Geological factors in the design and construction of the Ffestiniog Pumped Storage Scheme, Merioneth, Wales

Abstract

The Ffestiniog Pumped Storage Scheme, in Merioneth, the first in Britain, is situated on Upper Cambrian and Ordovician sediments and metasediments, on Ordovician volcanics and fine-grained intrusions and on a granite laccolite. The nature and structure of these rocks and the glacial history were major factors in the choice of site and in the design and construction of works which include two large dams, deep vertical shafts, pressure tunnels and penstocks, and a power station of 360 MW capacity.The presence of a glaciated rock-lip of Ordovician igneous rocks located the upper dam site, but the effects of glacial drag necessitated deeper excavation than expected; faulting also had to be taken into account. Twin 640 ft vertical shafts pass down through the igneous rocks into Ordovician sediments. Four pressure tunnels and the power station site were excavated in flaggy Upper Cambrian rocks, subject to a varying degree of metamorphism; in spite of well-marked bedding and cleavage and the presence of several faults (forecast from site investigation) these strata proved satisfactory both mechanically and hydraulically. Hornfelsed sediments provided sound foundations at shallow depth for the Lower Dam; deeper excavation was necessary in granite cut by minor faults and intrusions and affected by weathering.Site exploration included nearly 4000 ft of core boring.No new geological views are put forward but observations are recorded bearing on the relation of the granite to its aureole, on the nature of the Ordovician volcanics and intrusions, on the Cambrian/Ordovician boundary and on the fault and joint systems.Upper or Stwlan Dam. Gravity buttress dam 1200 ft long and 120 ft above foundations; top water level 1650 o.n. Pressure Shafts.Two shafts each 14 89 ft in internal diameter and extending 640 ft vertically.Pressure Tunnels and Penstocks. Four tunnels, each about 3800 ft long, 10 ft 8 in (concrete-lined) in internal diameter in upper half-length and 9 ft 3 in (steel-lined) in lower; four penstocks each 7 ft 6 in in internal diameter and about 800 ft long.

New litho‐ and biostratigraphic evidence for a Mid‐Ordovician hiatus in southern central Snowdonia, North Wales

“…A similar microconglomerate at the base of a mudstone sequence is also recorded in the boreholes and tunnel sections of the Ffestiniog Pumped Storage Scheme (Figure 4). This microconglomerate horizon, about 150 m above the base of the Arenig succession, was mistakenly identified by Anderson (1969) as the Garth Grit Member. The section along NO2 Tunnel (Anderson 1969, fig 4, p. 189) (Figure 4) indicates two microconglomerate beds with the lower bed occurring in spotted sandstones and siltstones.…”

The Contribution of Discontinuous Rock-Mass Failure to Glacier Erosion

“…Excavation for the upper dam foundations of the Ffestiniog pumped-storage hydro-electric scheme, constructed on the bedrock threshold of a glacial cirque, revealed considerably disturbed, hard, un weathered rhyolite dislocated along pre existing discontinuities to a depth of 13 m across a front 150 m wide, which necessitated design modifications. Anderson (1969:193) considered the dislocation to have been caused by glacier drag across the threshold: “ ... facilitated by the presence of five faults in the part most affected and by joints almost at right angles to the rock-lip ... The affected zone does not tail off but ends abruptly on both sides.…”

The role of the Engineering Geologist during construction

“…Very similar discontinuities have been found during construction at Llyn Brianne darn site. Anderson (1970) has also described the effects of glacial drag as demonstrated in the foundations of Stwlan Dam in the Ffestiniog Pumped Storage Scheme and such documentation is invaluable in the assessment of future engineering works in similar environments.…”