As an Engineering Team, we often have peer review discussions on the needs for underpinning, and if established, where to stop and start. We have built a system of checks and balances to insure that multiple points of view utilized. I have written a series of blogs on common human biases. One of the ways to minimize human biases is to get multiple opinions on a case; we call this process peer review.
We, as a group, have had many discussions on underpinning requirements. As I point out in my earlier blogs on The Rules of Thumb of Diagnosing Foundation Performance; every rule seems to have exceptions.
One of the first things we look at is does the foundation exceed the allowable limits of the FPA (Foundation Performance Association) and PTI (Post Tensioning Institute) … see link for a discussion of tilt and deflection (https://www.foundationaz.com/blog/deflection-vs-tilt-whats-the-difference) . This is a good place to start. While there is some uncertainty of the original elevations. We can look at the signs of stress and make some inferences on post construction movement.
After all 16 factors in diagnoses are analyzed and an opinion of settlement beyond the allowable limits has been established, the next step it to determine where underpinning is needed, and what the starting and stopping points are.
Following our rule of thumb that the best predictor of future movement is documentation of past movement, we try to recommend underpinning where significant movement has already occurred.
One of the first things to do is to determine the hinge points of movement. This can be done by noting signs of stress. This is not always the exact same place as the actual deflection as the structures, particularly frame structures, have a tendency to be somewhat elastic with some tensile capacity that shifts the point of stress further away from the hinge point.
One tool is to look at is the spacing of the topographical lines. Usually tight spacing (ie steeper slope) indicates movement. The topo lines spacing out indicates a grade break, also known as a hinge point.
Sometimes, we see evenly spaced topo lines from one end of the house to the other. In other words, no grade breaks and no deflection - just tilt - like a big raft. In this case, the entire house transverse to the topo lines needs to be underpinned.
Fully understanding the foundation problem will allow us to recommend the appropriate solution(s).