Bored piles for bridges, precast for an embankment, wick drains and surcharges are all part of work on the Sittingbourne relief road in Kent. Adrian Greeman reports.
The Sittingbourne relief road on the Kent coast is not a big project - it is just over 1km from end to end.
But it nevertheless packs in a variety of ground works and foundations challenges for contractor Jackson Civil Engineering to tackle.
The combination of structures has been subject to several changes which have usefully value engineered some cast savings for the project’s client, Kent County Council, either directly or by getting around difficulties that would have needed last-minute changes at extra expense.
The road runs to the north of the town close to a still operating paper mill.
When complete at the end of the year it will much reduce heavy lorry movements through the clogged town centre.
But the alignment has to run over some of the marshy ground typical of the north Kent coast.
It also has to cross an old landfill site.
Crossing the route is also an unusual narrow gauge railway.
There is also a bridge across Milton Creek which runs into narrow Swale channel, which separates the Isle of Sheppey from the mainland.
“The significant ground issues on the site are the landfill and the marshland, though in fact the bridge abutment work was the place where we were able to offer the biggest saving,” says Jackson contracts manager Andy Nailor.
At Milton Creek, an existing jetty retaining wall was tied back to a buried structure at the future abutment location, which could not be accessed for maintenance once it was built. It meant new support was needed for the jetty wall.
“One of the issues was whether there would be obstructions in the fill, since it contained waste concrete and the like, but in the event there were very few refusals”
“The plan was to build a new H-pile wall in front of the old one with a cantilevered support toe,” explains Nailor. “But we offered a cheaper solution instead simply with new ties running back to the abutment itself.” He thinks the change helped win the job.
Larger scale groundwork was needed on the 500m long section between the creek bridge and another crossing for the railway further up.
In this zone the route runs across a landfill which meant there was a risk of unknown settlement problems in the future and possibly some other environmental problems.
“It is not a housing refuse site, however, being mainly used for building waste and the like,” says Nailor. The 9m deep landfill closed in the 1980s and was capped with a 1m thick clay layer, he adds.
The road will run on an embankment over this section which is piled. One of the earlier jobs for subcontractor Aarsleff, in spring last year, was to install 1,500 driven precast concrete piles. These were of varying length through the 6m to 9m deep fill and into firmer gravels beneath.
“Aarsleff used single piles up to 14m long and in a few places went as deep as 18m for which it put a jointed pile in place,” says Nailor. “One of the issues was whether there would be obstructions in the fill, since it contained waste concrete and the like, but in the event there were very few refusals.”
Even where around half dozen of the piles “bounced”, they proved to be usable, he says: “We tested them for load capacity and found them suitable.”
Despite the nature of the fill there was some danger of waste gas escaping from the site.
So before piling could begin, the top was further sealed with a geosynthetc liner, a kind of clay “sandwich” in geofabric.
The piles obviously punched through this, so clay granules were placed around the resulting hole.
These expand in contact with moisture to create a seal around the pile.
The finished piles, at roughly 1m centres, were individually capped with either 600mm square or 900mm square tops.
Basetex geofabric was laid over these in two layers to distribute loads, one lengthwise and one across.
“On top of that the embankment is built up between 2m and 7m high,” says Nailor.
A small saving here was to use surplus off-cuts from the concrete piles for fixing the edges of the geofabric.
They replaced some of the gabions which originally were to run each side of the embankment with the fabric wrapped around it to prevent it being dragged inwards as the fill is placed.
“We suggested the alternative as a way of reducing imported fill and at the same time reducing the need to remove waste from site and that was accepted” says Nailor.
A final issue is that drainage for the road and services, now being installed have to be kept above the level of the fabric.
“That is not such a problem for the smaller things, but we are installing a big water main for Southern Water as part of the works which would usually go a bit deeper, and care is needed,” explains Nailor.
Further along the route different work is being undertaken to build the road up on 250m of marshland.
To force settlement of the soft ground, 7,000 wick drains were installed and a careful regime of embankment building was carried out, including a small surcharge.
“We built the Class 1A fill in 300mm lifts,” explains Nailor. “We were obliged to do no more than a 2m increase in any one phase, waiting four weeks before adding another layer. Up to three layers were placed, and then a final 1m to 2m with the complete embankment allowed to sit for 36 weeks.”
Substantial instrumentation was installed to monitor the settlement by Sussex firm Instrumentation, Testing & Monitoring.
To fit with the schedule, the excavation of a cutting section at the top end of the site, which provided some fill, was done intermittently, so there was no need to move too much material around.
“We were obliged to do no more than a 2m increase in any one phase, waiting four weeks before adding another layer”
Between the embankment sections there are two major structures.
The first crosses the narrow gauge Sittingbourne & Kemsley railway.
“It was once used to connect two paper mills carrying materials and workers, though one of those is closed and these days the railway is more of a tourist attraction,” says Nailor.
Despite its small size, a reasonably large span of 42m was needed to take the road over the rail line because of a nearby gas main which would have been expensive to divert.
Piles for the abutment were to have been bored but were changed at Jackson’s suggestion to CFA because of running sand layers in the area, for which the fully cased CFA was more suitable.
“The specified bored piles were used for the crossing of the Milton Creek however,” says Nailor.
Though this has a centre span of 41m its overall length is greater with two 35m side spans.
For the piling work, jetties were formed either side of the river with tubular piling, to give access to the middle of the river where sheet piled cofferdam piling platforms were made at the pier locations.
Driving the piles was quite difficult, says Nailor, as the underlying gravel was harder than thought.
“The time we lost has been caught up, however,” he says, not least because the dry spring allowed work to progress.
Piles for the piers and the abutments were bored at 900mm and 1,050mm diameter under a polymer bentonite by Morgan Sindall Piling.
Once the pile cap was in the ties for the old jetty wall could them be made back to the new structure “which certainly is less likely to move than the old buried wall tieback”.
Nailor says the jetty solution was an obvious one but made problematic because the new bridge is owned by the country council, while the jetty is the responsibility of the local Swale District Council.
“We had to do a bit of communication between them to get it sorted out.”
A big 1,000t mobile crane was on site in March to lift in steel beams for the bridge, working from a prepared platform at the sides.
“The abutment piles form a ‘u’ around the precast embankment support and these meant the loads could be handled without extra piling,” explains Nailor.
On the far side, the ground is better anyway, he says.
Last of the embankment fill was being placed in early summer with only the road construction itself now to be completed by the autumn.