Use of monopiles for one of the major structures on the A14 upgrade called for collaboration when ground conditions proved to be more variable than expected.
Upgrade of the A14 between Huntingdon and Cambridge is Highways England’s largest scheme underway at £1.5bn and involves in excess of £20M of piling work. One of the major structures calling for piled foundations is the 750m long viaduct carrying the road over the River Great Ouse flood plain.
Design of the viaduct piles started a year before the scheme moved onto site but despite the planning, the A14 Integrated Delivery Team (see box) still faced a number of construction challenges. Ground conditions that were more variable than expected was the main issue but the team also had to deal with flood risk and overhead cables restricting access, as well as the scale of the structure.
“The Great River Ouse Viaduct is formed of 17 spans with a composite steel and concrete superstructure,” says A14 Integrated Delivery Team (IDT) construction manager Nick Whyment.
Piling work started on site in March 2017 after several months of enabling works and involved installation of 84 monopiles with a 1.8m diameter that extend to depths of between 30m and 38m.
“The monopile design was selected for efficiency through a value engineering process,” says IDT piling joint venture project director Ian Lovett.
The initial design was for shorter 2.4m diameter piles.
The preparatory work was essential as the viaduct spans three old gravel pits, which were used as fishing lakes, as well as the river. The East Coast Main Line railway also crosses the route of the new road just beyond the river but the bridge over the rail line is being built as a separate structure with an earth fill embankment between.
Formation of the piling mat and construction platform called for 120,000t of 6A1 material to be brought into the site and a Bailey Bridge has been installed to give the construction team access over the river during the work too.
“The material will be used to landscape the area at the end of the project,” says Whyment.
“We are working in a flood plain so any material place within the flood plan had to be allowed for so we had to create two flood compensation ponds.”
Test piling was carried out in December 2016 and 22MN force was applied to a 1.8m diameter pile with movement of less that 5mm. “It performed well,” says Lovett.
Despite the test pile performing well, Lovett describes the working pile phase as “a journey of discovery”.
“At the western end there is a variable glacial till that comes in at about 5m and extend to depths of up to 30m before we encounter limestone,” he says. “Essentially, we are working in an infilled glacial valley.
“The limestone also contains faulting and is very difficult to pile through and we general got refusal after 1m or 2m. We tried to carry out Schmidt Hammer Tests but could not get samples to test.
“The ground investigation described the glacial till as cohesive and the pile design was based on skin friction with 12m casing use to take the bore through the backfill material.
“We started on the western side and installed the first three piles with no issues but the fourth one, which was only 3.8m from one of the others, became unstable at 12m due to variation in the glacial till.”
The challenges were overcome tanks to the collaboration within the team, according to Lovett.
Whyment adds: “The monopile approach is more efficient but it means that you only have one shot to get it right.”
“We backfilled the problem pile and moved to the next location and started carrying out probing ahead of the piling work,” says Lovett.
Use of big vibrators to put in deeper casing was also considered as a solution but Lovett says that bentonite was the best approach.
“Using bentonite in the flood plain was an issue through and we had to locate the plant itself outside the flood plain and pipe it up to 450m to the pile locations,” he says. “We drew on experience gained during construction of the Amsterdam metro to pump the bentonite in 30m long sections.”
Work on site paused at this stage to allow time for the designers to re-assess and this resulted in the pile lengths being extended by up to 8m to mobilise more skin friction.
“Originally the piles were planned to be 18m to 24m in length and found in the glacial till and extension took the pile base into the limestone,” says Lovett. “Most of the piles hit the limestone at 30m to 34m and we brought in cross cutters to drive the pile toe into the limestone.”
Inspections of each pile were carried out to allow the pile lengths to be determined by penetration rather than design lengths.
“This approach was successful until we reached the river,” says Lovett. “There the glacial till thins and we encountered the Oxford Clay. Trials meant that we were able to use open bores from there until we reached the eastern abutment but the bentonite plant was kept on site just in case there was an issue.”
This proved to be a good move with issues encountered on the third to last pile and the bentonite was put into action again.
In total 50 out of the 84 piles were undertaken using bentonite support.
The lead in time for the re-design took three weeks and overall the piling operation took 12 and half weeks longer than expected.
“We were initially aiming to do seven piles a week but we actually undertook four,” says Lovett.
Whyment adds: “The work here shows that you can do lots of ground investigation and still have variable ground with rapidly changing ground conditions.”
Fortunately, the viaduct was not on the critical path for the overall project.
Piling work and construction of the columns is now concluded on the viaduct and the steel work is scheduled to be completed by May this year with the whole structure on course to be finished in September.
Nonetheless, there is still a considerable amount of piling to be undertaken on the overall scheme with a total of 976 piles planned so piling rigs are expected to be a feature on the site until 2019.
The challenges presented by monopiles has not put off Highways England which is hoping to use the experience gained on other structures on the A14, as well as future schemes too.
The £1.5bn project is being delivered by the A14 Integrated Delivery Team – or IDT for short – which is formed by contractors Balfour Beatty, Costain and Skanska with consultants Atkins and CH2M delivering the design elements. Carillion was also part of the team but since the company’s liquidation, staff have been seconded to other businesses so the project remains unaffected.
The new 33km route between Huntingdon and Cambridge is being upgraded to three lanes in either direction, including a new 27km bypass to the south of Huntingdon with four lanes in either direction between Bar Hill and Girton.
“The new bypass and widened A14 will open to traffic by the end of 2020, although some finishing work such as the removal of the A14 viaduct in Huntingdon will continue beyond that,” says IDT construction director Jim McNicholas.
The project calls for 10M.m3 of earthmoving and construction of 34 structures, including a 750m viaduct over the River Great Ouse, and 28 of those are built on piled foundations. In total the project will see construction of 976 piles with a linear length of over 20km and 35,000m3 of concrete.