Durability of soil-cement columns in coastal areas

Global warming and sea level rise have become major concerns of the modern world with the Intergovernmental Panel on Climate Change (IPCC) reporting that sea levels may rise by 52-98 cm in the 21^st century. At the upper end of predicted sea level rise, 50% of the world’s population will be affected, with 33% of coastal land lost. As the majority of buildings and transport infrastructure are concentrated in these coastal areas, it is very important to understand of the longevity of these structures in the face of sea level rise. Soil-cement columns are a geotechnical solution used for ground improvement in coastal areas. However, after long periods of exposure, the strength of these columns may decrease to below their designed safe bearing capacity ultimately resulting in failure. In this study, needle penetration resistance tests, uniaxial compression tests, thermogravimetric analysis, chemical and image analyses were applied to determine the extent of deterioration in scaled soil-cement columns exposed to synthetic seawater. The effects of high sulphate concentrations (100%, 200%, 500% and 1000% that of seawater) on the durability of soil-cement samples were also studied. The experimental results show that the effects of seawater (sulphate) are significant on the outer surface strength development. For samples exposed to seawater, inhibition of the portlandite and formation of gypsum and ettringite are the main reasons leading to the destruction of soil-cement samples. Moreover, the deterioration is strong at the surface and develops inward with time. An analytical model has been developed and calibrated using the experimental data to predict the deterioration depths and total strength change of the soil-cement columns as a function of time and sulphate concentration. Results show that for the 0.5 m diameter column exposed to 200% SW, the strength will fall below the minimum design strength after 75 years. For higher sulphate environments (500% and 1000% that of seawater), the same column would never reach the minimum design strength requirement. Consequently, this has significant implications to stabilising soils in high sulphate environments such as those containing pyrite which makes up approximate 95,000 km² of the Australian coastline.