A warehouse extension near the old Wet Dock hit a snag last year: the boreholes showed 6 metres of loose fluvial sand overlying the Gault Clay, and the client needed a bearing pressure of 150 kPa. Standard pad footings weren't going to cut it without serious settlement—we estimated 40 to 50 millimetres differential within the first five years just from self-weight. Vibrocompaction became the logical path. We designed a triangular grid at 2.2-metre spacing, ran a pre-production trial monitored with CPT testing to confirm the target relative density of 70 percent was achievable, and then tied the acceptance criteria directly to BS EN 1997-2 for the production phase. Ipswich has a long history of building on made ground and river terrace deposits; the difference between a straightforward job and a claims nightmare is almost always the quality of the pre-treatment investigation. When the soil grading curve falls within the treatable band—fines content below 15 percent, D10 above 0.2 millimetres—vibrocompaction delivers a level of reliability that dynamic compaction simply cannot match in built-up areas.
A well-designed vibrocompaction grid in clean Ipswich sands can double the SPT N-value in a single pass—but only if the energy transfer is matched to the grain size distribution.
