The A86 West forms the final link of the A86 outer ring road around Greater Paris, most of which was constructed in the 1990s. It passes through an environmentally sensitive area of forest parkland, historic sites (chateau de Versailles) and residential areas and is thus being constructed in tunnel. There are two toll tunnels in the project, the 10km long 10.4m diameter eastern tunnel and the 7.5km long 10.9m diameter western tunnel, to be built later.
The innovative eastern tunnel will be a double-deck tunnel for light traffic with two separate one-way carriageways, each with 2.55m headroom. Each three-lane carriageway will comprise two traffic lanes and a hard shoulder. A 70km/h speed limit will be imposed; usage is expected to be between 35,000 and 54,000 vehicles/day with a peak capacity of 4600 to 6400 vehicles/hour. The tunnel has a maximum grade of 4.5% and will include an intermediate underground interchange with the A13 motorway.
Safety measures in the tunnel will include traffic lights, temperature sensors at frequent intervals, CCTV cameras with automatic detection of stationary vehicles, pushbutton alarms, emergency panels and emergency alcove fire fighting equipment. Pressurised refuges will be constructed at 200m intervals and emergency shafts will be located at 1.2km intervals. In the event of an accident, all traffic lights would turn red, and vehicle occupants would evacuate to the refuges, each of which can accommodate up to 100 people on two levels (Figure 2).
The western tunnel will be a conventional single carriageway two-lane tunnel with a 4.5m clearance, designed for all vehicles including HGVs, with a maximum grade of 2%.
The French State granted the concession for the A86 to Cofiroute, who appointed Socatop as consultant engineers and contractors, with a remit to include construction work. Socatop is an association that has brought together three of the biggest construction and road industry companies in France, VINCI, Eiffage Construction and Colas.
Preparatory work on the project started in 1996, but came to a halt in 1998 after a complaint was filed with the EU, which forced a re-tender. Work resumed at the end of 1999 and the first TBM parts arrived on site in June 2000. Tunnel boring started in December 2000, with breakthrough on 14 October 2003.
The design was revisited in August 2000 in line with a French government decree for enhanced fire safety re-design (after the Mont Blanc fire). Final approval of all operational safety measures was granted in October 2003. The Tunnel ventilation system will include fresh air and exhaust ducts in the quadrants above and below the two carriageways, with upcast and downcast ventilation shafts located at intervals.
TBM selection
The east tunnel horizon is located within limestone where possible, but the requirement to rise midway for the A13 intersection results in it passing through chalk, soft clay layers and water bearing sands, representing all geological formations found in the Parisian region. A large diameter multi-mode machine was needed to operate at varying depths of cover below the water table in such mixed strata.
The project team had concluded that an EPB machine would be compatible with cohesive ground and with soft rock. It would also permit open mode working and higher outputs, but would be difficult to drive in sand. A slurry machine would be compatible with the unstable formations and offer better face control, thus limiting settlement, but progress would be limited in the stable formations. Access to the face would be available whatever the geological formation.
Stability trials with a test rig at Storebaelt had demonstrated the difficulty in confining Fontainebleau sands with an EPB machine. The project team therefore opted to use a Herrenknecht Mixshield machine, using the modes listed in Table 1.
The cutterhead of the 11.6m diameter TBM required an extremely high torque and could be turned under full power in reverse to stabilise the shield. The screw conveyor was, at the time of machine erection, the largest in length and diameter. A gate at the front of the screw allowed its removal for maintenance.
Tunnel boring was south from Rueil-Malmaison in a single drive with an intermediate breakthrough into the cut and cover A13 interchange structure at Vaucresson (4.56km). The drive then resumed the remaining 5.55km to Pont Colbert and the connection with the A86.
Tunnel fire on 5 March 2002
On 5 March 2002, the TBM had progressed some 2100m from the Rueil-Malmaison portal, mostly upgrade. A fire broke out at 22.30hrs on the locomotive engine of the supply train on its uphill approach to the TBM; directly beneath the leading edge of the lower carriageway slab, which followed some 550m behind the face. Construction of the slab entailed placing of precast beams and in situ infill concrete with a 20t overhead crane. The Peri formwork for this operation included some timber elements.
The train was stopped by its fire detection system and tunnel personnel attempted to extinguish the fire using hand held extinguishers. However, the fire quickly spread to the locomotive’s fuel tank and then to the spoil removal conveyor belt, generating a considerable amount of smoke. The tunnel personnel retreated to the TBM, having started the water curtain, located some 400m from the face. The fire destroyed the crown ventilation duct, some smoke penetrated the water curtain and the 19 tunnel personnel on the TBM took refuge in the airlock. All personnel were rescued by emergency services personnel after the fire had been brought under control at 06.00hrs the following day. A step by step account of the fire and rescue was published in the November 2003 edition of T&TI, p42.
Safety programme before fire
Ventilation comprised 35m³/sec through a flexible crown ventilation duct and 90m³/sec through the access way below the lower carriageway slab. Thus some 125m³/sec flowed back up the main part of the tunnel. As part of emergency planning, four fire scenarios were assessed, namely fire on the TBM, fire between TBM and the carriageway slab and fire either above or below the carriageway slab. Appropriate ventilation regimes and fan settings were determined for each scenario. Various fire precautions had been developed in association with safety authorities and the fire brigade. These measures included:
Inquiry and report
A judicial enquiry was convened, and initially tunnel access was forbidden, from 5-12 March, for collection of forensic evidence. The enquiry appointed a panel of four experts to examine the evidence.
Their preliminary report in November 2002 concluded that the size of the fire had been 40MW, with temperatures reaching 600°C. The main fuel had been the conveyor belt and the service train fuel. The cause of the fire had been a leak of hydraulic oil from a turbo on the locomotive’s propulsion system.
Repairs
Repairs comprised resurfacing the segments above axis level, which were spalled to a maximum depth of 60mm over a 150 length. Tunnel plant in the area of the fire (service train, conveyor belt, cables, ventilation duct, overhead crane and formwork) were damaged beyond repair, but none of the TBM back-up was affected, having been protected by the water curtain.
Lessons learned and enhancements made
Communication to the face was lost early in the fire and steps were taken to achieve secure communications in the tunnel. The emergency electrical supply was improved, as was ventilation to ensure improved smoke extraction. The TBM water curtain was enhanced and anti-smoke goggles introduced to supplement the breathing apparatus. A rapid intervention vehicle was provided at the request of the insurers. The emergency plan was revised and its fire scenarios simplified. Fire training was extended across the workforce, together with training in the use of rapid intervention vehicles.
Eventually all of the workers rescued from the fire returned to their posts, after being given counselling. For one, it was his second tunnel fire.
TBM drive beneath the railway
The tunnel passes beneath a busy electrified railway line with only 11m cover, in water bearing sandy ground. It was not possible to close the line, so measures to permit tunnelling under the live railway had to be adopted. At the crossing point, the railway is in a 20m deep cutting, some 25m from the portal of a tunnel constructed between 1880 and 1884. The ground water level is just below rail level, and thus the head of water at the TBM face was up to 25m. The ground comprised fine sand, requiring TBM slurry mode operation.
Measures adopted at the crossing included the installation of a new set of wire brush seals on the TBM, a large number of boreholes in the vicinity, some with extensometers, supplementary rails to stiffen the track, frequent track monitoring and a speed restriction for trains. Modifications to the spoil handling system were also made, to prevent clogging and enable tunnelling to proceed through the crossing zone without stopping. Tunnelling under the railway was achieved with no more than 10mm settlement.
The presentation concluded with a video of TBM breakthrough to the accompaniment of stirring music from Mozart’s Requiem.
Related Files
Figure 1 – Map of Paris, showing the alignment of the A86 West and East tunnels
Figure 3 – Longitudinal section of the A86 tunnel showing the TBM’s modes of operation through the varying geology
Figure 2 – Cross section of the A86 East tunnel at a refuge point
