At an engineering team that I once owned, we often had peer review discussions on the need for underpinning and, if established, where to stop and start. 

We built a system of checks and balances to ensure that multiple points of view were utilized. I have written a series of blogs on common human biases… which you can find here: Forensic Engineering. And one of the ways to minimize human biases is to get multiple opinions on a case. Over the years, we hashed out the following guidelines. 

Before we can develop recommendations and specifications, we need to ensure that we understand the mechanisms of failure, assess the likelihood of future movement, and take into account the goals and risk tolerances of the customer. You can read more in my blogs on this subject, which cover 16 areas of information to be investigated.

Identifying When and Where Underpinning Is Needed

One of the first things we look at is whether the foundation movement exceeds the allowable limits of the FPA (Foundation Performance Association), PTI (Post-Tensioning Institute), or the newly developed radius of curvature method. See more here.

After all 16 factors in diagnoses are analyzed and an opinion of settlement beyond the allowable limits has been established, the next step is to determine where underpinning is needed and what the starting and stopping pile location points are on the structure.

Recognizing Hinge Points and Grade Breaks

One of the first things to do is 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 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 to look at is the spacing of the topographical lines. Usually, tight spacing (i.e., steeper slope) indicates movement. As the topo lines begin to space out… that indicates a grade break—aka hinge point. Further spaced topo lines (flatter), especially in the low areas, usually point to less movement since settlement around a large perimeter (other than initial) is rarely uniform.

In the example below, the topo lines start to be more spaced out near the red line, indicating that the area is the grade break location. Accordingly, piles should start and stop along this line. Notice that it coincides with the stress damages.

Understanding Where Settlement Begins and Heave Ends

Sometimes, it’s not as simple as shown above. Our experience in a drier climate with more heave potential led us to look more closely and try to understand where the topo lines from the heaving end and the topo lines for settlement begin.

For a more in-depth discussion on upward movement from expansive soil heave, you can check my blogs about soil myths and mechanisms of settlement and heave.

Topo line patterns help us distinguish between heave and settlement when taken in context with all the other 16 areas of data accumulation. See my blog here to learn how to use topo lines to see the difference between heave and settlement.

Now, look at the illustration below and see my notes and comments embedded in it.

Look at the gray areas. As my box above says…. Are the topo lines in these areas part of the settlement or part of a heave pattern? Well, it’s a good question. Look at how uniform all the corners are in elevation compared to each other. Three of them are similar, with the northwest corner being substantially lower than the other three. 

It’s not normal to expect settlement to be uniform around the perimeter. So, by Occam’s Razor, the simplest answer is that these areas are the original grade, and therefore heave dominates the gray areas in this particular case. Of course, in real life, topo lines are not the only thing to be considered. Damages, soils, precipitation, drainage, exterior foliage, and many other of the 16 factors must be considered. 

The bottom line is that it takes considerable skill to use all the tools to distinguish where to stop piles placed for settlement. If piles are stopped too soon, then there could be a risk of having a larger area of future settlement overwhelm the end pile, as pointed out further down in this post in scenario 1 and scenario 2.

Addressing Severe Tilt with Little Deflection Foundation Repairs

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.

See the example below. In this particular case, almost all of the data pointed conclusively to settlement only. Accordingly, even though the topo lines are further apart in the low areas, we concluded that since it is at the bottom of a continuous tilt, the entire north end moved downward, requiring piles over the entire length of the home. This is another instance where repair plan logic ensures we make precise recommendations based on data.

In summary, deflections require piles up to the grade breaks, and failures in tilt require piles over the entire length of the tilted area.

Budget-Driven Repairs vs. Engineering-Driven Solutions

It turns out that contractors lately have been offering various solutions to homeowners in order to “right-size” the project to fit their budget. At first, this may sound quite appealing… listening to the homeowner and fitting the project to what they can afford. Great!

Let’s say we have an area that is settling, and we put in piles to support it. Great. 

Now, what if we put in piles to only support half of that area? The piles are engineered to support the structure above them, but typical single helical or push piles will not support the whole house… or even half of it… or maybe only a quarter of it… or less. How much will it support?

See the diagram below:

This is where engineering skills become important. If a large area adjacent to a pile settles, could it pull the pile down next to it? Perhaps. How big is the area, and how much is it likely to move?

In very simplistic terms, let me try to put this into basic engineering concepts. Pile 3 has 1½ arrows of weight to support. Pile 2 has two arrows (one in the middle and ½ on each side that it is sharing with the piles on either side).

Look at Pile 1. If we believe that all of the areas are going down (based on geotechnical engineering analysis), then look how many arrows Pile 1 needs to support… 9½ arrows! Way too many! If the house continues to settle, Pile 1 will fail.

Also, we need to understand the area of the roof loads. Corners only have half the load of a pile in the middle of the wall, which is again half the load of a weight-bearing column in the middle of the house, as they share the roof loads with other supports.

See the diagram below.

The colored areas represent the percentage of area passing down onto the piles as they share the roof load with other piles or walls. Notice how the corner pile has the smallest area, and hence, the smallest weight bearing down on it. Of course, the gable ends need to be factored in, as the trusses bear most of the weight on the non-gable ends.

When Is It Okay to Only Pier A Small Area

Now look at the next diagram:

In this diagram, the topo mapping shows that most of the left portion of the house is still mostly flat, showing little downward movement, so future downward movement is not likely, especially if the red area is heave. 

In this case, Pile 1 still only has two total areas of weight arrows pushing it down, so all of the piles are likely to perform well in the future. Also, in this depiction, this is the gable end, so it has less weight.

How do engineers know how to tell the difference between Scenario 1 and Scenario 2? It’s done by carefully looking at the topographical mapping, identifying hinge points or grade breaks, and synthesizing this with all the other 16 points of data. 

It’s important to understand that, often, the floor slab where we are taking the readings is not physically connected to the footings. This means that it is possible (though not likely) for the slab to move separately from the footing.

The Role of Engineering Judgment in Repair Plans

Not only is it important to understand where likely settlement of the footing starts and stops, but also to know the difference between soil heave and settlement. Floor slab topography can also display upward movement from heave that has nothing to do with footing settlement. 

It’s not nearly as simple as many foundation repair salesmen think it is… hence the need for complete, detailed information and engineering judgment to interpret it.

Engineers must understand not only soil mechanics and how the soil at each specific location is affecting the structure but also how that structure is reacting to those forces by gathering the 16 areas of data, analyzing them, and weighing them to reach conclusions.

To have sales personnel produce various solutions that are arrived at with anything other than engineering processes is likely to produce results that are questionable at best. 

So, as it turns out, the Cadillac, Chevy, or Hugo solutions may not be in the homeowner’s best interest unless the decision is grounded in data-driven analysis and professional engineering judgment.