Which Piers Don’t You Need?
By Christopher E. Carter, P.E.
A few years ago, at a previous place of employment, I saw one of my co-workers limping around with his leg wrapped in a splint in obvious pain. I asked him what had happened. He said he was doing an inspection, and tripped over a foundation pier sticking out of the ground. I asked him how the leg was. He said it would heal in a few weeks, but the pier suffered most – he knocked it over!
I know you inspector types out there are shaking your heads right now saying, “Yep, I’ve seen a few 12 inch deep piers in my day.”
So how can this be, you ask? It can happen when the most important components of a drilled pier and beam foundation system, the piers, are not fully inspected, i.e. 100 percent drilled hole, ready-to-pour, steel-in-place inspection.
Drilled pier and beam foundation systems are common in the Denver-metro area. They are designed and specified for home sites where existing soil conditions such as expansive soil, collapsible soil, or poorly compacted fill need to be by-passed to support the foundation down to a deeper, more stable material.
The average tract home constructed with such a system could have as many as 20 to 25 piers under its foundation. To properly construct each pier, many engineering specifications have to be met. For example, to construct a drilled concrete pier designed to resist expansive soil, the pier must be: drilled to a certain diameter and minimum depth, having a small tolerance for groundwater infiltration or side-wall sloughing, else be cased, with a minimum penetration into bedrock, then reinforced with a specified amount, size, and alignment of steel, and finally, poured with concrete in a certain way having a specific strength – all this just for one pier. Now think of a neighborhood with 120 homes. If only half of them have pier and beam foundations, that’s about 1,350 piers.
So who verifies that piers are properly constructed and inspected? All local municipalities require that piers be inspected by a Colorado registered professional engineer, but not all agree on regularity. For example, the city and county of Broomfield requires at least 25 percent pier inspection. Jefferson County requires at least 50 percent (dry hole) inspection. Most others leave inspection frequency to the inspecting engineer’s judgment.
Ultimately, it is the home builder who is accountable for its finished product. One improperly constructed pier that fails in an expansive soils environment can be the first domino to fall, and lead to a whole neighborhood of perceived problems. Pier inspection is kind of like what my dentist always tells me, “only floss the teeth you want to keep.”