RNLI STATION, BARRA

In April 2022 Central Alliance (CA), were instructed by the RNLI (Client) on behalf of Royal Haskoning DHV (RHDHV), to undertake a Ground Investigation (GI), at Castlebay on the Isle of Barra in the Outer Hebrides (see map below). The purpose of the GI was to provide ground data for the construction of a new RNLI station.

The fieldwork was delivered using a collaborative approach with RSK over two separate phases, with a hiatus of two weeks between each phase. Phase One (April 2023) comprised a Geophysical Survey of the site to determine the top level of bedrock and thickness of superficial deposits, as well as the presence of possible historical buildings/foundations and ground water levels. The Geophysics (delivered by RSK Geophysics), comprised Ground Penetrating Radar (GPR), which showed a layer interpreted to be the bedrock, at different depths across site and the absence of any buried structures. The Geophysics data was used by RHDHV to inform the location of the exploratory holes and refine the specification for the GI in Phase Two.

Due to the remote nature of the site, the GI was delivered over a continuous two week period in May 2023 and comprised, 1No. Rotary Cored Borehole to 15m bgl with Packer Testing, 4No. Hand Held Horizontal Diamond Cores (1m length), and 6No. Hand Dug Inspection Pits. The borehole was installed with a 50mm ID Groundwater Monitoring standpipe and equipped with 2No. Groundwater Monitoring Divers, thereby providing resilience with remote data collection.

Following contract award the CA Project Manager (PM), arranged regular check-point calls with the client team to finalise arrangements for the GI. Due to the remote nature of the site, it was imperative for a robust delivery plan to be agreed by all from the project outset. CA worked with the RNLI team who provided local contacts to support our operations. The tendered price by CA was robust and made allowances for delays, associated with ferry cancellations and inclement weather.

The Drilling Supervisor (DS), from CA was involved from the project outset to determine the optimum drilling methodology, along with a robust kit list to manage any breakdown of plant and unforeseen ground conditions. The Outer Hebrides are primarily formed from very hard ancient (3000 my old), metamorphosed igneous rocks known as Lewisian Gneiss. We mobilised multiple drill bits and alternative casing diameters to provide options for borehole construction of a sufficient diameter to facilitate Packer Testing.

The Packer Testing crew (as provided by Structural Soils Limited – RSK), was mobilised to site following the completion of the borehole. Both a Single and Double Packer system was implemented with pressure transducers placed above, below and within the test section with the capture of real time digital data. 5 No packer tests according to BS EN ISO 22282 were undertaken with raw data processed on site to provide a real time approximation of permeability.

The GI was supervised on a full-time basis by a Geotechnical Engineer, with over ten years experience of describing both metamorphic and igneous rocks across the UK. The Geotechnical Engineer produced a typed log with associated photographs and test schedules on a daily basis, using a tablet equipped with logging software. The CA and RHDHV Engineer worked closely to manage the GI along with sub-sampling on site to improve efficiencies with Laboratory Testing.

CA provided daily updates of progress supported with images of operations and recovered samples, thereby allowing RHDHV to make real time decisions with associated on site efficiencies. This challenging GI was delivered to the required specification and within the agreed budget and programme without incident and demonstrates the importance of collaborative working and clear lines of communication from the project outset. CA are proud to have been involved with this project and to be the first contractor to construct a borehole on the Isle of Barra.