Groundworks for new housing in Kings Langley include reworking an old mill race and diverting a side branch to the Grand Union Canal. Words and pictures by Adrian Greeman
When Galliford Try site manager Tom Willoughby heard that a new housing project would centre on a central waterway feature through the site, formed as a side branch to the Grand Union Canal, he knew that he had to work on the scheme.
“It is not every housing development that involves some interesting civil engineering and groundworks,” he explains. “It is a chance to show a bit more of what we can do. The work covers quite a range of techniques too so it was an opportunity to do it all.”
The Main Project
The main project is for 380 homes near Kings Langley in Hertfordshire with a focal water feature that is designed to increase the desirability of the development while dealing with some difficult existing hydraulic elements.
The housing is in two separate lots on land acquired jointly by Crest Nicholson and the Galliford Try-owned Linden Homes.
Each is building its own section and both brought in Galliford Try to undertake substantial piling and some significant water course diversion and reconstruction works in a £4M contract.
The housing development is on a 3.5ha brownfield site, formerly the location of a large printing works which closed in 2006.
The printing house was a development of a paper mill located at the site since the late 18th century which in turn displaced a various water-powered flour mills reputedly dating back as far as the 11th century.
The old building on the site, Nash House, built at the turn of the 20th century and a listed structure, is also being renovated.
“It is a chance to show a bit more of what we can do”
But the big issue is the water courses and drainage of the site.
When the Grand Union was built next to the site in 1819 the mill race was restructured as an inlet from the canal, giving boat loading access to the paper mill.
“The water flow from the mill discharged further along below the second of two locks on the main canal,” explains Willoughby.
It was carried in two 900mm diameter pipes “running three-quarters full most of the time”.
The side-arm and mill race also had an overflow system to balance water rises when the locks were operating and this ran across the centre of the site to connect to the 6m wide main canal, in between the two locks. It comprised open channels at either end linked by an underground pipe.
“That sat under a 450mm concrete slab so that the heavy lorries, carrying 20t paper rolls for the printworks, could pass across it,” says Willoughby’s assistant site engineer Matt Reeves.
The civil works, designed by consultant URS-Scott Wilson, involved the installation of a new underground connection to replace the intermediate overflow.
Once that was in place it would provide emergency capacity for water overflow, allowing the longer pipe connection across the site to be removed and replaced with a surface channel, which would provide a major water feature for the site.
Early work included moving and diverting a significant range of services, including fibre optics and power lines.
Then the first part of the civils proper was to reshape the end of the mill race inlet and build two parallel sheet pile end walls.
The first of these will be a permanent weir for the project in the future and the second is a temporary cut-off to divert water into the new underground overflow channel.
This will be removed eventually when the main channel is filled.
“A water table sits just 3m down and there was a lot of pumping to do during the excavation both here and for the other works.”
- Matt Reeves, Galliford Try
The underground side channel was installed with 1m x 1.5m precast culvert elements craned into an excavated channel.
That itself required careful work as there is a chalk layer 8m below ground “and you start getting into aquifers at that point”, says Willoughby.
Above the chalk is gravel topped by a 2-3m thick layer of friable clay. “A water table sits just 3m down and there was a lot of pumping to do during the excavation both here and for the other works,” says Reeves.
A couple of Siltbuster units are used to clarify the pump runoff, which will continue until the end of the civil works.
With the channel installed and a concrete head section completed, including a concrete overflow, the flow through the mill race was safely diverted and excavation could move on for the main channel.
This was built as a 5m wide concrete trough within a 3m deep trench excavation.
The excavation is battered with a 1:2 slope on the western side to create a pleasant effect along the housing areas.
A waterproof membrane retains the water and there will be planting and landscaping.
On the other side there is a vertical reinforced concrete retaining wall the cantilevered toe of which forms the base of the channel.
The now brick-clad wall bounds a phase two section of the project where eventually apartments and a care home will be built.
The Next Step
“Phase two is an additional 2ha to the site and is in a kind of island formed by the new water course. Therefore connections are needed,” says Willoughby.
“The biggest units weighed 26t and had to be lifted in with a 350t crane which took some planning and organisation”
Three bridging points have been made in the new channel’s reinforced concrete culverts.
Precast units from Hanson make up the crossings, each stretching 7m wide across the channel.
For the main crossing the culverts are 2.5m from top to bottom.
Seven pieces, each 1.5m wide sit side by side and the road goes on top. Slightly smaller units were used on the other crossing points.
“The biggest units weighed 26t and had to be lifted in with a 350t crane which took some planning and organisation,” says Willoughby.
A firm platform had to made for the crane which needed several wagons to arrive at site.
Although there is only one route into the site, the M1 is not far away and so logistics are not too difficult “as long as everything does not all arrive at the same time”, says Willoughby.
The slightly smaller units for the smaller bridge points weighed 18t and 21t respectively; four of the smallest were required for the second crossing and six for the last one.
While this work was under way during spring and summer, there has been significant foundations work to get on with.
The soft friable clay across much of the housing site has been improved using stone vibro columns installed by Balfour Beatty Ground Engineering.
Some 600 columns up to 5m long and 500mm in diameter were installed, work which started right at the beginning of contract in November 2010 and ran on until the spring.
Rock & Alluvium began work in December 2010 on the installation of just less than 200 CFA piles, mostly around 13m long which takes them down into the chalk.
Some of these are used to support cantilevered foundations for the houses on a top corner of the site.
This is to protect an existing 2m square reinforced concrete culvert which runs across - it belongs to Thames Water and carries storm water.
That existing culvert will be used to carry water output from an overall SUDS drainage design for the whole site.
Features of this include “porous” road paving using Hanson Formpave, over a formation that includes a 63mm layer of non-interlocking stone to allow water through.
The road itself is tanked and drains out into the central channel via two 150mm diameter outlets and two more into the corner culvert storm drain.
“There are flow-restricting vortex orifices and other features to create a slow discharge over time,” says Reeve.
Much of this civil engineering was over by October this year, though the channel is still to be filled and finishing work to be completed.
Housing work on the sites is moving on apace.