Tom,

There are places where the ground swells and shrinks over the year depending on moisture content and/or freezing.

Freezing ground with lots of moisture in it causes swelling.  I think it’s called frost swell – or something like that.

Another ground condition that causes seasonal rising and falling is a high clay content.  Apparently clay swells when it’s wet and shrinks when it dries.

Huston, TX is famous for seasonal ground rising and falling due to moisture content.  I understand the ground in Huston can rise and then fall at least a FOOT depending on moisture content.  And this goes for buildings, parking lots, and single-dwelling houses.  And weight means nothing.  If the ground wants to swell and rise – 100,000 lbs. of concrete is nothing.

And here in Southern California we have earth quakes – which moves the ground up, down, and even sideways.  After a really big quake, some of the guys in my club with permanent piers have to re-adjust P.A.

So there are several possible causes of PA changing.

Woody