Vibrocompaction Design for Slough Sites

For a standalone design package covering a plot up to 800 m², including pre-treatment CPT profiling, laboratory grain-size curves, the design report, and post-treatment verification, the fee typically falls between £1,170 and £3,670. The spread depends on the number of CPT soundings required, the depth of the loose zone, and whether pore pressure dissipation modelling is needed for higher silt fractions.

They generally densify well because the gravel clasts are sub-angular and strong, with relatively low silt content in the cleaner horizons. The challenge is the interbedded silt lenses: if a continuous silt layer overlies a gravel unit, vibration energy reflects back and the gravel beneath remains untreated. We map those lenses with closely spaced CPT soundings before finalising the probe grid, and sometimes switch to a bottom-feed stone column approach where the silt seam is thicker than 400 mm.

BS EN 1997-1, via the UK National Annex, requires direct verification that the design relative density or cone resistance has been achieved. The accepted methods are CPT, SPT, or a combination of both, with a minimum of one test location per 200 m² of treated area, positioned at the centroid of the compaction grid where improvement is statistically weakest. We also run zone load tests when the foundation imposes high bearing pressures.

It can, but the vibration monitoring protocol becomes the controlling document. We set peak particle velocity limits at 5 mm/s for listed structures and 10 mm/s for modern framed buildings, measured with triaxial geophones fixed to the nearest foundation. The compaction sequence is staggered so no probe fires within 8 metres of the building until the far-field grid is complete, creating a densified buffer that attenuates ground-borne vibration.

If the CPT and SPT data already exist, we can turn around the design package in three to four working days. When we need to mobilise our own CPT rig in Slough, add one day for fieldwork and one day for data processing. Complex sites requiring pore pressure dissipation analysis or stone column hybrid design may extend the timeline to seven working days in total.