Full Success for CSM Bessac First EPB TBMJust less than nine months have been required to complete the last downstream section of the VL10 sewer, which will bring wastewater into the Valenton treatment plant south of Paris. The owner is the SIAAP, the Paris metropolitan area sewage authority. This 1,830 m-long 4 m ID sewer has been driven at a depth of 30 m under 25 m head of water. The job broke through on 27th January, 2005.CSM Bessac intervene as main contractor of the JV which carries out the contract, as well as manufacturer and supplier of the TBM, back-up, muck train and ancillaries. For the purpose of this contract, CSM Bessac designed and built its first earth pressure balanced TBM. Renowned as the developer of the compressed air pressure balanced boom head shield process, CSM Bessac is a key player on the urban tunnelling market for water, wastewater and utility tunnels. The company offers today to its clients a wide know-how for all TBM types under all confinement methods. Thanks to its double status of TBM manufacturer and main contractor, the company is in a good position to develop tunnelling equipment well suited for the market of urban tunnels with diameters under 6 m. Thus, the TBM built for the VL10 sewer has been designed to cope with the specificities of this market characterised by tunnels in urban areas, to be built from reduced working site areas, in difficult grounds, and with variable lengths ranging from a hundred metres to several kilometres. The complete chain of the process is organised in line with this logic with the TBM, the back-up and the muck train built in CSM Bessac's plant in Toulouse, France. The TBM has therefore been designed to allow for quick set-up, from small shafts. It is not necessary to build a front tunnel for the TBM assembly. The construction of a rear and launching gallery is only required for long projects. The TBM is compact. All the power necessary to its operation is installed within the shield. It is lowered into the shaft as a single piece but the cutterhead can be dismantled to reduce the load that must be hoisted. This allows to start up extremely fast since the assembly phase has disappeared. In such a TBM, the length of the back-up will depend on the tunnel length. The TBM can be operated using only the first sledge, where the pilot cabin, the electric power transformer, the segment feeder and grout batching tank are located.The EPB shield for the VL10 sewer has a length of 8.75 m and weighs 175 tonnes. The excavation diameter is 4,850 mm and the outer diameter is 4,810 mm. The cutterhead, equipped with drag bits, disc cutters and a copy cutter, is hydraulically driven by 12 cylinders. The total thrust is 20,000 kN. The maximum torque is 3,125 kN.m. The total installed power is 930 kW. The shield / tailskin assembly is an articulation, with easy movement to allow the TBM to drive curves. This articulation is manoeuvred by eight cylinders, with a total capacity of 7,000 kN. Work took place in a maximum operating pressure of 3.5 bar. A motorised roller segment conveyor feeded the tunnel with the segments (5 + 1 key per ring), installed by a segment erector with hydraulic screw drive. Each ring is 1.3 m wide. The 300 mm-thick segments were produced by Stradal. Visit www.stradal.frAt the VL10 sewer jobsite, the length of the tunnel required a mucking-out capacity of 40 cubic metres per muck train trip. Consequently, the train comprises of seven sledges which support the belt conveyor which ensures the loading of the four muck cars. The spoil is removed by a 650 mm-diameter screw conveyor behind the cutter head, with a maximum mucking capacity of 220 cu m/h prior to being transferred to this 40 m-long belt conveyor. The sledges carry other ancillary equipment necessary to the adequate logistics of the TBM such as the foam processing and water cooling device, etc. The muck train consists of a loco which carries one 10 cu m integrated skip plus three additional 10 cu m skips. Such capacity is necessary to evacuate all the spoil removed during a 1.3 m advance, which is the width of a segmental ring. This train also includes one segment car and two 2 cu m tanks/mixers for the grouting. The muck cars are unloaded into skips located at bottom shaft, being quickly tilted upside down thanks to a purpose-made integrated jacking device. This process reduces the stand-by of the muck train in the working shaft. A single train is thus sufficient. The bottom muck skips are hoisted out of the shaft by a tower crane and disposed in a storage area. The segments are lowered into the shaft by a gantry crane. The whole process is organised through a 15 m-diameter working shaft with a 12 m-long rear gallery. In the case of short tunnels, it is possible to use only one muck skip thereby suppressing the need for such rear gallery. The shaft diameter can also be reduced to 11 m.The tunnel mostly crosses marls with gypsum layers in its downstream part, then crosses Beauchamp sands and goes through Saint-Ouen marly limestone in the last half of the alignment. The hydrostatic pressure is approximately 25 m. The tunnel alignment includes 300 m radius curves and passes under several sensitive structures such as high speed rail tracks, a RATP building, existing sewers, etc. The settlements measured on the rail tracks reached a maximum of 3 mm.Tunnelling was carried out in two 7-hour shifts. A third shift is dedicated to maintenance of the equipment. This third shift is also responsible for drilling probe holes ahead of the TBM. These probe drills permit to investigate the presence of voids along the tunnel alignment. A slightly conical aureole consisting of five probe holes is drilled in the top heading, through starting points in the structure of the TBM, two holes are drilled at the invert and one through the cutting wheel following the axis of the tunnel. The probe drills measure approximately 30 metres. The voids located during this investigatory phase are systematically filled with grout through the drill rods. The tunnelling/grouting crews have been highly mobilised when ground areas with great quantities of voids have been encountered. This jobsite organisation has allowed to drive as many as 95 metres of tunnel per week. However, the machine encountered an area with swelling ground which obliged the muck train to do a second trip to muck out a fifth skip of muck. This caused the output to decline.CSM Bessac took the benefits out of its large expertise in compressed air tunnelling which allowed to use the EPB shield in mixed mode, earth pressure or compressed air pressure. Indeed, the compressed air production plant at the worksite permits to combine permanently the air pressure with the earth pressure on an important part of the excavation chamber (the third or half of it) to stabilise the face. This mixed confinement method brings many advantages: reduction in the cutting wheel torque, and therefore less stress to the mechanical components; better confinement of the upper part of the chamber; better compensation of the pressure of the water table; possibility to view the chamber (camera and lighting of the chamber); better safety for interventions in the chamber thanks to significant flows of air available at any time; less quantity of special products (foam); and production of much less water-bearing spoil. Click fr/41. Visit www.csmbessac.com 13/05. Click to enlarge Click to enlarge Click to enlarge