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Creating a Geotechnical Baseline Report for Project Hobson, Auckland


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Author H Asche, M Sheffield and M Sheffield


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.

The $118 M project will
offer several benefits to the owner, Watercare Services Ltd, and the local

  • provide capacity to meet projected growth in the

  • 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.

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.

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.

This paper describes the general principles behind the use of a GBR, the
development of the GBR used in Project Hobson and some lessons