Another factor to consider when running randomly on soils is a condition in which one considers that little or no damage will occur, and particularly to the subsoil.
Typical at harvest on clay soils is extensive cracking due to dry conditions, as exampled in the photo.
Often the cracking tends to divide the soil up into big blocks with cracks extending to perhaps 0.5 m deep. Imagine a harvester with a wheel load of around 12 Mg running over one of these blocks which could be completely isolated from its neighbours.
Although dry at the surface, moisture in the subsoil could still be quite high and effectively the load is transmitted directly from the surface through the isolated column down to the subsoil. Out of sight and often completely out of mind, this has the potential to create significant subsoil compaction, which may be at a depth which is uneconomic to repair.
The moisture seeking roots of wheat for example, will often penetrate to 1.5 m but compaction above this may limit rooting or indeed prevent water from penetrating to this level.
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