To gain access the tunnel portal, a track which will be primarily for construction purposes only,
is required from the crossing of the Glen Gyle to climb at approximately at a grade of 1:8 to the tunnel
construction area through similar regenerating hardwood bush as that on the banks of the Creek downstream.
(Simpson P 07.08).
This section of track / bench may be utilised for the placement of pipes from the tunnel
depending on the outcome of detailed geophysical and hydraulic design. Temporary culverting of the creek
immediately downstream from the tunnel portal is required to facilitate a construction platform and the removal of
tunnelling tailings by truck to a designated filling site or for roading purposes.
Environmental Impacts / Mitigation
These are expected to be similar to those of the roading activities in the Parapara Catchment and similar
procedures will be adopted to minimise effects.
Additional use of traps and settlement ponds with a regular,
set maintenance schedule will be required to minimise sediment run-off from watertables and exposed cut faces
in this catchment due to the recent placement of the alluvial materials making up the valley base and immediate sides.
This is a result of the extensive and widespread sluicing activities in this catchment where the whole hillsides on
both sides of the catchment were washed down slope and most of the material down stream into the Parapara.
The new roading will allow the public to access by vehicle the sluice ponds of Appo's Flat and the lower
Parapara River below the gorge and walking access to the upper Glen Gyle Catchment. This access will encourage hunting
activities particularly of pig which have a significant effect on the soils of this catchment.
7) Tunnel re-opening
Investigation work is required to establish the most appropriate technique for reopening the PHSC's tunnel
through to Richmond Flat.
A internal diameter of 1.2m is envisaged to replace the 4' x 5' cross section of
the original tunnel.
Pipe jacking is the currently preferred option. This will be confirmed by the re-opening
of the existing portal using small earthmoving equipment in a cut and cover operation until the depth of excavation
exceeds eight metres or the basement rock is encountered.
Pipe jacking involves the thrusting of pre-joined
sections of steel pipe of suitably thick wall section to not deform under the compressive load, into the
required tunnel face under the action of a number of large, long throw hydraulic jacks. Additional sections of
pipe are added when required and the material at the cutting face of the tunnel is removed by mechanical means
to the thrusting pit where it is loaded out onto trucks for disposal as road making material or to a designated
fill site. The tunnel is thus both cut, and shoring provided, in the one operation. Thrust distances of in excess of
700 metres required here are possible using this technique. The tunnel through the Waingaro Schist in the centre of
this section may not have collapsed and not require this form of shoring.
In order to establish tunnelling equipment and to load out the cut material from the tunnel a working area will be
required to be established on the TRHS of the Glen Gyle at approximate GR 811.6497.2. This area has a bush cover
with trees probably 50 years in age. Storage areas can be laid out on the side of the access road and the work area
limited to 200 m2.
With the completion of construction the work area will be reduced in size to the minimum necessary for maintenance purposes.