The Forsyth Barr stadium - the world’s first permanently-roofed stadium with natural turf
"The Forsyth Barr stadium is the world's first permanently-roofed stadium with natural turf, where not a single drop of rain will ever fall on the surface"
Mention the expression 'all weather' to most sports administrators and players and they will automatically think of artificial turf.
However, those who followed the England team during the recent Rugby World Cup in New Zealand will have noticed that their first three pool games were played on a very different type of surface - tucked away near the bottom of the South Island is the world's first permanently-roofed stadium with natural turf, where not a single drop of rain will ever fall on the surface.
This impressive stadium is the 30,000 seat, Forsyth Barr Stadium in Dunedin, the most southerly professional stadium in the world. Built for a fixed cost of approximately £100m, the new stadium is very different from the other grounds around the country that received upgrades in the lead-up to the event.
Not only was the stadium a massive challenge from a political and budgeting perspective, but significant challenges were also raised by its design in terms of engineering and turf requirements. The stadium opened on 1 August 2011, barely a month before the beginning of the biggest sporting event ever held in New 
Zealand, and it took just over five years to complete the project from when the original concept was first put to paper.
This article provides an insight into how the stadium came to be and the many agronomic challenges that needed to be overcome.
Dr Richard Gibbs, Technical Director of Sports Surface Design & Management, explains how the stadium offers the best of both worlds!
Back in August 2006, a replacement stadium for Dunedin's ageing Carisbrook Stadium was still very much in its infancy. With the 2011 Rugby World Cup already confirmed for New Zealand, the Carisbrook Stadium Charitable Trust had just been formed and two possible scenarios were being investigated for a new stadium in Dunedin - redevelop the existing Carisbrook Stadium or construct a completely new one.
A new stadium was seen as the better option, because it could be combined with a proposed Otago University development. It could also be designed from scratch to accommodate a variety of non-sporting events, making the stadium truly multi-purpose.
A preliminary report, completed by the Trust in October 2006, concluded that a new multi-purpose stadium was a feasible option that warranted further investigation.
By February 2007, the Trust had produced a masterplan and feasibility report which had investigated four redevelopment options at Carisbrook and two options for a new stadium.
Dunedin City Council voted to proceed with the preferred multi-purpose stadium option in association with the university. The challenge was that the stadium brief required a design for a permanently-closed roof, under which natural turf was expected to grow, a feat that had not been achieved successfully before.
Nevertheless, the report had highlighted that the development of clear cladding and roofing materials, such as ethylene tetrafluoro ethylene (ETFE), represented a realistic option for constructing a fully enclosed stadium that could support natural turf.
Over the next two years, a formidable design team, assembled by project and delivery managers Arrow International, set to work on turning this concept into a real building using a feasibility driven process. The design team included joint venture architects Populous and Jasmax, structural engineers SKM and services/civil engineers Aurecon. Sports Surface Design & Management (SSDM), the consultancy division of Recreational Services Ltd, was engaged as turf consultants. Stakeholders for the project were Dunedin City Council, Otago Regional Council, The University of Otago and The NZ Community Trust.
Research and design phase - early research and trials work
One of the early turf feasibility reports listed so many challenges that it was clear turf trials work would be required using a specially constructed ETFE test rig. A proposed methodology was determined with input from Vector Foiltec (the leading manufacturer and supplier of ETFE) and submitted in June 2007 for consideration by Arrow International and the Trust.
Turf trials work commenced in July 2007 with the building of an ETFE test rig at Carisbrook's turf nursery (Image 1). SSDM was engaged to carry out the trials in conjunction with the then Carisbrook head groundsman Coryn Huddy. Two formal trials were conducted in the test rig, the first one ending in March 2008 (spring and summer trial) and the second one ending in September 2008 (autumn and winter trial).
From October 2008 until September 2009, testing continued in the rig, but at a more informal level. By this time the first two formal trials had validated the concept that turf could grow successfully under ETFE, albeit with some challenges, and the final informal trialing carried out by Coryn Huddy allowed various maintenance techniques that were likely to be required in the real stadium to be tested (eg. fertiliser management).
The trials also supplied two other important pieces of information for the design team. The first was the supply of data that was used to help model levels of 
photosynthetically active radiation (PAR) likely to be found in the real stadium (Image 2); the second was that the trials helped guide the pitch design process which started in March 2008.
Pitch design stages
The pitch design process was split into three discrete stages to mirror those carried out by the wider design team, who had started designing the stadium structure in July 2007.
Preliminary design - The preliminary design stage was a robust 'bridging' report that reviewed potential pitch designs, and which brought together the results of the ETFE test rig work, the light prediction modeling work, and budget and construction timing issues.
The initial pitch design stage also reviewed the availability of pitch construction materials in the Dunedin region, a comparison of pitch designs used by other stadia around the country, a review of artificial lighting and turf reinforcement systems, plus liaison with the key stakeholders to ascertain pitch performance objectives.
The outcome of the above analysis was a decision making process that addressed specific design challenges for the project from a turf perspective.
Three conclusions were particularly important:
- The pitch design should focus on growing turfgrass in the stadium as opposed to relying on any turf replacement for areas that might get worn out (it was also at this point that the Desso GrassMaster turf reinforcing system was first recommended)
- The pitch should be designed with a perimeter of artificial turf to eliminate the significantly worse turfgrass growing conditions that would be found around the edge of the pitch
- The pitch construction budget (which had been set well in advance of any turf work commencing) was realistic
Developed design - Preliminary design was completed in May 2008 and moved straight into developed design. This stage required much more coordination with the wider design team to select the most appropriate pitch design option commensurate with the available budget. This was clearly no ordinary 'off-the-shelf' pitch - its design did not need to follow those used for conventional outdoor stadium pitches - and some fairly important design questions had to be asked, including:
- What type of rootzone, irrigation, and drainage systems would be best suited to the permanently-roofed stadium design?
- How would the environmental conditions within the stadium bowl be monitored and linked to pitch maintenance?
- Would the proposed design stand up to international peer review?
By the end of the developed design stage in October 2008, design components for building the pitch including subgrade and surface shape, subgrade drainage design, rootzone selection and profile design, irrigation system design, turfgrass species selection and establishment were complete. Serious consideration was also given to establishing the surface using turf harvested from Carisbrook as a contingency in the event of construction delays, but a decision was made in late 2009 to establish the surface on site from seed.
There were two other important components of the developed design stage. The first was international peer review, both at a scientific level and at a practical turf management level.
This review included using the Sports Turf Research Institute in Bingley to benchmark the predicted light levels for the stadium against northern hemisphere stadia known to suffer from turfgrass decline during the winter period. The second important component of the developed design stage was a visit to the Etihad Stadium in Melbourne to gain a better appreciation of turf management challenges in a stadium with a retractable roof, which was then followed by a visit to the Eden Project in Cornwall, one of the most successful facilities growing plants under an ETFE structure.
Detailed design - The final section of the pitch design stage involved the preparation of specifications and schedules for tender, which were completed in February 2010. By this stage the stadium was about eight months into the building contract. The essential design components of the pitch were:
- a 300mm deep sand profile placed over a drained subgrade, with the top part of the profile being made up of a sand/soil/compost rootzone specially blended off-site
- a strategically located zeolite-amended sand rootzone layer
- sub-surface irrigation as well as a conventional pop-up irrigation system
- buried soil moisture sensors for optimum soil moisture management
- Desso GrassMaster turf reinforcement - the first stadium in Australasia to use such a system
- a 3m wide perimeter boundary of artificial turf that was to marry seamlessly with the natural turf (total pitch dimensions are 132m by 81m)
Construction facts and figures
The space age EFTE roof was originally developed by German chemical company Hoescht in the 1970s as a film for solar collectors, because of its unique properties and resistance to UV radiation and atmospheric pollution. The oldest ETFE structure in the world is the Mangrove Hall at Burger's Zoo in Arnhem, Netherlands, which was completed in 1982.
ETFE roof structures are made up of inflated 'cushions'. The material is extruded into thin films and supported in an aluminium frame, which itself is supported on the building frame. A typical ETFE cushion weighs 2-3.5 kg/m2 and the cushions are inflated to about 220 Pa to give them structural stability. The material is acoustically transparent, so net effect of using an ETFE roof is that it is similar in feel to an open air venue. 
ETFE has 1% of the weight of glass, is able to bear 400 times its own weight and can stretch 3-4 times its length before breaking. It was used at the 2008 Beijing Olympics National Aquatics Centre and has also been used for cladding the new grandstand at Eden Park in Auckland. A total of 20,500m2 of ETFE has been used at the Forsyth Barr Stadium, which includes side walls as well as the roof.
The roof itself is supported on five arch trusses, each with a 105m span (Image 3). A main truss, 130m in length and weighing 390 tonnes, supports one end of the five arch trusses along the southern grandstand, with the other end of the arch trusses connecting directly to the top of the north stand. The placement of the main truss that supports the roof along the southern grandstand took six hours to complete and used three of the biggest cranes in the country. The internal roof clearance is 37m from the surface of the pitch - out of reach of the boot of a rugby player!
Pitch construction sequence
Stadium construction commenced in June 2009 and took just over two years. Pitch construction commenced in September 2010, with work confined initially to main drain installation, as the playing area was still required for access to the roof. Once the first three roof trusses were in place, it was possible to start preparing the subgrade and installing drainage and irrigation lines (Image 4).
The pitch was built from west to east, the same direction as the roof arch truss installation. However, because of the layered design of the pitch and, in particular, the need for the sub-surface irrigation system to run the full length of the pitch, the profile could not be built in multiple sections. Instead, rootzone material had to be installed in layers, which initially caused some concerns from a timing perspective, as it meant the full pitch subgrade area had to be handed over for this purpose. I
n the end, it was possible to achieve this requirement by leaving only a small section of the subgrade at the eastern end of the pitch available for roof construction access (Image 5).
With the sowing deadline rapidly approaching, progress increased once the last roof truss was in place and the bulk of the pitch subgrade was available to the pitch contractor. Unencumbered by having to allow access to the roof, the rootzone layers could be spread with ease to the very tight tolerances specified. Almost overnight, a full size pitch began to take shape (Image 6).
Turf establishment and testing
The bulk of the pitch was sown with a blend of three perennial ryegrass cultivars on 24 January 2011, with the final 3m wide eastern section being sown nearly two months later. The turf cultivars used were selected from eight industry standards evaluated in the wear and recovery trials carried out in the ETFE test rig.
Turf establishment was rapid and a full and dense ground cover was achieved by 30 March 2011, the date of the first of two surface performance tests (Image 7). At that time, the pitch had not received any physical treatment and was understandably hard and very dense at depth. In order for the planned GrassMaster turf 
reinforcement installation to proceed, the pitch was verti-drained to 180mm depth, a few centimetres above the sub-surface irrigation pipe.
GrassMaster installation
Installation of GrassMaster turf reinforcement fibres commenced on 21 April 2011, and had to be completed by 9 May 2011 when all machinery was required to be off the pitch to allow the sports lights to be set up and tested.
Two GrassMaster machines had been shipped to New Zealand from South Africa where they had been involved in reinforcing pitches used for the 2010 Football World Cup. The machines worked for sixteen hours a day, in two shifts of eight hours, in order to meet the deadline. Results were dramatic, with the artificial fibres instantly providing confidence that the surface 
would take the punishing scrums that were about to take place on the surface (Image 8).
Ongoing maintenance
With the pitch completed and turf established, the task of implementing an on-going maintenance programme began. Using results gained from the ETFE test rig, a detailed maintenance specification was prepared by SSDM, in conjunction with Coryn Huddy, who had conducted an overseas stadium study tour in January 2009 in order to gather valuable maintenance experience.
The maintenance programme has focused on taking into account the low light levels in winter, and significantly reduced air movement and higher humidity levels in the stadium bowl that are likely to contribute to potential disease outbreaks. Actions, such as a preventative rather than a reactive programme to disease management, have been specified, at least initially until more experience with the management of the turfgrass surface is gained. 
Other aspects of ongoing maintenance include routine turfgrass clippings removal, prevention of moisture build-up at the surface by regularly removing dew, carefully timed watering programmes to prevent excessive periods of leaf wetness and specialist fertilisation management using both foliar and granular applications. Ongoing maintenance shall be reviewed as growing conditions become better understood.
A key component of the pitch construction specification was the design and supply of a customised environmental monitoring system that could measure various characteristics, such as temperature, humidity and photosynthetically active radiation. A portable wireless weather station, located in the centre of the pitch (aptly dubbed the lunar landing module), buried soil moisture sensors and external light capturing sensors are the basis of the system. This monitoring system will help establish how close the light modelling predictions have been, as well as help build an understanding of the actual environmental conditions that directly affect the growth and performance of the turfgrass surface (Image 9).
Handover
Handover occurred on time and on budget on 1 August 2011 following a very successful grow-in by Coryn Huddy. Two weeks earlier the pitch had received its second performance testing but, more importantly, a full-on training session by the All Blacks (Image 10), who gave nothing but praise for the new pitch.
Pitch usage commenced in early August, first with a local university college rugby game, followed quickly by a second tier provincial club rugby game. However, it is somewhat ironic that the first major rugby game scheduled for 17 August (ITM Cup) had to be postponed until 30 August because of heavy snowfall conditions - but it wasn't the stadium that was out of action because of the snow - it was the airport, and teams couldn't get to the venue!
It was, therefore, not until 20 August that the stadium's ability to cope with a large crowd was first tested, this being the 15,000 people who attended Dunedin's biggest professional football match since the U17 World Cup in 1999.
And the rest, as they say, is history. The stadium hosted four fabulous pool games for the 2011 Rugby World Cup and has just held a very successful community concert attended by 20,000 people. At the time of writing, the stadium is again going to be fully tested with a sold-out Elton John concert in late November.
Conclusions
This has been a visionary project. Throughout the entire planning stages of the stadium, the Carisbrook Stadium Trust and the Project Delivery Team, led by Arrow International, ensured that the turf remained at the forefront of the design in this 'whole stadium' solution. 
The stadium was built around the turf rather than the other way round. This was a project where the design process was driven by feasibility from day one, and it was a rigorous and challenging process with the words "no more time, no more money, find another solution" being a common phrase to the design team.
Not surprisingly, in terms of turf consultancy, this project has been one of epic proportions. The huge time commitment, requirement to meet non-negotiable deadlines with clear decisions and recommendations, and overall technical challenges that may not be seen again in a stadium in this country for some time (although rumour has it that the earthquake-damaged AMI Stadium in Christchurch may be a roofed stadium rebuild).
It has been a most rewarding experience to have worked with such a dedicated team of project and delivery managers, consultants, contractors and individuals. As with any brand new stadium, there will be challenging financial targets to meet, which will need to be carefully balanced against events planned on the pitch.
Hopefully, managers of this new stadium will tread very carefully as experience is slowly gained with its unique management. And will we see more of these designs around the world, with a permanent roof to protect spectators and players from harsh winter elements? I suspect we will.
Acknowledgements: Dr Richard Gibbs is the technical director of Auckland-based Sports Surface Design & Management, the consultancy division of Recreational Services Ltd (www.ssdm.co.nz). The author wishes to thank Lale Ieremia and Mike McCleery of Arrow International Ltd. and Coryn Huddy of Dunedin Venues Ltd. for their assistance in the preparation of this article.