TBM recovery success for new DTL2 contractor 2 July 2014
Peter Kenyon, TunnelTalk
Excavation of the twin running tunnels on Contract C917A for Singapore Downtown Line 2 (DTL2) is completed by the replacement contractor brought in to free the TBMs and finish the drives left when the original contractor, Alpine Bau, went into insolvency.
Australia-based McConnell Dowell was brought in by the project owner, Singapore Land Transport Authority (LTA), to adopt a caretaker role on two contracts involving four tunnel drives that were affected by Austrian contractor Alpine’s demise.
Singapore Downtown Line 2 (red)

Singapore Downtown Line 2 (red)

The first of these was Contract C917 for construction of King Albert Park and Sixth Avenue stations and the parallel twin running tunnels between them, while the second was for the running tunnels out of Tan Kah Kee Station as part of Contract C918.
For a period of seven weeks the two Herrenknecht EPBMs for C917 (TBM Sophia and TBM Grace) and the two Herrenknecht Mixshields for C918 were left in the ground while the LTA negotiated the caretaker role with McConnell Dowell and a team was installed to manage both construction sites and ensure that damage to all four TBMs was minimized while a long term solution was found.
During this caretaker period, McConnell Dowell managed to get the TBMs unstuck, repaired and restarted, with Sophia advancing 41 rings (56m), and Grace advancing one ring (1.4m). The Australian contractor played a similar caretaker role, which lasted for a further five weeks, on the C918 Contract. Both Mixshields on that drive were also freed, repaired and restarted, during which time competitive tenders for both contracts were issued, priced and awarded by the LTA. McConnell Dowell won the C917 contract, now renamed C917A, while the new C918A contract was awarded to SK Engineering of Korea.
Sixth Avenue Station job site

Sixth Avenue Station job site
Photo: Nethaniel Foo, DTL blogspot

“After contract award, McConnell Dowell rapidly mobilised additional resources to supplement the caretaker team and seamlessly recommence full TBM tunnelling operations for Sophia from 9 September 2013, and for Grace from 19 September 2013. This was a remarkable effort to manage, tender, finance, achieve commercial agreement, and employ, mobilise and train a significant site team in this very short period,” said McConnell Dowell in a statement.
As a result of standing idle underground while the LTA negotiated for completion of the contract, McConnell Dowell crews first had to carry out a week of manned interventions into the excavation chambers of both machines to carry out essential cutterhead maintenance.
Worn cutting tools were changed out, and clogged material inside the excavation chamber that had effectively seen the machines become stuck, was removed. The first ring of the caretaker contract, ring 33, was set after McConnell Dowell advanced the TBM just over 1m to create enough space.
McConnell Dowell excavated DTL2 Beauty World drives

McConnell Dowell excavated DTL2 Beauty World drives

TBM Grace fully resumed tunnelling on 19 September (2013), reaching the planned stop location at Ring 53 (82m) on 11 October in order to carry out the main drive changeover, the second stage of TBM assembly where the feed lines from the surface are removed and the remaining TBM back-up gantries are assembled, lowered into place, and transferred into the tunnel to connect to the remainder of the TBM. After this 6-week stop, Grace was fully commissioned and ready to commence tunnelling at the programmed rates. Tunnelling recommenced on 15 November (2013) and apart from regular manned interventions for cutterhead maintenance and a short stoppage for work on the screw conveyor, the TBM completed tunnelling 209 days later on 11 June, without any major issues arising. Sister machine Sophia broke through the Sixth Avenue Station headwall in May, just a few days earlier.
The breakthroughs mark completion of the excavation works of the metro tunnels with the remaining tunnelling works on the project now limited to completion of cross passage excavation and completion of the ventilation shaft adit tunnel at King Albert Park Station beneath Bukit Timah Road (approximately 18% excavated).
“We experienced extremely difficult ground conditions, difficulties associated with the inherited TBMs, and up-skilling of the inexperienced foreign workforce taken over from the previous contractor,” said a McConnell Dowell spokesman. “These were in addition to the intense time pressures from the client, who needed to make up lost time from the previous insolvency to keep to their DTL2 opening on schedule.”
McConnell Dowell has worked on previous DTL2 projects, having successfully completed construction of the Beauty World Station and its associated running tunnels, also using a pair of 6m diameter Herrenknecht machines.
Austrian tunnel contractor files for bankruptcyTunnelTalk, June 2013
Singapore replaces bankrupt contractorTunnelTalk, October 2013
Downtown Line blogspot

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