Advanced Analysis Of Track Formation On Coode Island Silt

The design of rail formation in Australia is largely dictated by railway standards using a ‘cookbook’ approach. Generally, railway standards define the required thickness of the ‘track formation’ (capping layer and structural fill) based on the soaked California Bearing Ratio (CBR) of the subgrade. In the docklands area of Melbourne, very soft to soft clay deposits of Coode Island Silt (CIS) are present close to the existing ground surface level. This material has a soaked CBR value of less than 1% and therefore falls below the soaked CBR values for which design guidance is provided within the rail standards.

This paper presents the methodology adopted for the design of track formation on subgrade that falls outside conventional railway standards. The advanced Plaxis 2 dimensional (2D) and Plaxis 3 dimensional (3D) analyses demonstrated that the very soft subgrade was not overstressed and the approach was also used to provide an estimate of the ongoing settlement rates in the CIS. Constructability issues considered during the design process due to constructing on very soft clay are also discussed. The design solution developed provided a stable formation on which to construct the tracks. It did not attempt to remove the ongoing issue of creep settlement within the CIS.

This approach could be adopted for designs on similar soft subgrades to demonstrate an acceptable track formation.