UFC 3-220-01N
15 AUGUST 2005
8-7.1
General . The applicability and essential features of foundation soil
treatments are summarized in Tables 8-7.1 and 8-7.2 and in Figure 8-7.1. The depth of
stabilization generally must be sufficient to absorb most of the foundation pressure bulb.
The relative benefits of vibrocompaction, vibrodisplacement compaction, and
precompression increase as load intensity decreases and size of loaded area
increases, soft, cohesive soils treated in place are generally suitable only for
low-intensity loadings. Soil stabilization of wet, soft soils may be accomplished by
addition of lime; grout to control water flow into excavations to reduce lateral support
requirements or to reduce liquefaction or settlement caused by adjacent pile driving;
seepage control by electro osmosis; and temporary stabilization by freezing. The range
of soil grain sizes for which each stabilization method is most applicable is shown in
figure 8-7.1.
8-7.2
Vibrocompaction. Vibrocompaction methods (blasting, terraprobe, and
vibratory rollers) can be used for rapid densification of saturated cohesionless soils
(Figure 8-7.1.) The ranges of grain-size distributions suitable for treatment by
vibrocompaction, as well as vibroflotation, are shown in Figure 8-7.2. The effectiveness
of these methods is greatly reduced if the percent finer than the 0.075 Micron (No. 200)
sieve exceeds about 20 percent or if more than about 5 percent is finer than 0.002
millimeter, primarily because the hydraulic conductivity of such materials is too low to
prevent rapid drainage following liquefaction. The usefulness of these methods in partly
saturated sands is limited, because the lack of an increase of pore water pressure
impedes liquefaction. Lack of complete saturation is less of a restriction to use of
blasting because the high-intensity shock wave accompanying detonation displaces
soil, leaving depressions that later can be backfilled.
8-7.2.1
Blasting. Theoretical design procedures for densification by blasting are
not available and continuous onsite supervision by experienced engineers having
authority to modify procedures as required is essential if this treatment method is used.
A surface heave of about 150 mm (6 in) will be observed for proper charge sizes and
placement depths. Surface cratering should be avoided. Charge sizes of less than 1.8
to more than 27 kg (4 to more than 60 lb) have been used. The effective radius of
influence for charges using (meters = lb) 60 percent dynamite is as follows:
R = 3M1/3 (feet)
8-7.2.1.1
Charge Spacings. Charge spacings of 3 to 7.62 m (10 to 25 ft) are
typical. The center of charges should be located at a depth of about two thirds the
thickness of the layer to be densified, and three to five successive detonations of
several spaced charges each are likely to be more effective than a single large blast.
Little densification is likely to result above about a 1 m (3 ft) depth, and loosened
material may remain around blast points. Firing patterns should be established to avoid
the "boxing in" of pore water. Free-water escape on at least two sides is desirable.
8-7.2.1.2
flooding of the site is desirable. In one technique, blast holes about 76 to 88 mm (3 to 3
in) in diameter are drilled to the desired depth of treatment, then small charges
8-80
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