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the project team successfully delivered a river pipeline through an
environmentally sensitive riverbank, protecting the cultural heritage
of the area and ensuring that the final flood-resilient solution
was fit for purpose. These outcomes were achieved through a
willingness to innovate, a commitment to resource recovery and an
intent to reduce or eliminate material use.
Elimination of offshore dredging
Through optimising the alignment, chainage and support of the
discharge pipelines, all offshore dredging was eliminated. This
approach had benefits beyond the requirement for disposal
of dredged material. Excavation of the riverbank and offshore
dredging would have necessitated placement of scour rock, with
associated quarrying, transport and placement implications on the
environment. This approach meant that no dredged spoil needed
to be disposed of and no scour rock was required to be supplied
or placed offshore as the existing seabed was not disturbed.
Optimisation of pipeline design
The design of the discharge pipeline was optimised by utilising
two pipelines, as opposed to the original single pipeline concept.
This resulted in a reduction to half the required number of diffuser
por ts and one third of the required number of offshore supporting
piles. Fur thermore, BRC offered surplus lengths of GRP pipeline
from an earlier contract. BMC and SMEC were subsequently able
to redesign the de-aeration structure, which enabled re-use of
this surplus pipe.
Twin 800-millimetre diameter pipelines replaced a single
1200-millimetre diameter pipeline, which had been indicated
in the original concept design. These were set parallel, each
ending with half the required number of diffuser ports. As well
as reducing the required number of offshore supporting piles
by one third, this arrangement enabled the completed facility to
better deal with a staged capacity as the WWTP inflows will grow
steadily in the future.
With a smaller pipeline diameter, the solution used trenchless
technology to enable the pipelines to be installed to an exit point
in the river without impacting on the existing shoreline. This
prevented the need for shoring and construction access through
the riverbank, eliminated any damage to the natural shoreline
and associated flora, and eliminated any requirement for scour
protection after laying the pipe.
The soft alluvial soils present below the seabed were not
suitable for HDD (horizontal directional drilling), so the adopted
trenchless technology used a pneumatic pipe rammer to
drive steel envelopers to the required grade and position. The
application of this technology was at the limit of its previous use,
requiring the team to optimise the design and adopt a number
of specific techniques to ensure success. The use of trenchless
technology for the shore crossing eliminated any disturbance of
the existing riverbank and subsequent remediation.
In addition, a redesign of the onshore structures enabled the
overflow pit to be located at the end of the previously laid
incoming pipeline, thus preventing the need to dig up the sealed
access road to the boat ramp. The excavation of a three-metre-
thick layer of basalt over the length of the onshore trench
produced suitable rock for crushing and grading for re-use in
backfilling the trench, as well as for a temporary diversion to
maintain public access to the adjacent boat ramp. This temporary
diversion road was so successful that it ended up remaining as a
permanent legacy item.
These examples highlight how embracing a strategy of resource
reduction, re-use and recovery from the early stages of the
design process, through to delivery of the works, produced an
optimal solution that met all project objectives.
The new WWTP and discharge have greatly improved water
quality within the community. During project execution, an
important factor in reducing the impact of construction on the
local community was to maintain access to the public boat ramp
(a client preference but not a contract requirement). This was
achieved by constructing a short diversion road around the area
required for construction. The diversion road proved so successful
that it was left in place to enhance access in the long term.
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