Putting the Geo into Geotechnical – The Role of the Engineering Geologist During Construction of the Eastern Distributor Tunnel, Sydney, Australia

The 1.5km long Eastern Distributor Tunnel, to the east of Sydney’s CBD, is Australia’s first piggyback tunnel. The tunnel is excavated through a highly urbanised area with limited rock outcrop, hence the geotechnical model formulated for design purposes was heavily reliant on borehole information. During construction, detailed engineering geological mapping served to ‘proof check’ the assumptions and interpretation made in the design model.

This paper briefly summarises the design geotechnical model, the role of the Engineering Geologist during construction and the Geologist’s position in the overall context of the construction team. The post construction model, formed on the basis of in-tunnel mapping, is then presented.

It is concluded that the mapping undertaken during construction largely confirmed the preconstruction model. Where encountered tunnelling conditions were different to those anticipated in the design model, the results of engineering geological mapping allowed appropriate support redesign in ‘real time’.

The paper therefore provides a working example of the concept of ‘putting the “geo” into “geotechnical’.