## 1 Introduction
### The Role of Natural Airfield pavements  
Air transport has undergone a huge transformation at the turn of the last three decades. The increasing take-off mass of aircraft, their construction, and continuous development resulted in the transition from airports with natural/turf to artificial surfaces. Nevertheless, this specific type of surface is still present at civil or military airports and landing zones which play a very important role in air operations. Natural airfield surfaces are responsible for ensuring safety in relation to the rejected take-off or delayed landing maneuver and possible taxing out off the runway. However, until recently, the requirements and methodology for assessing the load bearing capacity of natural airfield pavements were not regulated, which meant that they were partially neglected, and this directly affects the safety of air operations. Accidents, air incidents related to the above-mentioned situations are registered in the international base of air accidents and incidents, sometimes with very tragic consequences, and an emergency runway exit particularly dangerous in the event of inadequate load bearing capacity of the natural pavement within the runway. According to [34], the natural airfield pavement is the airfield pavement created by proper surface preparation in order to ensure safe operation of military and civil aircraft. Currently, there are two types of natural surfaces, i.e., soil and turf surfaces [35]. Soil surfaces are made of properly prepared soil--they do not have a turf (grass) layer. On the other hand, the turf surface is a soil surface with a layer of properly selected and developed grass vegetation [36]. Depending on the type of airport and its function, natural airfield surfaces fulfil specific tasks. Currently, the natural airfield surfaces are airport functional elements, such as (Figure 2):  
* emergency runway shoulders (shoulders), which protect the aircraft against damage in the event of an emergency runway exit. In case of a possible aircraft exit from the runway, natural surfaces should be prepared or constructed in order to destroy neither aircraft nor underground infrastructure, and guarantee quick restoration of the airport's operational capacity by efficient removal of the aircraft by airport services [28; 29];
* runway end safety area (RESA), which guarantee that the maneuver of aborted take-off or delayed landing and possible taxing out of an aircraft from the runway can be carried out without damaging it [28; 29];
* unpaved runways at smaller aerodromes or an emergency runway (AFE at military airfields) that are part of the runway intended for the take-off approach and take-off run, and for a touchdown and landing runway. International requirements for the load bearing capacity of natural airfield pavements are presented in Annex 14 to the ICAO Convention on International Civil Aviation and in Regulation (EU) No. 139/2014 of 12 February 2014 that consists of the requirements and administrative procedures for airports in accordance with Regulation (EC) No 216/2008 of the European Parliament and of the Council [37]. On the other hand, the method of assessing the load bearing capacity of natural airfield pavements and the minimum requirements for ensuring the safety of air operations by aircraft have been regulated by the provisions of the defence standard NO-17-A503: 2017 Airfield pavements. Natural airfield pavements. Load capacity testing [34]. Currently, the above-mentioned standard is used by military and civil airport services.