The case of the mole and the disappearing caisson
June 9, 2004
The largest mechanical mole to be seen in North Shore City, a spoil removal system and a three-storeys-high disappearing
caisson will be the stars of a $12.5m Browns Bay engineering project aimed at improving the city's beach water quality.
The mysteries of these more unusual engineering techniques are now revealed, with work about to start on a major
wastewater (sewerage) upgrade in the busy seaside centre.
North Shore City Council engineers have chosen these methods to keep disruption to a minimum while they enlarge the
wastewater system in the area's extensive catchment.
Works and environment committee chairperson, Joel Cayford, says the council has consulted extensively with local
business people and residents who may be affected by the project. The engineering methods used will ensure that the job
will be done as quickly and efficiently as possible, he says.
"The methods have been used in New Zealand and elsewhere before. They provide major benefits over digging trenches to
lay the pipes because disruption is minimised.
"The work is being done as a part of North Shore City's Project CARE programme to reduce wet weather sewage overflow
events from an average of 12 a year to two. We are making steady progress toward those goals," says Councillor Cayford.
The increased diameter of the sewer pipes will serve not only as a wastewater storage facility to cater for high wet
weather flow, but also to provide for greater trunk sewer capacity to meet the needs for future growth. More
North Shore City's wastewater network projects manager, Dave Woods, says the caisson method eliminates the need for
sheet piling and a much larger excavation.
The work will start this week and will be finished by September next year. It will culminate in reinstatement of the
Anzac Rd/Beach Front Lane area to its original condition.
The mole, a tunnelling machine with a 2.1 metre-wide bite, will burrow its way at a depth of four to five metres along
Anzac Road and later Beach Front Lane, and new 2.1 metre diameter concrete pipes will be jacked right in behind it as it
moves forward.
The spoil removal system will then move the spoil back along the tunnel, and the spoil is then brought to the surface
and trucked away. Special provision will be made to prevent trucks from leaving dirt on the road.
But before the tunnelling machine can be dropped into the hole, a 10-metre diameter jacking pit will be constructed
using the 'caisson' method. This involves the building of a three-storeys high (7.5 metre) by 10 metre-diameter concrete
cylinder, to be built in 12 curved pre-cast panels, erected above ground and joined on site. Over a period of a month or
so, spoil inside the caisson will be removed. The caisson, a weather-tight chamber equipped with a cutting blade around
its bottom edge, will slowly sink into the excavation under its own weight to form the pit to take the tunnelling
machine.
Only three pits are needed for the two separate tunnels or drives. The caisson will be placed in the first pit in the
beachfront car park at the corner of Anzac Rd and Beach Front Lane. A second pit at the end of the 600-metre drive along
Anzac Rd, (in Glencoe Rd) will be sunk to recover the tunnelling machine, and the third pit will be at the northern end
of the 500 metre section along Beach Front Lane. It will be used to recover the machine after that drive.
Associated with the sewer project is an upgrading of the existing wastewater pumping station at the end of Browns Bay
Road on the south side of the Taioatea Creek. This involves enlarging the collection well to cater for the increased
flow from the upgraded sewers as well as to allow for long term growth.
The first stage of the work is to construct the caisson and main jacking pit at the corner of Anzac Rd and Beach Front
Lane. As the pit will be the main working area for the tunnelling operation, a site working area of approximately 45
metres by 60 metres will be levelled out and fenced to provide a work area and to safely contain the tunnelling
machinery (including a crane, excavator and spoil extraction machinery).
This will mean a temporary reduction in the size of the car park, and the car park entrance will be relocated to Beach
Front Lane near the petanque court.
ENDS