The grass on athletic grounds, particularly in large stadiums, is not only an important concern for sports but also one of the largest cost drivers. Therefore, the monitoring and care of the grounds takes on special significance. PlantCare has installed a soil moisture monitoring system as part of the redesign of the entire playing field at St. Jakob-Park stadium in Basel. The system measures soil moisture and temperature at various positions and transmits the measured data by radio to the technical room located behind the grandstands.
St. Jakob-Park stadium, home ground of FC Basel, was opened in 2001. It has received the UEFA 4-star award and cost about CHF 250 million. With more than 38,000 seats, it is the largest stadium in Switzerland.
Varying degrees of shading over the pitch area has, over the years, given rise to recurrent irrigation problems. Since parts of the pitch were too dry while others were too wet, the operator decided on a complete renovation of the entire playing surface. At the same time, a new irrigation system was installed.
To obtain a better overview of soil moisture and to maintain its gradient differences, PlantCare was contracted to install a monitoring system that measures soil moisture and temperature at given depths at the centre spot and the four corners. As a result, the greenkeeper always has an up-to-the moment overview of the moisture conditions in the playing area.
The contracted work involved some very specific problems. Sensors and electronics had to be installed so that nothing protruded above the surface. This meant that the electronics had to be buried, rendering wireless connection impossible, as wet soil is a very strong absorbent of radio waves. Cables were therefore needed to connect the sensor points to the electronics, while the sensor electronics had to be installed at one location - the cabinet to which television cameras are connected. This requirement made it necessary to have cable lengths of up to 250 metres. Since the values measured by the PlantCare sensors do not, fortunately, depend on cable length, the requirement could be easily met.
In total, over 400 metres of cable had to be installed.
Another challenge was the fact that the sensor electronics of all five sensors had to be housed in a metal electrical cabinet in order to protect against possible vandalism. The radio signal must therefore be transmitted from the cabinet through the existing reinforced concrete grandstand to the technical room behind the structure.
This problem was also successfully solved: the electronics for the five sensors was mounted on the right inner casing wall of the electrical cabinet. A repeater was installed on the opposing wall to receive the signal from the sensors and to retransmit it at higher power. For the sake of further external transmission, an external transmitter was connected to transmit the radio signals to the technical room.
In the technical room, there is a second repeater, which receives the signals and transmits them at another radio frequency to the controller with the display screen.
A special problem was also caused by the special substrate used as turf base. This substrate must naturally have high water permeability; otherwise puddles would immediately form during rain. The high water permeability means reduced field capacity of the soil and therefore requires that the felt used in our sensors must also have a correspondingly low field capacity. The most suitable felt for this demanding application was selected by means of laboratory tests. Measurements indicated retention field capacity of approximately 65% (100% = full saturation).