It is here, in this leafy, affluent suburb of south west London, that Dew Piling is supporting the foundations for a state-of-the-art residential property, complete with a basement to house a swimming pool and leisure suite.
Dew is delivering the project for main contractor Francis Construction on behalf of a private client, who has demolished an existing house to make way for the new property.
The high water table in Barnes and the close proximity of neighbouring properties in Queens Ride created a need for significant ground support during the basement construction. There were additional complications due to the design of the proposed house, which required the 18m by 9m basement to follow a number of tight corners.
The ground conditions consist of about 1.5m of made ground overlying Kempton Park Terrace Gravel – a medium-dense sandy gravel – beneath which lies a layer of firm London Clay approximately 7m below ground level.
These factors led Dew to consider a number of solutions at design stage. The first option was a temporary partial cofferdam, constructed in steel. However, this was discounted due to the significant propping and framing needed, which could interfere with the follow-on construction of the basement.
A second option was to build a full steel cofferdam with sheet piles installed to about 6m below ground level, supported by full bracing and framing during basement construction. The sheet piles could then be removed afterwards.
Following discussions with the main contractor and designers, a third option was selected as the most economic. Site workers would install sheet piles to a depth of 8m to create a steel cofferdam to act as a cantilever, which would later be incorporated into the permanent basement.
This was also in preference to a partial cofferdam, after an Anderson Shelter (a Second World War air raid shelter) was discovered buried beneath the property during demolition. This resulted in larger scale excavations than originally anticipated.
Although this was a more expensive choice compared with the other piling options, the savings in time because of a lack of disruption to the follow-on activities, more than compensates.
"Due to increasing global pressures on steel sheet pile supply, we have invested heavily in steel stocks. On this project our in-house design team was able to use existing steel stocks, providing the client with a keen price and also guaranteeing availability to suit the contract programme," says Dew director Alec Courts.
The contractor is building the sheet pile cofferdam using 8m long VL605 sheet piles – 91 in total – manufactured in the Czech Republic. These act in cantilever during the excavation process.
Given the potentially high water table indicated in the site investigation, the clutches of the piles will be welded to provide a dry environment for the excavation and concreting works.
Out of respect for neighbouring properties in Queens Ride, and as a requirement of the party wall agreement, the contractor is using a low vibration pile technique to install the sheet piles with a newly acquired Kowan WP150 Stillworker pile press.
The equipment uses the reaction against previously installed piles to hydraulically press the next sheet pile into position and exert a pressing force in the region of 150t. On projects where extraction is required, the Stillworker can pull to a maximum of 160t.
The acquisition of another Stillworker is part of Dew's ongoing investment programme to expand its plant fleet. In recent months it has invested in new piling rigs and equipment following its acquisition last year by Belfast-based Lagan Holdings – a civil engineering and construction materials business.
Dew commenced work at Queens Ride in early September and completed the basement project within two weeks to enable construction of the main property to begin.
"Our project in Barnes is typical of an increasing number of basement schemes where steel sheet piles are being used as a permanent solution to wall construction," says Courts. "The market for residential basements and car parks has increased significantly over the past three years and we anticipate a steady and continued rise in demand well into the future."