Highway schemes have already benefited from Balfour Beatty’s evolution of sheet piling but it looks to be the turn of the rail sector to recognise the advantages of King Sheet Piling in 2014
Necessity is said to be the mother of invention and that was definitely true when it came to development of the King Sheet Pile (KSP) retaining wall method for the £1bn M25 widening. “The scheme called for 1.6km of road to be widened every month during the three year construction period,” explains Balfour Beatty major projects chief engineer Nick Boyle. “That was twice as fast as had previously been achieved.”
Value engineering at the tender stage led to the KSP concept and Balfour Beatty major projects KSP inventor and manager David Baker believes that the solution is the first real revolution in sheet piling in 120 years.
“On site the KSP solution was up to four times faster and 40% cheaper than the conventional approach, as well as having safety and sustainability benefits,” he says. Four years after its introduction, Baker believes that some still do not fully understand the difference between the KSP technique and a staggered sheet pile wall.
“In the KSP system, the king piles are connected by a shorter pile. But this section is not actually there to act in vertical bending - we call it an intermediate because it bridges the load horizontally between the two king piles it connects,” he explains.
“In a staggered sheet pile wall the bending forces are taken through both the longer and shorter piles with staggering introduced only at depth where lower bending permits.”
Boyle says that the KSP solution is a simple concept, but its innovation - and reason the system gained a patent in 2013 - is that it ties two ideas together. “Most sheet pile selection is done on the basis of the thickness required for driving, so the theoretical size needed for the bending forces is
actually thinner,” he says. Baker adds: “Looking to economise the design and seeing that there was spare capacity in bending, I realised that there was no reason why we couldn’t apply the principles of king post wall systems to sheet piles with vertical driving.”
Although it was a new concept in 2008, Baker had little difficulty convincing the designers, sheet piling contractors and the Highways Agency that the solution would not only work on the M25 widening but be safer, faster and cheaper.
“The issue came more from our own site team,” adds Boyle. “People like innovation, just not on their projects especially at the start of a fast-track scheme - they prefer
proven techniques for greater certainty. The M25 project was a high profile scheme with much at stake, so you can understand the reservations.”
Nonetheless, once the scheme moved into construction, the benefits were quickly seen and the use of the technique was expanded. As a result of the innovation,
Balfour Beatty and its joint venture partner Skanska constructed 25km of KSP wall on the widening between junctions 16 and 23, saving £10M in the process and playing a key role in delivering the project on time. The site team routinely installed 60 linear metres of sheet piles a day in the Chalk sections, which Boyle and Baker believe is four times faster than conventional techniques.
Design detail The success of the system also comes from removing issues that commonly affect the construction of sheet pile walls.
“Use of panel driving with sequenced driving calls for lots of equipment movements that can add safety risks, but the pitch and drive approach normally results in the wall going out of alignment due to the clutch friction problem,” says Baker.
“The KSP approach eliminates the issues associated with both styles of driving a conventional sheet pile wall.”
The shorter connection between the kings and intermediates means almost no clutch friction between the piles during driving, so they are easier to drive using hydraulic techniques reducing the need for percussive driving. Baker says that noise levels on the M25 were substantially reduced by KSP because only 50% of the piles were full length and could be driven much further using hydraulic driving.
The system is also safer than conventional reinforced concrete solutions with all components prefabricated to the right size off site. “The installation system is equipment-orientated and reduces the people/plant interface, which is safer,” says Boyle.
“The use of a steel capping beam also minimises working at height issues, further improving the safety aspect.”
Since the first use of the system on the M25 smart motorway between junctions 16 and 23 in 2009, KSP walls have been used on three other sections of M25 upgrade, the M4/M5 managed motorway and in a pumping station on the A421 improvement scheme.
The technique has also been used as a temporary works solution on the A11 Th etford project.
The system is now being adapted for its first rail application. Design work is now underway to use the system at Abbey Wood on Network Rail’s work for Crossrail where the KSP technique will be used for combined temporary and permanent support for a culvert in very soft ground to avoid the need for piling.
“The system is being used to support the temporary works, but shear studs fi xed to the king piles will allow reinforced concrete for the permanent work to be cast onto the KSP wall,” says Baker.
The patent, which can be used under licence, allows for the system to be used as a cantilevered wall, anchored wall, propped wall and in basement applications.
But Boyle believes that the technique has significant potential to benefit the rail sector and, in addition to the technique being used at Abbey Wood, Balfour Beatty has presented it to Network Rail’s supply chain innovation and suggestion scheme.
According to Baker, the system is suitable for any ground where a sheet pile would normally be considered and has pushed sheet piling to its limits.
“The piles needed to go through flint bands within the Chalk on the M25 and on the M4/M5 we were piling into weak rock, which the lack of clutch friction facilitated,” he says.
“Our steel supplier Arcelor Mittal actually said aft er the initial M25 work was completed that the size of the flint bands in the Chalk were such that it doubted that a conventional continuous sheet pile wall could be driven through.”
While it is clear that the KSP solution has already delivered a step-change for sheet steel piling, there are also future advantages yet to be realised. “You have to wonder whether the M25 will still exist in its current form in 100 years’ time,” Baker says. “Using sheet piling techniques means that the piles can be recovered and the steel recycled.”
The government also recognises the benefits that the KSP solution can offer in terms of sustainability as it was selected as one of the case studies in the recently published Infrastructure Carbon Review.
In the foreword, commercial secretary to the treasury Lord Deighton and minister of state for business and energy Michael Fallon state that “pursuing a low carbon agenda stimulates innovation” - something both Boyle and Baker fi rmly believe KSP has achieved.