Just hours before messrs Harry Kane, Dele Alli and Hugo Lloris graced the turf at the Tottenham Hotspur Stadium for the first time, New Civil Engineer went behind the scenes for an in depth look at the UK’s first retractable football pitch.
The £1bn stadium in north London is the first in the UK to have a moving grass pitch, which can be retracted into a purpose-built garage located under the stadium carpark.
Almost 2m beneath the grass pitch is an articial playing surface, scheduled to be used for American football matches.
The unique stadium design requested by Tottenham Hotspur Football Club presented mechanical and electrical engineering firm SCX with a set of unique challenges.
Tottenham Hotspur head of moving structures Nick Cooper – who previously worked on the construction of the Channel Tunnel – explained that the first challenge was the pitch surrounds, which have to move up and down to remain in line with whichever pitch is being used.
“The first thing that has to happen [to move the pitch] is the surrounds and the approach ramps go down to allow the pitch to move over the top,” Cooper said. The surrounds must rise or fall by 1.4m and to minimise the chances of failure, it was decided that surrounds for each side of the pitch would move as one single piece.
“The surrounds house all the advertising, broadcast equipment and electronic systems we need to run a football match,” explained Cooper. ”Rather than have multiple sections that move, the solution was to have a complete 120m long truss with all the ramps and everything moving at once, so we can keep all our problem areas in one place.”
Once the surrounds have been lowed, the grass pitch can be moved. However, the south stand of the stadium, under which the pitch must move, has been designed and built as a single tier with the seating tier supported by two huge steel ‘trees’ with significant piled foundations. These obstacles mean the pitch cannot be moved out in one single piece.
Source: Tottenham Hotspur
Instead, the pitch splits into three 30m by 100m plates, each weighing 3,000t. This allows the pitch to move past the structural steel at the southern end of the stadium, with each plate running on 300 wheels and having to move in two directions to allow it to reach the garage.
Source: Tottenham Hotspur & SCX
The process to move the 9,000t of grass and steel takes anywhere from 30 minutes to an hour, but once in the garage, the live grass can remain there for up to five days due to a full “life support system” inside featuring 500W of LED growing lights, air ventilation and dehumidification systems.
The various irrigation and groundkeeping systems provided another level of challenge for SCX engineers, Cooper said.
“Where we have the pitch joints, they are 100m long, and because we have under-pitch heating, we have hot and cold water and an oxygen blowing system, you get temperature differentials between the sheet plates, if one side gets five degrees hotter than the other, the plate will bow and when the pitch comes together [there will be a gap].”
The joints themselves are formed of two rubber lips that overlap, the grass itself is ripped and pulled apart when the plates split to leave a rough edge, so that when the plates are reunited, the grass is better able to grow back together.
The whole system has been engineered to give it as long a life as possible. Bearings in the wheels, which need to support 25t each, are rated up to 100t.
This helped overall construction of the stadium, with 90t cranes positioned on the pitch to help with other aspects of construction.
Cooper was keen to see that all components of the moving pitch came from the UK, with every bit of steel used in the moving pitch made using UK suppliers in Yorkshire and Sheffield, and concrete being sourced from Northern Ireland.
SCX projects director Danny Pickard said the pitch had originally been a big concern, but quickly proved completely sound.
“The Tottenham pitch went from being perceived as the highest risk part of the project – with all the moving parts, the turf joins and an engineering concept that had not been tried before – to something neither we nor the client had to worry about,” he added.
The overall stadium project was delivered by Mace.
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