Vibro-compaction also known as vibro-flotation is a technique in which the density of granular soils are increased by the insertion of a heavy vibrating poker to desired depth. This vibrating poker is also known as depth vibrator or a vibroflot or simply flot. It is one of the most common ground improvement technique using vibration. Water flushing is done during the insertion process.
Vibroflotation is most suitable for very loose sands submerged under water table. Efficiency of densification reduces with increase in silt and clay content and penetration rate reduces in case of dense sands with deep water tables.
The vibroflot imparts horizontal vibrations into the ground to breakdown the frictional contacts and effective stress between soil structures. Thus ground settles into a denser structure as shown in image.
Soil structure is breakdown under dynamic stresses of around 0.5 g. When acceleration is increased to about 1-1.5g the shear strength is decreased and ground becomes fluidised. Further increase to 3 g, will lead to dilation of the soil.
Accelerations imparted by the vibrator decrease with increase in distance. The level of compaction is lesser near to the compaction point. For successful compaction, the ground should drain freely. Minimum recommended permeability is 10-5 m/s. With increasing silt content in ground, the shear stresses are not effectively transferred or are damped affecting the efficiency of compaction. Similarly, higher fines reduces degree of improvement achieved.
Vibro flotation Compaction points are usually at spacing of 1.8 to 3.0m on square or triangular grids.
Vibrating Poker Description
The vibrating poker or vibroflot consists of
- Vibrating unit;
- Follower sections;
- Lifting head.
An eccentric weight mounted at bottom of shaft connected to hydraulic or electric motor is the source of the vibration.
- Power developed: 35 kW to 100 kW, even 160 kW for some machines;
- Vibration frequencies: 30 Hz or 50 Hz;
- Amplitudes of 5-10 mm, up to 23 mm.
Penetration of flot during initial boring is supported by compressed air or water jets. Compaction of the ground is done in vertical steps of about 0.3 m providing sufficient time at each stage to maximize degree of compaction.
Applications of Vibro-Compaction or Vibro-flotation
Vibro-flotation was first used in saturated natural fine sands. Application of vibro compaction are as follows:
It is applied to improve hydraulic fills
Vibro-flotation reduces risk of liquefaction during seismic event.
Vibro-compaction is used over water.
Depths of 30 m can be treated and it can be upto about 50 m.
Multiple vibrators can be coupled together in groups.
Vibro compaction is used in conjunction with surcharging.
Limitations of Vibro-Compaction or Vibro-flotation
Vibro-compaction is not effective where the fines (silt and clay) exceeds 20 per cent.
Coarser grain size limit is applicable to vibro-compaction.
Cannot be used in low permeable soils.
Vibro-Compaction or Vibro-flotation Facts
- Spacing of treatment points are determined by means of empirical graphs and influence coefficients.
- Vibro-flotation treatment must extend outside the planned area of loading.
- Cone penetration testing and SPTs are used to test density before and after process.
- Control of degree of compaction is governed by spacing, depth of compaction, vibration energy and duration of vibration.