Management of geotechnical risks for some tunnels in the Wellington Region

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Published 01 September 2008
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Management of geotechnical risks for some tunnels in the Wellington Region

Rock falls and lining failures are potential hazards in tunnels. The current paper presents three case histories of the management of tunnel risks in the Wellington Region. Rock fall inspections are required by Land Transport New Zealand in the Orongorongo water supply tunnel to allow use of the access railway through the tunnel. Inspections enable potential rock fall sites to be identified and allow mitigation measures to be recommended if required. Inspections at the Kaitoke water tunnels enable identification of actual or potential rockfall sites that could adversely affect the capacity of these key water supply tunnels. The Carey’s Gully stream tunnel runs under Wellington’s Southern Landfill. Sections of this tunnel were found to have substandard lining making the tunnel vulnerable to collapse. The risk of blockage with associated potential for overtopping of the landfill during a flood event was considered to be unacceptable. A programme of periodic inspections and drilling investigations has been carried out to facilitate management of the risk. Investigations in the Carey’s Gully tunnel found that significant sections of roof concrete lining were inadequate to support long term design loadings. The drilling investigations provided confidence to implement a cost effective retrofit solution involving installation of steel beams to support the roof.

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Issue 1
Volume 34
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ISSN 0111-9532