Design Hindhead Tunnel

1 design

1.1 assessment of proposals
1.2 geology
1.3 access , old a3 road
1.4 tunnel description

design

by 2000 a3 between southfields , havant had been built dual carriageway standards entire length apart 5.5 km (3.4 miles) hindhead section (there still sections of single carriageway within london , portsmouth). termini of northern , southern sections of dual carriageway @ altitudes of 180 m (590 ft) , 190 m (620 ft) above sea level respectively , single carriageway linking them followed winding route around devil s punch bowl, reaching altitude of 260 m (850 ft). hindhead tunnel project shortened route 300 m (980 ft) , routed road through tunnel northern , southern portals @ altitudes of 190 m (620 ft) , 200 m (660 ft) above sea level respectively.

assessment of proposals

devil s punch bowl seen old a3 (before closure)

the need improvements a3 through hindhead had been recognised many years route study being undertaken between 1970 , 1976. in 1983 9 alternatives a3 investigated department transport, assessment showed 1 went around north , west side of punch bowl (the red route ), crossing smallbrook valley viable. public consultation on route met opposition , 2 alternatives suggested both of drew less support proposed route in 1988 became preferred route. subsequent environmental surveys showed route have substantial adverse impacts resulting in proposal adapt earlier scheme including tunnel avoid sensitive parts of route. scheme entered government’s targeted programme of improvements in 2001.

a public inquiry held in september 2004 hear objections , consider alternatives proposal. among alternative proposals 1 surface route following more westerly line avoid building tunnel (an adaptation of red route ). despite being more expensive building surface road, tunnel preferred after 2 alternative surface schemes rejected on environmental grounds. decision put part of road in tunnel pushed cost of project £371 million, underground section costing £155,000 per metre (£142,000 per yard) making section of road second expensive road in uk per unit distance, after limehouse link tunnel.

geology

geology of hindhead tunnel

the hindhead tunnel runs through sequence of fine grained strata of lower greensand group laid on wealden clay during (part of wealden group) during lower cretaceous period (70–140 million years ago) on margins of subsiding weald basin. greensand group consists of hythe beds, overlay 10 20 m (33 66 ft) layer of atherfield clay. hythe beds themselves, host tunnel, consist of upper hythe bed, has 4 substrata identified letters d , , lower hythe bed, has 2 strata identified letters , b . whole area covered burgate beds, when weald uplifted, burgate beds eroded away.

most of tunnel passes through upper hythe c , d layers , lower hythe layer described weak, locally weak moderately strong, clayey fine medium sandstone occasional thin beds of clayey/silty fine sand , typically has uniaxial compressive strength (ucs) values of 2–5 mpa. rock heavily fractured , has mean fracture centres varying 190 815 mm (7.5 32.1 inches). southern end of tunnel passes through less competent upper hythe , b layers described medium dense thinly bedded , thinly laminated, clean silty , clayey fine , medium sand subordinate weak strong sandstone, cherty sandstone , chert . of tunnel above predicted water table.

route map of hindhead tunnel scheme


access , old a3 road

access new section of road north via original a3 access old a3 being provided thursley junction, completed in 2005, 1 kilometre beyond northern limit of hindhead tunnel works. new road can accessed south using original a3 new access points being provided new junctions @ hazel grove.

after construction completed, section of old a3 a287 intersection in hindhead southwards renamed a333, 250 m (820 ft) section intersection northwards national trust car park has been retained, , section follows devil s punch bowl contour has been returned nature. remainder of old a3 either converted cycle track/bridlepath or being used local road.

tunnel description

panorama seen northern viewpoint.



open day 14 may 2011



the old a3 being returned nature. view taken close gibbet hill, directly above tunnel. october 2012

in order meet predicted traffic capacity, each tunnel required 7.3 metres (24 ft) wide carriageway 5.03 m (16.5 ft) high traffic gauge , 1.2 m (3 ft 11 in) wide verges on either side. in addition 250 mm (9.8 inches) area had left above traffic gauge protect overhead equipment loose ropes, tarpaulins , like.

the tunnel excavated using mechanical diggers rather tunnel boring machine (tbm) tbm have been cheaper if tunnel longer 2.5 km (1.6 miles). result, excavated part of tunnel horse-shoe in shape rather circular, , amount of spoil removed 20% less have been case had tbm been used. each tunnel has approximate excavated diameter of 11.6 m (38 ft). 200 mm (8 inches) shotcrete primary lining provided initial support tunnel. @ southern end, tunneling passed through lower hythe sands, primary lining supplemented 4 m (13 ft) pins provide more support.

emergency interconnecting cross passages located @ intervals of 100 m (330 ft) facilitate movement of pedestrians between tunnels in case of emergency.

ancillary works include deep cutting south of tunnel new junction hindhead , hammer @ southern end. equestrian , pedestrian bridge, miss james bridge, crosses cutting between tunnel , junction, , includes heathland planting link habitats on either side of cutting.