As part of the extensive, ongoing power upgrade of the London Underground (Package 4), MGF had been selected by Bam Nuttall to assist in designing a support system for the construction of a reinforced concrete substructure on Chapel Road.
MGF’s scope was to design and supply a structurally efficient temporary works propping scheme whilst not compromising site buildability. Due to the deep and high-risk nature of the temporary works (adjacent to a live LU track), MGF’s design was subject to a rigorous Cat III check prior to obtaining LUL/Network Rail approvals before works could progress on site.
A challenging as well as varying site topography combined with complex cofferdam geometry presented many design issues beyond that normally considered with a traditional sheet piled cofferdam. With existing ground level differences of up to 5.6m across the cofferdam combined with non-parallel cofferdam walls, this resulted in double-raked flying shores. Well beyond the scope of any modular format shoring systems, MGF’s propping scheme comprised a blend of off-the-shelf modular hydraulic propping systems and bespoke elements to provide our client with the best value whilst maintaining the benefits and versatility of modular hydraulic systems.
London Underground Cofferdam
BAM Nuttall
Providing safety solutions to a site in an urban built environment
Structural detailing required consideration of horizontal as well as vertical shear restraint details for the walings and props. This was to ensure a stable cofferdam and temporary works support system were maintained. Horizontal shear restraints comprised steel shear keys welded to the backside of the walings, which were then cast into concreted sections within designated sheet pile pans, or where this was not practical, steel plates welded between the walings and sheet piles were used. At raked flying shore positions, vertical shear restraints were required to offer transitional and rotational restraint to steel walings. Prop-to-waling connections comprised MGF’s modular 2500kN swivel base assembly.
Due to a very congested site with virtually no working space outside the cofferdam, MGF’s design was based on a staged propping installation approach mirroring the client’s construction sequence. With the propping installation only nearer completion as excavation works had progressed across the site, the temporary support frame could only be treated as an open/incomplete frame, and therefore additional horizontal restraint steelwork to some of the walings was required to achieve a stable cofferdam and temporary works support system. Once all of the walings and props had been installed, a more traditional closed/complete cofferdam propping system was formed.
The use of MGF’s hydraulic propping systems mitigated construction/fitting tolerances during the installation stages. Although preloading of the propping system was not required by design, the inherent preloading capability of MGF’s hydraulic propping props provided our client with peace of mind that the propping installation would be to a very high standard, removing any ‘slack’ inherent within any TWs installation prior to excavating and actual earth pressures loading the system.
This helped reduce ground movements and the obvious consequential risks of ground settlement given the locality of the site (built urban environment) and its close proximity to a live LU rail track/asset.
UniShore® Heavy is a heavy-duty, modular shoring and propping system with class leading performance for the support and jacking of …
UniShore® Medium is a medium-duty, modular shoring and propping system that can be handled manually when necessary, combined with bending …
UniShore® Light is a lightweight, modular shoring and propping system that balances the need to minimise component weight, reducing risk …
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