Design Considerations for Station Boxes and Shafts in Swelling Rock

Bringelly Shale rock is a sedimentary rock in the Sydney Basin which contains reactive clay minerals and is thus susceptible to swelling. This poses many issues with the design and construction of permanent structures such as services shafts in Bringelly Shale, considering that swelling is a time-dependent behaviour that depends on stress relief and groundwater changes. This paper covers the design considerations of two shafts located in Bringelly Shale, including modelling techniques to take into account the swelling behaviour, several design optimization and risk management strategies to manage the swelling behaviour, reduce the associated construction risk and reduce construction and material costs. Some examples are the use of a compressible layer to allow for the swelling to happen without placing undue stress on the internal primary structure, the use of free length in the base slab tension anchor design to reduce the stiffness and thus reduce the swelling-induced loads, the combined use of a Plaxis 3D models to take into account 3D effects and Plaxis 2D models to take into account the swelling behaviour using the UDSM Swelling Rock model.