Down by the Riverside
With London’s finance district, Canary Wharf proving to be a popular location for some of the largest international businesses, it is hardly surprising that the area is home to a number of ambitious building projects which will add to the London’s skyline. One such project is taking place at the Riverside South site, where Bachy Soletanche Limited (BSL) is constructing a considerable 610 m diaphragm wall for two high-rise structures.
The site is one of the last river fronted areas remaining at Canary Wharf, located on the west side of the Isle of Dogs. The two buildings, proposed to be the second tallest in the Wharf, are specially designed to compliment the skyline without compromising the views of the famous three buildings – One Canada Square, HSBC Tower and Citigroup Centre – that have stood for several years.
The leading geotechnical firm is working in a joint venture with Costain under a contract with Canary Wharf Contractors Limited. The project includes the sophisticated diaphragm wall technique, as Ben Hardy, Bachy Soletanche Limited’s Contract Manager for the Riverside project, explained:
“We were awarded the ground works project to construct a basement wall around the perimeter of the site, having proposed the diaphragm wall technique in lieu of the conforming secant pile scheme. The superior watertightness of a diaphragm wall for the excavation of the three floor basement was a big selling point.”
A diaphragm wall is constructed in panels with the Riverside South project requiring a massive 96 panels to complete the 610 m wall. However before the panels can even begin to be installed, a guide wall trench is being created to maintain the accuracy of the wall construction.
The ‘panels’ are a combination of 1000 mm and 1200 mm widths, and are constructed using 2.8 m long rope grabs. They consist of three “bites” – a “virgin” bite, a “stop-end” bite and a “centre bite”. When these three bites are complete, each panel is up to 7 m in length.
In order to form watertight joints between the panels, stopend casings containing a double sided water bar are placed at both ends of the first ‘panel’ prior to concreting. The stopend is designed so that half of the waterbar is cast into the panel and half is protected/contained within the structure of the stopend. These stopends are then withdrawn once the adjacent panels are excavated, and the second half of the waterbar is left behind to be encased in the concrete of the adjacent panels.
Ben Hardy continued:
“In normal circumstances we would just construct the diaphragm wall, but with the site being on a large bend in the River Thames, the Riverside site has a long history of boat building and boat maintenance. Dry docks have been built over the top of the other for centuries before finally being crudely backfilled in the middle of the twentieth century. A diaphragm wall grab is not designed to go through these timber, stone, concrete and steel structures, and therefore the project involves a significant amount of rotary coring.”
The old docks are cored out using 1500mm diameter coring equipment and rock augers down to a depth of 12m. The hard cored out material is then replaced with a soft slurry mix which the grab can easily ‘bite’ through after it has set.
Ben Hardy concluded:
“We have a systematic approach to this part of the project ensuring that the docks are fully cored out on the alignment of the panels. For the diaphragm wall operation to run smoothly we can’t afford to have a big lumps of material obstructing the grabs”
When the wall is complete in mid-spring 2008, the ground will be excavated by Costain with staged installation of ground anchors.