?The main purpose of the US$1.18bn A41 Highway extension currently under construction in France is to provide an un-interrupted connection between the French city of Annecy and Geneva in Switzerland (figure 1). Along with the A43, the A41 forms the last motorway link that will connect all the major cities in the Haut-Savoie region; namely, Grenoble, Chambery, Annecy and Geneva.
The project will also provide improved access to the high-speed rail lines between Paris-Bellegarde and Paris-Dijon as well as more convenient connections to and between the airports at Lyon and Geneva. Upon completion of this section of the A41, users will be able to travel between Annecy and Geneva in less than 30 minutes and traffic will be greatly reduced along the national road RN201.
The project has been awarded as a 55-year concession, which includes the design, construction, financing, operation and maintenance of the 18.8km of highway. The concession was originally awarded to the public service company ATMB, but soon after this a change in the law concerning the make up of concession projects forced a new Request for Proposals (RFP).
It was in May 2003 that the Ministère des Transports/Direction des Routes (Department of Transportation), published a second RFP, following which ADELAC was chosen. At the time of putting together its proposal ADELAC recognised that toll revenues would cover construction costs and therefore sought to differentiate itself from other bidders by promising to deliver the programme 10 to 12 months faster than its competitors.
The ADELAC consortium consists of motorway concession company AREA (a subsidiary of Autoroutes Paris Rhin-Rhône), several subsidiaries of Bouygues Construction (Bouygues Travaux Publics, GFC Construction, DTP Terrassement, Losinger Construction), Colas, design consultants SETEC, and local financial institution Caisse d’Épargne et de Prévoyance des Alpes. The contractor for the project, the GIE A41, is a JV led by Bouygues Construction. The designer, i-LAC, is also a JV of Bouygues and SETEC.
Financing for the overall US$1.18bn project cost was put together by the concessionaire. AREA provided the majority share of 49.9%, with Bouygues making up the next 46.1%. The remaining 4% was put up by SETEC and Caisse d’Epargne with a 2% stake each. The cost of the tunnel is estimated at over 40% of the total construction costs of US$835.6M. The sources of finance come both from capital, US$235M, and loans, US$939M.
Like any high-risk project, there are considerable penalties associated with this contract, ranging from US$20,000-55,000 per day, payable to the Department of Transportation.
The project has been divided into four independent sub-projects: the roadway, the toll area, four viaducts (to be constructed simultaneously), and the Mont Sion tunnel.
The Mont Sion tunnel is a key component of the extension. It involves the construction of twin tubes approximately 3km long, each 12m in diameter, four vehicular cross passages, four pedestrian cross passages, 20 emergency recesses and 15 fire equipment recesses. Each vehicular cross passage is 3.5m wide by 3.5m high and alternate with the pedestrian cross passages, which are 1.8m wide by 2.2m high.
Fast-tracking the schedule
Considerable risks had to be taken by the contractor to achieve the extremely tight project schedule. Design of the TBM and instigation of the TBM contract, intensive geotechnical investigations as well as fast-tracking of all the engineering stages including administrative authorisations (CNESOR, CETU, RCA and other government agencies including environment, fire brigades, etc.), were all carried out before the final design had gained approval.
Of all the risks taken by the contractor however, possibly the most commercially significant was placing an order for a 12m diameter TBM with Herrenknecht before the project’s financial close. This was done under a two-part contract. The first part, from July to October 2005, saw the TBM design undertaken and securing of the principal machine parts, such as the main bearing. The second part of the contract involved the actual fabrication of the machine, which was ordered the day after the project’s financial close.
As stated, a substantial risk was taken by forging ahead with the acquisition of the TBM, especially considering its size, as it was done before the tunnel’s final design had been approved by the French government’s tunnel study centre, CETU (Centre d’Etudes des Tunnels).
As a result of the tight scheduling, the contractor also undertook concurrent engineering work for the planning approvals, design and site preparations.
Considerable time and expense was invested into the site investigations with three different geological studies performed by three different companies. Based on this very intensive programme, it was established that tunnel boring would take place almost exclusively through layers of molasses (except for the portals where the studies found poor quality moraines).
Fortunately, the molasses proved to be impervious and subsequently not connected to local groundwater sources, meaning excavation would have little impact on the region’s groundwater table.
Unfortunately, the ground investigation also indicated that the tunnel alignment was expected to pass through 12 major fault zones where groundwater could be encountered. However, since significant water ingress was not predicted, grout injections were not deemed necessary.
Tunnelling set up and TBM design
The pre-cast tunnel lining consists of five segments plus a key. Each traditionally reinforced segment is 2m wide and 450mm thick. The internal diameter of each tunnel, 10.7m, provides for a two-lane road with 0.6m wide sidewalks on both sides and a clearance of 4.75m to allow for heavy trucks.
In order to insure the continuous supply of segments to the TBM, quick response to machine advance rates, and quality of the tunnel lining, the casting of the concrete segments is being carried out on site.
The 180m long, 12m diameter, open face TBM tips the scale at 2,200 tons – the largest TBM presently in French soil. It is composed of cutterhead, tailskin, conveyor and 11 gantries. The TBM uses guidance software called Pyxis, created by Bouygues Travaux Publics, which allows the operator of the TBM to monitor excavation movements such as jack pressures, speed of the cutterhead, alignment of the machine and any deviation.
Cooperation with Herrenknecht began very early in the project. As with the design and construction processes, Bouygues had a strong presence during the development of the TBM. The company prescribed the technical specifications, detailing of the fabrication and also the architecture of the machine. An important aspect, which is often overlooked during the design of a TBM, took centre stage: ergonomics.
Engineers tend to focus all their attention on results and optimising the system to obtain desired performances. For this reason ergonomics and safety are often an afterthought. However, this means miners have to adapt to awkwardly positioned workstations and it is up to them to manage the risk. Adopting a radically different approach, Bouygues TP focused the TBM and equipment design around ergonomics and safety. As far as the contractor is aware, this is the first time ergonomics have been prescribed in TBM specifications.
Passageways, access between the gantries, operator tools, and maintenance procedures have all been designed to maximise ease of use and safety. For example, by applying a concept called “5th Avenue”, a broad central alley was created for easy access to all equipment – whereas on most ordinary TBMs, workers are obliged to follow narrow and difficult to access routes. In addition, practically all the ladders have been replaced by stairways.
Important modifications were also made to the most sensitive work stations, in particular that of the ring erector operator. Several months were necessary to design a platform that would allow the operator and his assistant to work without risk while increasing efficiency and precision.
Excavation
The TBM was delivered in August 2006. Launch and reception chambers were built prior to launch, mostly due to the poor quality of the soil at the portal locations. These chambers comprise of diaphragm walls built for the upper portion of the tunnel alignment, with the TBM excavating the remainder.
Break-in of the first tube took place on 10 October 2006, from the northern side of Mont Sion. After eight months of boring at an average rate of 30m/day and at depths of up to 150m, the TBM saw light of day on 12 June 2007.
Upon reaching the fault zones, big blocks were encountered and at one point caused breakages to the conveyor belt. As a way of preventing this, a hopper with a grid was added, to limit the size of blocks landing on the belt.
Inclined probe drilling was undertaken prior to arrival at the predicted fault zones to check the ground (water presence, etc) and software called CATSBY (created by Bouygues) was used in order to monitor the parameters of the TBM in real time. If the software found abnormal behaviour, such as sudden jumps in speed and torque, an alarm would sound.
The TBM was also equipped with grout injectors capable of treating the upper portions of the excavation in case significant amounts of water were encountered. However, water met during the first drive was minimal and no such injections have taken place so far.
The machine is currently being disassembled and transported back to the northern side of Mont Sion. The tailskin and cutterhead are being transported by road. The cutterhead, weighing at 220 tons, will be transported as a single piece in order to save time. The rest of the TBM, gantries and conveyor belt have been towed back through the tunnel itself.
The estimated date of break-in for the second tube is September 2007. The average progress rate, without the learning curve, is expected to be 32m/day, which will allow the second tube to be bored in less than seven months.
Tunnel lining
One of the key features of this project is the technique used for the installation of the lining. In order to meet the tight construction schedule, the contractor decided to use an experimental method. Based on a practice often used by the Swiss, the method consists of retracting all of the hydraulic jacks simultaneously and installing the ring in direct contact with the tailskin of the TBM, before pushing them back against the previously built rings.
The main difference between the two methods is that with the Swiss the segments work as a temporary lining, with the final lining being constructed by cast in-situ concrete. The Mont Sion segments however, form the final lining. They have been designed with a thicker concrete cover to the reinforcing steel in order to withstand the long-term ground loading.
Considerable time is gained by directly installing the final lining. Speed in fabrication, transport and installation is also attained by using a fewer number of segments. In addition, this method allows for earlier installation of operational equipment such as lighting. On the other hand, due to the large dimensions, more attention is required during handling and installation in order to prevent cracks and corner damage.
Project scheduling
October 2005 – August 2006: Environmental studies and administrative procedures (10 months)
November 2005 – July 2007: Final design (20 months)
May 2006 – November 2006: Preliminary works / site preparation (6 months)
August 2006 – November 2008: Highway construction, including the tollbooth area (15 months)
May 2006 – June 2008: Construction of all 4 viaducts (24 months)
October 2006 – December 2008: Tunnel construction and E&M (27 months)
Important dates
24/11/2005: Conception of construction contract between ADELAC & GIE Construction
28/04/2006: Order to Proceed, OS (Ordre de Service) to start preliminary works
12/06/2006: Preliminary design, APOA (Avant Projet d’Ouvrage d’Art) approved by CNESOR & CETU
28/07/2006: Commencement of tunnel construction
10/10/2006: Break-in of the first tunnel
12/06/2007: Break-out of the first tunnel
28/12/2008: Deadline for operational use
Fig 1 – Project location The 12m diameter Herrenknecht TBM is prepared for its first drive The first drive holes through The excavated tunnel with its cast on-site segmental lining The TBMs back-up gantry, showing its unique ergonomic design
