Design award for Hermitage Quarry tunnel

Gallagher pick up a KDDA award for cut and cover tunnel structure at their Kentish Ragstone site

GALLAGHER have won the ‘Infrastructure and Renewables’ category in the Kent Design and Development Awards (KDDA), announced on 14 December, for their cut and cover tunnel at Hermitage Quarry, near Maidstone, Kent. The awards are held every two years to reward and celebrate construction excellence across the county.

The privately funded tunnel was constructed in 2014 to link the existing quarry to a new westerly extension. Planning permission for the extension was granted in 2014, thereby guaranteeing another 25 years’ supply of Kentish Ragstone – an important and longstanding building material for the county.

The tunnel, which is 100m long, offers 15m of clear height and is wide enough for two large dumptrucks to pass each other comfortably, represents a unique piece of engineering utilizing simple techniques on a large scale.

First, the rock was progressively removed by a sequence of controlled blasting, then a shelf was formed along each side of the tunnel and bearing beams cast 20m above the base of the tunnel.

A total of 102 precast beam sections, each measuring 20m in length and weighing up to 50 tonnes, were then placed to form the tunnel roof, with a 300mm in-situ waterproof concrete slab tying the beams together and forming the top surface of the tunnel.

As the new quarry extension is situated on one side of a public byway with all the processing plant and highway access on the other side, the byway had to be temporarily diverted for nine months.

The pathway was subsequently reconstructed, with 4m of soil replaced over the tunnel, trees planted, hedges replanted and fast-growing sweet chestnut stools planted along each side to form an instant screen.

When it reopened, people using the byway were unaware that they were travelling above a tunnel.