Piers Go Down to Bedrock Because the Clay Above It Keeps Moving
The clay under homes here swells when it is wet and shrinks when it dries out. A footing resting on that clay rises and drops with it, which is how a corner settles.
Bedrock does not move with the weather. Once the footing rests on piers that reach it, the clay can keep swelling and shrinking without taking that corner of the house along.
