We arrange for the highest Helix to be at least 2.5m below ground where there is no soil movement, and other Helices will be deeper.
The comparatively slender pile shaft may damage a very small portion of the roots, but the Aeration to the soil round the roots may well actually improve tree viability.
An average pile will be around 4m minimum length, so if we assume that the top 1.5 m of shaft is in soil which moves (up or down) the skin friction will be cancelled by that acting on the lower 2.5m, so resulting in no net movement. Pile capacity does not depend on the skin friction on the pile shaft, it rests entirely on the helices.
Generally structures will be designed to be above the ground, so movement of the soil does not affect the building itself.
Screw piles have both Tension and Compression capacities (about 20-25% more in compression) so can resist any uplift.
Each pile is driven to a specified Torque, and hence known capacity, unlike all other foundations which can only be tested after installation.