Vibro compaction densifies clean, cohesionless granular soils with a downhole vibrator. Keller has performed 100’s of projects using vibro compaction.
- Increase bearing capacity
- Decrease settlement
- Mitigate liquefaction
The vibrator is typically hung from a crane and lowered vertically into the soil under its own weight and vibrations. Penetration is usually helped by water jets integrated into the vibrator assembly. After reaching the bottom of the treatment zone, the soils are densified as the vibrator is raised in lifts. During vibro compaction, clean sand backfill is typically added at the ground surface to compensate for the decrease in soil volume from the densification process. The vibratory energy reduces the intergranular forces between the soil particles, allowing them to move into a denser configuration, typically reaching a relative density of 70 to 85%. The treated soils have increased density, friction angle and stiffness. Compaction is achieved above and below the water table.
The improved soil characteristics depend on the soil type and gradation, spacing of the penetration points, and the time spent performing the compaction. Generally, the vibro compaction penetration spacing is between 6 feet and 14 feet, with centers arranged on a triangular or square pattern. Compaction takes place without stresses in the soil exceeding the overburden stress, which ensures permanent densification.
The use of clean sand backfill during vibro compaction allows the original site elevation to be maintained. However, on sites where the planned final grade is below the existing grade, no backfill is added, resulting in lowering of the site surface elevation during compaction.
Vibro compaction allows the use of economical spread footings with acceptable settlements and design bearing pressures generally in the range of of 5 ksf up to 10 ksf. The process also reduces the seismic liquefaction potential. The required treatment depth is designed for each project and is typically in the range of 15 to 50 feet with a maximum depth of 120 feet.
Vibro compaction rigs can be fully instrumented with an on-board data acquisition system. Data from the system, such as amperage and lift rate, are recorded and displayed in real-time alongside specified target values on an in-cab monitor. Monitoring allows the operator to correct any deviations in real-time during the construction process to keep the vibro compaction within project specifications.