Digger ready to tunnel under Tauranga streets

THE biggest tunnelling machine built in New Zealand is only a week away from starting work to eliminate the recurring nightmare of the floods which devastated dozens of homes in Pillans Point and Bureta two years ago.
Months of preparations peaked yesterday when the custom-made machine was lowered one section at a time into the drilling pit beside the lowest point of Tauranga's most vulnerable catchment.
The devastating storm of May 2005 exposed the shortcomings of Pillans Point's stormwater system, causing torrents to cascade through pensioner flats, homes and shops on its overland flow path down Shelley St, past the Bureta shops and into the harbour by Kulim Ave.
Residents, including a big turnout from the neighbouring council pensioners' village, watched in quiet satisfaction as the massive drilling head and gearbox, followed by the motor, were slotted into place.
The machine will spend 2 1/2 months drilling a 400m tunnel, big enough to hold a small car. Half the cost of the $8 million project was the tunnel.
Drilling will run directly below the most easterly section of Pillans Rd, emerging onto the Waikareao Estuary between the original Pillans family homestead and the landmark pink house of Tauranga National MP Bob Clarkson.
And if you thought a bunch of burly miners will accompany the painstaking progress of the machine through difficult soft soils, then think again.
Council's storm recovery manager Angus Emslie said the tunneller consisted of one man sitting behind a computer - no one entered the tunnel during drilling.
It is a gigantic hi-tech version of the modern system of thrusting pipes underground, instead of disruptive trenches.
A 2.9m diameter cutting head of hardened steel teeth drills the huge hole into which concrete stormwater pipes are inserted every three metres.
And with the laser-guided tunnelling machine hoping to achieve a rate of about 8m a day over a 12 hour day, five day working week, the pipe was expected to break through the bank beside the estuary by the second week in October.
The project was due to finish in February, once the estuary outlet structure had been built and the approach pipes to the tunnel were linked.
Mr Emslie said another popular misconception was that tunnelling conditions would be easier than hard rock. In fact soft collapsible ground made for harder tunnelling, with the added risk of encountering ancient tree trunks.
Tree trunks were a tunnelling nightmare because they clogged the cutting heads, with the potential to stall the machine and severely set back progress.
One of the few advantages of tunnelling through soft ground was that people should not notice their homes vibrating as drilling progressed - with the limitation on hours imposed by the noise of generators.
The tunnelling has its own electricity substation, sucking enough power to run the equivalent of a small town of nearly 500 homes.
A lucky break for project planners was the proximity of Rutherford Park which was a hive of engineering activity. Yellow silos contain the bentonite drilling mud which is mixed with water and piped through to the drilling head. The mud stops most of the groundwater from getting into the drilling operation.
The park also holds the centrifugal plant which takes the waste slurry from the drilling head and separates out the solids from water, allowing the water to be reused.
Pensioner village resident Grace Erikson, 88, who was evacuated from her flat on the day the floodwaters hit in May 2005, said she would be really glad once the problem was fixed.
She will never forget watching floodwaters steadily rising and getting closer to entering her unit, trapping her. "It got awfully close."
Although others lower down her block of flats were not so lucky, the council evacuated her for one night just to be on the safe side.
The four months taken to reach this stage of the project has provided an interesting diversion in the lives of the village residents.
Pensioner Joyce Salt said she had been watching progress every day. "It is wonderful, especially if we get rid of the floods."
Her neighbour Doris Leal expects the project will improve the frequently sodden conditions on the village's lower lawns. Mr Emslie appreciates the interest shown by residents: "We have had an inspection committee every day."