Identification and mitigation of risks in an Auckland tunnelling project

A thorough understanding of ground conditions and associated risks is one of the primary roles of geologists on engineering projects, and allows clear communication and mitigation of potential risks. A wide spectrum of risks was encountered on the St Marys Bay and Masefield Beach Combined Sewer Upgrade project, consisting of 3 shafts, a storage tunnel, and a marine trench and outfall in a high-profile suburb of central Auckland. Although much of the terrestrial alignment is expected to be constructed in good East Coast Bays Formation rock, key ground risks include lenses and layers of harder volcaniclastic conglomerate in the marine environment and an alluvial channel and sources of contamination along the main storage tunnel. A further challenge is the communication of the risks to a concerned and influential public and the fact that parts of the project area are close to or within sites of significance to local Maori tribes. Existing infrastructure dating back to the middle of last century and documentation of design and as-built structures pose another potential risk in the project area. A review of historical data, combined with 2 phases of ground investigation, lab testing and hydrogeological analyses, all visualised in a 3D model, has enabled us to represent and communicate a range of complex risks and aid in their mitigation.