GEOTECHNICAL CONTRACTOR CAN recently finished work on what is claimed to be one of the UK's largest ever tied-back face stabilisation projects, using soil nails with a heavily reinforced concrete facing.
Ballylumford on the north east coast of County Antrim in Northern Ireland is the site chosen for a new combined cycle gas turbine power station. Its position on the northern tip of Island Magee exposes it to some of the worst North Atlantic weather.
Because the site is bounded by the sea and another power station, there was no option but to cut into the steeply rising hillside for the footprint of the new structure.
The project's primary subcontractor, Enelpower, produced a conceptual design involving excavation of the stiff boulder clay and underlying basalt to form a horseshoe-shaped 70infinity backwall 320m long and 23m high.
Anchor design required the formation of 300mm diameter holes up to 14m long to achieve a maximum design load of 240kN.
Because of the potential difficulties in achieving this through an anticipated mixture of saturated clays, large cobbles and basalt, all of varying degrees of weathering, CAN offered an alternative proposal based on the installation of hollow selfdrilling soil nails.
The technique avoids having to case the holes, by using simultaneous drilling and grouting. CAN says it also provides better pullout capacity than conventional methods because of grout penetration into loose material around the borehole.
The proposal was accepted and the first task was to install trial nails into a variety of ground conditions using Ischebeck Titan's 40mm diameter bars. Several anchors were tested to failure, yielding 'remarkable' results according to CAN operations director Andy Wingfield.
'We achieved average bonds in excess of 60kN/linear metre in clay, while those in weathered basalt resulted in bar or coupler failure, ' he says.
Much of the hillside was removed by the earthworks subcontractor Farrans Construction before nailing started. This enabled critical path construction of the heavy foundations to proceed in parallel with the stabilisation works.
Wingfield explains that excavation and stabilisation work was far from straightforward, due in part to the varied ground conditions. This added to the difficulties of working on a narrow bench with steel fixing and wet sprayed concrete operations following on the heels of the nailing crews.
'Keeping all our crews working, when it was often impossible to relocate equipment along the berm, required constant vigilance and close liaison between all disciplines, 'Wingfield adds.
Three grouting stations behind the crestline provided a constant supply of grout to the drill rigs 100m below, while radio communication was used to co-ordinate the switch between low strength flushing and high strength anchor grout.
The concrete bulkhead consisted of 200mm thick wet sprayed concrete reinforced with twin mats of 12mm and 14mm diameter reinforcement, incorporating planting boxes at regular intervals.
CAN installed more than 1800 nails and 4300m 2of concrete bulkhead over 15 weeks, forming what has become known locally as the 'Great Wall of Ballylumford' It is visible to passengers on the ferry to Larne and, once the flowers have taken root, the project team joke that it might be Ireland's answer to Cornwall's Eden Project.