Assessment of the use of sodium alginate for soil improvement in coastal applications

Rising sea levels and intensifying coastal erosion necessitate sustainable soil improvement methods to protect vulnerable coastal zones. Previous studies have demonstrated that crosslinked sodium alginate can enhance soil strength. Nevertheless, for coastal applications, it is essential to characterise the effect of increasing alginate concentrations to identify an optimal dosage and to evaluate the stability of the calcium crosslink when exposed to seawater wet-dry cycles. This study explores the potential of sodium alginate as an environmentally friendly technique for improving the stability and performance of granular soils in coastal regions, with the overarching goal of assessing its durability under repeated seawater wet–dry cycles. The experimental work focused on quartzitic, poorly graded sands, beginning with comparisons between different sodium alginates, varying sodium alginate concentrations (1.4–10%) and two mixing methodologies (dry and wet) for treatment. This was followed by an assessment of the effect of seawater wet-dry cycles on the integrity and mechanical behaviour of treated specimens. Unconfined compressive strength (UCS) tests demonstrated a linear increase in strength up to 2.3% alginate concentration, with an optimal UCS achieved at 4.6%. Higher alginate contents led to specimen deformation and reduced strength. Durability assessments involving up to 28 wet-dry cycles with artificial seawater revealed that although calcium alginate membranes maintained chemical stability, specimens exhibited 26–37% reductions in UCS, primarily due to membrane tearing from repeated expansion–contraction and crystallisation pressures from precipitated salts. SEM-EDS analyses confirmed structural damage and the accumulation of sodium chloride, calcium chloride, and calcium sulphate crystals. The results suggest that sodium alginate treatment can effectively improve clean sand strength but supplementary measures are necessary to reduce the permeability of the treated specimens, enhancing their durability under harsh coastal conditions.