Transformation of an ash pitch into an artificial turf playing field – is everything just routine?
by Dipl.-Ing. Bernd Schnabel, Landscape Architect AKH BDLA
Planning Office S. Lukowski + Partner
Following a magistrate’s decision, FC Kalbach was to acquire an artificial turf playing field by means of reconstruction of the existing ash ground. A requirement for the planning office S. Lukowski + Partner from Darmstadt, commissioned by the Parks and Gardens Department of the City of Frankfurt, was the creation of a playing field adhering to the usual dimensions, for the city of Frankfurt, of 90 x 60 metres, plus they also had to conform to standards relating to unobstructed areas in sports field construction.
In the middle of April 2010, Strabag Sportstättenbau GmbH, which was commissioned with the construction work, set up the building site. In the course of the construction work, the whole infrastructure of the playing field was to be modernised and brought in line with the latest technology. Because of the required adjustments to the playing field in the alignment and the elevation, the ball catch fences and the masts for the training lightning also had to be renewed, among other things. A further request to be considered was the shielding from the surrounding mass rabbit population.
The reconstructed playing ground is wedged in between the functional building and the access road to the stadium in the south, the adjacent agricultural area and a continuous power line in the north as well as a tree population worthy of protection in the west. In addition, there is a radio mast for a mobile station on the northeast corner, whose maintenance mode had to be kept. In coordination discussions with the mobile network operator, it was agreed that the artificial turf coating should be brought right up to the base of the mast.
Stabilisation of the foundation
Despite the existing base of the ash playing field made from lava gravel, there was insufficient stability on the surface. Experimental tests and the inspection with plate-loading tests showed that additional stabilisation of the whole foundation was inevitable. The in-situ ground and also the stability turned out to be very heterogeneous. A soil exchange in sub-areas would have meant too much uncertainty, connected to long delays and incalculable costs. Hence, the existing lava gravel was pushed into the adjacent areas and used as a base in the path areas. The remaining quantity, following stabilisation, was brought again to the sports ground as a sub-base. A chalk-cement mix was worked into the foundation with a high-speed milling cutter and a so-called mortar-mix was produced. The additional advantage of this was that the components were largely independent of the atmospheric conditions, as the ground also remained accessible after rainfall.
The playing field is situated lower than the adjacent acres and right next to the functional building. Thus, the surface discharge and infiltration of surface water, for instance in the hollows of the turf, was not possible. Previously, in instances of heavy rain, it led to water-logging on the surface of the playing field. In order to absorb the accumulated rainwater, a trough was provided around the whole playing area which simultaneously absorbs the water of the adjacent paved path areas. The foundation in the area of the stadium is almost watertight, and an infiltration over shafts or infiltration ditches is therefore not possible. The existing connection to the drainage system was maintained. The installation of a drainage system to drain out water from the playing field structure was necessary because of the foundation. The old drainage system was no longer fully functional because of the change in altitude and the slurry seal coating due to the back-water caused by heavy rain.
The existing ash coating was arranged in accordance with environmental research because of the stresses and strains with arsenic and sulphate in the category > Z2 in accordance with LAGA and had to be disposed of appropriately when it was removed.
As a result of the soil properties (fine coarse silt or clay), the material couldn’t be used for re-instalment. Hence, the trenching and base excavation material was completely discarded.
The existing concrete flooring of the track within the stadium between the new artificial turf grounds and the turf playing field was shabby and had to be partially forced open because of the changes to the drainage system. Renovation of the whole path area was already considered in the planning stage. The artificial turf playing field was framed with a cobbled path which was adapted, in breadth, to the available space. The cobbled areas serve both as maintenance and cleanliness stripes and as path areas or as floor space for spectators.
The path and subsequent adjacent areas had to be aligned height-wise to the new height of the playing field whereby in a playing field corner the existing cantilever wall had to be renewed.
In order to manufacture a more rabbit-secure enclosure of the playing field, all seams and gaps are limited to a maximum of 3 cm. The grid fences of the barriers and the bottom mats of the ball catch fences were thus designed with a 25 mm distance between posts. The barriers had to be fastened to all over the fences. Passages and connections are to be carried out meticulously and also the required downward slopes for the drainage must be considered in order to limit the ground clearance of the distance between posts.
If the many aforementioned special points are not sufficiently considered in the planning and execution of the construction work, problems in adhering to the budget and construction time quickly arise.
The playing field was supplied with a powder-filled artificial turf coating which was applied on an elastic base, manufactured onsite. Quartz sand and green TPE-granules were used as the filling materials. The unattached, new ballast substructure was manufactured in two layers. For the upper layer, gravel was used with the 0/11 granulation in order to be able to fulfil the high demands on the planarity.
The additional unobstructed spaces required for the safety zones are located, because of the very cramped conditions, partially outside the playing field coating on the adjacent cobbled areas.
Lighting and electricity
In the course of the construction work, the lighting system of the sports field, which was no longer the best available technology, was completely renewed.
The system includes vertical floodlights with asymmetrical reflectors, which focus the light on the sports ground and minimise the amount of scattered light. Thus, not only is the glare of the surroundings reduced, but, to the best possible extent, the nuisance factor of insects too. In addition, the flood lights were equipped with special discs which filter out UV rays and further reduce insects being attracted to them.
An electrically controlled countersink sprinkler system creates the possibility of being able to reduce the surface temperature of the playing field through spray. Furthermore, the play properties of the artificial turf with the rubber-granule-filled system are also improved by the humidification.
A sprinkler system which also feeds the adjacent turf playing field was available, but was quite old. Therefore, the modernisation of a booster station and changing the system from a hydraulic to an electric control system was necessary. The existing water supply was no longer standard and was equipped with a feed tank with unrestricted water intake. A key-operated switch on the outer wall of the garage, in which the sprinkler system is located, simplifies the system’s operation. The existing sprinkling cables couldn’t be obtained because of the comprehensive excavation for foundations, etc., and therefore had to be completely renewed.
The large turf playing field situated to the east - which had to remain usable during the building project - presented a restriction for the execution of the project. As the work was carried out in the summer months, the sprinkler had to remain operational, despite the reconstruction work. This problem was solved using a transitional, temporary, above-ground additional cable. The actual modernisation of the booster station was arranged in precisely such a way in the construction sequence that the system was completely out of operation for just one day.
Conclusion
The project clearly shows the necessity to carry out adequate and extensive pre-examinations and detailed planning, even with the seemingly uncomplicated conversion of an existing ash pitch into an artificial playing field, otherwise costs will quickly rise above the available budget, the construction time can take considerably longer or serious defects can quickly cloud the joy on the new surface.
As the builder, planners and the companies carrying out the work worked collaterally and responded immediately to all difficulties and any problems which cropped up, such as the discovery of old asbestos-containing pipes, the task could be completed within the planned construction time from the middle of April 2010 to the beginning of August 2010 and to the satisfaction of all involved.
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