TM 5-818-1 / AFM 88-3, Chap. 7
CHAPTER 16
STABILIZATION OF SUBGRADE SOILS
have been used. The effective radius of influence for
16-1.
General.
charges using (M = lb) 60 percent dynamite is as follows:
a. The applicability and essential features of
1/3
R = 3M (feet)
(16-1)
foundation soil treatments are summarized in tables 16-1
Charge spacings of 10 to 25 feet are typical. The center
and 16-2 and in figure 16-1. The depth of stabilization
of charges should be located at a depth of about two-
generally must be sufficient to absorb most of the
thirds the thickness of the layer to be densified, and
foundation pressure bulb.
three to five successive detonations of several spaced
b. The relative benefits of vibrocompaction,
charges each are likely to be more effective than a single
vibrodisplacement compaction, and precompression
large blast. Little densification is likely to result above
increase as load intensity decreases and size of loaded
about a 3-foot depth, and loosened material may remain
area increases. Soft, cohesive soils treated in place are
around blast points.
Firing patterns should be
generally suitable only for low-intensity loadings. Soil
established to avoid the "boxing in" of pore water. Free-
stabilization of wet, soft soils may be accomplished by
water escape on at least two sides is desirable.
addition of lime; grout to control water flow into
(2) If blasting is used in partly saturated
excavations to reduce lateral support requirements or to
sands or loess, preflooding of the site is desirable. In
reduce liquefaction or settlement caused by adjacent pile
one technique, blast holes about 3 to 3/2 inches in
driving; seepage control by electroosmosis; and
diameter are drilled to the desired depth of treatment,
temporary stabilization by freezing. The range of soil
then small charges connected by prima cord, or simply
grain sizes for which each stabilization method is most
the prima cord alone, are strung the full depth of the
applicable is shown in figure 16-1.
hole. Each hole is detonated in succession, and the
resulting large diameter holes formed by lateral
16-2. Vibrocompaction. Vibrocompaction methods
displacement are backfilled. A sluiced-in cohesionless
(blasting, terraprobe, and vibratory rollers) can be used
backfill will densify under the action of vibrations from
for rapid densification of saturated cohesionless soils (fig
subsequent blasts.
Finer grained backfills can be
16-1). The ranges of grain-size distributions suitable for
densified by tamping.
treatment by vibrocompaction, as well as vibroflotation,
b. Vibratingprobe (terraprobe).
are shown in figure 16-2. The effectiveness of these
(1) A 30-inch-outside-diameter, open-
methods is greatly reduced if the percent finer than the
ended pipe pile with 3/, -inch wall thickness is suspended
No. 200 sieve exceeds about 20 percent or if more than
from a vibratory pile driver operating at 15 Hz. A probe
about 5 percent is finer than 0.002 millimeter, primarily
length 10 to 15 feet greater than the soil depth to be
because the hydraulic conductivity of such materials is
stabilized is used. Vibrations of 7%- to 1-inch amplitude
too low to prevent rapid drainage following liquefaction.
are in a vertical mode. Probes are made at spacings of
The usefulness of these methods in partly saturated
3 to 10 feet. After sinkage to the desired depth, the
sands is limited, because the lack of an increase of pore
probe is held for 30 to 60 seconds before extraction.
water pressure impedes liquefaction. Lack of complete
The total time required per probe is typically 21/2 to 4
saturation is less of a restriction to use of blasting
minutes. Effective treatment has been accomplished at
because the high-intensity shock wave accompanying
depths of 12 to 60 feet. Areas in the range of 450 to 700
detonation displaces soil, leaving depressions that later
square yards may be treated per machine per 8-hour
can be backfilled.
shift.
a. Blasting.
(2) Test sections about 30 to 60 feet on a
(1) Theoretical design procedures for
side are desirable to evaluate the effectiveness and
densification by blasting are not available and continuous
required probe spacing. The grain-size range of treated
on-site supervision by experienced engineers having
soil should fall within limits shown in figure 16-2. A
authority to modify procedures as required is esential if
square pattern is often used, with a fifth probe at the
this treatment method is used. A surface heave of about
center of each square giving more effective increased
6 inches will be observed for proper charge sizes and
densification than a reduced spacing. Saturated soil
placement depths. Surface cratering should be avoided.
Charge masses of less than 4 to more than 60 pounds
16-1
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