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Anchors put up Braveheart barrier

ISCHEBECK TITAN grouted self-drilling rock anchors have been used as part of a rock slide protection system below the Wallace Monument near Stirling, Scotland.

The Monument to Sir William Wallace - more popularly known as 'Braveheart' - sits on the doleritic volcanic plug of Abbey Craig, overlooking Stirling. The dolerite is prone to weathering and the A907 road running below the steep outcrop is at risk from rock slides.

Existing protection on the south western flank of the outcrop uses torpedo netting but recent rock falls outside the area gave cause for alarm and it was decided that a new permanent structure was needed to give further protection.

Consultant Babtie Group and the Transport Research Laboratory in Edinburgh designed a multiple barrier system to stop falls. In the most exposed areas, this comprises two lines of sacrificial energy-absorbent fences together with a permanent fence running for over 300m along the toe of the slope.

The upper barriers are made of wire mesh supported by steel stanchions attached to large anchored base plates by steel cables. In the event of a slide, the wire mesh and cables stretch, 'bagging' the falling rock. The bottom fence has more substantial anchors and posts and is designed to arrest rock passing through the upper fences.

Two sizes of Ischebeck's hollow tendon injection anchors were used. 40mm diameter anchors fix the base plates through the scree layer into bedrock, while 33mm diameter anchors were installed for the intermediate sacrificial uprights.

The continuously threaded anchors combine drill rod, grouting conduit and reinforcement tendon, saving time by removing several installation steps. They are simply installed using pneumatic hammer drills and then grouted up by pressure grouting. The anchors are protected against corrosion by a Combi-Coat - galvanising, zinc phosphating and epoxy powder coating.

Results of computer simulations of the system were positive and were further supported by a field test, with the sacrificial fencing stopping a 3t boulder travelling at around 8m/s to 10m/s.

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