Tousands of stone columns combined with major contamination risks pose tough test for Keller on Colchester housing scheme.
Project: Rowhedge Wharf residential development
Developer: Bloor Homes
Contract value: £0.3m
Groundworks subcontractor: Keller UK
Groundworks consultant: RLT
Start date: February 2016
Completion date: August 2016
Like many towns and cities across the country, Colchester is suffering from the effects of the housing shortage.
In recent years its population has swollen thanks to the area’s economic success as well as the draw of the easy commute to and from London.
As the number of those living within the city limits has increased, so has the demand for housing across all rungs of the ladder. This has led to Colchester seeing new developments spring up on brownfield sites across its boundaries.
With pressure to increase the number of residential developments in the area growing, so those developers have looked outside the town and into the villages that surround it.
Bloor Homes is no exception and has found one such suitable site on the edge of the picturesque village of Rowhedge, overlooking the river Colne that gives Colchester its name.
History of variety
The Rowhedge Wharf scheme is a development of 170 new homes on a brownfield site which wends its way down to the riverside.
“In some locations there was concern over the downward passage of patches of contamination at depth”
Derek Taylor, Keller
Given its position on the banks of the tidal river Colne and its previous light industrial use, it was always likely that the ground might throw up some challenging conditions, so the developers called upon the expertise of Keller to help deliver the project.
“The site has quite a bit of history,” says Keller development director Derek Taylor. “It has been a gravel working site as well as general storage, wharfage, warehousing and a boat building yard and latterly a distribution site. We were called in to help Bloor Homes through the budgeting stage when there were a few concerns raised.”
Those concerns were primarily based around the topography and historical use of the site. Like most light industrial sites there were areas of contamination – nothing heavy enough to warrant a major muck shift or soil decontamination process, but rather an engineered solution.
“As with any former industrial site there is some contamination [near] the surface which the developer is dealing with by way of removal and capping,” Mr Taylor explains. “But in some locations there was concern over the downward passage of patches of mild contamination at depth.”
The site itself features a layer of made ground up to 4 m thick in places which overlays a layer of soft alluvial clays. This in turn sits above a layer of river gravels before reaching the London Clay bedrock.
From an early stage, the plan had been for the team to install a series of stone columns around the site to help support the roads and drainage system across the scheme. With some of the site requiring additional fill to be brought in to bring it above flood-risk levels, there was genuine concern that the weight of this extra fill could cause long-term differential settlement.
“Each column takes around three tonnes of stone. We needed around 450 tonnes of stone just to keep us going each week”
Derek Taylor, Keller
Colchester-based consultant RLT designed the stone column solution, which will help alleviate any pore water pressure build-up in the alluvial clays. These stone columns will act as pressure release valves and ensure uniform consolidation of the ground when loaded with the extra weight of the additional fill and road surfacing.
“The drainage system for the new estate is laid through the layer of made ground and there are very shallow falls across the site,” Mr Taylor says. “There were concerns that any differential settlement across the scheme could compromise the efficiency of that drainage system. The grid of columns will ensure uniform consolidation.”
But after detailed site investigation work it became apparent that there was a risk of downward migration of contaminants into the lower levels of the site’s underlying strata, with the band of river gravels at around 8 m being at particular risk.
Drinking water is extracted at this level, so to avoid any contaminants being washed through and into the gravel layer, the Keller team suggested a possible solution to the problem.
“We needed to plug that section of the vibro-stone column where it passes through the river gravel layer,” Mr Taylor explains. “We suggested grouting the first 1.5 m of each column to form a solid plug or barrier where the columns pass through the gravel and into the London Clay, sealing off any possibility of contamination migration.”
Precast piling right at home
Thanks to the nature of the site and the additional fill required to help raise the level of the scheme, the housing development at Rowhedge is being built on a series of piles designed and installed by the Keller team.
It has used 220 mm x 220 mm square section precast concrete piles installed to depths of 8-10 m across the scheme. This first phase of work will see around 250 piles installed in total. The piles are then tied together with precast ground beams, enabling the houses to be launched from this base.
“You would always use a displacement pile when possible on a site like this,” Mr Taylor explains. “It avoids having to remove any potentially contaminated arisings. There is no problem with driving a precast pile through the strata here, there is no migration and the precast system is clean and efficient.”
The team has used a cement and bentonite mix to form the plug across the 2,000 stone columns that have been installed, each measuring 600 mm in diameter and located at 2.8 m centres following the line of the roads across the scheme.
The grout line is welded to the vibrator that forms the hole before clean, imported natural aggregate, which comprises 20-40 mm-sized stone, is dropped into position. This stone is then vibrated and consolidated as the grout is pumped down the line to the required level before the column is drawn up to the surface, once again using natural clean aggregates.
The final 2 m is filled using 40-75 mm dimension recycled aggregate, which has been site-won by crushing the concrete slabs of the existing warehousing and storage yards.
The vibro-stone column installation is not the only ground engineering work the Keller team have been carrying out at Rowhedge.
The houses themselves are being supported on precast concrete piles (see box), while alongside the riverside itself a 300 m long new wharf wall is being constructed, replacing the existing sheet pile wall.
Here a 500 mm-thick reinforced concrete relieving slab will be installed behind the face of the new wall. Raking piles at 3 m centres will then be put in, tying the new wall to the slab and preventing any possible overturning, before the site is backfilled.
With 170 houses to be built across the scheme, the Keller team is making sure the new homes at Rowhedge are built on firm foundations.
Access obstacle leads to huge stockpile
It’s not just city centre schemes that have their access issues; even those projects in the heart of the English countryside can throw up their own difficulties.
At Rowhedge a restriction on the planning approval has meant the site itself can only be served using one access road in and out of the project. Although there was an existing haul road, the project team needed to build what will become the main 1.5 km access road to the site.
But with the development team at Bloor Homes keen to get the scheme up and running as quickly as possible, Keller was instructed to stockpile material so that it could continue its work as the access road was being built.
“The Bloor team needed four weeks to complete the new access road but we couldn’t stockpile four weeks’ worth of material on site,” Mr Taylor says.
“It just wasn’t feasible. We would end up with huge stockpiles of material with no way of working around it. We negotiated a two-week stockpile but even that meant huge amounts of material had to be moved onto site.”
To keep the job fed over the two weeks in which the access road was being built and improved, the team needed to bring in large quantities of stone, bentonite, precast piles, cement and fuel – everything that the project could need.
“We did think about bringing in a spare rig just in case the one we were using broke down but decided against it,” Mr Taylor explains. “We had around 300 pile sections stacked up on site as well as massive mounds of stone for the columns.
“Each column takes around three tonnes of stone. We needed around 450 tonnes of stone just to keep us going each week. It was a big task to get that side of the scheme organised, but we managed it and it hasn’t cost us any time.”