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Compacting Loose Sands

What Is Vibrocompaction?

Crane mounted vibratorVibrocompaction is a ground improvement process for densifying loose sands to create stable foundation soils. The principle behind vibrocompaction is simple. The combined action of vibration and water saturation by jetting rearranges loose sand grains into a more compact state.  Vibrocompaction is performed with specially-designed vibrating probes. Both horizontal and vertical modes of vibration have been used in the past. The vibrators used by TerraSystems consist of torpedo-shaped probes 12 to 16 inches in diameter which vibrate at frequencies typically in the range of 30 to 50 Hz.

The probe is first inserted into the ground by both jetting and vibration. After the probe reaches the required depth of compaction, granular material, usually sand, is added from the ground surface to fill the void space created by the vibrator. A compacted radial zone of granular material is created.

What Soils Are Suitable?

Vibrocompaction is ideally suited for clean sands where the silt content is less than 10 to 15 percent. Soils with appreciable silt content can not be densified by vibration alone. However, the use of gravel backfill with the vibroreplacement (stone column) method extends the range of soils that can be improved to silty and clayey sands, silts and clays.

How Much Improvement Is Possible?

The degree of densification that can be achieved by vibrocompaction is dependent upon many factors, including:

  • Soil type, plasticity, and gradation characteristics.
  • Spacing of compaction points.
  • Type of backfill.
  • Characteristics of vibrators.
  • Construction techniques used.

Relative densities in excess of 85% can be achieved, with relative densities of 70 percent being common. Allowable bearing pressures of up to 8,000 pounds per square foot (psf) are not uncommon with vibrocompaction. Minimal settlement results since the modulus of deformation is often increased to over 1,000 tsf.