The massive drilling programme, undertaken by Aurecon, was designed to provide information on the underlying ground conditions ahead of constructing an underground rail line that will link Britomart Station in downtown Auckland and the existing western line near Mt Eden via the city centre. When finished, it will allow trains to pass through most Auckland stations every 5 to 10 minutes at peak times.
More than 3000 metres of core samples have been removed from 127-cored boreholes drilled. If laid end to end, the samples would cross the 1020m Auckland Harbour Bridge three times.
The drilling programme is an important element of progressing and refining design on the CRL over the next 18 months. So far, it has revealed a mixture of volcanic rock, sandy sediment, reclamation material and groundwater unearthed along different parts of the CRL route.
Of particular interest, according to Aurecon geologist Philip Kirk, is the discovery of a 10m layer of hard sandstone under Karangahape Rd, thought to have been formed as a channel-fill deposit on a submarine fan 20 million years ago.
Philip thinks the channelised sandstone is likely to be hundreds of metres across and potentially kilometres long with its contents similar to another channel seen on Motuihe Island.
“The brittleness of the rock has left it riddled with cracks that are filled with water. These will have to be drained or isolated before a station can be built to a depth of 33m below ground. We need to understand what’s going to happen to that water when we drill shafts for the station.”
The Aurecon geotechnical team is arranging a groundwater pumping test to be undertaken early in the New Year that will answer more questions about the groundwater conditions in the Karangahape Road area.
The geotechnical information is required for Resource Consents for the project, and has been used by engineers to develop the reference design for inclusion in contract documentation to be used for the procurement phase of the project.
The team is now focusing more closely on key parts of the route.
Carrying out ground investigation drilling within the busy CBD area of Auckland posed particular challenges for the team. Tom Ireland, Technical Director and Project Director for the Auckland City Rail Link at Aurecon, said the drilling programme demanded an innovative solution using non-traditional excavation methods to reduce risk.
“There were major health and safety issues involved in undertaking intrusive ground investigations in an environment that contains a complex and dense network of buried services within the road and footpath areas and the high risk of striking one of those services.
Naturally, we were also extremely aware of the effect that striking one would have on the surrounding stakeholders.”
A portal was installed within the cleared zone that allowed geotechnical drilling contractors to safely undertake investigation work on the ground below the level of the known buried services. This innovation has allowed the geotechnical investigations to be delivered safely with no interruption to key business services in the Auckland CBD.
Aurecon also prepared 3D geological models for parts of the CRL alignment as part of a broader task for data integration and visualisation.
With Auckland City predicted to grow to more than 700 000 people over the next 30 years, the Auckland City Rail Link is a key project in an integrated transport programme to keep the city moving.