Gallagher wins KDDA for tunnel at Hermitage Quarry

  • Cut and Cover tunnel at Hermitage Quarry

Gallagher has won the Infrastructure and Renewables category at the Kent Design and Development Awards, announced on 14th December, for its Cut and Cover Tunnel at Hermitage Quarry in Maidstone, Kent.

The awards are held every two years to reward and celebrate construction excellence across the county.

The tunnel was constructed in 2014 to link the existing quarry to the new westerly extension. Planning permission for the quarry extension was granted in 2014, thus guaranteeing another 25 years’ supply of Kentish Ragstone. This privately funded project required detailed project management and investment in order to sustain local employment and access to an important and longstanding building material for the county.

The tunnel is 100 metres long, offers 15 metres of clear height and is wide enough for two large dump trucks to pass each other comfortably.

The tunnel is a unique piece of engineering, utilising simple techniques on a large scale. The rock was progressively removed by a sequence of controlled blasting. A shelf was then formed along each side of the tunnel and bearing beams cast, 20 metres above the base of the tunnel. 102 pre-cast beam sections 20 metres long and weighing up to 50 tonnes each were then placed to form the tunnel roof with a 300mm insitu waterproof concrete slab tying the beams together and forming the top surface of the tunnel.

The new extension was positioned one side of a public by-way with all the processing equipment and highway access on the other side, so the byway had to be temporarily diverted for 9 months. The pathway was re-constructed replacing 4 metres of soil over the tunnel, trees planted, hedges re-planted and fast-growing sweet chestnut stools planted along each side to form an instant screen. When it was re-opened, people had no sense that they were travelling above a tunnel.

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