Page 7 - TunnelTalk Annual Review 2014

First breakthrough for
Auckland mega-TBM
In September 2014, the 14.5m diameter Herrenknecht EPBM brokethrough at the end of its first drive for the NZ$1.4 billion Waterview highway connection in Auckland, New Zealand.
The project comprises 2.4km of twin running tunnels as part of a wider 5km x 3-lane highway project that completes the city’s Western Bypass and should provide much-needed relief to the congested city centre and the harbour bridge. The project is notable for hitting all schedules set out by a delivery team that incorporates the New Zealand Transportation Agency as owner, and its tunnel contractors McConnell Dowell, Obayashi and Fletcher, which are together known as the Well Connected Alliance.
The machine was powered up at the start of the first drive on the very day that had been planned nearly two years before, and first breakthrough was also achieved on schedule (Table 1).
This is despite the fact that four weeks were lost during the drive for two unscheduled stops. Construction Director Stefan Hanke told TunnelTalk from New Zealand: “We had a bit of a slower start than we thought while we monitored the performance of the TBM and checked settlement levels, and there were two unscheduled stops to replace the tail seals after we discovered leakages. That put us back a month, but we still managed the September breakthrough, as planned, so we remain fully on programme.”
“We did not rush to make metres,” said Ian Simmons, Tunnel Construction Manager on the NZ$1.4 billion project. We
always planned to do everything very carefully at the start. We went through live trials of running the TBM in full EPB mode and found the maximum required would be about 1 bar at the crown, but on a machine of this size that means about 2.5 bar at the invert. The machine has the design capability to reach a maximum thrust of 300,000kN but we managed through the first drive at about 60,000kN,” said Simmons.
Other than the tail seal issues, everything went smoothly. The geology was as predicted, comprising a mix of sand and silts of the Taurunga Group, unweathered weak siltstone and sandstone of the East Coast Bay Formation (ECBF), and with a small risk of meeting much harder (up to 150MPa) layers of cemented sandstone of the Parnell Grit type. Abrasive wear to the 19in cutters, manufactured for Herrenknecht in the USA and selected for their abrasion resistance qualities, remained within parameters, as did anticipated water inflow rates.
On all occasions the team was able to find competent ground in which to complete maintenance. As a result, no hyperbaric interventions proved necessary, said Hanke.
Fears about possible clogging at the TBM cutterhead and in the mixing chamber – a common experience in Auckland for previous mechanised drives – turned out to be ill-founded. This is attributed to the design of the TBM, which incorporates a larger than usual opening ratio of 45%,
Peter Kenyon, TunnelTalk compared to the more usual 30-35%, to
mitigate such risks. “As it happened we had very little problem with clogging. At the start of the drive we had a slight challenge after we went through the portal headwall, where some clogging was experienced, most likely as a result of a mixture of concrete, steel fibres and soil, but during the remainder of the drive we had no real issues in this regard,” said Hanke.
Water ingress at rates of up to 20 litres/ sec in some places, and at pressures of up to 3 bar, especially through sections of the harder layers of Parnell Grit, required careful balancing of face pressure.
Best excavation performance in a single 24-hour period was recorded at 29.8m, with a best monthly advance of 452m. “We expect to complete the second drive substantially faster as a result of lessons learned from the first drive about the geology, machine capability, and knowledge gained about optimised soil conditioning,” said Hanke.
Progress through what was predicted to be the most challenging section of the alignment through the saturated silts of the Taurunga Group, progressed at an average 10 rings/day, but was made more complicated towards the end by the fact
TBM will be turned into second drive across greased steel plates
table 1. Project programme milestone targets
Completion of the first bore
September 2014
Turn around period
Three months
Completion of all tunnelling
September 2015
Contractual delivery of the finished tunnels
April 2017
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