Innovation was the key to beating bizarre ground conditions and building Northern Ireland's first privately funded motorway junction.
Dave Parker reports from Lisburn.
If there was one major contributor to the success of the new US $11.4M Junction 8 interchange at Lisburn on the M1 south west of Belfast, it was the decision to go for the design and build route in procuring the main underpass. Consultant Kirk McClure Morton (KMM) senior associate Allan Chapple explains: 'We saw this as key to getting the most efficient traffic management scheme.
'Coning costs alone could have run into six figures, as the Department for Regional Development would have charged US $16,000 a week for traffic management. Whitemountain, the main contractor, came up with a solution that saved most of these costs - and saved a lot on earthworks as well.'
Constructing a four lane underpass under a live motorway is always a major challenge.
Earlier solutions have included diverting traffic on to temporary bridges, or box jacking. The challenge here included coping with ground conditions that were bizarre, as KMM project engineer Brendan Daly explains.
'Most of the site is on sand with a fine silt content ranging from 2% to 20% across the site.
When the moisture content is low it's fine, with CBRs over 50.
But when it reaches a critical level the sand suddenly becomes thixotropic (meaning that its viscosity reduces when stress is applied) - a quicksand in fact.'
Planning limitations dictated that the junction had to stay below the motorway. The two new roundabouts and the bridge footings would be 9m or more below existing ground levels - and up to 7m below the water table. Given that the M1 is built on the route of the old Lagan Canal, groundwater was bound to be a problem.
Minimising earthworks was ultimately as important as avoiding traffic disruption, Daly adds.
Whitemountain's innovative solution achieved both ends.
The contractor chose to build the sliproads to the west of the M1 first, link them with a temporary road and divert Belfast-bound traffic off the motorway for the duration of the bridge works. The contractor's structural engineer Gifford then plumped for top down construction for the new underpass, and - crucially - went for a twin span design.
'This reduced the deck thickness compared to a single span design by 750mm, ' says Daly.
'So there was 750mm less excavation right across the site and significantly reduced need for dewatering and wellpointing.'
Bridge construction began in November last year with the installation of three rows of 600mm diameter contiguous flight auger (CFA) piles across the closed half of the motorway.
Whitemountain then excavated down to 100mm below the soffit level of the new bridge deck and poured a 100mm thick sacrificial concrete slab on to the sand.
An epoxy resin debonding layer went down next, then the 700mm thick concrete deck slab.
The carriageway was reinstated, parapet railings and safety barriers installed, and the westbound traffic switched back. By the middle of December the second half of the deck was under construction and the final resurfacing was completed at the end of May.
While this was going on a pair of 'hard men from Cork' were 8m below the motorway further to the north, digging out a 1.35m diameter, 77m long thrust bore through the site's only significant area of clay. Such were their efforts that the whole operation took just 30 days. This tunnel will eventually take all the gravity surface water from the south side of the interchange into a nearby stream.
'Drainage design here is complicated by the fact that the sand isn't very permeable, because of its silt content, ' Daly explains. 'The drains have to be wrapped in geotextile to keep the silt out - but we'll still have 40 litre/s coming into the pumping station even in dry periods.'
A mixture of filter and blanket drains keep the interchange dry, while extensive use of geogrids and geotextiles also features.
Situated in the middle of the northern roundabout, the pumping station is made up of 3m diameter precast concrete sections extending 9m down through the sand to the underlying clay.
Like everywhere else on the interchange site extensive wellpointing is needed during the construction phase. When complete the pumping station will pick up the northern network and lift the water into the convenient stream.
Earthworks subcontractor Brian Herron had to tackle 300,000m 3of excavation during the winter. Whitemountain contract manager Nigel Dugan says the site team soon learned not to fight the baffling ground conditions. 'You couldn't beat it, you had to respect it, ' he says.
'When we first saw water boiling up from 7m below as the earthmovers went by 30m away we were taken aback. But once we got the wellpointing going we found that when the sand was dry enough it was a very good material to work with.'
Another pleasant discovery was that the original geotechnical survey was extremely accurate, and no unexpected problems occurred.
Everyone involved with the project praises the latest generation of GPS surveying technology. Whitemountain site engineer Dominic Flanagan effectively controls all setting out and earthwork operations on site armed with a 'pogostick' - a GPS system containing all the design information.
'This one engineer basically does the job of at least four using old style equipment, ' says KMM resident engineer Douglas Hill. 'He needs no chainman, and the earthmovers need no banksmen - there's no batter rails or pegs on this site.'
Cost savings have reached six figures, Hill says. Currently Flanagan is controlling the excavation for the northern pumping station.
Spoil from the works, like all the rest, is being spread over the area of the new retail park. On their way there the earthmovers pass under the new underpass, which will eventually have its raw CFA 'columns' clad in cosmetic blockwork.
Dugan is convinced the top down construction technique was the right choice. 'It almost eliminates working at height, which is a real safety benefit.
And there's no need for deep temporarily supported excavations, or excavations close to live traffic, and a much reduced requirement for dewatering.'
With most of the really demanding sections of the works complete, the project is under budget and ahead of programme. Scheduled completion date is November.
For many years through traffic between Belfast and Dublin has clashed uncomfortably with shoppers heading for the retail park at Sprucefield south of Lisburn.
The pinch point was the interchange between Northern Ireland's M1 motorway and the A1 to Dublin, which was also the retail park access. When joint venture developers Stannifer and Snoddons Construction proposed an even larger retail park at the junction, actually between the M1 and the A1, it was obvious that major road improvements would have to be an integral part of the project.
The revised layout as devised by consultant Kirk McClure Morton (KMM) takes Belfast-bound traffic off the A1 at a new roundabout and up the new A101 dual carriageway to Northern Ireland's first 'dumbbell' roundabout beneath the M1. From there traffic will join the motorway around 1km further west than before. Shopping traffic will continue to use the existing Junction 7 interchange to access both old and new retail parks.