Vibro compaction technique is applicable and effective for the improvement of loose natural fine sands as well as the compaction of Fills. Relative density is improved, potential for liquefaction from a seismic event is reduced, improvement depth up to 20.0 m or even much deeper can be achieved with this technique depending on the size of equipment used. Vibro compaction technique cannot be employed effectively if the silt content exceeds 20%.
Setting out of the compaction points location shall be carried out by a qualified surveyor based on approved probe layout drawings. The points shall be marked by fixing steel pegs into the ground, keeping 30 – 40 cm above the ground in order to keep it visible by the operator and foreman, the pegs will be also marked with working tape.
The densification of essentially granular soils will be carried out using heavy duty vibrating pokers. The vibrating poker is cylindrical, and has within its bottom section an eccentric weight, which is rotated using hydraulic motor. The lower working section of the vibrator is separated from the upper “non working” section of the poker by a vibration isolator whose objective is to restrict vibration to the lower section only. Rotation of the eccentric weight imports vibration in a horizontal plane to the lower section of the poker.
The vibroflot is suspended from a tracked crawler crane. Penetration at each compaction point to the required depth is achieved by a combination of the vibratory action of the poker (vibroflot) and the jetting action of high-pressure water from nose cone jets. The high-pressure water jetting causes temporary liquefaction of the soils surrounding the vibrator, which also assists penetration. On reaching the required depth the water pressure on the nose cone jets is reduced and sometimes-additional lower pressure side water jets are then utilised to assist in the compacting phase.
The vibroflot is extracted in stages, with appropriate pauses at intervals to allow the vibratory action of the vibroflot / poker to compact the soils around the point of insertion. The objective of the additional side jets (if used / required) is to encourage erosion of the granular soils around the bore and assist its introduction to the zone of compaction. By this means the vibroflot is gradually brought to the surface to provide a zone of compacted ground around the point of insertion. Additional sand may also be added from the surface, and the rate of extraction would be varied to suit conditions actually encountered to provide the correct degree of densification.
The technique is generally only used in clean granular soils with less than 15% fines (silt / clay). Because the vibro compaction is rearranging the sand particles to a denser state there will be a reduction in volume, which will be evident at the surface as a cone of depression. This would normally be filled with sand and compacted with the vibroflot in stages to the surface. After the vibro-compaction post treatment levels will be taken, Client’s supplied sand will be used to compact the surface to the required level. The installation technique is shown in Appendix.
On completion of the treatment foundations may be constructed at normal depths (in excess of 0.50 m), while,(depending on loadings) floor slabs may be constructed, as normal, after proof rolling of the surface.
Water supply for the Contract is to be pumped from the nearby sea.
TREATMENT SPACINGS AND DEPTHS
The normal procedure is to estimate the required bearing pressure, from which an appropriate spacing of compaction may be determined. This may be considered either in terms of area treatment, or specific treatment beneath the foundations. Based on the trials carried out at this site we propose the following treatment regime. The vibro compaction will be carried out in a triangular pattern in the area provided to us.
After 6 – 10 days from vibro compaction works completion, post treatment static cone penetration test will be carried out in order to assess the performance of the treated ground.
Drawings will be prepared which give locations of all compaction points, related to known reference points/guides. Each compaction points will be assigned a unique number.
Daily record sheets will be prepared which will include the following information for each compaction point (probe).
- Probe no.
- Date installed.
- Penetration depth
- Time started
- Time completed
- Hydraulic pressure range during penetration
- Hydraulic pressure during extraction and compaction
- Water pressure of the tip nose jets during penetration
- Water pressure of the side jets
- Quantity of sand fill
- Level of original ground level before treatment
- Level of ground level after treatment