Some might say that using structural sprayed concrete is a risky business given its association with the Heathrow Express tunnelling collapse in 1994. The material was used to support the vast underground excavation which failed at the centre of the airport (NCE 27 October 1994). The technique is expected to come under the spotlight when the Health & Safety Executive launches its prosecution of HEX contractor Balfour Beatty in October.
Undeterred by the Heathrow experience is consultant John Grimes Partnership. The firm has designed a spray concrete retaining wall for a small but vital road on the remote island of Lundy. When complete next month the structure will also have to withstand the impact of stormy seas.
The £500,000 road is the first phase of a project launched by the island's owner Landmark Trust. When complete the 300m long road will link the steep track between the island's harbour and the 40 or so properties on the clifftop above.
The road will run across the harbour beach to a new jetty which will be built next year to enable small ships to dock at the island. Currently ferries between the island and north Devon have to drop anchor in the harbour before using smaller craft to ferry people and freight to the beach.
Lundy's inaccessibility, plus the tight space constraints at the foot of the cliffs at high tide had a major influence on the decision to use sprayed concrete for the retaining walls.
'Anything the island needs is a problem to land,' says JGP senior partner John Grimes. As a result he decided on a construction method which kept plant and materials to a minimum. An insitu reinforced concrete structure was rejected out by the Landmark Trust on aesthetic grounds and because it would have been too expensive to ship out the necessary materials and plant from the mainland.
An anchored, sprayed concrete structure was considered more practical, allowing smoother, more aesthetically pleasing finishes and requiring less material and equipment. Rock anchors also meant concrete thicknesses could be kept down to 250mm, saving further on materials.
Grimes believes the sprayed concrete, applied by specialist contractor Professional Gunite Services is ideal because it can produce a seamless structure without cracks or joints. This is important for a wall which will have to withstand the aggressive action of salty seawater at high tide when large sections of the wall will be under water.
Space constraints were one of the main reasons for choosing dry mix sprayed concrete rather than the more cumbersome wet mix method. Dry mix requires little back up plant, as cement is combined with water in the spray nozzle. The wet method needs a mixing plant, as cement, aggregate and water have to be combined before the mix is pumped. But as the sea comes right up to the cliff at high tide, 'we didn't have room to set up a mixing plant,' says Grimes.
Sprayed concrete subcontractor Professional Gunite Services has been working steadily on the project since it began in February last year. Supervised by Grimes, the company is using a 1:3 or 1:3.5 mix, with aggregates complying with the old-style Zone 2 grading. This is shovelled into an ancient-looking 'cement gun' which uses air pressure to force the mix down a hose to the spray nozzle where it is mixed with water to produce a 0.35 water cement ratio mix.
Rapid curing times are vital to keep the work on programme because the PGS team has to stop just before high tide starts to lap against the bottom of the wall. Using Sika 4a accelerator in the mix keeps curing times down to 20 minutes.
Grimes believes that much of the success of the dry mix method depends on choosing an aggregate that drains well if it gets wet before use. 'You have to have an aggregate that dries well,' he says. Damp aggregates can cause hoses to clog, upsetting the mixing process at the nozzle.
Keeping together a well trained spray concrete team is also vital, and Grimes admits that because Lundy is 16km off the north Devon coast, 'maintaining a competent workforce has been a bit of a problem.'
Commuting from the mainland by boat is out of the question because ferries take too long and arrive late in the morning. Helicopters are too expensive to use regularly. As a result site workers have to stay on the island for a gruelling six week stint, working whenever the weather and tides allow.
Evening social activity on the island is limited to the small pub which also doubles as general store and restaurant. Such isolation has hampered efforts to recruit and retain workers for the project. This, combined with bad weather, has delayed completion of the work by about a month.