Vibrocompaction Methods.

Vibrocompaction methods (blasting, terraprobe, and vibratory rollers) can be used for rapid densification of saturated cohesionless soils (fig 16-1).  The ranges of grain-size distributions suitable for treatment by vibrocompaction, as well as vibroflotation, are shown in figure 16-2.  The effectiveness of these methods is greatly reduced if the percent finer than the 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.

Applicable grain-size ranges for different stabilization methods.
Figure 16-1.  Applicable grain-size ranges for different stabilization methods.

Range of particle-size distributions suitable for densification by vibrocompaction.
Figure 16-2.  Range of particle-size distributions suitable for densification by vibrocompaction.

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.

a. Blasting.

(1) Theoretical design procedures for densification by blasting are not available and continuous on-site supervision by experienced engineers having authority to modify procedures as required is esential if this treatment method is used.  A surface heave of about 6 inches will be observed for proper charge sizes and placement depths.  Surface cratering should be avoided.

Charge masses of less than 4 to more than 60 pounds have been used.  The effective radius of influence for charges using (M = lb) 60 percent dynamite is as follows:

Charge spacings of 10 to 25 feet 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 3-foot 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.

(2) If blasting is used in partly saturated sands or loess, preflooding of the site is desirable.  In one technique, blast holes about 3 to 3/2 inches in diameter are drilled to the desired depth of treatment, then small charges connected by prima cord, or simply the prima cord alone, are strung the full depth of the hole.  Each hole is detonated in succession, and the resulting large diameter holes formed by lateral displacement are backfilled. 
A sluiced-in cohesionless backfill will densify under the action of vibrations from subsequent blasts.  Finer grained backfills can be densified by tamping.

b. Vibratingprobe (terraprobe).

(1) A 30-inch-outside-diameter, open- ended pipe pile with 3/, -inch wall thickness is suspended
from a vibratory pile driver operating at 15 Hz.  A probe length 10 to 15 feet greater than the soil depth to be stabilized is used.  Vibrations of 7%- to 1-inch amplitude are in a vertical mode.  Probes are made at spacings of 3 to 10 feet.  After sinkage to the desired depth, the
probe is held for 30 to 60 seconds before extraction.

The total time required per probe is typically 21/2 to 4 minutes. Effective treatment has been accomplished at
depths of 12 to 60 feet.  Areas in the range of 450 to 700 square yards may be treated per machine per 8-hour shift.

(2) Test sections about 30 to 60 feet on a side are desirable to evaluate the effectiveness and required probe spacing.  The grain-size range of treated soil should fall within limits shown in figure 16-2.  A square pattern is often used, with a fifth probe at the center of each square giving more effective increased densification than a reduced spacing.  Saturated soil conditions are necessary.  Underlying soft clay layers may dampen vibrations.

c. Vibratory rollers.   Where cohesionless deposits are of limited thickness, e.g., less than 6 feet, or
where cohesionless fills are being placed, vibratory rollers are likely to be the best and most economical means for achieving high density and strength.  Use with flooding where a source of water is available.  The effective depth of densification may be 6 feet or more for the heaviest vibratory rollers (fig 16-3a).  For a fill placed in successive lifts, a density-depth distribution similar to that in figure 16-3b results.  It is essential that the lift thickness, soil type, and roller type be matched. Properly matched systems can yield compacted layers at a relative density of 85 to 90 percent or more.

Sand densification using vibratory rollers.
Figure 16-3.  Sand densification using vibratory rollers.

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