Limited space and difficult ground led contractors installing 32 gantries on the M4/M5 interchange to use an innovative piling solution.
With over 140,000 vehicles a day using the intersections between the M4 and M5 motorways, the crossroads is a key strategic link between London, Wales, the South West and the Midlands, as well as serving local traffic. Under plans to improve traffic flow in the area, sensors have been placed in the road to monitor traffic and congestion. The speed and availability of lanes canthen be managed via information signs in a bid to minimise congestion.
The £77m scheme will allow hard shoulder running at peak times and use variable speed limits to manage traffic flows. The scheme is due to be completed in early 2014.
With no additional land available to accomodate the scheme, the Highways Agency needed a retaining solution to creating the space to construct the necessary overhead gantries and refuge lay bys. However, the ground conditions meant the answer was not straightforward. Hard ground ruled out the use of conventional sheet piling and the alternative gabion basket solution was time consuming and labour intensive. Instead, Balfour Beatty worked with Dawson Contract Piling to use the King Sheet Piling (KSP) system to overcome the problem.
In total the project called for the construction of 32 super-span gantries between junctions 19 and 20 of the M4 and junctions 15 and 17 of the M5. These will carry the 197 information signs that will inform drivers of the speed limit and lane use during peak traffic periods.
According to Balfour Beatty, using the KSP retaining wall system allowed a change to a form of construction that simplified the work, substantially increased the safety of the workforce and minimised the removal of valued vegetation
barriers that screen nearby residential communities.
The ground conditions consisted of firm, stiff and very stiff to hard clay over weak – ranging to moderately strong – mudstone and sandstone with occasional strong limestone bands. These strata are part of the Blue Lias Formation, Penarth Group, Mercia Mudstone, Blue Anchor Formation and Redcliffe Sandstone Formation.
Balfour Beatty has said that the minimal clutch friction between the king and intermediate sheet piles in the KSP system meant that it could cope with the challenging ground conditions. Dawson also planned to use pre-augering techniques in areas where hard ground was expected, so as to stop the work being delayed.
Trials of the KSP solution were successful and 10 structures with a total length of 550m were converted from the gabion design to the KSP alternative, although the gabion design was still used in locations where the motorway is on an embankment and infilling was required. Estimates of construction time for a 140m long, 2.2m high gabion retaining structure were in the region of 10 weeks but the alternative could be delivered in three and without the issues related to bringing large volumes of aggregate onto site.
The KSP work was carried out during daytime from the hardshoulder only, with all lanes on the carriageway open to traffic.
The work involved constructing retaining structures for the scheme with the retained height varying from 1m to 3m. Balfour Beatty used king piles driven to 2.5 to 5.5m below ground level, while the intermediate piles were driven to 1m. In total 716 z-piles were driven during the work.
Dawson installed the sheet piles using a Liebherr LRB12 5XL piling rig fitted with a Muller 1100 H high frequency vibrator. Piles were installed by vibratory methods to full depth or refusal and, where required, were later driven further to full depth by a Dawson HPH 2400 hydraulic impact hammer mounted on the rig. Pre-augering was carried out with a Hoylunds CA100 3Tm auger drive fitted to the Liebherr LRB12 5XL.
According to Balfour Beatty, the cost difference between the gabion solution and the KSP alternative is minimal but there were significant gains in terms of productivity and sustainability, as well as minimising risks to the workforce and road users during the construction phase.