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  • Writer's pictureJedidiah Williamson

"I have a sinking feeling about this!"

Updated: Feb 26, 2020

Some people believe engineering is a boring profession. As a structural engineer who works on existing buildings, I would beg to differ! Even after a decade of existing building work, I still come across something new on a pretty regular basis.

It was nearly a year ago that I got a call from a motel owner. He was quite worked up because the building department had red-tagged a portion of his building. He said a sinkhole had opened "near the foundation" of his motel and the authority having jurisdiction was not allowing him to rent out the two rooms nearest the sinkhole.

A holiday weekend was approaching and he was anxious to have his motel at full functionality for the impending busy season. Understanding that time was of the essence, I scheduled a site visit as soon as possible to assess how the sink hole was affecting the structure. I think I had fixated on the words "near the foundation" in the owners description of the situation. I was somehow envisioning a depression in the native soil close to the foundation of the structure and I was going to be evaluating whether the sinkhole was allowing the soil under the foundation to settle. But just to be sure I would have all the information I needed to assess the situation properly, I had told the property owner to engage a geotechnical engineer as well. I was very glad that I had made that request.

When I got to the property, the geotech (Todd Schlittenhart with Yeh and Associates) and I were led to the back of a single story portion of the structure. The hole in the photo below is quite obviously adjacent to the foundation, but I was happy to see that it was not too wide. It was worse than I had first envisioned, but the foundation did not appear to be spanning very far over a narrow, deep hole.

However, my understanding on the narrow width of the hole was short lived. The property owner informed us the exterior hole was dug as a way to investigate the "rest of the sinkhole". Mr. Schlittenhart and I were led into a janitors closet that had a crawlspace access hatch. As it turns out, the sinkhole was not only adjacent to the foundation, it was under it! The view of the sinkhole from the crawlspace is below.

The sinkhole measured 8.5 feet along the foundation, 5 feet away from the foundation, and 9 feet deep! As you can see in the photograph the crawlspace was wet from plumbing leaks. Soil movement happens when there is a change in water content; active clay swells when when wet and shrinks when dried; karst soils can literally dissolve in the presence of water; or, as what may have happened here, according to Mr. Schlittenhart, finer-grained topsoil can wash down into the voids of underlying gravels and boulders.

No matter the cause of the sinkhole it was necessary to shore the structure as soon as possible! After a discussion with Mr. Schlittenhart, it was decided flow-fill, a low-strength, self-leveling type of concrete, would be a good option to serve as a fast and possibly permanent shoring solution. Yeh and Associates had used this approach to stop other similar sinkholes in the area. I was glad to have competent and knowledgeable geotechnical help!

The property owner was also directed to fix all the plumbing leaks and other water intrusion sources to prevent a repeat performance of this scenario. He was also given the task of monitoring the flow-fill to watch for settlement. The "shored" scenario is visible below.

The property owner was able to rent out the two rooms soon after the flow-fill was installed and he has been monitoring the flow-fill to be sure it does not move. It appears that this solution is holding well!

After reading this, is anyone going to say that engineering is boring? I think not.

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