Advance work for the UK's first box jacking under a live motorway involved installing huge temporary support for the jacking pit excavation. Max Soudain reports.
Jacking a new trunk road underpass beneath a live road without stopping traffic is a enormous undertaking at the best of times. But when it is being done for the first time in the UK and the 'live road' is the M1, with 112,000 vehicles passing overhead each day, the pressure is even greater.
Before the more 'glamorous' aspects of the Highways Agency's plans for Junction 15a of the M1 begin in October, intense activity has been under way on the jacking pit next to the motorway.
Huge yellow hydraulic struts from shoring firm Groundforce Mechplant span the excavation above workers casting the jacking slab on the pit floor.
The A43 is the main link between the M1 near Northampton and the M40 near Towcester. Its length has been upgraded to dual carriageway, except where it passes under the M1.
Here, the road narrows to two lanes, forming a bottleneck, says Rob Yates, site agent for main contractor Edmund Nuttall.
Roundabouts on either side feed the motorway slip roads, the Rothersthorpe services and the new Swan Valley light industrial estate. As a condition of the original planning consent, developer Swan Valley undertook to increase the junction's capacity by providing a second underpass and remodelling the roundabouts.
Nuttall began work in January.
First, 15.5m and 7m long steel sheet piles were driven to form the headwall and sidewalls of the 65m long, 16.2m wide cofferdam on the north side of the M1, which is 11m deep at the motorway end and 5m deep at the other.
The pit was excavated through a variety of glacial deposits, with sandy gravely boulder clay down to 1m above the excavation floor and a very stiff grey sandy clay with gravel below. Limestone is at depth, although at one stage during excavations the team thought it had hit bedrock - 'But further site investigation showed that it was just a glacial erratic, ' Yates says.
'Luckily it was sufficiently weathered that we could dig it out.' The water table lies 2m below the pit floor and is 'fairly constant', says Yates. 'The excavation has remained dry throughout, although we have had some water ingress from the motorway embankment, ' he adds.
Groundforce's modular support system comprises one level of its Mega Brace waling system installed along the three sheet pile walls, 5m above formation level and braced using six huge hydraulic struts at 8.5m intervals along the excavation.
'The system is designed to provide support to the walls, while giving enough space between the struts for construction, ' explains Groundforce technical services engineer Neil Bleakley. Groundforce worked with Nuttall's temporary works department to develop the excavation procedure and propping design, with the bracing system installed as excavation progressed, he adds.
'Five of the hydraulic struts have 80t capacity, while the four corner struts and the last perpendicular strut have 150t capacity to cope with the higher loads and the larger retained height, ' says Bleakley.
Each strut is 15.5m long and has a hydraulic unit at one end with a 1.2m stroke. The Mega Brace steel waling beams, made up of 1m, 7m and 10m long sections, were hung over the side of the excavation from a crane.
The struts were then offered up to the beams and the hydraulic rams pressurised to 2000psi [13,790kN/m 2] and pumped out to push the beams against the sheet piles, bracing the excavation.
At the headwall, a top level of specially made steel props was installed at the start of excavation and at the main propping level four knee braces with 150t capacity span across the corners at 3m spacing, a pair each side.
Along the headwall one 10m and one 3m Mega Brace section have been used with a hydraulic ram to provide hydraulic flexibility. The knee braces are prevented from slipping along the waling beams by shear stops.
'The alternative would have been to use steel welded braces throughout, ' says Bleakley. 'But the modular system is easier to install and remove and is also more economical than welded steel bracing.'
Yates agrees - installation went 'extremely well', he says.
In the reception pit on the southern side of the M1, less support is needed: 7m long sheet piles will be installed here and bracing will only be in place for a short time, says Bleakley. On both sides of the motorway the sheet piles nearest the road will be left in, finished in concrete and will then form the underpass wing walls.
Construction of the jacking slab in the base of the excavation was well under way at the beginning of July. The bracing system will soon be removed to allow construction of the box structure, with Nuttall working its way back from the headwall.
The underpass consists of a 3500t, 45m long, 8.5m high and 14.5m wide concrete box, with its top just 1.2m below the road.
The box will have a steel cutting shield on the front and will be pushed forward by jacks in the base of the pit in 150mm increments, with excavation taking place on two levels. The embankment is made up of clay fill on the northern side and PFA on the southern side.
Jacking will proceed 24 hours a day without closing the motorway. No ground treatment will be used, says Yates. Movement will be controlled by bentonite injection around the box and the same anti-drag greased rope system Nuttall used to jack a box culvert under a railway near Maidenhead in 1999 (GE October 1999).
The motorway will be given a camber at 90° to the tunnel (installed during night-time lane closures) to counteract the predicted maximum settlement of 60mm at the centreline of the box.
Long term ground movement will be prevented by back grouting the box roof, walls and floor to fill any voids left by the small over-cut.
'We will be monitoring the road for two months during and after jacking, ' says Yates.
Jacking is set to start in October and expected to last three to four weeks. Junction remodelling is due to finish in spring 2003.