There are several reasons for house foundation settlement to occur.  The biggest underlying problem in Texas is the expansion and contraction of clay soils.  Some soils shrink when they dry up and then expand when the rain comes.  I was working southwest of Houston for a few years and the cracks would open up big as anything.  I saw cracks that were 3/4" across at the surface.  One day I was curious and was able to stick my tape measure three feet into the ground before it I couldn't push it any further.

This can be a problem for the builder or remodeler.  What does it take to stabilize a foundation in unstable ground?  It depends on the soil that is there.  It takes an engineer to properly size the foundation.  Historically, I would guess that most people didn't worry about it.  Older houses were typically pier and beam foundations, not poured concrete.  The house would shift and cracks would appear.  Some times of the year some windows and doors would be easier to open and close than at other times.  The house was up off the ground with the plumbing suspended underneath.  So the plumbing would just move with the house.  Not a big deal, just what was.  Any foundation problems were relatively easy to fix because you could get under the house.

Then along came concrete slabs.  The plumbing was underneath or in the concrete and the house is often very close to the ground.  Now when it moves, there still may be problems with windows and doors, but there are other issues as well.  The pipes can break and cracks in the concrete can allow water to come in.  Termites can follow the cracks and get to the framing without being seen. 

I once owned a house that instead of rebar in the concrete had post-tensioned cables.  The clay soils would expand and contract.  At the rear of the house, the ground sloped away from the house.  The slab cracked and the rear of the house dropped about 5".  That was very noticeable walking across the living room.   Fortunately, all the plumbing stayed intact and did not break.

The solution was to add piers under the house to support the foundation.  My memory is about 22 piers and a cost of around $10-12,000.  Not cheap and not easy.  No one could live in the house during work because those piers were not only under the perimeter of the house, but they were in the middle of the slab as well.  I think there were three in the living room alone.  What a mess! 

The end result was to stabilize the slab.  Raising it to nearly level was not a good option.  If we had done that the plumbing pipes would have broken and sheetrock on the walls would have cracked and buckled.  That was more cost than I could bear.

The house I still have has a concrete slab.  The grade at the back of the house slopes up and away from the house.  The problem with that is that dirt and debris have been washed down against the house, raising the level of the dirt against the back of the house.  In correcting that I discovered that there is a rock ledge about a foot or two under the house.  Between that and a well constructed slab, it has held up very well and it was built in the '50's.

Knowing the soil where a foundation is put is critical for proper engineering.  Good materials and proper workmanship round out the necessary ingredients for a stable slab.  That's how to avoid house foundation settlement.