Sloping Rubble Seawalls Founded on Soft Soil Improved by Stone Columns-Paper

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The paper presents design and construction guidance for sloping rubble marine seawalls founded on soft soil improved with stone columns. This “zero-dredge” approach avoids removing seabed deposits and offers environmental benefits, while stone columns can improve strength, drainage, settlement performance and slope stability. Past projects indicate they can remain effective despite substantial settlement and lateral movement.<br /><br />Successful performance depends on adequate column confinement, staged construction and careful control of deformation. The authors recommend pre-consolidating very soft upper soil with vertical drains and preload, using a stone-column replacement factor generally of at least 15%, and allowing pause periods for the soil to gain strength between filling stages. Reclamation fill should advance obliquely to the seawall, rather than perpendicular to it, to limit lateral displacement. Designers should account for soil smearing around columns and consider larger column diameters in softer upper layers. High stress concentration on columns can increase horizontal movements and should be assessed carefully.<br /><br />Treatment depth should reflect bearing conditions and potential failure surfaces, but field conditions may differ from those inferred from limited CPT data. Accordingly, refusal criteria should incorporate construction feedback, including vibroprobe effort. Offshore installation quality control should record depth, equipment amperage, gravel-tube pressure and material consumption at intervals no longer than one second, helping identify defects.<br /><br />Because seawall behaviour in soft ground is difficult to predict, the authors advocate an observational approach. Instrumentation and in-situ testing should be used to compare actual performance with design predictions and trigger levels. Vane shear tests can verify soil-strength gains; inclinometers and extensometers track movement and settlement; settlement plates and piezometers provide supplementary information. A project risk register should identify hazards and specify timely mitigation or remedial actions, such as extending pause periods, adding drains or constructing a stabilizing toe berm.<br /><br />Overall, stone-column-supported seawalls can be reliable and cost-effective when design, construction staging, quality control, monitoring and risk management are integrated.

Keywords

stone columns

sloping rubble seawalls

soft soil improvement

zero-dredge

marine seawalls

staged construction

soil stabilization

settlement performance

quality control

observational approach