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The reasom it does not brek the kite string is as follows.
Take a breaking strain of 20kg. The string is say 1mm thick and is tied arround a 100mm branch.
this means the string is 314.2mm long and (ignoring the round profile)it has a surface area of 314.2mm.
This is .0003142 of a sqared metre. 1 pascal is the force exerted by 1 kilogram over 1 square metre. we
now have the root applying 105,000 pascals over .0003142 squre metres or 32.991kg of force. The string will
cut in until it deteriorates or is engulfed.
The situation is now different with the path. A 2 metre root 50mm wide exerts force over .1 square metres.
This is 10,500kg (about 10ton) of force. Some may be absorbed by the soil but that is enough to lift the path!!!
You are right however about some movement from the expansion and contraction or slumping of the soil.
For those who wonder. Assuming the same root above under a strip footing 500mm wide it will exert a force capable of lifting
2.5 tonne. As a building tends to act as a ridgid member the root does NO damage.
Mark