A longside the pretty River Weaver in Nantwich, Cheshire, a small site is getting an overhaul.
Dew Piling is completing a £250,000 piling contract for client and main contractor Brighouse Homes. The new apartments will be built on top of a permanent steel sheet-piled cofferdam, which as well as forming a basement level car park will also act as load bearing foundations.
The strata on site comprises 1m of made ground above 1m to 5.5m wet sand overlying good clays going into dense clays.
Because of this the design has made sure the sheet piles will be of a sufficient depth into the clays so they act in cantilever to retain the soil surrounding what will eventually be the underground car park.
This means that the majority are LV604 – which when connected side by side form a pair 600mm wide – steel sheet piles installed at 9m depths.
A Bauer RTG RG 19t rig with an MR 100V-03 vibratory centrifugal hammer is shaking these into the ground to sit 4m in the clays. These are skin friction piles with no toe load-bearing capacity and workers are aligning each one with a laser line to help with precision (and ultimately to ensure water is kept out) of the structure.
Dew works manager Mick Little says they are designed this way because the bottom of the sheet pile is like a knife's edge resting in the clay. So, with skin friction alone, each pile will take a load of 200kN.
The intention is that this cofferdam will work on three levels: to hold the soil away from the car park; to provide the building's foundations; and to form a watertight box to protect the property against threats from the nearby river.
This sits about 30m away and has a tendency to flood. Dew general foreman Tim Hopkinson says it happened as recently as one week before GE visited the project. Fortunately for the team, on that occasion the water spilled over the opposite riverbank to that which flanks the site.
But to help protect future residents and their homes, Dew is making certain that the building remains as watertight as possible. It carries out welding work to seal together each of the piles along the inside seam of each of the clutches.
This section of work is done in two phases. Site workers weld the top 1m of pile first, shortly after the piles have been driven to full depth.
This is the initial stage of securing the structure because Brighouse will then cast at the top of the piles a 700mm2 steel-reinforced concrete capping ring beam to secure the cofferdam.
The ring beam needs a minimum of seven days to ensure the concrete has time to cure. This beam effectively holds the box together while the developer's 21t excavator digs out to formation level, which will be 4.2m down from the top of the concrete.
Dew will then return to continue welding the remaining visible seam between each pile down to formation level.
Hopkinson says: "It's getting the seal and the corners right – that's the important part because it needs to be watertight – so we've had to make the piles fit the building." In some cases this means the team has to cut piles to make junction piles to fit the corners. One other vital element is getting the seal right between all piles.
This is done with the help of a 10mm diameter steel round bar that is inserted along the clutch between each pile and welded on either side.
Hopkinson says: "Originally we did try to weld the sheet piles directly together [without this steel bar] but we ended up with gaps along the seam." While it was important to get a tight seal using the bar, this also gave them a neater looking weld, which Little says helps because the inside of the sheet piles will form the wall of the car park – once it is blasted and painted.
A second measure will be taken to protect against flooding should the building be subject to water infiltrating via the ramp down to the car park. Brighouse is installing a sump to help with dewatering once the building is in use.
The plan to install this sump is less than surprising because the groundwater is already making its presence well known before
the excavation has begun. Brighouse has kept the piling platform higher than it needed because of this threat as well as the potential for the river to flood.
In addition, the firm's site manager Rob Paulson says he expects to have to dewater when excavating begins because there is a substantial amount of groundwater sitting above the clay.
In total about 300 piles will create the cofferdam. And with the help of a 50t Hitachi Sumitomo crane, which lifts each pile to the Bauer rig, work is being done at a rate of up to 40 piles per day.
This is one of the defining factors for why the client opted for this foundation design. "If the loads allow it, this [using sheet piled foundations] is quick and straightforward and is the most cost-effective method," Paulson says.
The piling work was due to finish by the beginning of February and on completing the development Brighouse will have built 47 apartments.
These are designed in two styles – half in a traditional style and the other half to appeal to a more modern buyer. The modern block of 23 apartments will come complete with ground source heat pumps to provide under floor heating (see "A modern method – ground source heat pumps").
Paulson says he is increasingly seeing the two features of a sustainable heat source and underground car parking as a major selling point. He says that although local house buying has slowed, six of the modern block apartments sold within the first two weeks of going on the market.
A modern method – ground source heat pumps
Brighouse Homes has decided to give its homebuyers the choice to move towards more sustainable living by installing ground source heat pumps for half of the 47 new properties at Barony Road in Nantwich.
It has subcontracted Geocore Site Investigations to carry out the £40,000 work to drill 12 boreholes that will take ground loops supplied by IVT and designed and specified by ICE Energy.
Geocore site workers are using the firm's newly purchased Commachio 450-P rig with a pneumatic rotary drill to bore these 150mm diameter holes down to 100m. However, ground conditions (see "Site strata") mean where there are layers of sand, the hole needs to be kept open with the help of temporary steel casings. Therefore site workers are inserting 1.5m lengths of casing to reach well into the marls
at a maximum depth of 35m. This is because the initial 10m or so comprises quite weathered rock.
Once this stage is complete, the ground loops go in. These are 40mm diameter pipes containing fluid that is warmed by the heat from surrounding soil as it is pumped through. This ultimately feeds into individual heat pumps installed in each of the apartments.
Brighouse site manager Rob Paulson says this effectively works like a fridge in reverse. The fluid is put through a series of heat exchangers and a compressor in the heat pumps, which concentrates the heat. The system will then be able to provide under floor heating for these units as well as hot water for domestic use.
As well as Paulson's expectations that the market for deep basement construction and geothermal heating methods is likely
to grow, Geocore's new Commachio has brought its geothermal drilling fleet total to four rigs. Director Adam Woodhead says each of these purchases has been made only in the past 15 months and represents the increase in geothermal drilling work the company has experienced – something he predicts is likely to continue.