In non cohesive soils (granular soils), the optimal depth of surface compactor and vibratory roller is restricted to a few meters under ground level and large depths can be attained with deep compaction method having depth vibrators. This method is described as Vibro compaction.
The depth vibrator is curtailed into the ground under its own weight supported with water flushing from jets which are placed adjacent to the tip of the vibrator (i.e. bottom jets).It is found that penetration is very suitable when a high water flow rate is utilized, in place of high water pressure.
After achieving the optimal final depth, the bottom jets are closed and flushing sustained by water from jets positioned near the top of the vibrator. These jets lead water completely outside, facilitating the adjacent sand to run into the space around the vibrator. The vibrator is retained at the final depth unless either the power consumption of the vibrator attains pre-determined amperage or the pre-set time intervals are lapsed, generally+ 30–60sec, whomever is the sooner.
When the amperage/time criterion is fulfilled, the vibrator is elevated to a pre-determined height, generally 0.5–1.0m, and again is retained in position for the pre-set time or unless the amperage attains the target level, whichever is earlier.
The vibrator is then elevated for the next compaction step and this procedure goes on stepwise unless the vibrator attains the surface.
Due to Vibro compaction, the occurred settlement may vary from 5% to 15% of the compaction depth based on the original density and the required density. A schematic that demonstrates the step-by-step installation process of Vibro compaction is provided.
If Vibro compaction is utilized for bigger areas, it is usually executed by either a triangular or rectangular grid pattern having probe spacing in the range of 2m to 4m c/c.
The spacing is based on various factors, along with the soil type, backfill type, probe type and energy, and the level of improvement necessary.
For more information, go through the following video tutorial.