References
Gautrain
SOUTH AFRICA, 2008-2009
Large diameter piles in dolomite for the foundations of a railway viaduct.
Gautrain is a high-speed train project (80 kilometres - including 15 underground) between Johannesburg and Pretoria, built by the Bombela Civil Works Joint Venture. Dura Solétanche Bachy has been awarded the foundations subcontract for 13 piers of a viaduct located in the north of Centurion. The contract involves 76 large diameter piles anchored in dolomitic rock.
Geology
Ground conditions in the dolomitic area located to the north of Centurion are extremely heterogeneous and, in places, the dolomite may have a resistance of 600 MPa. The dolomite substrate has a pinnacle morphology: pinnacles are very irregular columns that can be more than 50m in height. This substrate is covered with heavily weathered ground comprising boulders and voids. The risk of void formation is very serious and capable of generating major subsidence.
Preliminary boreholes
A preliminary site investigation was carried out between October 2006 and May 2007. Up to 5 drilling rigs were mobilized to carry out approximately 450 boreholes (up to 5 boreholes for the same pile). The aim of these preliminary surveys consisted in optimizing the type of foundation required for each pier and, for those built on piles, the position and length of these piles.
Piles anchored in the rock
The foundation slab that bears each pier is founded on 1,300mm diameter piles drilled and anchored into sound rock. Pile depth ranges from 20 to 50m with various anchoring lengths for the same pillar. All piles are equipped with sonic and coring tubes which are used to check pile quality and, subsequently, to ensure good contact is made between the concreted base of the pile and sound rock.
Drilling
The equipment used includes a BG 36 drilling rig capable of applying a 40 tonnes force and reaching a rotary torque of 36 tonnes-meters. The pile drilling rig is equipped with several types of drilling tools used according to the conditions encountered (progressive rock auger, classic corers, various buckets). In the case of rock that has a hardness of up to 600 MPa, a coring tool is used. This is a tungsten tipped coring tool with rotary cone cutters and bearings that travels over the rock surface and cuts by crushing and grinding on the pile perimeter. The pile is bored inside a casing oscilated into position (1.5m in diameter). Once the casing is in place and drilling is completed, the reinforcement cage is lowered into position and the pile cast in concrete.
Techniques
A pile is a structural element driven into the soil for transferring loads and prevent deformation. Its slenderness ratio is not limited.
Pile shafts can be uniform and rectilinear, telescopic and belled out.
Piles can be installed either separately or in groups. They can also form a retaining wall, a mixed curtain wall, contiguous piles, secant piles and composite curtain walls, such as Berlin walls and similar. Piles are also used as precast beams to be placed in the structure of the building they support.
Ouvrages
Soletanche Bachy has developed a number of techniques for use in railway works; cut and cover tunnels, highways, underground stations, viaduct etc.
Voyage au coeur de l'ingénierie de pointe
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