An Innovative Soil Mixing Treatment to Underpin Existing Buildings-Paper

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Three residential buildings in Civitavecchia, Italy, experienced damaging differential settlement because their shallow footings rested on variable, moisture-sensitive manmade soils over stronger Flysch. The resulting cracks prompted investigation of a less disruptive alternative to conventional reinforced-concrete underpinning.<br /><br />The adopted treatment used a specially designed soil-mixing tool with blades that open and close to vary the column diameter. Small openings were cored through existing foundation plinths, allowing the tool to pass through and form larger cement-stabilized soil columns beneath the footings, in direct contact with the structures. Depending on footing size, two or four columns were installed per foundation. The work could be carried out in restricted basement spaces with limited disturbance.<br /><br />Field trials guided tool selection and operating parameters. Soil was mixed with Portland cement slurry, targeting a binder content of about 500 kg/m³. Quality assurance included trial columns and unconfined-compression testing of samples collected using a cased-pipe sampler inserted while the treated soil was fresh. The sampler produced representative, in-situ-cured specimens; test results showed relatively little variation with depth and were consistent with those from preliminary trials. Visual inspection also confirmed contact between the enlarged columns and the existing slab.<br /><br />The authors conclude that variable-diameter soil mixing offers a practical, comparatively economical and rapid method for improving and equalizing foundation performance. Real-time monitoring of construction parameters helped adapt the process to ground conditions and support consistent column quality. Long-term monitoring was still underway.

Keywords

differential settlement

underpinning

soil mixing

cement-stabilized columns

variable-diameter tool

shallow footings

foundation remediation

moisture-sensitive soils

Civitavecchia

quality assurance testing