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Gnat's the way to do it

INNOVATION IN GEOTECHNICS

New style fireworks are providing a quiet bang for rock excavation in sensitive areas. David Hayward reports from a peaceful Northumbrian village.

Dubbed the silent explosive, a new rock blasting technique is fast winning friends in projects where noise, vibration and flying debris are banned.

Rockchop, a low acceleration deflagrating (intense burning) agent developed by demolition firm Gnat International, has just completed its first commercial contract in the sleepy Northumbrian hamlet of Waren Mill.

Blasting a shallow sewer trench through hard igneous rock down the centre of the holiday village's cottage-lined main street, less than a metre from front doors, proved such a successful anticlimax that few inhabitants realised it was under way.

Even more important to client and contractor, not a single local complained.

The new style explosive is likened more to a firework than dynamite. The resulting 'display' is a damp squib compared with its conventional cousins.

'It produces a dull thud, with no fly rock, no shock waves, no vibration and no damage to neighbouring property, however close, ' says Gnat International director Nick Turnbull. 'We are now investigating its use in a wide range of environmentally sensitive blasting operations.' This list of negatives claimed by Turnbull were central to the brief from client Northumbrian Water for excavation of the 1m deep sewer trench.

Waren Mill is a cluster of holiday cottages near Bamburgh on the Northumbrian coast. The cottages have no mains sewerage so the seasonal population of about 30 relies on septic tanks. The most practical layout for a new water authority sewer was down the middle of the only street, only metres from most cottages and routed through one of the hardest igneous rocks, dolerite.

There could be no noise, damage to property - either from flying debris or structurally through foundations - or even extended inconvenience for the inhabitants. Many properties are rented to holidaymakers, so the compensation bill could have soared.

These considerations ruled out conventional explosives and high powered rock peckers. Hydrodemolition or hydraulic bursting were impractical on grounds of cost, time and inconvenience.

Trials of the only conventional possibility, stitch drilling, proved less than successful. Holes were difficult to form and, with large peckers still needed to break up the rock, noise levels would quickly have exceeded the prescribed 95dbA maximum level at cottage doors. Fortunately Gnat arrived with its gentle firework.

For several years the company has specialised in removing concrete from inconvenient locations where neighbourhood-friendly explosives were essential.

The North Yorkshire firm has met challenges such as breaking out chloride-attacked concrete from congested bridge piers where surrounding reinforcement was still needed, or removing machinery base slabs while factory work continued unaffected.

The dull thud technique involves electrically igniting a high temperature gas which burns rather than explodes to expand and crack surrounding material such as rock or concrete.

Energy dissipates immediately, leaving cracks or open fissures sufficient for a conventional - and relatively quiet - excavator or small pecker to remove the weakened material.

Several products exist to crack rock, but Turnbull claims none proved reliable. 'They either regularly misfired or had no consistency in their strength grading system, ' he says.

So Gnat approached major explosives designer Nobel Enterprises, an ICI subsidiary.

Together they produced Rockchop, a high energy to weight ratio, rapid burning product.

At Waren Mill the 200mm long Rockchop capsules were installed in a similar way to conventional explosives. The tubes were placed in the bottom of three, 1m deep holes, spaced at 200mm centres across the trench line.

Capsules were connected to an electric detonator and the 32mm diameter holes stemmed with a sand packing to help direct the force sideways rather than up the hole. Once ignited, the high temperature gas expands quickly -though its expansion rate of 18m/s is more 60 times slower than for a high explosive.

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

The resulting shattered rock was broken up using low powered peckers and removed by back hoe excavator. Gnat achieved excavation rates of up to 2m 3/day during formation of the 80m long trench.

The only problem occurred when water trapped in the rock entered the holes. This made them difficult to stem with sand, allowing the explosive force to blow up the hole which reduced its cracking power. The solution was to crack the rock in two half depth horizontal benches with holes drilled only 500mm deep to avoid water ingress.

Both telephone cables and mains water supply pipes lay across the trench route. Shielding the services with steel sheeting allowed the cracking operation to continue around them without damage.

Martin Ellis, agent for main sewer contractor Seymour, says: 'This excavation technique has proved the ideal and only feasible method, avoiding the risk of potentially horrific vibration and consequently phenomenal compensation claims.

'None of the properties suffered any damage and we had no complaints from residents.' Gnat has now successfully trialled the technique on a shaft sinking contract in Bristol.

Improvements to the city's sewerage system required an inclined 8.5m diameter shaft to be blasted through hard limestone only 2m from a major live sewer.

Fears that the high explosives used to blast most of the shaft could damage the sewer led to its replacement by Rockchop in the potential danger zone.

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