Sluggan Bridge, a Cairngorm pink granite Scheduled Monument with a 20m arch spanning the River Dulnain in the Scottish Highlands, is being restored thanks to a national cycle network being created around Scotland.

The bridge was built by the Jacobean English army as part of a 400km network of roads to carry troops and wagons into the Scottish Highlands.

Nearly 300 years later it is now being preserved with the help of some of a £43.5million grant from the Millennium Commission money which has been allocated to creating the cycle network.

In order to carry the traffic expected to use the route safely, the bridge needed strengthening, a job for which Sustrans, a charity set up to promote sustainable transport that is heading the creation of the cycle network, have employed stone and conservation specialists Cromarty Conservation.

Stabilizing the bridge has so far included drilling through 5m sections of continuous rock construction and areas of fragile, fragmented walling to insert stainless steel tie bars.

Mark Buchanan, Cromarty\'s managing director, says: We are dealing with a Scheduled Monument here. The use of percussive tools would cause further destabilisation and is completely out of the question.

The obvious solution was to use diamond core drills, which is what they have done, calling on diamond tool company Nimbus to provide the solution.

What they came up with was one of their front-line core drill specifications, the X232, specially modified with a softer matrix and reduced diamond concentration to achieve free cutting performance in the granite that has a high quartz content.

Nimbus also supplied a 2.8kW, three-speed electric drill rig to drive the 46mm diameter core bits and 5.6m long mining barrels.

Rob Buchanan, Cromarty\'s operations manager, said: The use of mining barrels became virtually mandatory to prevent the collapse of debris into the bore. Bearing in mind the age of the structure involved we obviously had no prior knowledge of the precise construction and what we might find in the way of infill.

The work so far carried out as phase one of the project is little more than investigational. Next year, more work will be carried out to protect and preserve this important ancient structure.