Vibro Stone Columns

The use of vibro replacement (stone columns) relies heavily upon the soil properties of the material to be improved. The soil must be appraised in terms of:

Type (cohesive/granular/intermediate)

Soil Description (Clay/Sand/Gravel/Peat/Fill)

Strength/density (ability to provide lateral stability for the stone column)

Swell/Heave potential

Water Content/Standing Water Level.

 

 

Granular Soils

Treatment – vibration increases the density by reduction of the void ratio of the granular soils. The vibrations act to overcome the frictional contacts (effective stress) between the soil particles, and forms a state of greater relative density

 

Cohesive Soils

Treatment – reduces pore water pressure therefore increasing effective stress (strength). The cohesive soils also confine the stone column and this passive resistance allows the dense granular column to reinforce the soil.

 

Limitations

  • Very soft Clays (cu <15kN/m2) caution for clays with cu <30kN/m2
  • Peat thicker than 600mm (Ciria 573), 300mm (NHBC)
  • Recently placed cohesive fill (self weight settlement)
  • Close Proximity to trees with sensitive clays (see NHBC guidance note 4.2)
  • Obstructions/Hard ground – generally results in incomplete penetration, and will require grubbing out (obstruction) or Pre-boring or pre-loosening depending on depth (hard ground)
  • Filled ground of single sized material or large obstructions – can form large voids in the ground and can not be improved by vibro techniques.
  • Loose fills susceptible to collapse settlement or inundation or rises in groundwater – the column acts as a drain, and if left open to the elements can cause washing of the soils into the column voids, or if changeable water levels this can have the same effect on silt/clay and fine sand deposits.
  • Edges of filled pits/quarries – can form high differential settlements
  • Near crest or toe of slopes – can reduce slope stability
  • Close Proximity to existing Buildings/Foundations

     

     

  • Methodology

    • The Vibro Rig displaces the soil by vibrating a mandrel into the ground to the required depth or refusal, whichever is achieved first.
    • The mandrel is withdrawn and the subsequent void filled with a clean stone. The stone is generally a 40mm – 75mm clean quarried material, however other sources of materials can be used (crushed concrete, screened railway ballast).
    • The mandrel is then re-introduced to the in-filled void and taken down to close to the base of the previously formed void, displacing the stone laterally into the surrounding soil. The process is repeated with subsequent filling and compaction of the stone to form a vertical stone column, with close interlocking into the surrounding soils.
    • This method provides the ground with the ability of increased load bearing characteristics.