Engineers Australia

Effects of preloading on soft clay improvement using deep soil mixing

Behzad Fatahi, Dirk Engelbert, Sanjin Mujic and Hadi Khabbaz


Poor performing soils, particularly soft clays, are more prevalent around lakes and coastal environments, where demand for construction is generally higher. It is therefore critical that suitable ground improvement techniques be developed and refined to ensure these sites perform satisfactorily under applied structural loads. There is a clear trend in geotechnical construction to further develop technologies such as Deep Soil Mixing (DSM), using cement, lime, fly ash or bottom ash, with an aim to improve the mechanical properties of problematic soils. In this study, the influence of surcharge (10 kPa to 120 kPa) applied during curing on soil-cement columns is investigated using two different clay types, namely kaolinite and bentonite. Preload provides confinement and pre-compression during curing, which in turn increases the bearing capacity of the treated ground. The results of unconfined compressive strength tests are analysed to illustrate how the mechanical properties of the clays composed of differing cement content are influenced under varying surcharges applied instantly after mixing. The results indicate that mechanical properties of cement treated soft clays, including strength and stiffness can be enhanced through the application of surcharge immediately after construction, during the curing phase. This could potentially provide a cost effective and environmentally friendly alternative by reducing the required cement content being added to soil to achieve a given strength.