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tracks that have a clay sub-soil.  In poor weather they easily become soft and waterlogged so that even a few vehicles can cause rutting and damage. The remedy is to completely replace the surface with a more suitable one, a local authority responsibility.  Much damage is caused/amplified by water, and the failure of local authorities to maintain drainage.  In lowland areas poor drainage allows routes to become waterlogged and always being wet, vehicles have more impact than would normally be the case.  Irresponsible vehicle users who look for the biggest holes cause massive damage by spinning wheels and digging out vehicles that have become stuck.  Similar damage is often caused by the attachment of vehicular winches to trees to pull vehicles out of large holes. 
In upland areas water running down tracks accelerates erosion especially if the covering layer of vegetation has been removed through the over use of vehicles.  Examples of damage to tracks.

              Poorly drained clay soil susceptible to rutting and waterlogging.  (Oxfordshire)

Where poor drainage is combined with shade, rutted and waterlogged conditions may remain throughout the summer.  (Surrey, summer 1996)

(DETR, 1998, p7)

Soft peaty soil susceptible to rutting and waterlogging.  (Lake District National Park)

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