Assessment of Impact Rolling for Ground Improvement on A Site With Variable Soil Condition-Paper

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This case study evaluates impact rolling as a ground-improvement method for a commercial development in Marion, South Australia. The site had variable subsurface conditions: fill over soft to firm sandy and silty clay, with localized granular layers and groundwater encountered at about 5 m. The project aimed to improve the upper soils sufficiently to support shallow footings and avoid deep foundations.<br /><br />Impact rolling was conducted in five areas using a four-sided roller. Each area received 20–30 passes, with rolling continued until settlement during five consecutive passes was typically below 5 mm. Cone Penetration Tests (CPTs) and Dynamic Cone Penetrometer (DCP) tests were performed before and after treatment to assess changes in soil resistance.<br /><br />Results showed measurable improvement mainly in granular soils. CPT and DCP resistance increased in the near-surface layers, with the treatment’s influence generally limited to the upper 1.5–2 m. Surface settlements during rolling ranged from 20 to 200 mm, and granular zones showed the strongest response. Cohesive clay and silt areas improved less, likely because their low permeability limited dissipation of pore pressure and particle rearrangement under dynamic loading.<br /><br />Proof rolling identified localized rutting and deflections greater than 5 mm, particularly in the western part of the site. These soft spots were addressed through excavation and replacement with engineered fill, followed by compaction. The study also recommended DCP testing at footing locations and ongoing monitoring for softening or instability.<br /><br />Impact rolling helped establish a building platform suitable for shallow foundations in most areas, but its performance varied with soil type. The authors conclude that it can be effective for shallow improvement in granular soils, while fine-grained or persistently soft areas may require supplementary treatment. Detailed pre- and post-treatment testing and site-specific remedial measures are essential for reliable outcomes.

Keywords

impact rolling

ground improvement

commercial development

Marion South Australia

shallow foundations

CPT testing

DCP testing

granular soils

soft clay

proof rolling