When a
vehicle gets in contact with the forest floor, its weight bears on the ground. One
part bears on the solid phase like stones, sand, clay, and roots. But the
forces are transmitted to the soil pores, too, which can be filled with air or water.
In case of water, this liquid cannot be compressed and transfers the load in
all directions.
Directly on
the surface two additional effects occur:
• The cohesion describes the binding
forces of a body, for example when the wheel is caught by a thorn vine.
• And there is a certain adhesion that
depends on the electromagnetical coherence between two units, here between
wheel and soil surface (but this force is very week).
All these
forces together form the resultant force.
The
resultant force can be expressed by two components:
• the normal force, which works
perpendicular to the contact surface and
• the shear force, which works rectangular
to it parallel to the surface.
Together with
the reaction of the ground, they form a power triangle: When the triangle is
closed, the soil is stable enough to keep the wheel. But when the potential of
the soil is lower, then the triangle is not closed and the soil will be
compacted.
Since this
compaction will make the soil stronger, the reaction force increases. When the
reaction force is equal to the resultant force, the compaction stops. But
a rut remains; we call it plastic deformation.
