Hindhead Tunnel
View of the Northern Portal circa 2011
For further information on the scheme visit www.hindheadtogether.org.uk or contact Rob Fairbanks on 01372 220 650 or rob.fairbanks@surreycc.gov.uk
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HINDHEAD TUNNELS -CUTTING BENEATH THE BOWL 4 Sept 2008 Digging, shifting, checking and spraying - the tunneling cycle at Hindhead is non-stop.
Few projects in the UK at the moment share the same level of excitement as the f 37 l M Hindhead Tunnel in Surrey.
The project is large-scale civil engineering at its finest - burying a busy section of the as dual carriageway so that it no longer cuts through the village of Hindhead and an area of outstanding natural beauty.
It is also one road project that locals and environmentalists have few objections to because it will reinstate large sections of National Trust Woodland while reducing traffic and pollution.
A natural deep depression in the ground known as the Devil's Punchbowl meant that, to bypass it, the original Portsmouth to London road cut around it and through Hindhead. Building a bridge across the Punchbowl was considered too expensive, so a tunnel became the only option.
But there is only a narrow layer of good ground between the surface and the water table in which the tunnel can be excavated without undermining the Punchbowl itself. Consultant Mott Macdonald investigated ground conditions to prove it could be safely built as far back as 1 993.
Contractor Balfour Beady is currently a third of the way through excavating the two l .83km tunnels for client the Highways Agency and expects to finish in March 2009.
The tunnels run from the north portal in a south westerly curve to the south portal. This means the tunnel to the north is slightly shorter, being on the inside curve of the route, than the one to the south.
Tunneling began in January, starting at the north portal where ground conditions are more predictable as the structure is well within the good ground - a weak sandstone seam - and above the water table.
''The main challenge is keeping everything working 24/7 and continually maintaining the plant to make sure things stay that way's explains Balfour Beatty project director Paul Hoyland.
Tunneling advances with construction of the heading, or top section, first, followed by the bench, or bottom section.
Two-thirds of the l 00m2 tunnel cross section is excavated as part of the heading. The rest, known as the bench is taken out later on.
The tunneling operation runs on a four hour cycle during which time a header or bench section is excavated.
For the first 90 minutes, excavation work takes place. Over the next hour, spoil is scooped up, broken down and fed onto a conveyor belt bolted to the side of the tunnel and transported out. Digging stops once the tunnel has advanced 1.6m.
''We can excavate up to 2m at a time, but we're typically not exceeding l.6m because this seems to be the optimum advance rate for supplying the Shotcrete (sprayed concrete explains Balfour Beatty tunneling manager Roger Bridge.
After the excavation of a new section of tunnel a geologist checks the condition of the fresh tunnel face for anomalies. Engineers and surveyors follow, checking the profile of the unsupported tunnel section to make sure the position and shape is right. Finally, 90 minutes are spent spraying the steel fibre reinforced primary concrete lining.
After the primary, structural sprayed concrete lining has been applied, and outside the four hour construction sequence, the bottom am of the tunnel walls are covered with waterproof membrane sheeting, ready for a layer of insitu concrete. The top section is also sprayed with a waterproof membrane ready for it to receive a sprayed concrete secondary lining.
The secondary sprayed concrete lining is reinforced with 1kg/m3 of polypropylene fibres which are intended to prevent the concrete from shattering in a fire. A sprinkler system will not be used as the fire resistance of the fibres is considered sufficient protection for the lining.
The contractor is hoping to replace all the steel fibre reinforcement in the project with polypropylene to save money.
The tunnelling process is standard along the tunnel length apart from where ground conditions vary.
''We expected weak sandstone and that': what we got - only with slightly more faultsj'' says Mott Macdonald, site based geologist and civil engineer Bethan Haig describing the ground conditions near the north portal.
To keep the ground intact, at weak points on the tunnel route, fibre glass spices had to be installed to support the ground ahead of tunnelling. Fifty of these 4m long, 32mm diameter dowels were inserted at between 250mm and 300mm spacings around the tunnel profile in the direction of tunnelling and ahead of excavation to knit the fault lines together and prevent chunks falling off.
''It was always in our continence plan to use more spices in the tunnel if we found more faulted grounding adds Balfour beady project director Paul Hoyland. ''Basically, we have daily meetings to assess progress and decide which solution, depending on the ground conditions, is most appropriator's Tunnelling proceeded slower than anticipated near the north portal to safely accommodate the faults. In some instances, where there was a risk of falling rock, the tunnel face was excavated in smaller advances, for example lm advances instead of l .6m at a time.
Tunnelling also had to proceed with caution - and incorporate spites and extra monitoring - where it passed under the as at two locations and where the ground cover is just 20m compared to a maximum of 65m elsewhere on the route.
The tunnel is constantly monitored to check for settlement. But so far, movement has been well within the lomm maximum allowed even where the Punchbowl was deepest.
Although tunnelling was slower than anticipated to start with because of the faults, work has speeded up as site operatives have become more experienced with the machinery and techniques - and because they have a bonus scheme based on the time they take. Work is now ahead of programme.
The twin tunnels are being excavated from both portals. So far, excavators working from the north portal have completed 395m of heading and 138m of bench on the southern bore and 430m of heading on the northern bore.
Here, work on the bench has yet to begin.
By comparison, restrictions on working hours and more difficult ground at the south west end of the tunnel have slowed progress. Teams working from the south west portal have completed just 4l m of heading on the southern bore and 62m on the northern bore while none of the bench has been worked on yet.Tunneling from the north portal is advancing at 35m per week, while from the south portal the rate is only am as a result of the need to accommodate more difficult ground conditions.
NCE visited the north portal tunnels last month and what grabs you on reaching the tunnel face is not just the scale of the project - the tunnel diameter l lm - but the fact that a burly, 44t excavator is rather elegantly nibbling away at the sandstone rock to create the tunnels.
Unlike a normal excavator, the Liebherr R944C roadheaders move more like robots enabling the cutting tool to swivel left and right as well as up and down. They are also more cost effective than tunnel boring machines.
A drill probes the ground to check its hardness prior to excavating each advance. If there is concern, porthole samples can be taken and cameras can be placed down the hole to investigate further.
Tunneling from the southern portal tackles additional issues. Being nearer housing has meant that work can only take place between 7am and 7pm Monday to Friday and only half a day on Saturday. This means that the tunnel cycle must be completed by close of play every day with an extra supporting structure erected to stabilise it overnight.
The ground here is also sandier with fewer rocks, which makes it inherently less stable. ''At the south (west) end we have more sand and less rock. The layer of rock ants as roots holding the sand lenses together,'' explains Bridge.
Without it, the sand is liable to collapse.
The overnight support required to make the tunnel safe coincided with the need to support the ground more extensively during construction due to the weaker sandy ground. This comes in the form of a permanent pipe canopy which is made up of l2m long, 140mm diameter circular hollow steel sections installed perpendicular to the surface before excavation.
Two Liebherr roadheaders are working from the north portal, but only one machine is needed at the south west end, switching between bores with the pipe canopy following behind fresh excavation work, since when it is working in a tunnel, the pipe canopy is being installed in the other. Then the operations in each tunnel can swap overnight.
The tunnels are relatively crowded with ventilation ducts, do-dusting equipment and an electrically powered muck- shifting conveyor to comply with new health and safety regulations for air quality in tunnels. This means that electrically powered machinery is used where possible - a factor which added 20% to the original project cost.
As well as building the tunnel, Balfour Beatty is undertaking extensive groundworks for the 30m of cut and cover tunnel sections at each portal, as well as building the rest of the road and a host of footbridges and underpasses.
The new A3 is expected to open in 201 l , but work does not stop there. The following nine months will see the original contours of the Punchbowl reinstated, a new cycle track built and new vegetation planted. Eventually no one will ever know that the A3 once cut around the Devil's Punchbowl.
