Boscastle, Bossiney and Tintagel Sewage Treatment and Pumping Stations
The three villages of Boscastle, Bossiney and Tintagel, located in an area of outstanding natural beauty on the North Cornish Coast, are currently being upgraded with a new combined sewage treatment scheme. A new treatment plant is being constructed half way between Boscastle and Tintagel at the hamlet of Trevalga, with 5 new pumping stations required to pull the sewage up from the villages, including a 120m hydraulic lift out of Boscastle.
Geotechnics were employed by South West Water under the direction of Consulting Engineer, Pell Frischmann, to carry out site investigations along the pipeline route and at the proposed treatment works. A three phase investigation was undertaken which included 37 trial pits to maximum 4.80m depth and 12 rotary open-hole and cored boreholes to up to 15m depth, with in situ variable head permeability tests.
The main constraints on the project were associated with the location. The site was located in an area of outstanding natural beauty and access to the locations needed careful consideration. In particular, one pumping station site was located adjacent to Tintagel Visitors Centre with only a narrow track normally used by the centre to transport tourists to and from the Castle by Landrover. A small tracked rotary rig was mobilised, and ground guards were used extensively throughout the site to prevent the already wet fields from deteriorating due to tracking.
Given the nearby location of Tintagel Castle, a watching brief was carried out by local archaeologists while several of the trial pits were excavated.
A range of geological materials were encountered during the investigation including foliated slates, phyllites and micaceous schists. The proposed treatment works at Trevalga included the construction of an 8m deep, 20m diameter activated sludge tank. Detailed rock core logging by Geotechnics identified the tank area to be underlain by weak to medium strong sub-horizontally dipping slate with generally very close to medium spaced fractures. This enabled the temporary slopes to be designed with 1:1 gradients and the tank base to be cast directly onto the fractured slate bedrock with no piled foundations and only minimal steel mesh reinforcement.
The project is currently under construction by BAM Nuttall and is due to be completed in Spring 2010.