Holiday disruption minimised

Brownsea Island is owned by the National Trust and sits at the entrance to Poole Harbour, Dorset. Both the harbour and the island are sites of special scientific interest.

The island boasts a castle visitor centre, several cottages, church, restaurant and National Trust offices. The only water supply, however, was from a borehole which was proving inadequate. It was decided a mains supply from Sandbanks on the mainland would have to be installed. Avoidatrench was asked by Halcrow to assist in feasibility and subsequent design and installation of the new 900m water main.

This was not a straightforward crossing. The launch site was to be within the gardens of a National Trust house along Panorama Road. It had restricted access, and the road has some of the most expensive real estate in the UK.

Launch location

The path of the crossing when viewed in plan was S-shaped in order to avoid land issues on launch (private jetties), staying clear of the salt lagoon and missing the foundations of the castle. The exit point was to be on the island green, adjacent to the summer open-air theatre and in front of the church. At its deepest point, the installation would be 8m beneath 14m of water.

Due to restrictive and drawn-out planning issues regarding mature trees at the launch site, and obvious noise concerns from adjacent homeowners, the works were to commence at the height of the tourist season in late July.

The 6m lengths of product pipe (100mm diameter SDR 11) and sleeve pipe (180mm diameter SDR 11 HPPE) were delivered along with all island-based construction plant via barge and off loaded by crane at the visitors quay. Activities on the island were carried out in close proximity to the public and, despite a high level of interest, no accidents or near misses were recorded in the seven-week programme.

The sleeve and product pipe were butt fusion welded into continuous 900m lengths, the product pipe was then winched inside the sleeve pipe. The pipe at this time was placed alongside a nature walk on the island awaiting completion of the pilot shot.

Manoeuvring the American Auger DD140, 12m mud recycling plant, control cabin, mud pump, 9m drill rods, bentonite and so on into the restricted garden (with protected trees) 24m x 9m took several days. While this was happening, the Avoidatrench engineer was surveying in the proposed drill path.

Work was limited to normal working hours and subject to restriction in the planning approval.

Drilling began on 8 August 2003. There could be no breakout in the busy and protected shipping channel entrance to the harbour. The run could not pass under neighbouring land, including jetties. The team had to avoid undocumented castle foundations and restrict any breakout under ornamental hotel grounds on exit.

The first 100m of the drill was through gravel, various mixes of bentonite were used until it was felt full returns would be maintained through this section and for the rest of the crossing. Progress along the whole pilot shot was slow with each rod being steered accurately on line and level and then each rod being swabbed several times to ensure returns.

With the depth of the channel being 14m the pilot shot was to have 8m of cover, to limit the chance of breakout and an environmental disaster to the absolute minimum. With the restricted working hours and the ground changing from gravel, soft silts and sands to compact sands, clay and sandstone it took until 29 August to complete the 900m pilot shot.

The only breakout, which was less than 230 litres, occurred near the end of the crossing in the castle gardens. It was decided to cut down the mud flow to a minimum and reduce amount of steering and drill the last 40m. This resulted in punch out being about 2m to the left and 2m short. This, however, was an acceptable exit and a fair trade off to prevent any further breakout in the castle grounds.

Saturday 30 August was spent removing tri-cone bit, bent hosing and non-mags these were replaced with reamer and swivel awaiting the pipe string.

A reception pit of 2m x 2m x 1m deep was then excavated around the drill rods in order to contain the bentonite. Another pit in an old quarry area was lined with polythene to become a reservoir for the bentonite which would be transported via tractor and trailer during the pipe pull from reception pit.

The pipe string was towed across the green and connected via the heavy-duty sealed towing head to the oversized reamer. The pipe pull then began at 10.30am because the hole had not been pre-reamed and swabbed as would be the norm for this length of crossing. Pull back speed was kept to a minimum ensuring good returns were maintained and a clean hole was cut.

Pipe pulling continued all day, with the reaming and pipe exiting into launch pit at 1.30am on Sunday.

Having completed the trenchless installation, all equipment was removed from site and connections made at each end of the installation, the garden was then re-instated and site cleared.

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