# The Impact of Incorporated Nonwovens on the Surface Roughness of Sport Pitches
## 1 Introduction  
There are 35,993 sport pitches for soccer in Germany [1]. Sport pitches are intensively used and highly maintained. Normally, they are constructed by a layer-by-layer structure, according to the German standard DIN 18035-4 [2]. This structure consists of three layers as described from the top: rootzone, drainage and subgrade. The lowest layer is the subgrade, which consists of the existing subsoil. A drainage layer is installed when the subgrade is water impermeable. The turf grows on the root zone layer. The described construction method is comparable to that of the United States Golf Association (USGA), which is used on golf courses in the United States of America. The USGA drainage layer is made of gravel with a particle size distribution of 65% between 6.4 and 9.5 mm. The high water permeability is necessary to drain excess water from sports surfaces. At the same time, supplying turf grasses with sufficient water is a major challenge for the planning and construction of sports pitches [3]. Therefore, artificial irrigation is necessary. To reduce the irrigation costs of a sport pitch built according to DIN 18035-4 [2], an improvement of the standard construction method was developed [4]. A vertically incorporated biodegradable nonwoven should raise water from the drainage layer to the root zone. To vertically incorporate the nonwoven in existing sport pitches, a device was developed that precuts the turf vertically, opens a slit, places the nonwoven vertically into the slit, closes it up and finally compacts the root area again [5]. This device literally causes a change in the surface due to the high interference with the turf. A low roughness of the surface is important for the playability of the sport pitch. If the surface profile is not level, the players can be injured and the desired moving height cannot be fulfilled. In addition, an increased roughness is associated with a decreasing ball rollinglength. Roughness is defined by the deviation from the ideal surface of the object. On sport pitches, the roughness is the deviation of the profile from an ideally leveled sport surface. However, reviewing literature showed no significant research on the surface roughness for sport pitches with natural turf. This does not apply to artificial turf. Here, the roughness of the surface is of high importance for the playability and the injury rate in sports [6; 7]. Much research exists on the measurement of road or field profiles. Description and classification of road surface profiles are standardized by ISO 8608 [8]. According to this standard, the fast Fourier analysis is used to calculate the power spectral density. This power spectral density is used to classify the roughness of a road in eight classes, but the standard does not specify an explicit measurement method [9]. The power spectral density evaluation is strongly dependent on the measurement method and the measurement frequency [10]. The international roughness index was established by the World Bank and is calculated by using a measured profile. It is smoothed by a quarter-car model of a so-called golden car with a speed of 80 km h\({}^{-1}\)[11]. Both indexes are used to assess the riding quality on road pavements. The presented study deals with the roughness of sport pitches that can be better compared to agricultural areas rather than to roads. The power spectral density according to ISO 8606 is even used to describe country roads or grass fields to evaluate the fatigue life of agricultural machinery [12]. However, according to the German standard DIN 18035-4, the evenness of a sport pitch is measured with a 4 m long leveling bar and a measuring wedge with a measurement resolution of 1 mm [2]. The unevenness of the sports pitch surface must be less than 20 mm over a length of 4 m [2]. This method is normally used for the acceptance of the work performed on a sport pitch, but not in research. In laboratory studies, the roughness of soil surfaces is often measured with a pin meter or a roller chain, when using contact devices [13; 14].