structural properties of the frozen ground to
in amounts of 5 to 10 percent of the dry soil weight, is
include the strength and the stress-strain-time behavior,
best for use in sandy soils. Compressive strengths in
and thermal considerations to include heat flow, transfer
excess of 200 kips per square foot can be obtained in
of water to ice, and design of the refrigeration system.
hese materials. Lime is effective in both expansive
plastic clays and in saturated soft clay. Compressive
moderate temperatures, e.g., 1000C+, can cause drying
strengths of about 20 to 40 kips per square foot are to be
and accompanying strength increase if subsequent
expected in these materials. If overlapping piles are
rewetting is prevented. Heating to higher temperatures
formed, a mix-in-place wall results.
can
result
in
significant
permanent
property
b. Vibroreplacement stone columns.
A
improvements, including decreases in water sensitivity,
vibroflot is used to make a cylindrical, vertical hole under
swelling, and compressibility; and increases in strength.
its own weight by jetting to the desired depth. Then, 1/2-
Burning of liquid or gas fuels in boreholes or injection of
to 1- cubic yard coarse granular backfill, usually gravel or
hot air into 6- to 9-inch-diameter boreholes can produce
crushed rock 3/4 to 1 inch is dumped in, and the vibroflot
4- to 7-foot-diameter strengthened zones, after
is used to compact the gravel vertically and radially into
continuous treatment for about 10 days. Dry or partly
the surrounding soft soil. The process of backfilling and
saturated weak clayey soils and loess are well suited for
compaction by vibration is continued until the densified
this type of treatment, which is presently regarded as
stone column reaches the surface.
experimental.
c. Strips and membranes.
(1) Low-cost,
durable
waterproof
16-7.
Miscellaneous methods.
membranes, such as polyethylene, polypropolylene
a. Remove and replace. Removal of poor soil
asphalt, and polyester fabric asphalt, have had
application as moisture barriers. At the same time, these
and replacement with the same soil treated by
materials have sufficient tensile strength that when used
compaction, with or without admixtures, or by a higher
in envelope construction, such as surrounding a well-
quality material offer an excellent opportunity for
compacted, fine-grained soil, the composite structure
producing high-strength, relatively incompressible,
has a greater resistance to applied loads than
uniform foundation conditions. The cost of removal and
conventional construction with granular materials. The
replacement of thick deposits is high because of the
reason is that any deformation of the enveloped soil layer
need for excavation and materials handling, processing,
causes tension in the membrance, which in turn
and recompaction.
Occasionally, an expensive
produces additional confinement on the soil and thus
dewatering system also may be required. Excluding
increases its resist- ance to further deformation.
highly organic soils, peats and sanitary landfills, virtually
(2) In the case of a granular soil where
any inorganic soil can be processed and treated so as to
moisture infiltration is not likely to be detrimental to
form an acceptable structural fill material.
b. Lime treatment. This treatment of plastic
strength, horizontally bedded thin, flat metal or plastic
strips can act as tensile reinforcing elements.
fine-
Reinforced earth has been used mainly for earth
retaining structures; however, the feasibility of using
reinforced earth slabs to improve the bearing capacity of
granular soil has been demonstrated.
16-16
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