Heathrow baggage tunnel 01 Oct 1994

Although used on the Continent for some time, steel fibre reinforced precast concrete segments are finding their first use in the UK on the 1.5km long x 4.5m diameter baggage tunnel under the runways of Heathrow Airport in London. As part of its design and construct contract, Miller Civil Engineering has seized the opportunity to use steel fibre as opposed to rebar cages to reinforce the segments of the tunnel's precast concrete expanded wedge block lining and to introduce this new technology to Britain.

The tunnel is being driven using a 4.8m o.d. Dosco shield fitted with a telescopic roadheader boom and a vacuum segment erector

"Reinforcement in the lining is not required to provide extra load bearing capacity of the lining", explained Colin Eddie, Chief Tunnelling Engineer for Miller Tunnelling. "We are passing through good quality London Clay at Heathrow and unreinforced precast concrete segments have been used on several projects to line tunnels in London Clay, most notably, the recent work for the Thames Water Ring Main. But for the baggage tunnel, we have designed a 1m wide ring of fewer and thinner segments which require reinforcement principally for handling purposes and to withstand the thrust pressures of the tunnelling shield. Where a 1m wide ring of unreinforced segments for a tunnel of similar diameter (4.5m i.d.) would require 11 or so segments plus a key and be some 200mm thick, the steel fibre reinforced ring for the baggage tunnel is 25% thinner at 150mm thick and comprises seven segments and a key."

Since this is believed to be the first use of steel fibre reinforced tunnelling segments in the UK, it is difficult to make meaningful comparisons between the cost of producing different types of precast concrete lining. With no domestic producer of steel fibre in the UK, fibre is being imported from the Continent and production costs for the baggage tunnel project also include the costs of the moulds and set up of the production line at the factory. However, on the Continent where steel fibre reinforced segments have been used for several years, the cost of production compares favourably with that for both unreinforced segments and segments reinforced with rebar cages. Although rebar is less expensive than steel fibre by weight, there are significant savings to be had by eliminating the cost of making the rebar cages and taking them out of the mould set up and production line. The most tangible economic advantage at Heathrow is that with fewer segments/ring, each ring is quicker to build, providing faster tunnelling rates overall. In the good quality London Clay and using an open faced tunnelling shield from Dosco, Miller hope to sustain and better a target average of 100m/week over the 1.5km long tunnel, working two 10h shifts/day, five days/week.

Contract award

The Heathrow baggage tunnel is a £40 million project financed and managed by Heathrow Airport Ltd, part of BAA, the owner and operator of most of Britain's largest airports. The project is being provided for British Airways, the largest airline operating from Heathrow. When in service, baggage in transit from flights arriving at Terminal 4 and leaving again from Terminal 1, 2, or 3, and vice versa, will be transferred through the tunnel rather than by road as at present. Small unmanned trains driven by electromagnets will rise and fall on 10m spirals to drop off and pick up bags without stopping. Laser scanners and bar codes will steer the bags to their correct flights.

Plan of Heathrow baggage tunnel

The civil contract for the project was bid in late 1993 with Miller Civil Engineering receiving a letter of intent in December 1993. With a tender price of £6.5 million, Miller believes it was innovative design which gave it the edge over its five competitors. Ove Arup & Partners is lead design consultant for BAA, and Bovis Construction is supervising the construction phase.

As set out by Ove Arup, the tunnel is divided into two drives, both of which must be driven from an Airside central access shaft near Terminal 4 on a space where wing tip clearance is afforded between the main runway on one side and a taxiway on the other. From here the two tunnel drives comprise a short run of 250m back to Terminal 4 and a longer drive of 1,200m under the main runways to the Central Terminal Area (Fig 1).

When the Dosco shield arrived in April 1994, it set off in early May to complete the short 250m drive first. After achieving modest learning curve advance rates and a best of 13 rings (13m) in one 10h shift, the shield broke through into the reception shaft on 23 June 1994. After a speedy turn of only 16 days, the machine was relaunched from the central working shaft on 12 July to complete the 1,200m drive.

To late September, the tunnel had advanced 967m and was expected to breakthrough on schedule in mid October. According to Chris Hughes, manager of Miller's tunnelling division, the drive average has remained consistent at about 100m/week with a shift best of 16 rings (16m) in 10h. The tunnel had also successfully passed under the Piccadilly Line with no significant movement recorded in the existing structure.

Segment production

Being part of a design and construct contract, Miller has broken with tradition in the design and procurement of its segmental lining. Normally, tunnelling contractors in the UK purchase their segments from one of the three of four well established precast concrete segment manufacturers.

Mucking out and transport of segments and supplies using a 16m³ capacity selfpropelled Shuttlecar

On this occasion Miller has teamed up with Crendon Concrete of Aylesbury, who have over 40 years experience in the production of standard and specialised precast concrete elements for surface structures, and are eager to expand into the field of tunnel segment manufacture. With major contracts such as the Twickenham Rugby Stadium, the Merrhill development in Birmingham and a wide range of civil engineering concrete products in its resume, this is Crendon's first job producing tunnel segments.

In the meantime, Miller had approached Bekaert of Belgium, manufacturers of Dramix , for the supply of the steel fibre. Bekaert welcomed this move by Miller to introduce a well known technique from the Continent to the UK, and contributed valuable advice, support and technical data to the project. It is also supplying the necessary steel fibre through its UK agent, Tinsley Building Products in Sheffield.

Among other points, Bekaert was instrumental in answering doubts about the durability and possible corrosion of the steel fibres in the precast segments over time. In fact, experience on the Continent has shown that, for various reasons, steel fibre reinforced precast concrete suffers less from corrosion attack than segments reinforced with rebar cages. The path of possible corrosion is not as defined as the path offered by rebar through concrete and the area of concrete deterioration around a corroding steel fibre is many times less than that around the cross-section of corroding rebar. Together, Miller with Crendon and Bekaert have advanced the design and production of steel fibre reinforced segments in the UK and have created a lead which others are sure to follow.

"There are no codes or standards of practice for steel fibre reinforced segments in the UK, so we have worked with the Building Research Establishment at Watford to produce the design for the lining", said Eddie. Together with Mott MacDonald, who provided the check on the ring design, a programme of full scale load tests was devised to verify the performance of the ring. Through calculation and tests, the necessary performance specification was met using 30kg of steel fibre for each cubic metre of concrete. An equivalent of about 80kg/m² of rebar would be required to meet the same specification. The design also calls for a minimum concrete strength of 50N/mm². Crendon is maintaining a consistent 60N/mm2 in its segment production. In another time and labour saving modification, the central drainage channel is cast into the invert segment of the lining eliminating the need to build this as a second pass in-situ concreting exercise.

Over the length of tunnel built to date, damage to segments either during handling at the factory, during transportation, or during ring build or shield pushing has been low and well below industry averages. The knuckle joints between segments in each ring avoids spalling of the edges as the ring is expanded and packers between rings ensure even distribution of thrust pressures. The one key in each ring is located in the crown as opposed to other expanded rings of similar size which have one or two keys at knee or axis position. The "axis" key design is favoured on some projects as it avoids the possibility of crown keys falling out should they become loose over time. However, experience shows that the "axis" key rings are more difficult and are time consuming to build. The gaps left by the shorter wedge keys in the expanded lining are filled with high strength non-shrink mortar as part of the finishing works.

Testing rig at Crendon Concrete in Aylesbury, where the fibre reinforced segments were cast

As it is, the seven segments and crown key in each 1m wide x 4.5m i.d. ring at Heathrow are being erected in about 10 minutes on average. The 4.8m o.d. Dosco shield is fitted with a 360° vacuum segment erector and it applies about 3000kN of its total 7500kN thrust capacity to advance each 1m stroke. To accommodate the thin (150mm thick) edges of the lining, the hoes on the shield's 15 thrust rams are offset to direct the pushing forces squarely into the body of the lining.

The tunnelling equipment chosen for the job is a simple, robust and well proven set up. The non-articulated tunnelling shield is 5m long and has an adjustable bead on the leading edge to assist machine steering and efficient ring build. The competent London Clay is readily cut by the shield's telescopic roadheader boom and lifted by its chain conveyor back to the muck transfer belt and so into the Shuttlecar hired from Specialist Plant of Wellingborough for the job. The self-propelled diesel driven Shuttlecar (fitted with "greedy boards" for this contract) has a carrying capacity of 16m³ and takes two trips to muck out each 1m ring or stroke. The car remains stationary under the muck conveyor as its chain conveyor moving floor draws back the load until full. At the shaft, the car off-loads into the 22m³ shaft hopper which is hoisted to the top for emptying. The shuttle car carries the segments into the tunnel on its return journey. "Waiting for the Shuttlecar on its round trip is unlikely to create a delay in the cycle time until the far end of the longer 1,200m drive," said Eddie.

Shafts and NATM

Besides the central working shaft, there are five further shafts on the project, two at either end of the tunnel and an on-line ventilation shaft. With the exception of the ventilation shaft, all five shafts are 10.3m i.d., are between 18 and 21m deep and have been sunk using a combination of traditional and NATM techniques. Through the top 5m of wet gravels, they were sunk as jacked caissons using eight x 80t hydraulic thrust jacks instead of kentledge to apply the downward pressure. The jacks, with a 1,200mm stroke, force each 1m deep ring of bolted shaft segments through the ground and the core is removed by an hydraulic excavator. Once into London Clay, the shafts are continued down in 1.2m steps using sprayed concrete as the primary and permanent lining.

This sprayed concrete, which is being applied as a subcontract for Miller by Sprayed Concrete Ltd of Lenham, Kent, is of the wet mix method and is being applied in several layers. On excavation, a flash 50mm thick layer of 40kg/m³ steel fibre reinforced shotcrete is followed by a 150mm thick layer of bulk shotcrete which is also reinforced with a similar quantity of Dramix steel fibre. The following day, a finishing coat of 75mm thick plain shotcrete is applied over wire mesh reinforcement. A maximum advance of 1.2m was specified by Miller for safety and settlement reasons.

The working shaft is within the airport boundaries where crane heights and movements as well as site access is strictly controlled by wing tip clearance and high security procedures

NATM work now continues at the bottom of the two shafts at either end of the tunnel where large chambers are required for the operation of the baggage transfer equipment. These 4.5m long x 4.5m diameter chambers will also be lined with the same shotcreting system as used on the shafts for the primary and permanent lining.

This use of NATM and steel fibre reinforced shotcrete as the permanent lining in London Clay is also at the leading edge of tunnelling technology in the UK. For its application as part of the design and construct contract at Heathrow, Miller has drawn on its joint venture partnership with Beton-und Monierbau of Austria for technical and practical know-how.

Settlement

Surface settlement above the baggage tunnel is a major consideration given its 12 to 25m deep alignment beneath the main runways and taxiways of one of the world's busiest international airports. A tolerance of 20mm maximum is allowed and settlement is monitored extensively. The tunnel also passes under the Piccadilly Line of the Underground with only 11m between each (from axis to axis) or a clearance of only 1.5 times the tunnel diameter. The expanded lining, erected as it is directly against the ground and expanded by the key for a tight contact with the ground, effectively limits surface settlement. The short 5m length of the Dosco shield with its small bead also ensures that the lining is applied quickly giving the ground little time to relax, thus limiting face loss and subsequent surface settlement.

Although Miller is far from suggesting that steel fibre reinforced segments will take over from other segment designs, it does believe that under certain circumstances and particularly in London Clay, the technique has economic advantages over the alternatives. Having led the way on its relatively small baggage tunnel contract, Miller is now seeing certain techniques being picked up by contractors working in London Clay on the Jubilee Line Extension and the Paddington-Heathrow Express Railway Link. NATM has been approved for the construction of underground stations on the Jubilee Line and Heathrow Express projects and both are now investigating the use of steel fibre rather than wire mesh reinforced shotcrete. In addition, proposals to use steel fibre to reinforce the segments of the alternative expanded lining designs for the Jubilee Line running tunnels in London Clay are being examined using the experience on the baggage tunnel as a bench mark.

Miller expects to hand over the completed job in February 1995. The transport equipment will then be installed by the American company BAE of Dallas with the facility scheduled to go into operation in Summer 1996.