The Tideway project is a 25km super sewer, currently under construction, that will run from Acton in the west to Abbey Mills in the east. The huge tunnel mostly runs underneath the tidal section of the river Thames and will collect, store and convey to treatment works downstream almost all of the raw sewage and rainwater that currently overflows into the river.
Cremorne Wharf includes the Lots Road Pumping Station, an early 20th century grade II listed building. Enough raw sewage to fill more than 400 Olympic-sized swimming pools currently spills into the Thames from this pumping station every year, as the site simply does not have enough capacity to cope with modern demands.
Main contractor Ferrovial Construction/Laing O'Rourke JV (FLO), is carrying out this phase of the project to connect this site to the new super sewer tunnel, thus reducing this pollution by more than 90 per cent.
The contractor has sunk a 30m-deep secant-piled shaft down from the surface to link up with the Tideway tunnel below. And next to this another excavation, which extends right up to the wall of the shaft, has been created to house new air-treatment equipment.
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It was for this excavation that Groundforce Shorco supplied equipment to brace the sides and provide safe access for people working in the excavation.
The irregular shape of the excavation – and its proximity to both the new shaft and the historic pumping station represented a very challenging design requirement.
The excavation was roughly square in plan, measuring 12.5m by 13.5m but with the already-completed circular shaft intruding into one corner, reducing the two adjoining sides to 8m and 6.5m in length.
Additionally, the 12.5m-long side of the excavation ran parallel with, and only millimetres away from, the foundations of the grade II-listed pumping station.
The excavation was lined with Larssen interlocking steel sheet piles around the perimeter, braced at three levels with Groundforce Shorco’s Mega Brace hydraulic units and extensions.
The depth of the excavation ranged from 4.3m down to 6.9m in one corner but the site's close proximity to the river meant a relatively high water-table at around 4.8m below ground level.
This, together with soft clay soils to a depth of almost 8m, created challenging ground conditions which necessitated the use of 10m-long sheet piles despite the excavation itself being only about half this depth.
MP125 hydraulic struts were installed as knee-braces across the corners of the excavation to provide lateral support to the Mega Brace units while allowing a 10.4m x 8.4m clear opening in the excavation.
Marcos Fernandez, senior engineer with FLO, commented: "Groundforce, in its field, is one of the best subcontractors I have worked with. They give prompt answers and solutions to my queries and offer a big range of options and a proper and specialised team."
At the Falconbrook Pumping Station site Mr Cropper, supplied a Mega 4 Jaw Cropper and Power pack to crop concrete piles which were 10M deep in a chamber.
The Cropper was lowered in and suspended by a large crane which was already on site. Power was supplied to the cropper via a power pack which eliminated the need to have a large excavator in the chamber, which reduced risk, but also lowered the amount of exhaust gasses in the confined area.
Groundforce Shorco also supplied products from its SiteSafe Safety range at Cremone Wharf, around the edge of the excavation provided by the modular EdgeSafe system and Davit arm, and Ladder access to the excavation was via the Groundforce Premier LadderSafe platform, a heavy duty, temporary, gated access platform which uses a conventional ladder to allow access and egress for medium-depth excavations typically up to 6m deep.
