Temporary works design at Crossrail's first-opened station

Source: Rail Technology Magazine June/July 2014

Martin Reader, sector director of engineering services at Morgan Sindall Professional Services, describes the complex temporary works design that facilitated the completion of East London’s newly opened Pudding Mill Lane DLR station.

Pudding Mill Lane (PML), while not being a Crossrail station, has been a necessary part of the Crossrail project. It has transformed one of the smallest stations on the Docklands Light Railway (DLR) line to the largest, allowing increased capacity following the area’s Olympic-fuelled regeneration and now greater capacity for future events and football matches.

Crossrail’s new alignment and tunnel portal cuts directly through the old PML station, which meant the new station was constructed approximately 50 metres south of its original location.

Morgan Sindall was awarded the C350 Crossrail PML contract in April 2011, the scope of which included the construction of the new station (including the permanent way, station fit-out, railway systems, communications equipment and integration) and controlled removal of the existing one. The scope of the project also included the construction of a 300m long tunnel portal, a 120m long cut and cover approach section, an elevated section of the DLR to provide access to the tunnel portal and a number of other bridge structures.

Crucial to the delivery of this project was the temporary works that were designed by Morgan Sindall Professional Services (MSPS) under an inter-company sub-contract. The Crossrail project also launched the tunnel boring machines (TBMs) from PML that will create the Crossrail tunnel to Stepney Green. Accommodating these machines within the vast excavated box was unique to the project and required innovative structural solutions.

The temporary works design consisted of two main elements: the design of the temporary excavation supports and the design of the headwall tunnel eye, which forms the portal for the new tunnel to Stepney Green. The purpose was to mitigate against structural collapse, where the depth of the excavation meant the forces at play were considerable; to manage the poor ground conditions; and to allow for re-use of materials and sustainability.

Headwall tunnel

The headwall tunnel formed the opening for the TBMs to start tunnelling. It was therefore critical that the opening was at the chosen 7.55m and with the supports in place to ensure the TBMs were driven into the ground at an even depth. This was achieved with temporary support systems at precise locations beneath the portal and a temporary structure designed to the exact dimensions to form and support the opening.

Temporary excavation supports

The temporary excavation supports used both walers (horizontal beams that support the width of the excavation) and props, which are used to secure and retain distance between the walers. Also important in the design of the excavation supports was the careful co-ordination with the adjacent project to install and launch the TBM. Practically, this meant allowing enough working space for the TBM to be lowered into the excavation, whilst retaining the full integrity of the designed load and stress capabilities.

Critical to MSPS’ role was determining the required sizes and strengths of the walers, props, lifting eyes and end plates.

The twinned walers employed were some of the largest ever used, at 1,016 x 305 x 487kg/m, with 250mm x 40mm flange plates. The distance between the walers was also a key consideration; they were 4.5 metres apart and meticulous design was required to mitigate against a ‘prop-out’ eventuality, where if a support prop was accidentally removed, the temporary works would have the required strength to prevent structural collapse.

Ground conditions

The ground conditions were the biggest challenge to the project because the water table is notoriously high in London. Solutions to the additional pressures exerted by the water bearing ground were complex, and required a detailed understanding of London’s water table. A dewatering system was utilised to reduce the groundwater by four metres and walls were designed to hold back 7m depth of ground.

Re-use and sustainability

When not possible to use environmentally sustainable materials, the next best practice to improve the sustainability of a project is to choose design structures that can be re-used in their primary application elsewhere in the project. The walers were secured with specifically-designed props to ensure ease and safety of removal, and the length and position

of joints on the walers were calculated to ensure they were also suitable for future projects. This reduced the carbon footprint of bringing new materials into London and the creation of Crossrail’s temporary structures as a whole.

The temporary works element of the PML project provided an elegant, robust and safe solution that not only mitigated against structural collapse and ensured the safety of those working within the excavation, but also took significant measures to ensure the project was forward thinking in its sustainability. Its success marks it out as a significant and rewarding project on MSPS’ temporary works portfolio.

About MSPS

MSPS is a sustainable engineering and design consultancy that specialises in complex infrastructure, process and built environments. The business offers engineering and design consultancy across the full project life cycle. Cost and risk are reduced by applying their technical expertise in a fully BIM compliant environment.

It is part of Morgan Sindall Group plc, a leading UK construction and regeneration group with revenue of over £2bn and which operates through five divisions of construction and infrastructure, fit out, affordable housing, urban regeneration and investments.

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