Impact of soil consolidation and solution composition on the hydraulic properties of coastal acid sulfate soils

Abstract

Acid sulfate soils (ASS) are distributed worldwide on coastal floodplains, presenting a great challenge to coastal development and urbanisation. Upon oxidation, these soils become stratified with visibly distinguishable soil strata that are progressively less oxidised with depth. In this study, the geotechnical properties, quantified by hydraulic conductivity and consolidation coefficient, of an ASS profile from the Tweed River floodplain, north-eastern New South Wales, Australia, were investigated at a laboratory scale and compared with results obtained from the field. Measurements were conducted with a Rowe cell (or hydraulic consolidometer) by controlled compressive and pore water pressures. The results indicated that hydraulic conductivity and consolidation coefficient values gradually decreased with increasing consolidation pressure or decreasing void ratio, but were significantly higher for the more oxidised ASS horizons. These results suggest that controlled soil consolidation along ASS drainage banks may prove to be effective at reducing acid discharge. Passing low pH (pH 3) or high cation concentration (50 mm CaCl2) solutions through intact consolidated potential ASS samples did not induce changes in the hydraulic conductivity or consolidation coefficient of this material indicating that ASS soil ripening involves more than acidification reactions, and the practice of flushing drains with high ionic strength estuarine tidal waters is unlikely to induce soil subsidence as a result of ASS structural change and clay flocculation.