A depth vibrator is essentially a long steel tube with an eccentric weight spinning inside it, suspended from a crawler crane. In Dublin, our team deploys electric-powered units that reach 25 metres below ground level, penetrating the loose sands and silts of the Liffey valley. The vibrator is lowered under its own weight, assisted by water jets at the tip that fluidise the soil temporarily. Once it hits target depth, the operator retracts it in controlled lifts—usually 0.5 to 1.0 metre steps—while the horizontal vibrations rearrange the grain skeleton into a denser configuration. The result is a compacted column of ground that extends radially from the insertion point. We often combine this with a CPT test before and after treatment to quantify the improvement in cone resistance, and in projects where the fill is too coarse for CPT, we use sand cone density tests to verify compaction between columns. For sites near the quays where soft estuarine clays appear, we also cross-check with MASW surveys to confirm that shear wave velocity has increased across the treatment zone.
A well-designed vibrocompaction grid in Dublin's alluvial sands can reduce post-construction settlement to under 15 millimetres for a five-storey structure.
