Portsmouth sits on a lot of soft ground. The harbour silts, the reclaimed land around Portsea Island, the tidal clays, they all create real problems for foundations. We see it time and again. A site investigation comes back with low blow counts. The bearing capacity is marginal. Traditional footings would need to go impossibly deep. That is where stone column design makes the difference. Rather than excavating and replacing metres of poor material we install compacted stone columns through the soft layer. The columns transfer load to firmer strata below. They densify the surrounding soil. They accelerate drainage. For warehouses near the M275. For apartment blocks in Southsea. For industrial units in Hilsea. The technique fits the local geology. CPT testing helps us map the soft zones precisely before we commit to a column grid. And when we need to verify improvement after installation, in-situ permeability testing confirms the drainage paths are working.
Stone columns do two jobs at once in Portsmouth: they reinforce the soft silts and they drain the water that keeps them weak.
Process overview
Local context
Portsmouth is home to over 200,000 people, squeezed onto a flat island where buildable land is scarce. That pushes development onto marginal sites. Old landfill near Tipner. Silted channels in the Camber. Soft clays across much of Fratton. The risk is differential settlement. One corner of a building settles more than the rest. Cracks appear. Services fracture. The cost of remediation after occupation dwarfs the cost of Improvement during construction. We have seen it. Stone column design directly addresses this by creating a stiffened composite mass. The columns share load with the soil between them. Settlement becomes more uniform. The risk drops. A slope stability assessment often runs in parallel when the site is near a quay wall or an embankment. Portsmouth's sea defences and dock edges are not places to take chances with weak ground.
Visual overview
Reference standards
BS EN 1997-1:2004 (Eurocode 7: Geotechnical design), BS EN 14731:2005 (Execution of special geotechnical work – Ground treatment by deep vibration), BS 5930:2015 (Code of practice for ground investigations), ICE Specification for Ground Treatment (2012), Priebe, H.J., 'The design of vibro replacement', Ground Engineering (1995)
Additional services
Feasibility and Preliminary Design
Review of ground investigation data, assessment of soil suitability for vibro techniques, initial grid layout, replacement ratio estimation, and settlement analysis using Priebe method. Delivered as a design report with recommendations for site trial.
Detailed Design and Construction Support
Full 3D finite element modelling where required, column specification and spacing, load test specification, acceptance criteria based on post-treatment CPT, and on-site technical support during the trial and production phases.
Typical parameters
Quick answers
What ground conditions in Portsmouth suit stone columns?
Soft alluvial silts and clays with undrained shear strength above 15 kPa respond well. The marine deposits around Portsea Island, the harbour approach channel infill, and weaker zones of the London Clay are typical candidates. Very organic soils or peat require careful assessment. A site investigation with CPT profiling is the starting point.
How much does stone column design cost for a Portsmouth project?
For a typical Portsmouth site, the design package ranges from £1,070 for a small preliminary assessment to £3,960 for a full detailed design with FEM modelling and construction phase support. The final fee depends on site complexity, treated area, and the required deliverables.
Does stone column installation cause vibration that could damage neighbouring buildings?
Vibro-displacement methods do generate vibration. We assess the risk for each project. In sensitive areas, such as near historic structures in Old Portsmouth or adjacent to occupied buildings, we can specify low-vibration vibro-replacement techniques or combine the treatment with a grouted isolation perimeter.
How do you verify the stone columns are working after installation?
We specify a combination of post-treatment CPT testing between columns to measure densification, load tests on individual columns to confirm stiffness, and settlement monitoring during construction. The acceptance criteria are written into the design before work starts, aligned with BS EN 14731.
Can stone columns be used under raft foundations in Portsmouth's soft ground?
Yes, and that combination is common here. The stone columns improve the bearing stratum beneath the raft, reducing total and differential settlement. We design the column grid to work compositely with the raft, often using a load transfer platform of compacted granular fill between the column heads and the underside of the raft.