Warren Mill Northumberland

Rockchop cuts an 800mm wide trench into a solid rock bed through a village where noise and vibration are banned.

200mm Rockchop capsules were placed in the bottom of three 1 metre deep drill holes, spaced at 200mm centres across the trench line. Capsules were attached to an electronic detonator and the 32mm diameter holes stemmed with a sand packing to help direct the force sideways, rather than up the hole.

The aim was to crack a roughly 500mm thick block of dolerite to the full 1 metre depth and 800mm width of the trench. Rubber sheeting restricted the movement of any rock to less than 300mm, with negligible fly rock.

The resulting debris was further broken up by low-powered peckers and removed by conventional backacter. Gnat achieved excavation rates of up to 2 cubic metres a day during the formation of the 80 metre long section of trench. The trench route lay across telephone cables and mains water supply pipes, but shielding the services with steel sheeting allowed cracking to continue round the live obstructions without causing them damage.

"This technique has proved ideal and the only feasible method," says Martin Ellis, agent for main sewer contractor Seymour. "None of the properties received any damage and no complaints were received from the residents."