DSI Rockbolts on Gautrain ProjectDSI South Africa, the South African subsidiary of operating construction technology group Dywidag-Systems International, secured a ZAR21 million contract last year with the Bombela consortium for the supply of 70,000 double corrosion protection (DCP) rockbolts for the Gautrain development, over 30 months. In early February, 18,000 DCP rockbolts had been supplied. Visit www.dsigroundsupport.com/products/speciality-bolts/dcp-bolt.htmlThis particular type of bolt is ideal for complex engineering structures and aggressive rock conditions thanks to the special double corrosion protection technology, which provides long-term defence against the groundwater within the rock.In the 16 kilometres that the Bombela consortium has to tunnel underground, there are a variety of geological conditions, including sandstone rock that causes unstable ground conditions. Conditions at the Rosebank site also are very sandy and these problems are compounded by the high water table. Orders for lengths of the bolts have often had to be changed, at short notice, which has presented a considerable challenge. View pictures of the Gautrain site here.Developed by DSI in Europe and Australia, the rockbolt is a pre-tensioned and post-grouted steel bolt manufactured from a hot-rolled, high-tensile bar. It is manufactured and supplied at various lengths, from 2 m to 10 m, and in differing material forms, depending on the rock types and conditions, as well as the design requirements of the customer. The main length of the bolt is covered with a corrugated, high-density polyethylene sheath that provides permanent corrosion protection, maximum load distribution, and ensures grout flow inside and outside the sheath.It is tapered, allowing for easy installation through wire mesh, and allows for instant ground support using a claw-like expansion shell, allowing construction teams to advance without immediate grouting. The bolt assembly reduces friction, resulting in a higher load capacity. A slotted anchor nut at the base of the bolt also allows for high load transfer and grout flow, while a grout check valve or bell minimises grout draining or loss.Holes, measuring 43 mm to 45 mm in diameter, are drilled into the tunnel roof and walls and flushed clean to remove any possible impediments. A washer plate is fitted at a perpendicular angle to the base of the bolt as the bolt is inserted into the hole to its full length without rotation. The bolt is then rotated to engage the expansion shell, spreading the claws to the required tension. The grouting adaptor is attached for anchorage.The grout is pumped up and into the inner annulus to the bolt head and returns along the outer annulus, forming a double layer of grout, separated by the polyethylene sheath. When the grout is at the top of the sheath, it spills out on the outer side, providing a third layer of corrosive protection. Some 30 mm of bolt thread is left protruding below the nut to allow for standard pull testing equipment to be attached to the bolt.DSI South Africa has also developed tunnel linkbolts for the Gautrain project, adding stability to the walls between the various tunnels being drilled. 29/08.