Cut and cover tunnel at Hermitage Quarry
Following the granting of planning permission to extend Hermitage Quarry, which guaranteed another 25 years of production, a cut and cover tunnel needed to be constructed to link the existing quarry and its processing plant to the new quarry extension. The tunnel was necessary in order to preserve in the long term a public by-way, equine gallop and informal pathway. Whilst the tunnel was being constructed, existing public byways and footpaths were temporarily diverted.
Gallagher Ltd was employed to construct this new cut and cover tunnel on a design and build basis. It had to be of sufficient size to comfortably allow two large dumptrucks to pass each other and to provide natural daylight.
To construct the tunnel, 64,000 tonnes of rock had to be carefully blasted and removed. The tunnel is 100m long by 16m wide and 15m clear internal height. It was constructed by stitch drilling along each side then loosening the rock by a sequence of small controlled blasts with material progressively removed by dump truck to the processing plant. During construction, a large solution feature was encountered. This required 400m3 of ready mix concrete in a concrete wall, 15m high formed with a single side shutter.
102 pre-cast concrete beams, each weighing between 13 tonnes and 50 tonnes, were manufactured in Ireland to form the tunnel roof and reinforced with a concrete cover slab.
Up to 4m of soil was then placed over the tunnel roof, with hedges and trees replanted to return the area to the pre-existing levels and the original footpaths and byways reinstated.