The International Information Center for Geotechnical Engineers


 Construction Procedures

The vibroflot is inserted into the ground and typically can be used to improve soil up to depths of 150 feet. Vibroflotation utilizes water and the mechanical vibrations of the vibroflot to move the particles into a denser state. Typical radial distances affected range from 5 to 15 feet (Bauer Maschinen GmbH, 2012).

The vibroflot is suspended from a crane and seats on the surface of the ground that is to be improved. To penetrate the material, the bottom jet is activated and the vibration begins. The water saturates the material to create a “quick sand” condition (i.e. temporarily liquefying the material), which allows the vibroflot to sink to the desired depth of improvement. At that point, the bottom jet is stopped and the water is transferred to the upper jet. This is done to create a saturated environment surrounding the vibroflot, thereby enhancing the compaction of the material. The vibroflot remains at the desired depth of improvement until the material reaches adequate density. The density of the soil is measured by using the power input (via the electric current or hydraulic pressure) as an index. As the material densifies, the vibroflot requires more power to continue vibrating at which point an ammeter or pressure gauge displays a peak in required power.

Once this point is reached, the vibroflot is raised one lift (generally ranging from 1 to 3 feet) and compaction ensues until the peak amperage or hydraulic pressure is reached once again. A figure of the successive steps is provided in Figure 5. The peak power requirement can be correlated to the density of the soil, so an accurate measurement of the in situ density can be recorded.

Figure 5: Vibroflotation construction sequence (Bauer Maschinen GmbH, 2012)

Figure 5: Vibroflotation construction sequence (Bauer Maschinen GmbH, 2012)

As the procedure continues, a large crater is created at the surface which must be backfilled, as seen in Figure 5. Roughly 5 cubic feet of backfill material is needed for every foot that is compacted by vibroflotation (D’Appolonia, 1954). Acceptable backfills include gravel or sand with minimal fines content, or material from onsite with a fines content of less than 6% (Bauer Maschinen GmbH, 2012). Slag has been used in some cases, and can be an economical option if there is a large supply available. The gradation of the backfill is the most important factor controlling the speed at which the backfill reaches the void created by the vibroflot. The suitability number gives an index for the quality of the backfill material. It is calculated as:

Equation 1: Suitability number (Brown, 1977)

Equation 1: Suitability number (Brown, 1977) 

Where D50, D20, and D10 are the grain size diameters (in millimeters) at 50%, 20%, and 10% passing, respectively. Brown described suitability numbers in the range of 1-10 to be excellent, while numbers greater than 50 are considered unsuitable (Brown, 1977).

Coarser material generally makes better backfill material, however, if the particle size is too large, it can become stuck between the crater and the vibroflotation apparatus, preventing it from reaching the desired depth. It should be noted that vibroflotation generally does not work for surface materials (uppermost 2 to 3 feet of material) and instead a roller is needed to attain equivalent compaction.

Figure 6: Crater created due to vibroflotation method (Bauer Maschinen GmbH, 2012)

Figure 6: Crater created due to vibroflotation method (Bauer Maschinen GmbH, 2012)

Close attention and observation of the process is critical throughout the implementation of vibroflotation. If the addition of backfill material is stopped or reduced, the vibroflot may become “starved.” When this occurs, the vibroflot vibrates in the hole without contacting the surrounding material, and thereby reducing compaction effort. This can happen when a hole collapses and cuts off the supply of backfill material to the vibroflot, workers stop moving backfill into the hole, the probe is extracted too quickly, or when the wash water flow is too great and prevents the backfill from falling (Brown, 1977).

Quality Control

During the process, it is important to ensure that the technique is operating efficiently and effectively so that low soil densities are not discovered after completion of the site improvement. Like many construction activities, quality control is very important during construction. Several aspects can be monitored during implementation, including penetration depth, penetration rate, withdrawal rate, proper probe location, volume of added backfill, backfill gradation, ammeter or hydraulic pressure peak, and vibroflot operating frequency.

Upon the conclusion of vibroflotation activities, densities are usually checked to ensure that adequate compaction was achieved. While the standard penetration test (SPT) was the most used and available method for doing this, it gave a poor measure of bearing capacity and relative density. Today, CPTs are most commonly used for verifying relative density.  Relationships have been developed which correlate CPT results to relative density.

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