Polperro outfall reaches final phase
As South West Water reaches the end of its mammoth 20-year programme of coastal works, Natasha Wiseman reports on an outfall project in Cornwall
The final phase of South West Water (SWW)’s 20-year Clean Sweep programme is coming to an end with the imminent completion of a sewage screening station and outfall in Polperro, Cornwall.
Polperro is a pretty 13th-century fishing village situated on the South-east coast of Cornwall and is popular with tourists.
Halcrow carried out the civils design for the £5M scheme, which will improve sea water quality and provide environmental protection to the area, and works began at the end of June 2010. The general public have been able to follow the works via a live webcam on the utility’s website.
They includes installation of a screening plant with ancillary and control equipment and welfare facilities housed in a reinforced concrete building at Scilly Cove. Treated sewage is now discharged through a new 375m sea outfall laid from the existing outfall chamber in a generally southerly direction to a discharge point 330m offshore.
The main permanent access to the new screening building is via a newly constructed inclined lift, running on rails attached to the cliff face at the east end of the gully at Scilly Cove.
The lift car is housed within the building at the base of the cliff when not in use. At the top of the cliff, a platform has been constructed to park a quad bike and trailer, needed to give access to the lift itself, and rock netting has been fixed to the cliff face to prevent rock falls onto the lift and the screening
Due to the exposed, coastal nature of the site, the first task of contractors Dean & Dyball Civil Engineering was to mobilise a jack-up barge and 100t crawler crane to help build a steel platform into the English Channel. This enabled materials and equipment to be delivered by sea to the location of the new screening building, and also provide a safe, level platform for the workforce and crawler crane.
In July, works on the new sea outfall started, with a 330m-long trench dredged into the seabed. The outfall pipeline itself was delivered by sea from Norway, using tug boats, and was brought ashore at Par Docks.
Installation of pre-cast concrete protection weight collars for the outfall was undertaken at Par, while scaffolding works to enable safe access to the end of the existing outfall were also carried out. The new 385m-long sea outfall pipeline, together with an attached floatation pipe, was launched from Par Docks in mid- September 2010 and towed into position. It was then installed in the dredged sea bed trench with the aid of divers.
With the temporary steel platform in place, foundation works for the screening building could get under way. This involved breaking out areas of rock for precast foundation beams to be placed and, where necessary, additional ground improvements undertaken to ensure the structural stability of the site. Pre-cast concrete units, fabricated in Truro, for the new screening building were transported to site by sea barge.
Major cliff stabilisation works had to be undertaken by subcontractor Vertical Technology when a section of cliff was identified as inherently unstable. Cornwall Council had to close the South West Coast Path for several months until the remedial work was completed, just before Christmas.
Erection of the new screening building had to start on the southern section to avoid the cliff stabilisation works.
Once foundations to the easterly section could be undertaken, both site-mixed concrete and pre-cast concrete units were used. Limited on site storage meant that the pre-cast concrete units were transported piece-by-piece as space permitted.
Once the screening building itself was complete, the building was made watertight to enable the installation of the pumps and screening equipment. During March, the large process and screening equipment was lifted inside the building and Wanless PK began construction of a new electricity substation for Western Power Distribution (WPD) to supply the screening building.
At the head of the cliff, a new lift top station has been built by the coastal path to enable access to the screening building 24m below. Piles were driven into the cliff and structural steel columns anchored to the cliff face to support the lift car rails.
A team of industrial abseilers was employed to help install the supporting structural steelwork, undertaking rock trimming and drilling works. The coastal path had to be closed again in March and April while the rails were fixed to the support steelwork and utility connections were installed. With the main structure in place, Davy Markham undertook installation of the lifting mechanism and lift car itself. This was under way by June.
An existing 85m long tunnel runs back through the rock headland, from the location of the new screening building. Flows had to be diverted through a temporary over-pumping system and safe scaffold access was established while cleaning works were undertaken inside the tunnel and modifications made to the inlet and outlet chambers at either end.
Once the tunnel had been cleaned out, survey work and the installation of new pipework could be undertaken. Divers were used to install the intermediate section of the new pipework from the outfall chamber, through the sea gully, to meet the new long sea outfall pipe.
Working in the inter-tidal zone, the divers installed 65m of pipework along the seabed. A concrete covering was provided for the intermediate section of the gully outfall pipeline to provide added protection.
The weather and rough sea conditions delayed works at times and Cornwall Council granted a dispensation to allow weekend work to help the diving team take advantage of windows when tide and weather were more favourable.
Works to rehabilitate the main sewer from Polperro’s Roman Bridge to the screening building began at the end of February with jetting and cleaning of the sewer. Following surveys, a detailed repair scheme was drawn up and temporary pumps installed to enable the sewer to be relined with plastic pipe and manholes to be rebuilt.
A temporary bypass pipeline was set up in the inner harbour to allow the lining of the existing sewer to be undertaken.
As WWT went to press, commissioning and operational trials of both the process plant and lift car system were being undertaken. Final finishing and snagging works to the screening building were being completed to enable the external scaffolding to be removed.
A permanent waterproofing system was being applied to the roof to enable topsoil to be placed and a green roof planted by Landmark Living Roofs. Using the low tides, work was continuing on the outer harbour sewer refurbishment, between the Princess of Wales Pier and the screening building.
Commissioning of the process plant, lift and other equipment was complete, as was the the outfall pipe in the gully. The entire 475m long outfall pipeline from the screening building, through the existing tunnel, to the offshore diffuser had been successfully pressure tested.
Graham Murphy, South West Water’s director of engineering said: “We are delighted that our innovative sewage treatment scheme at Polperro is nearing completion. The severe difficulties of landward access presented by the narrow streets of this ancient fishing port required us to come up with an innovative solution.
“This entailed the construction of a complex plant on a rocky headland outside the harbour, the majority of work being serviced by sea, using nearby Par docks. This unique and lowcarbon solution will help improve seawater quality and provide environmental protection to the area for many years to come.”
He added: “South West Water has invested £2B in the Clean Sweep programme, which has modernised sewage treatment all around the peninsula, removing almost 250 crude outfalls and transforming the quality of the region’s bathing waters. The scheme at Polperro is one of the final pieces of the jigsaw.”
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