For 118 years, White Hart Lane, home of Tottenham Hotspur Football Club, has been one of the most famous addresses in football history and the stadium has hosted some of the team’s most dramatic moments. At the very heart of the stadium, the pitch has been transformed from a simple patch of grass into an ambitious engineering project. As well as being home to Tottenham Hotspur Football Club, the stadium will now host NFL games in the UK, and to accommodate both sports, the stadium boasts the world’s first dividing sliding pitch.
Matches at the new stadium will be played on the world’s first dividing, retractable grass football pitch. Mounted in three enormous trays, the grass surface can be rolled away beneath the south stand to reveal an artificial NFL American football field beneath. The retracting pitch means that the stadium can quickly convert from hosting a football match on the grass pitch to an NFL game on the artificial field in just a few hours. The football club turned to Sheffield-based engineering firm, SCX to help deliver the retractable pitch. SCX specialises in complex kinetic architecture.
The columns that support the terracing underneath the south stand prevented the pitch from being rolled out from beneath as a single entity. Instead, SCX opted to split the pitch into three lengthwise sections that divide roughly along the outer edges of the penalty box. Each section is mounted on rails that are hidden beneath the NFL pitch, which allow them to be rolled in and out of position through a complex set of manoeuvres from under the south stand.
Joining the pitch
The whole pitch weighs over 9000 tons, more than the Eiffel Tower. Each of the three sections needs to fit together seamlessly so that the fans and footballers cannot tell that matches are happening on trays mounted 1,6 metres above solid ground. Achieving the accuracy required to get this kind of join along the entire 118 metre edge was one of the most difficult aspects of the project. Each section, composed of 33 metal trays welded together from 16 mm steel plates, needed to line up perfectly when brought together.
The project team discovered that the best way of ensuring a reliable and repeatable joint was to have the trays touch via a three-ply rubber strip running down the entire length of each section of pitch, just beneath the turf. The engineers at SCX also tested how best to pull the pitch apart again because when the sections are joined, the grass can become entangled as roots and leaves grow. The team found that the best approach was to simply let the turf tear naturally as the sections pulled apart, allowing it to split along its own weak points. When complete, the trays were lined with a waterproof layer before matting, gravel and soil were placed on top. Then the turf itself was rolled out over the top.
On the move
Every second tray in each section is powered by 68 electric motors, with the ones on the centre rail doing most of the work, and the motors on the outside rails powering opposing wheels, running diagonally opposite. Together, they give each section a top speed of 7 m/min. Once the goals, advertising boards and pitch surround are removed, it takes two people just 25 minutes to roll the pitch away.
To switch from one event to the other takes four hours. As the pitch is rolled away, one operator walks in front of the moving section while another, the watcher, observes the pitch’s progress. Moving the pitch sections requires both operators to continually press down their safety buttons. An automatic slow-down system is triggered as the huge pitch sections trip sensors when the sections are 75 centimetres apart. This slows the movement down to a tenth of its top speed – 7 cm/sec – before triggering a second and third sensor that brings the whole structure to a halt. The motors themselves can be used to slow and brake the pitch while it is moving. Once it has stopped, a parking brake is applied to clamp the structure in place. The outer sections then move inwards to meet the centre of the pitch with the help of 32 hydraulic cylinders on the steel structure underneath.
Once the pitch is in place, hydraulic cylinders raise the sidelines before the flaps are lowered to rest on the edge of the turf. Ramps on each corner of the pitch also pivot upwards with the touchlines to give access from beneath the stands. Then the entire structure is locked into place. A third set of flaps hides a series of growlights and sprinklers that can be wheeled out over the pitch between games to give the grass some extra light and water. There are six giant trusses, each carrying 126 growlights, that span the entire width of the pitch and weigh just under 20 tons each, which can be rolled out on a rail hidden underneath the touchline. Hydraulic ramps on either side allow the height of these to be adjusted so that the amount of light the grass receives can be tuned.
Flexible but stable
With something of this size raised off solid ground, there was a danger that it could behave very differently from standard football pitches. Running on a large steel structure suspended above the ground will cause it to bounce, so the team faced the challenge of making sure the whole structure did not turn into a giant trampoline. The team fitted a series of accelerometers around a football field during a game to monitor vibrations and find out how a normal pitch behaved. Widening the main structural beams allowed the engineers to tune the pitch to the right frequency, so that it felt like solid ground.
While the structure needed to be solid, it also had to be flexible. Changes in temperature could cause the steel frames to expand and contract, meaning the pitch can grow or shrink by up to 45 millimetres in all directions. To cope with this constantly changing size, engineers designed the 598 wheels that support the pitch sections so that they do not slip off the rails as it moves. Each section of pitch runs on three rails, and the wheels on the central rail have a flange on both sides to keep the trays running in a straight line.
The front wheels of each pitch section were also fitted with steel scrapers and brushes to clear the rails of any dirt or debris that might get washed down onto the rails. While a waterproof liner seals the pitch trays to prevent any sand or soil spilling down onto the rails, bad weather can wash debris under the pitch, meaning this cleaning mechanism is essential. An integrated watering and drainage system in the pitch ensures that the grass can get the nutrients it needs to grow without making the ground beneath waterlogged. The systems can be quickly disconnected when the pitch needs to be moved. When the pitch is stored under the stands, LED lights provide the grass with just enough light to stay alive but not enough to cause it to grow. A ventilation system also pushes air over it to prevent it from becoming too hot, cold or damp.
Over the years, SCX has learned that an over-engineering strategy is the key to success in this sort of large moving project, because it is hard to know exactly what obstacles might need to be overcome when it is operating. It pays dividends to have extra capacity in hand.
SCX finished installing the pitch in October 2018 – just over six years since it first started working on the project – and the first matches are set to be played in 2019. This feat of engineering may mean that the stadium will become almost as famous for its world-first pitch, as it is for its football team.
This article was originally published in issue 77 of Ingenia, the Royal Academy of Engineering’s quarterly magazine (www.ingenia.org.uk).
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