In taking forward the UK’s development of permanent sprayed concrete lining (SCL), the construction of the A3 Hindhead twin bore road tunnels, in southern England, is also presenting new approaches to flexibility at strategic and tactical levels, such as the use of Early Contractor Involvement (ECI), single gang working and adapting to materials cost inflation.
The need for programming flexibility was also set to be tested earlier this year when, just getting underway with the main drives from the North Portal, contractor Balfour Beatty found the ground to be poorer that expected. Fortunately, the conditions did not last. Over recent months, the excavations and permanent SCL placement have made good, and steadily improving, progress with the targeted first breakthrough on the prioritised 1740m long heading of the northbound tube set for the end of the first quarter of next year. By mid-2009 all major tunnelling work on both tubes should be completed.
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As a major traffic congestion relief scheme by the Highways Agency (HA), the project calls for excavation below an area of protected natural beauty including the head of a valley called the Devil’s Punch Bowl. The new route is to be operational by August 2011 (T&TI, June 2007, p23).
Developing the solution
In October 2002, Balfour Beatty Major Projects began work on the ECI contract, which was on a Target Cost basis. The bid basis was 40:60 on cost-to-quality. Working with Mott MacDonald as a subconsultant, the contractor led the development and refinement of the scheme to planning and public enquiry stages, over 2004-5.
The tunnels have a reverse-curve alignment with the southbound tube, the longest drive being the 1790m bored section of the southbound tube. On completion, each tube will have a two-lane road and verges within a 10.6m finished width and height of 5.28m. The tubes will be linked by 16 cross passages. The client gives the project budget, discounted to 2002 levels, as US$580M. This figure includes HA’s costs and tax. Construction works are budgeted at US$422M (2002 rates).
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By GlobalDataEarly work, on design evolution, was cost-reimbursable, including for extra site investigation as requested by the contractor. Spend on geotechnical investigations totalled US$2.3M – 2002 rates, or 0.6% of construction costs, two-thirds of which was tunnel-related.
The geology is dominated by the sandstone of the Hythe Beds, and mostly above the historically-observed water table. The strata are variably sorted, highly glauconitic, variably bioturbated and cross-bedded sand and sandstones, and the alignment passes through the more competent units with UCS typically 2MPa-4MPa, though heavily fractured. The ground was known to be weaker at the south end of the tunnels.
A key challenge was establishing the ground behaviour model in weak sandstone with up to 20% interbedded soil layers, and especially with the rock being weaker than typical when assessing strength and stiffness by empirical methods – which also don’t allow for soil, especially in stiffness calculations. Site investigation included sonic, triaxial and pressuremeter tests – the latter proving the most reliable. Stiffness was estimated using a small strain stiffness model, and strength was modelled using a Mohr-Coulomb strain softening model.
Cover to the tubes varies significantly, most extremely from 16m-58m over only 130m. The vertical alignment is steeper at the southern end to minimise the length of drive through the weaker strata, and to maintain cover.
Project design and planning
Project development looked at merits of sequential excavation versus using a TBM, the latter having offered an alternative to avoiding vehicle emissions and generation of respirable crystalline silica from sandstone excavation. However, the advent of low emission diesel plant onto the market plus electric plant and the BTS guidance note on NOx exposure, out in 2006, presented the possibility for a modified, more economic, sequential lining and shotcrete support system.
The ground, and its relatively dry condition, has allowed the designer, Mott MacDonald, and the contractor and client to undertake the most extensive, and complete, use yet of permanent sprayed concrete – a design and construction approach that has made the most of major advances in the UK over the last decade, avoiding the need for lattice arches and steel mesh to help achieve permanent, stable ground support, whilst maintaining the required tunnel profile.
With an arch design, the lining is supported on wide footings – “elephants feet” – and the permanent SCL does not attract any tensile loads. Steel reinforcement is only therefore required at junctions, such as for cross passages. However, structural fibres are employed to provide a ductile lining giving safety to the tunnelling teams during construction. The permanent SCL will be made watertight using spray-applied waterproofing membranes specially developed for such applications. Like the sprayed concrete, the membranes are applied with robotic equipment.
The use of the permanent SCL system, as used at Hindhead, is its first major application in the UK, and in particular, a UK road tunnel. More generally, the system can be suitable for many applications in metros, rail and road tunnels where there is only modest water ingress and predictable ground conditions, which for example are not always found in Alpine tunnelling environments.
Construction gets underway
On project approval, in late 2006, the contractor geared up its small pre-construction operation led by project director Paul Hoyland and tunnel manager Roger Bridge. The payment basis moved to cost plus margin in relation to the target, and by mid-2007 major construction activities were underway.
In planning the works, which were to be neutral on cut and fill, a critical element is to hole through one heading to enable off-road spoil transfer from a cutting for embankment construction. Consequently, with the majority of excavation being pushed round-the-clock from the North Portal, the northbound tube has been prioritised and the wall-mounted conveyor will be kept active as bench excavation is finished. At the south end, the need to minimise disturbance for local people, progress is at a slower pace through adoption of a single, 12 hour shift and five day working.
Tunnelling began in February with four faces to be opened up, a pair at each end, but almost immediately there were ground problems at the north end. During portal cutting for the northbound bore, two fault zones were found, which were then met by the staggered southbound heading. Missed by boreholes, the undetected problem was due to an anticline with its near-vertical planes having fault zones, each 10m-20m long. Contingencies were explored, such as more excavation being done from the South Portal. Fortunately, the weak zones ended just beyond Boundless Valley and before the approach to the A3 at the Devil’s Punch Bowl. Since then, the ground has been as envisaged, if not better, for excavation. By the middle of last month, the job had enjoyed good progress rates for almost four months.
A total of almost 47% of the heading in the northbound tube had been driven, the advances being 694m from the north end and 119m from the south. The comparable progress for the southbound heading was nearly 44% with 685m bored from the north end and 95m from the south. Given its importance to the Critical Path, excavation of the northbound heading is now more than 50m ahead of the southbound bore.
The greatest allowable settlement was calculated to be 23mm when passing beneath the A3. Actual settlement, so far, at the location is 13mm.
With greater priority for the southbound for bench excavation to be pushed – enabling it to be first to complete the full profile and proceed to secondary lining – that work is almost 22% complete compared to only 6% in the northbound tube. The earlier delay held back the bench excavations, which began in July and August, respectively.
Excavation
The heading excavation is nominally 11.6m wide, and the face area is 96m2. Three Liebherr 944 machines are advancing the full face headings, two at the north end bores and one alternating between the bores at the south end. With dust generation to be minimised, ripper buckets were specified but as hard bands were also anticipated there were also breakers and cutters sourced. The excavators were required to meet a progress rate of 40m3/hr and the ground is proving quite abrasive for the bucket teeth.
Face advances were seen as being 1m-2m. In the most of the drives, beyond the fault zone, the advance has settled at 1.5m-1.6m to suit geological behaviour and the optimum load distribution take-up by the primary lining. Excavation and installation of support for either heading or bench advances are typically working on four-to-six hour cycles, including one hour set time.
The job has three Sandvik Tamrock Axera 8 rigs, each with two drill booms and a basket boom – two rigs at the north bores, one alternating between the bores at the south end. The rigs are for probing the ground and also to install the 4m long, 32mm diameter, self-drilling, glass reinforced plastic (GRP) dowels where needed. Combined with face sealing shotcrete, this is the extra support mainly used. In the fault zones, at the north end, there were up to 50 grouted GRP dowels installed at 250mm-300mm centres, which took up to three hours to install. In those stretches, advances reduced to 1m and grout stabilisation of looser ground was also done.
One rig is fitted with the Alwag Automated system for the heavier, pipe canopy installation at the south end. It also permits standard holes to be drilled for grouting or probing. The canopy is made of 140mm diameter, 12m long circular steep pipes in four sections. Typically, there are on average 25 pipes, but the number has varied from 19-31, less being required approaching rockier ground. The heading advances are approximately 8m, which is when the canopy is installed.
Geologists provide shift cover to inspect and log the faces, and help determine the most suitable support regime. The client’s consultant on the scheme is Atkins, which is operating as HA Project Manager’s Representative and Supervisor.
Tight survey control is vital to accurately excavate the profile and then construct the unreinforced, permanent SCL. At Hindhead, two systems are being used – a bespoke system being developed with VMT to examine 3-D control, and reflectorless total stations. The new system involves a total station tracking a prism on the excavator and which should, combined with feedback from inclinometers on the machines, enable software to execute real-time monitoring to help profile control.
The bores have extendable continuous conveyors, rated for 400 tonnes/hr, from Continental Conveyors. The initial plan was for them to follow the bench excavation but to minimise air pollutants they are behind the headings.
Each bore has a vent duct supplied by a twin 250kW fan set. Given the importance of limiting respirable crystalline silica there are four air filtration units of 25m3/s capacity plus a smaller unit. Dust monitors are used to check eight-hour weighted average for exposure.
Support and linings
In each bore, a BASF Meyco Logica robotic spraying unit applies the permanent SCL in progressive layers to the crown and walls, and as a single mass for the elephants feet to avoid the risk of rebound entrapment or lamination. There are three units on the project – one more than initially planned, which was hired (unlike the others at the north end) as more work was undertaken from the south end.
The shotcrete has a required strength gain in line with the Austrian J2 curve, and strength of 40N/mm2 but is achieving 50N/mm2 – 60N/mm2. The lining design is 200mm thick but can be up to 300mm thick where there is overbreak.
While the mix design is relatively standard, the advances that enable its application as permanent lining come in the superplasticisers and alkali-free additives, explains Ross Dimmock, Motts’ technical director for tunnels.
With the non-structural benefits of fibres for application of the mix, the shotcrete was specified to have approx 30kg/m3 of steel fibres. However, rising commodity prices has led to contractor to switch to sourcing structural PP fibres, though steel fibres will be kept onsite in reserve. A similar volume of PP fibres is used.
After the lining is applied, the heading is approximately 11m wide by 6.5m high, leaving just over 3.5m of bench to be excavated. The job has two Terex ITC Schaeff 210 excavators for bench and cross passages work.
Delivery and completion
Working across all faces, benches and lining activities is, unusually, a single gang. Their bonus is set on overall performance on the works. Ironically, the poor ground at the outset only helped to bed down the new, flexible working approach. Allowing for cross passage work, the gang advances the headings from the North Portal approx 10m/day (24 hours).
In the second quarter of 2009, work will move on to the non-structural secondary lining – again, 200mm in the design, but up to 300mm to allow for construction tolerances. Before then, a spray-on waterproof membrane (Masterseal 845A/855) will be applied, in effect creating a bonded “sandwich” of layers.
Watertightness has been a significant hurdle for permanent SCL linings, notes Dimmock. While being a relatively new approach, spray-applied membranes have a proven record to meet this need, and their use at Hindhead reflects a new design approach for SCL tunnels, and the provision of more economical, maintainable tunnels, he adds.
The overall design was also changed to make further use of the spray equipment and to benefit construction schedule and economics, resulting in the introduction of a 150mm thick SCL layer on the crown. Previously, the entire secondary lining was to be cast insitu but only the walls will be done this way, to give the aesthetics and smooth surface wanted.
Location of A3 Hindhead twin bore tunnels below area of natural beauty Profile control is a key feature of the successful use of permanent SCL Ariel view of the northern portal at Hindhead Sandstone dominates along the alignment Steel pipe canopy installation at weakest, south end of bores Heading and bench excavation, and use of permanent SCL
