Getting ground engineering experts on board early has proved beneficial for a leading housebuilder
New home owners in the picturesque village of Rowhedge in Essex may well be aware of its long and rich history, which includes smuggling, fishing, brewing and boat building. But what is less likely is that they will ever be fully aware of the range of geotechnical techniques that have gone into creating their attractive new waterfront environment.
Bloor Homes is currently building a new development of 170 homes alongside the River Colne at Rowhedge, a small village 4km south east of Colchester. The site has had a long history as a port, and has also been used for sand and gravel extraction, as well as for ship and boat building.
The port closed in 2001, and various development options were considered before Bloor started looking at it in 2011. At that stage the site was accessed be a concrete slab haul road winding through the adjacent woods, and most of the flat riverside area was topped with concrete slabs, access roads and warehouse buildings.
After demolition, it was clear that significant ground improvement and geotechnical input would be needed to make the wharf site suitable for new housing, and in 2012 the housebuilder brought in ground solutions specialist Keller and local consulting engineer RLT to discuss the various options.
Ground conditions consist of a layer of made ground and gravels above a weak clayey alluvium, with another layer of gravel beneath that before reaching London Clay at depths of between 7m and 9m below ground level. Perched water accumulates in the upper layers.
The key geotechnical challenges identified in those early stages were: providing support for the houses; finding a cost effective way to construct a new river wall along the development’s river frontage; and preventing undue settlement of the major roads and drainage.
The first of these challenges was a fairly simple decision to make: all the new homes are being built on 220mm square precast concrete piles, driven into the London Clay to depths of between 9m and 12m.
“On this site there are a lot of terraced blocks, with three houses per block, and we’re putting in six to eight driven piles to each block,” explains Bloor project manager Mark Quilton. “And we have deep ring beams – mainly 600mm by 600mm, but going up to 750mm – to spread the load.”
During the demolition, Bloor found two layers of concrete slabs in places, as well as buried basements and pumping stations. But Quilton says the company still did not want to take any risks that the driven piles might hit obstructions that had not been found during the site clearance: “We decided to bite the bullet and probe every pile location to a depth of 6m. Once we had set the piling positions out, we had a probe done on each position.”
Precast piles were preferred to in situ because of the site’s location and access. Bloor has taken up the old concrete haul road and is currently building a brand new road into the development on the same route, to adoptable standards. As this is the only way in and out of the site for construction deliveries and plant, it would not have been possible to have constant deliveries of concrete to service in situ pile installation.
“Had Bloor gone for CFA piles, there would have been lorry loads of muck away and concrete coming in,” explains Keller director Derek Taylor. “With the precast piles, one delivery can get a whole day’s piles in.”
Quilton adds that the decision “worked out almost perfectly for us”, as there was a two week period during the access road construction when no deliveries could come in or out.
So far Keller has installed 133 of the precast piles out of a total of almost 2,000, using a Junttan PM20 rig.
The second challenge is the river wall, which runs for approximately 300m along the frontage of the site. The existing steel sheet piled river wall and concrete capping beam are in a poor condition, and there is no record of what load it was designed to support.
As a result, a new sheet piled wall will be installed between 600mm and 900mm inside the existing wall, after which the old capping beam will be cut out and the piles removed.
The traditional technique would be to install a second line of steel sheet piles to support the new river wall, but with the piles having to reach depths of up to 12m, this would be an expensive option. The other two options would be long ground anchors – the current situation – or a relieving slab, and it is this latter option that consultant RLT has designed.
Once the new sheet piles have been installed and backfilled with engineered fill material, Bloor’s civils contractor Tamdown will cast a 400mm thick reinforced concrete slab up to 4m wide behind the new wall. Keller will then come in and drive raking piles to anchor the slab at angles of either one in three or one in four.
The 147 raking piles will be driven mainly in pairs by the Junttan PM20 rig, and will be spaced at 3m intervals along the length of the wall.
Work on the new river wall started this summer and is set to be complete by November, when Bloor will reopen the public footpath that runs alongside the river.
The company has also committed to monitoring noise, and making sure no operations that cause heavy noise and vibration are carried out between October and March, to prevent disturbance to overwintering birds that nest on this section of the River Colne.
The final geotechnical challenge to be overcome on the site was finding a way to ensure there would be no undue settlement to the main roads and drainage. The development is set out in a fan shape, with the river wall at the long top edge of the plan, and three sections of housing radiating out from a point at the base to the river. Roads and drainage run between the three phases, and the development’s main storm drain will follow the line of the footpath beside the river wall.
Drainage throughout the development is on a very shallow fall, so any undue settlement could cause serious disruption to the flow of storm water across the site to its eventual outfall through a headwall into the river. The storm drainage will be laid within the clayey alluvium and made ground layers, which would be susceptible to movement without some form of support or ground improvement.
In consultation with Keller, Bloor opted to treat the alluvium using vibro stone columns, installed through the made ground, upper gravels and alluvium, and into the lower gravel layer, which starts at around 6m to 8m down. The stone columns allow water to come out of the clay and the upper gravel level, and compress, as a result of settlement gets accelerated during construction, leaving any residual settlement within acceptable limits.
Keller was all set to install the 600mm diameter stone columns when Bloor’s contamination and geotechnical consultants RSK highlighted the possibility of contaminants from the upper gravel layer travelling through the columns into the lower gravel layer. Bloor was aware that mild contaminants – the remnants of the site’s industrial past – may be present in the perched water, and had made provision to treat them if necessary. But the potential that they might find their way into the lower gravels had to be eliminated – especially as water is abstracted from this gravel layer elsewhere in the Colchester area.
The solution that Bloor, the consultants and Keller came up with – and was approved by the Environment Agency – was to stick with the vibro stone columns, but insert a 1.5m to 2m deep bentonite grout plug at the base of each column. This way the columns can still act as they are intended, to treat the clay, but contaminants cannot travel between the clay and the lower gravel layer.
Keller is installing the vibro stone columns along the line of the main storm drain behind the river wall, as well as all the new roads on the development, using a purpose-designed rig that can deliver both stone and grout, which is mixed on site. They are set out in lines of three columns every 4m – a total of 2,000 columns, all with the grout plug. Much of the stone that is going into the columns is actually recycled material from the slabs and haul road that used to be on the site. Bloor is using this recycled material throughout the site for fill and for drainage layers.
The developer is already starting to build the houses in phase one, ready for the first occupiers to move in before Christmas. While the first home owners will see their neighbours’ houses being built, they will have little idea of the work that has gone into preparing the site for their arrival.