ABSTRACT
In constrained urban environments with development restrictions on building heights, there is significant opportunity to generate value through the creation of additional floorspace, achieved by subterranean excavation within the existing property footprint.
This paper examines the design and construction considerations implemented during the construction of a shallow pedestrian tunnel and lift shaft beneath a multi-storey terrace house on the edge of the iconic Sydney Harbour. The existing dwelling, constructed from brick circa 1900, was originally built on a narrow sloping block, retained by piles and elevated from the street level garage, with stairs providing the only access to the property. The new pedestrian access tunnel, immediately behind the garage, was designed to provide direct access into the living areas of the dwelling whilst providing additional valuable storage space for the client. The tunnel construction, through marine and reclaimed sands immediately below the footing structure, provided a very challenging environment, with significant potential for damage to the property and adjacent buildings.
There are several key learnings detailed in the paper including: the involvement and selection process of the Contractor and suppliers, pre-excavation ground support techniques in confined spaces, efficient methods for completing ground improvement in sandy soils for shallow tunnels, the construction sequence, and options for ground support mechanisms when hand mining and how the risks associated with residential tunnelling can be effectively managed. The paper will present the findings of a successful collaboration between stakeholders including lessons learnt and recommendations presented for future tunnels in similar environments.
