What kind of dirt...

[trilliwilli]

Just top soil is the best way to go. Sand is indeed very abrasive. Look at the videos on YT for The Stonebraeker Area, the indoor parcour in Germany or the channel from Nordbau AG. Lots of videos with digging in top soil and also the equipment of the scale Telestack conveyor belts.

Dry or just slightly moist works best, wet works with none of the materials in scale, you just get to big lumps to work with on scale.