Early August, the Bouygues-led JV building the twin road tunnels for the A41 highway in the French Alps plans to transfer the cutterhead of the 11.94m diameter TBM excavating the job as a single unit to meet the demands of the fast-track construction programme.
The JV – GIE Constructers A41 – has completed the first drive of the parallel, almost 3.1km long tunnels at Mont Sion in mid-June, advancing up to 42m per day through molasses with little groundwater despite passing through a dozen major geological faults zones. The open type Herrenknecht was launched in mid-October 2006.
Heavy effort was made in the site investigation stage between financial close on the contract award by the concessionaire, Adelac, in November 2005 and August 2006, shortly before the machine was launched. Data informed the JV of the geological faults and indicated there should be little groundwater.
The contractor had won the contract by outbidding rivals on the construction programme, which it bid to finish 10-12 months faster than its competitors, by 28 December 2008 under a 29 month schedule. The financial backers of the project are principally Bouygues, and Area plus contributions from Setec, Caisse d’Epargne and a few banks.
While at the preferred contractor nomination stage in the second quarter of 2005, the JV initiated a two-stage TBM order with Herrenknecht in July, working on the design and helping to secure strategic items such as the main bearing before all the site investigation data were available. When the contract closed in November 2005 the machine went into production.
The unusual approach was part of the JV’s risk management approach to helping to meet the fast-track programme, which also required environmental and tunnel authority approvals before digging could begin. On the project the JV has been performing all activities under concurrent engineering.
Launched last year, the TBM is excavating the first bores with up to 150m of overburden, erecting a 10.7m i.d. concrete lining with 450mm wall thickness, the rings being 2m long with five segments plus a key. The JV is using Bouygues bespoke Pyxis software to help monitoring of the works.
The TBM is to be re-assembled at the launch site to restart excavation in mid-September and the JV aims to complete all civil works on the contract by August 2008. In the meantime, M&E and signalling fit-out activities will get underway on the first tunnel as there will be only a few months left to complete the same works in the second tube after the TBM holes through next year.
Liquidated damages for late completion of the fast-track contract are daily rates up to a maximum of US$55.6M to Adelac, which has a 55-year concession on the section of A41 that includes viaduct and other road construction. Penalties of US$54,900-US$123,521 daily would also be due to the Transport Ministry. The potential bonus is capped at US$3.3M.
The 11.94m diameter Herrenknecht TBM who’s cutterhead will be transported as a single unit to keep to the tight schedule The 11.94m diameter Herrenknecht TBM The 11.94m diameter Herrenknecht TBM’s backup gear The Herrenknecht TBM’s backup gear
