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Geoscience for sustainable development

Fallago Rig Windfarm

Trial pit investigations for proposed wind turbines

Client: Holequest Ltd. Geotechnical Services for NBWP / Wind Prospect.

Objective: ongoing work to investigate the ground conditions at the proposed turbine locations and suitability for borrow pits.

Background: the extensive Fallago Rig site lies deep within the Lammermuir Hills, the eastern part of the Southern Uplands outcrop of Ordovician and Silurian rocks that extends across the south of Scotland. Existing geological information from the site (essentially dating from the 1870's survey) suggests little or no drift cover over a straightforward wacke succession with a few minor intrusions. However, more recent research in the western and central Southern Uplands suggests that the bedrock may be structurally complex.

Work completed: A walkover focussed on the principal river valleys where the wackes are exposed as steeply dipping, alternating units, several metres to tens of metres in thickness, of thick-bedded to massive, typically coarse-grained sandstone and cleaved, laminated to massive mudstone/siltstone. Aphyric or feldspar-phyric felsic dykes crop out locally up to 50 m thick.

Trial pitting was carried out in collaboration with the clients using 22 tonne excavators. Drift deposits several metres in thickness were proved to mantle most of the site, composed of variably silty sands and/or gravels overlain by peat up to 2 m thick. The bedrock proved extremely variable in character with widespread evidence of faulting (ranging from relatively narrow zones of faultrock to broad zones of 'crushed' mudstone) but also extensive, pervasive alteration (particularly affecting the massive sandstone units) and local thermal metamorphism adjacent to a potentially large granodioirite intrusion intersected in one corner of the site.

Products: a report of the site geology was provided with detailed logs of the trial pits and summary results in GIS format. Representative samples from the trial pits were taken for laboratory testing; felsite dyke outcrops were also sampled as a potential source of aggregate for the on-site manufacture of concrete.