Project Hobson involves the replacement of an existing 90-year-old above-ground sewer that crosses Hobson Bay in Auckland, New Zealand, and which is nearing the end of its economic life, with a 3 km long, 3.7 m internal diameter segmentally-lined bored tunnel._x000D_
The $118 M project will offer several benefits to the owner, Watercare Services Ltd, and the local community: provide capacity to meet projected growth in the region, virtually eliminate wastewater overflows into Hobson Bay and the Waitemata Harbour, and remove an obtrusive visual feature and open up the bay for recreational purposes._x000D_
The geology in the region is a sedimentary formation, which has been significantly influenced by volcanic activity; 49 volcanoes are present within 20 km of the nearby city centre. The geology along the alignment is dominated by two palaeovalleys which have been infilled with a combination of marine alluvium and volcanic deposits._x000D_
Watercare Services Limited has elected to manage and share tunnelling risk through, among other key measures, the implementation of a geotechnical baseline report (GBR); one of the first uses of this type of document in Australasia._x000D_
This paper describes the general principles behind the use of a GBR, the development of the GBR used in Project Hobson and some lessons learnt.