Digging Begins at HindheadResidents in Hindhead point out that they have been waiting 73 years for a bypass to be constructed around their village. Boring work finally got under way on the twin bore, two-lane road tunnel which will make the last remaining traffic bottleneck on the A3 between London and Portsmouth disappear and bring a GBP371 million economic boost to the district. Bulldozers moved in to dig underneath the Devil's Punchbowl beauty spot and a Liebherr 944 tunnel excavator started the boring work during a special ceremony at the north portal in Surrey on 1st February, 2008. Crews started digging the southbound bore which is the south easterly of the two.Most of the excavation work is taking place from the north end, away from the main population centre in Hindhead, and will continue for 1,500 metres. Tunnelling is expected to begin from the south in April and the two sides will meet up early in 2009. Each bore stands at about 20 metres with cross passages every 100 metres.From the north portal, both drives will be advanced with an appropriate stagger for safety reason (face stability). The drives from the south portal will both be advanced: one will be excavated whilst the pipe canopy and face dowels are installed in the other, then the activities will switch between the two drives from the southern portal every eight metres.Geology and hydrogeologyThe 2 x 1.8 km tunnel is constructed by Balfour Beatty through the Lower Hythe bed, which is mostly weak sandstone interspersed with more sandy layers. Spiles are steel props which provide extra support during excavation in the predominately weak sandstone. The pipe canopy will control the overbreak and provide more confidence for the excavation when it is left overnight. Geologists from Mott MacDonald will be present to check the condition of the excavation ahead of the tunnel advancing. The water table is also high in the area, so there is only a narrow strip of ground where it is favourable for tunnelling and would not hit water. Visit www.balfourbeatty.com or www.bbcel.co.uk and www.mottmac.comGround conditions for the first 150 metres from the south side differ from the rest of the project: the tunnel is shallower and will be constructed in the more sandy Upper Hythe beds. Here, 150 mm-diameter, 12 m-long steel pipes supplied by Alwag, spaced horizontally at 400 mm, will be used to support the ground ahead of excavation. Visit www.alwag.comSandvik supplies three DT820-AC twin boom drill rigs with basket boom. Two of them will be used for installing self-drilling GRP piles and probe holes, with the third unit used for drilling steel canopy support tubes under which the tunnel excavators will dig the ground. Click here. Visit www.sandvik.com Tunnelling methodOnce the steel pipe canopies are installed, the two bores are being constructed using the sprayed concrete lining technique. Initially the top half of the tunnel, the heading, is excavated in one and two metre lengths, the spoil is removed and then crews will install roof supporting bolts and apply a 200 mm-thick sprayed concrete lining on the exposed crown using a fleet of three Meyco Logica robotic spraying machines. The 40 N/sq mm strength concrete contains steel fibres and an accelerator which is applied at the point of spraying. The supplier of steel fibres has yet to be decided. The admixtures will be BASF's SA160 accelerator, Delvocrete Stabiliser and Glenium 51 Plasticiser. Visit www.meyco.basf.com/en/equipment/equipment-range/Pages/logica.aspx and www.basf-cc.com Excavation of the next stage, the bench, then takes place about 100 metres back from the heading face. Finally the bottom section, the invert, is excavated. When the tunnel is complete, a secondary concrete lining is cast to provide a smooth finish and a fire resistant layer. There will be one Liebherr 944 in each bore from the north portal to excavate the heading. There will be an ITC Schaeff TE210 to excavate the bench. At the south portal there will be one Liebherr 944 that will alternate between the two drives. Visit www.liebherr.com, www.itcsa.com and www.schaeff-cmt.comTo dig through the portals, Webster TD140 rockwheels will equip the Liebherr machines but generally crews will excavate the tunnel using a ripper bucket to minimise the generation of dust. The transverse cutter of breaker will only be used if there are problems associated with achieving the profile in areas of harder ground where the ripper plucks large pieces out. Visit www.webstertech.co.uk or www.websterequipment.comThe tunnel machinery is guided by a robotic theodolite which uses GPS (Global Positioning System). A network of survey systems has been established in the area. Four permanent survey monuments have been established at each end of the tunnel. The machinery will be orientated by sighting back down the tunnel, taking readings on the previous survey stations and then tracking targets on the tunnel excavator. This tells the excavator operator his position in real time and so where to dig. In this way it will control the tunnel alignment.Generally, the contractor will not install any support in areas where the ground conditions permit. Where the ground conditions dictate, Balfour Beatty will install either spiles or face dowels. Balfour Beatty are also still looking at options and have made no decision on the type although they are likely to adopt GRP bars.Mucking-out and ventilationThe spoil will be removed from the tunnel using conveyors to be installed in the drives from the north portal. They are manufactured by Continental Conveyors and supplied through Burrow Brothers. A sizer at the head of the conveyor will be located 20-30 m back from the face and will be loaded by Liebherr 566 loading shovels. Visit www.continental-conveyor.co.ukSurplus material from the south end will be transported through the tunnel to the north end, avoiding the need to use the existing A3. All excavated material will be reused in embankment formation and landscaping. There will be a 22 m-deep cutting at the southern end of the project and it is the spoil from this excavation that will be transported through the first bore to be completed and be incorporated in the embankments at the northern end of the project.The spoil arising from the tunnel works at the south portal will be removed from the tunnel by loading shovel as the drive length is 250 m maximum. Additional haulage with trucks might be adopted if journey times make the process inefficient. The spoil will be used to fill a small valley immediately outside the south portal.The tunnel is designed to be excavated in chunks to minimise the amount of airborne particles. This is a requirement of new health and safety legislation for improved air quality and has added 20% to the project's original cost. This was due to buying new machinery instead of hiring or buying older models and specifying the ones with the cleanest exhaust, using more electric and less diesel plant. The cost of ventilating the tunnel has gone up by a factor of 10 and Balfour Beatty have massive de-dusting equipment. The supplier of the ventilation fans and ducting are Elta and Flexadux. Visit www.eltafans.com or www.eltagroup.com and www.flexadux.co.ukThe ventilation equipment will also be used during application of the secondary lining, which consists of a 45 N/sq mm 200 mm-340 mm cast in-situ concrete lining with polypropylene fibres to provide fireproofing. To speed up construction and reduce costs there is a plan to only cast the sides of the tunnels, since the formwork for the in-situ lining is massive and costs GBP1.5 million on the project. The suppliers of the polypropylene fibres and cast in-situ formwork equipment are not yet decided.The crown would then be lined with sprayed concrete but it depends on whether the sprayed concrete will stick to the primary lining and if there is a lot of water in the ground. Perched water will be checked ahead of excavation and pumped out as necessary.Benefits of the projectWhen the A3 Hindhead bypass project is completed in a little over three years time, Portsmouth and London will be connected by non-stop dual carriageway for the first time and the nightmare of the Hindhead traffic jams will be gone, cutting journey times by 20 minutes. The twin-bore tunnel will remove 30,000 vehicles a day from the heathland site. The problem with the A3 at the moment is it divides the village at the traffic lights but this project will reunite it. The tunnel is expected to open to traffic in August 2011. Click here and uk/37. Read E-News Weekly 25/2007, 34/2006, 4/2006, 9/2005 & 2/2003. 10/08.