Undrained shear strength of variably pre-loaded Launceston Silt

Earth levees were constructed in the 1960’s as flood protection around the suburb of Invermay, located immediately north of Launceston’s CBD. In 2007 an upgrade program was begun by Launceston City Council (LCC) to provide 1 in 200 year flood protection, meaning that about 5km of levee have to be re-aligned and raised, with the new levee alignment partially over virgin ground and partially over pre-consolidated ground. This paper presents the results of testing and analysis to determine the spatially variable undrained shear strength profiles of the compressible Launceston Silt foundation. Relationships between undrained and drained shear strength were used to predict strength increase caused by preconsolidation, with good agreement with in situ measurements. The analysis allowed development of an innovative stability model that has vertical interfaces between foundation zones of different strengths.