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.

a. 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 3 to 6 feet 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 60 feet.  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.

b. Heavy tamping (dynamic consolidation).

(1) Repeated impacts of a very heavy weight (up to 80 kips) dropped from a height of 50 to 130 feet are applied to points spaced 15 to 30 feet apart over the area to be densified.  In the case of cohesionless soils, the impact energy causes liquefaction followed by settlement as water drains.  Radial fissures that form around the impact points, in some soils, facilitate drainage.  The method has been used successfully to treat soils both above and below the water table.

(2) The product of tamper mass and height of fall should exceed the square of the thickness of layer to be densified.  A total tamping energy of 2 to 3 blows per square yard is used.  Increased efficiency is
obtained if the impact velocity exceeds the wave velocity in the liquefying soil.  One crane and tamper can treat from 350 to 750 square yards per day.  Economical use of the method in sands requires a minimum treatment area of 7500 square yards.  Relative densities of 70 to 90 percent are obtained.  Bearing capacity increases of 200 to 400 percent are usual for sands and marls, with a corresponding increase in deformation modulus.  The cost is reported as low as one-fourth to one-third that of vibroflotation.

(3) Because of the high-amplitude, low- frequency vibrations (2-12 Hz), minimum distances should be maintained from adjacent facilities as follows:


c. Vibroflotation.

(1) A cylindrical penetrator about 15 inches in diameter and 6 feet long, called a vibroflot, is attached to an adapter section containing lead wires and hoses. The whole assembly is handled by a crane.  A rotating eccentric weight inside the vibroflot develops a horizontal centrifugal force of about 10 tons at 1800 revolutions per minute.  Total weight is about 2 tons.

(2) To sink the vibroflot to the desired treatment depth, a water jet at the tip is opened and acts in conjunction with the vibrations so that a hole can be advanced at a rate of about 3.6 feet per minute; then the bottom jet is closed, and the vibroflot is withdrawn at a rate of about 0.1 foot per minute.  Newer, heavier vibroflots operating at 100 horsepower can be withdrawn at twice this rate and have a greater effective penetration depth.  Concurrently, a cohesionless sand or gravel backfill is dumped in from the ground surface and densified.  Backfill consumption is at a rate of about 0.7 to 2 cubic yards per square yard of surface. In partly saturated sands, water jets at the top of the vibroflot can be opened to facilitate liquefaction and densification of the surrounding ground.  Liquefaction occurs to a radial distance of 1 to 2 feet from the surface of the vibroflot.

Most vibroflotation applications have been to depths less than 60 feet, although depths of 90 feet have been attained successfully.

(3) A relationship between probable relative density and vibroflot hold spacings is given in figure 16-4.  Newer vibroflots result in greater relative densities.  Figure 16-5 shows relationships between
allowable bearing pressure to limit settlements to 1 inch and vibroflot spacing.  Allowable pressures for "essentially cohesionless fills" are less than for clean sand deposits, because such fills invariably contain some fines and are harder to densify.

(4) Continuous square or triangular patterns are often used over a building site.
Alternatively, it may be desired to improve the soil only at the locations of individual spread footings.  Patterns and spacings required for an allowable pressure of 3 tons per square foot and square footings are given in table 16-3.

 Relative density as a function of vibroflot hole spacings.
Figure 16-4.  Relative density as a function of vibroflot hole spacings.


Allowable bearing pressure on cohesionless soil layers stabilized by vibroflotation.
Figure 16-5.  Allowable bearing pressure on cohesionless soil layers stabilized by vibroflotation.

Vibroflotation Patterns for Isolated Footings for an Allowable Bearing Pressure.
Table 16-3.  Vibroflotation Patterns for Isolated Footings for an Allowable Bearing Pressure.

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