A Case Study on Dispersive Soils – Saraji Mine, Dysart (QLD)

The presence of excess exchangeable sodium ions is one of the main causes of dispersive behaviour in clayey soils. However, the presence of excess exchangeable cations other than sodium can also promote dispersion in soils. In this paper, the effect of the presence of excess exchangeable magnesium and sodium ions in dispersive soil were investigated and an assessment on the effectiveness of using the Australian Standard Emerson Class Number (AS ECN) testing standard to identify dispersive soil was undertaken. This case study was based on geotechnical investigations undertaken for the design of Coolibah Dam and Drain at Saraji Mine, Dysart. Laboratory testing undertaken on various samples collected during investigations completed for the dam and drain confirmed the presence of sodic and abnormal levels of magnesic soils. Although it’s well established that sodicity is a main driver of dispersion, effect of magnesicity in soils is not well defined. Further laboratory testing suggested the presence of magnesic soil may also promote dispersion. Dispersive soil samples were treated with gypsum to reduce the amount of exchangeable sodium and magnesium based on the cation exchange capacity. Whilst gypsum treatments chemically stabilised the soil, they failed to satisfy the visual based criteria from ECN testing as defined in the Australian Standards (AS). The presence of excess exchangeable sodium and magnesium ions has been identified as a likely factor promoting dispersion. Furthermore, a review of the current AS ECN testing standard is suggested as it does not effectively amalgamate the chemical properties of soil which effect soil dispersion.