User:Martinvl/HindheadTunnel

The Hindhead Tunnel is a road tunnel that opened (southbound and northbound) on 29 July 2011, two days ahead of schedule, as part of the new Hindhead bypass for the A3 road in Surrey. It forms part of the 4 mile (6.7 km) dual-carriageway being built to replace the last remaining stretch of single-carriageway on the 68 mile London to Portsmouth road. The bypass is intended to improve road safety, reduce congestion and improve air quality. At 1.2 miles (1.83 km) long, the tunnel is the longest non-estuarial road tunnel in the United Kingdom, and takes the road beneath the Devil's Punch Bowl, a Site of Special Scientific Interest.

History
A naval dockyard has existed in Portsmouth since at least Tudor times, giving significant importance to the road linking that city with London. The original 73 mi route skirted the north-western limits of The Weald climbing the Devil's Punchbowl close to Hindhead. In 1826 the road, which in the 1920's was designated as the A3 was rebuilt to ensure that the gradient was no more than 5%.

By the start of the new millenium most of the A3 had been doubled; only the section that passed through Hindhead and the Devil's Punchbowl was still single carriageway. This section, which passed through an area of outstanding natural beauty, operated at or above capacity for much of the day and had an accident rate that was 40% higher than the national average for that class of roads.

Design
By the year 2000 the A3 had been built to dual carriageway standards for its entire length apart from the 5.5 km Hindhead section. The altitudes of terminii of the northern and southern sections of dual carriageway were 180 m and 190 m respectively and the single carriageway linking them followed a winding route around the Devils Punchbowl, reaching an altitude of 260 m. The Hindhead Tunnel project shortened the route by about 300 metres and routed the road through a tunnel whose northern and southern portals were are altitudes of 190 and 200 metres above sea level respectively.

Assessment of proposals
The need for improvements to the A3 through Hindhead had been recognised for many years with a route study being undertaken between 1970 and 1976. In 1983 some nine alternatives for the A3 were investigated by the Department for Transport, but assessment showed that only one which went around the north and west side of the Punch Bowl (the "Red Route"), crossing the Smallbrook Valley was viable. A public consultation on the route met with opposition and two alternatives were suggested both of which drew less support than the proposed route which in 1988 became the preferred route. Subsequent environmental surveys showed that this route would have substantial adverse impacts resulting in the proposal to adapt an earlier scheme by including a tunnel to avoid the most sensitive parts of the route. The scheme entered the Government’s targetted programme of improvements in 2001.

A public inquiry was held in September 2004 to hear objections and to consider alternatives to the proposal. Among the alternative proposals was one for a surface route following a more westerly line that would avoid building a tunnel (an adaptation of the "Red Route"). Despite being significantly more expensive than building a surface road, a tunnel was preferred after two alternative surface schemes were rejected on environmental grounds. The decision to put part of the road in a tunnel has meant that at a cost of about £142,000 per yard (£155,000 per metre) the underground portion of the new road will be the second most expensive road in the UK per unit distance, after Limehouse Link tunnel.

Geology
The Hindhead tunnel runs through a sequence of fine grained strata of the Lower Greensand Group that were laid over Wealdan Clay during (part of the Wealden Group) during the Lower Cretaceous period (70 - 140 million years ago) on the margins of the subsiding Weald Basin. The Greensand group is comprised of the Hythe Beds which overlay a 10 - 20 m layer of Atherfield Clay. The Hythe beds themselves, which host the tunnel, are comprised of the Upper Hythe bed which has a four substrata identified by the letters "A" to "D" and the Lower Hythe bed has two strata which are identified by the letters "A" and "B". Originally that whole area was covered by the Burgate Beds, but when the Weald was uplifted, the Burgate Beds were eroded away.

Most of the tunnel passes through the Upper Hythe C and D layers and the Lower Hythe A layer which are described as "Weak, locally very weak to moderately strong, slightly clayey fine to medium sandstone with occasional thin beds of clayey/silty fine sand" and typically has Uniaxial Compressive Strength (UCS) values of 2 - 5 MPa. The rock is heavily fracture and has mean fracture centres varying between 190 and 815 mm. The southern end of the tunnel however passes throught the "less competent" Upper Hythe A and B layers which are described as "medium dense thinly bedded and thinly laminated, clean to silty and clayey fine and medium sand with subordinate weak to strong sandstone, cherty sandstone and chert". Most of the tunnel is above the predicted water table.

Access and the old A3 road
Access to the new section of road from the north is via the original A3 with access to the old A3 being provided from the Thursely Junction, completed in 2005, about one kilometre beyond the northern limit of the Hindhead Tunnel works. The new road can be accessed from the south using the original A3 with new access points being provided by new junctions at Hazel Grove.

The tunnels themselvs are 1830 m in length - 1770 m were dug using Tunnel boring machines (TBMs) while the portals, each 30 m in length were constructed using a cut and cover technique. The tunnels have a 60 degree bend with a radius greater than 1100 m to accomodate the contours of the Deveils Punchbowl. Most of the tunnels lie in the Upper Hythe C and D, and Lower Hythe A units, but the most southerly 350 m part of lies in the geologially more challenging Upper Hythe A and B rock strata.

Ancillary works include a deep cutting to the south of the tunnel with a new junction for Hindhead and Hammer at its southern end. An equestrian and pedestrian bridge, the Miss James Bridge, crosses the cutting between tunnel and junction, and includes heathland planting to link the habitats on either side of the cutting.

After the construction was completed, the section of the old A3 from the A287 intersection in Hindhead southwards was renamed the A333, the 250 m section from the intersection northwards up to the National Trust car park is to be retained, the section that folows the Devil Punchbowl contour is to be covered up at returned to nature thereby making it impossible to use the original roads in an emergency should both tunnels be closed. The remainder of the old A3 is either being converted to a cycle track/bridlepath or is being used as a local road.

Tunnel description
In order to meet the predicted traffic capacity, each tunnel required a 7.3 m wide carriageway with a 5.03 m high traffic gauge and 1.2 m wide verges on either side. In addition a 250 mm area had to be left above the traffic gauge to protect overhead equipment from loose ropes, tarpaulins and the like.

The tunnel was excavated using mechanical diggers rather than a tunnel boring machine (TBM) as the TBM would only be cheaper if tunnel was longer than 2.5 km. As a result the excavated part of the tunnel was horse-shoe in shape rather than circular, and the amount of spoit removed was 20% less than would have been the case had a TBM been used. Each tunnel has an approximate excavated diameter of 11.6 m. A 200 mm shotcrete primary lining provided the initial support for the tunnel. At the southern end, where tunneling passed through the Lower Hythe sands, the primary lining was supplemented with 4 metre "pins" to provide more support.

Emergency interconnecting cross passages are located at 100 m intervals to facilitate movement of pedestrians between tunnels in case of emergency. Ref 30 - Hindhead tunnel geology & boring

Ref 32 - Hindhead lengths (Govt report)

Cross-section:

Elephant Feet pictures

Post building trade report

Extract from Converting
Cross section

The Hindhead tunnel layout comprises twin 2-lane bores with cross passages at 100m nominal centres. Each bore has two 3.65m lanes, with full batter curbs and 1.2m wide verges on each side of the tunnel. The verge width is sufficient to allow for sight-lines due to the horizontal curvature of the tunnel, to accommodate electrical services and also to provide wheelchair access to the cross passages and emergency points at 100m nominal centres along the tunnel (figure 3).

The vertical traffic gauge provided is 5.03m with an additional clearance of 250mm to the Equipment Gauge to allow for flapping tarpaulins and other transitory gauge infringements. These requirements result in a horseshoe shaped tunnel structure with a 10.6m i.d and an excavated diameter of 11.6m, with a face area of 96m2.

Tunnel excavation and support

The presence of the sand layers, in one location up to 2m thick, led to the selection of the Sprayed Concrete Liner (SCL) method whereby shotcrete is sprayed at the face following each excavation advance. Standard hard rock tunnel support techniques such as pattern bolting were not considered suitable due to the sand layers and the very low bond stress negatively impacting the effectiveness of rock reinforcement.

Four basic support types have been designed for the standard tunnel cross sections with minor variations required at cross passage junctions and Emergency Point niches. There is one main support type for the sandstone section, with three support types covering the section through sand and the transition from sand to sandstone.

Excavation and support types are specified based on tunnel chainage and have been designed to cover all expected ground conditions. It is not proposed that support types be selected based on geological inspection. The Hythe beds have 6 joint sets and an average joint spacing of less than 200mm. The UCS values are typically 2-5MPa. This material is expected to act as a continuum, and given the heavily fractured nature of the material, and presence of sand layers, meaningful variations in rock quality are expected to be difficult to detect. A suite of ‘additional measures’ discussed below have been designed to account for any local stability issues. Geological inspection and mapping of the open excavation, and monitoring results will be used to determine the advance length that may vary between 1m and 2m, with 1m advances specified at critical locations such as beneath surface structures and roads.

Support Type 1

At the northern end of the tunnel (Chainage 3120 to 4650), excavation is in rock (UHC/D, LHA) and Support Type 1 is specified throughout. The tunnel is generally excavated with a full-face heading followed at a distance by the bench excavation. Due to the generally stable nature of the ground and tunnel location above the water table, a closed invert is not required and the horse shoe shaped primary lining is supported on elephants feet (figure 4).

In addition to the sequences and support requirements for each of the support types, contingency ‘additional support measures’ have been specified. The requirement for the contingency measures will be triggered by geological inspection and mapping, and monitoring results, and will include:

Spiling - Self drilling GRP tubular spiles will be installed when ground conditions result in excessive overbreak, or instability in the crown. Spiling is detailed as mandatory for approximately 30% of the Support Type 1 excavation in areas of known potential crown overbreak such as where the tunnel crosses Fullers Earth bands, and where a 2m thick layer of sand and shattered rock intersects the crown.

Additional face support measures - Geological inspections will inform the selection of additional face support measures such as sealing layers, or face dowels.

Probe Drilling - Probe holes drilled ahead of the face have been specified for the entire length of Support Type 1 to relieve any potential hydrostatic pressure from perch water ahead of the face. If water is detected in the probe holes then additional holes will be drilled to drain any water

Grout Stabilisation - Microfine cement or chemical grouts will be used to stabilise running sand bands, or other local areas of instability caused by sandy layers. It is proposed to conduct site trials prior to construction to determine the optimum method and materials for stabilisation of sandy materials

Invert strut or rib bolts - Convergence monitoring will be undertaken during construction with a three stage trigger limit system. Unplanned convergence resulting from worse than expected ground conditions will be addressed by installation of an invert strut at bench level.

Support Types 2 - 4

At the South end of the tunnel (Chainage 2880 to 3120 (m001)), excavation is in sand (UHA/B) and support types 2, 3 and 4 are specified. The excavation will be carried out on dayshift only due to constraints on working hours and will be made stable with the use of a steel pipe umbrella and face dowels. The pipe canopy comprises 12m long, 114mm diameter tubes at 400mm centres with an overlap of 4-5m. The advance length is a maximum of 1m for these support types.

Support Type 2 has sandy material (UHA/B) in the heading only, with the heading elephants feet supported on the sandstone material (UHC/D). This means self-drilling GRP face dowels are required in the heading only, and the heading can advance ahead of the bench. The face dowels are 12m long with a 4m overlap and are installed with the same drill jumbo used to install the pipe canopy.

Support Type 3 has a full face of sandy material with the elephants feet of the bench supported on the sandstone material. As the heading elephants feet are not supported on sound material, the heading must be advanced with the bench, with a 2m separation provided to maintain face stability. Face dowels are required for both the heading and the bench (figure 5).

Support Type 4 has a full face of sandy material that extends below the tunnel, and therefore a closed invert is required. The heading must be advanced with the bench, with the invert closed a maximum of 6m behind the face.

Design and methodology

The excavation sequences outlined above are designed to control strains in the ground so that as much as possible of the ground load bearing capacity is used and the strains are maintained at levels that minimise yielding.

A principal innovation with the support measures is the design of primary lining as permanent. This is possible due to a number of advances in tunnelling technology in recent years. Firstly, non-alkaline accelerators are now available with no loss in shotcrete strength with time. A recent innovation is the use of 3-D scanning survey equipment that provides excellent shape control for both excavation and spraying, and allows shotcrete lined tunnels to be constructed without lattice girders. Historically the inclusion of lattice girders meant the primary lining had to be considered temporary due to the corrosion potential of the steel lattice girder within the primary lining. Spiling is envisioned in several locations due to adverse soil layers. This will be carried out with self-drilling Glass Reinforced Plastic (GRP) dowels, again with no adverse durability issues. The sprayed concrete will be reinforced with steel fibres as is required for safe installation, however the design does not rely on the flexural capacity of the steel fibres, and the lining is designed as plain concrete. This is possible due to the curved shape of the section with all moments resisted by axial forces within the lining.

Geology description from Geological map

Construction and opening
Advance works started in January 2007, and main construction works, including the tunnelling, started in 2008.

Excavation work from the north portal began on 1 February 2008. Two Liebherr diggers were employed, one on each tunnel with work being carried out around the clock, seven days a week. Work from the south portal began on 14 May using a single Liebherr digger to dig both tunnels. Since the south portal was close to housing, work was restricted to a single shift five days a week. The poorer ground at the southern end of the tunnel also slowed progress. The primary linings were sprayed onto the tunnel with the spraying operation following the excavation operation by about two metres when digging through standstone and about one metre when digging through sand. Breakthrough was achieved on both tunnels on 26 February 2009, 250 m from the south portal. The extraction rate for the southern portion was 1.2 m/day and for the northern portion 3.9 m/day.

Prior to breakthrugh, only the part of the tunnel - the semicircular cross-section above the bench was excavated. Excavation of the bench in the southbound tunnel began on 9 July 2009. This work progress much quicker than the inital excavation work. Again, the primary linings were sprayed onto to workings as digging progressed. The excavation and primary linings were completed on 31 March 2009 with a total of 737,000 cubic metres of spoil having been excavated.

The application of the waterproof membrance and the installation of the secondary lining only started once all the excavation and promary lining were complete. The waterproof membrane was sprayed onto the primary lining and the pre-cast invert and side section installed. Spraying of the secondary lining onto the crown of the tunnel was scheduled as part of the finalisation of the nnnn

On Sunday, 14 May 2011, one and a half months before the tunnel was due to open the contractors staged an open day when 7,000 pedestrians were able to walk the full length of the tunnel when local music groups performed at the north end of the tunnel. The opening ceremony itself, to which the public was not invited for safety reasons, was performed by the Secretary of State for Transport Philip Hammond on 29 July 2011.

Safety
Serious accidents in the Mont Blanc Tunnel and the Gotthard Road Tunnel in 1999 and 2001 respectively resulting in the closures of these tunnels led to a re-evaluation of road tunnel safety throughout Europe. A EU directive passed in 2004 laid down the minimum safety requirements for road tunnels within the EU and EFTA that exceed 500 m in length and that are part of the Trans-European Road Network (TEN). Although the Hindhead tunnel is more than 500 m in length, it does not carry part of the TEN road network so legally does not need to comply with the directive.

The tunnel approaches have cross-over systems that enable the police to direct all traffic through one of the tunnels should the other be close as a result of an incident or for maintenance. Due to part of the original A3 having been returned to nature, it will not be possible to use the old road in an emergency. However the South East of England has a comprehensive road network and using the M3 instead of the A3 will increase the Kingston to Portsmouth journey from 65 mi to 89 mi.

Cross-connection tunnels have been constructed at 100 metre intervals to allow for emergency evacuation to the other tunnel should there be an incident. Although the EU directive requires laybys at intervals not exceeding 1000 metres for TEN-T tunnels that do not have emergency lanes, the Hindhead tunnel does not have any laybys, but it does have 1.2 m verges on either side of the carriageway. The tunnel has two powers supplies, one via the north portal and the other via the south portal, each backing the other up in the event of failure. Ventilation is provided by fans mounted above the traffic, though in normal circumstances they will not be necessary.

Electronic equipment includes linear heat detectors, radar coverage and 104 CCTV cameras that can pinpoint incidents, intelligent lighting and LED cat's eyes and comprehensive fire-fighting systems. .

Environmental considerations
From the outset a tunnel was built rather than a cutting being dug to avoid spoiling a site of outstanding natural beauty and a Site of Special Scientific Interest, much of which is owned by the National Trust. Before digging started an environmental survey was carried out. Common lizards, adders and slow-worms found at Boundless Valley were relocated to National Trust land at Highcombe Edge while grass snakes were taken to Hurthill Copse.

Tree felling was scheduled to minimise disruption to nesting birds and to other wild-life and in certain instances, animals such as dormice were removed to similar habitats elsewhere. After the works were completed, 200,000 trees were planted on the route of the old road. The restoration of the old road to nature removed a barrier that prevented the migration of ground-nesting birds, such as woodlarks and nightjars from one part of the nature reserve to the other.

Among the artefacts found was an old milestone that was discarded during the rebuilding of the road in 1826. The 1811 Ordnance Survey map was used to identify the original position of the stone where it has since been re-erected. The pre-1826 road is now a pedestrian way to Gibbet Hill. Other artefacts of archaeological interest included two lime kilns found near the Thurseley side of the works dating from the 18th or 19th centuries. It is probable that lime produced by the kilns came from Petersfield 25 km away and would have been used to counteract the acid nature of the soil.

The main contractors received the Preservation Award at the 2011 Tunnels and Tunnelling Awards ceremony.