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Analysis confirms DLR station sequence

ADVANCED NUMERICAL analyses of complex construction and excavation sequences enabled a rapid evaluation of significant design modifications to the Cutty Sark station box on the Lewisham extension to the Docklands Light Railway.

The 60m long, 25m wide and 22.5m deep station box for the new DLR Cutty Sark station at Greenwich was originally to be fully constructed before the arrival of the TBM.

However early last year the construction programme was altered, which meant the TBM would arrive before the box was completed. By agreement between the Light Rail Group Contractors joint venture partners, Mitsui- Nishimatsu for the tunnel and John Mowlem and Company for the station, it was decided to incorporate a mechanism for allowing early entry of the TBM into the partially excavated station box.

The proposal, known as the 'soft eye' option, was devised by Mowlem in conjunction with con- sultant WS Atkins.

Essentially, this involved leaving out reinforcement from the lower part of diaphragm wall panels, on the southbound drive side of the station box end walls.

This change to the proposed construction sequence and reinforcement had significant implications for the behaviour of the station box walls and propping system. To assess these effects, the geotechnics and foundation division of WS Atkins developed its original 2D FLAC analyses to model the revised proposal.

Tunnel excavation and lining sequences were introduced into the construction sequence of the FLAC station box model. This enabled an assessment of the effect the reduced vertical stresses would have on the diaphragm wall bending moments and prop forces. The original geotechnical design analyses also included a ASAS 3D continuum model of the box and surrounding soil, but it was not considered necessary to re-run this.

The analysis determined the best excavation sequence around what would be an already constructed tunnel, and the effect that subsequent removal of the tunnel lining would have on the stability of the excavation base and the prop loads. Consideration was also given to whether ballast was required to provide additional weight within the tunnel, to ensure adequate base stability and passive restraint to the diaphragm walls.

The results of the soft eye analysis confirmed that early tunnel construction would result in an increase in diaphragm wall bending moments but showed that this increase could be accommodated by the steel provided in the original design. Prop loads would also increase, but again not sufficiently to cause any changes to the original propping proposals.

Excavation within the box, exposing the already in-place tunnel, is now well advanced. According to David French, Atkins' chief geotechnical engineer: 'Advanced numerical analysis, such as FLAC, provides an essential, pro-active and interactive design tool for rapidly evaluating design changes, which are characteristic of design and build projects.'

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