15 AUGUST 2005
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.
Vibrating Probe (Terraprobe). A 30-in-outside-diameter, open-ended
pipe pile with 9.5 mm (3/8-in) wall thickness is suspended from a vibratory pile driver
operating at 15 Hz. A probe length from 3.05 to 4.57 m (10 to 15 ft) greater than the soil
depth to be stabilized is used. Vibrations of 9.5 mm to 25.4 mm (3/8 to 1 in) amplitude
are applied in a vertical mode. Probes are made at a spacing of between 0.91 and 3.05
m (3 to 10 ft). After the soil sinks to the desired depth, the probe is held for 30 to 60
seconds before extraction. The total time required per probe is typically 2 , to 4
minutes. Effective treatment has been accomplished at depths of 3.66 to 18.29 m (12
to 60 ft). Areas in the range of 376 to 585 m2 (450 to 700 sq yd) may be treated per
machine per 8 hour shift.
Test sections about 9.1 to 18.3 m (30 to 60 ft) 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 as underlying soft clay layers
may dampen vibrations.
Vibratory Rollers. Where cohesionless deposits are of limited thickness,
e.g., less than 2 m (6 ft), or where cohesionless fills are being placed, vibratory rollers
are likely to be the beat 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 2 m (6 ft) or more for heaviest vibratory rollers or a fill placed in
successive lifts, a density-depth distribution similar to that in Figure 8-7.3 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.
Vibrodisplacement Compaction. The methods in this group are similar
to those described in the preceding section except that the vibrations are supplemented
by active displacement of the soil and, in the case of vibroflotation and compaction
piles, by backfilling the zones from which the soil has been displaced.
Compaction Piles. Partly saturated or freely draining soils can be
effectively densified and strengthened by this method, which involves driving
displacement piles at close spacings, usually 1 to 2 m (3 to 6 ft) on centers. One
effective procedure is to cap temporarily the end of a pipe pile (e.g., by a detachable
plate) and drive it to the desired depth, which may be up to 18 m (60 ft). Either an
impact hammer or a vibratory driver can be used. Sand or other backfill material is
introduced in lifts with each lift compacted concurrently with withdrawal of the pipe pile.
In this way, not only is the backfill compacted, but the compacted column has also
expanded laterally below the pipe tip forming a caisson pile.
Heavy Tamping (Dynamic Consolidation). Repeated impacts
of a very