Sinkhole Above Ground Pool Collapse
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This is part two of this story about a sinkhole that appeared in the bottom of a nearly new above ground pool. For the first part of this story start by reading this article about a sinkhole in an above ground pool which also includes a short video taken while the pool was still in the process of draining out - only moments after the sinkhole appeared!
This continuation of that article comes about six weeks later following the initial failure and inspection of the property. Insurance has denied all claims from the pool owner to date and so I offered to return to the site and conduct a more thorough examination of the property. The last time I saw this pool the liner was still in place, sitting exactly as it lay after ripping and allowing the pool to suddenly drain. Even at this time the holes looked amazingly deep. Now that the liner has been removed the full extent of the damage can be seen...and it is astounding!
Giant Sinkhole In Bottom Of Pool
The final measurement of the collapsed area is very close to 70" x 60" x 18" which is a little more than 1.6 cubic yards of material. In the hole is a 3/4" thin-walled black poly conduit of unknown origin. Inspection of the property revealed no source, or destination, for this pipe. It felt empty, but possibly has a wire inside it based on how it feels to shake it and move it around. I was not inclined to cut into the unknown pipe.
During one of the engineer inspections a terracotta pipe was found at the bottom of one of the deepest holes. It would appear that the line is no longer in service, but it can not be ruled out completely that water is still actively passing through this pipe, or at least was up until the total failure of the pool.
Terracotta drainage lines used to be used around houses in place of the weeping tile products that are commonly used today. The presence of this terracotta pipe is concerning as it might indicate an out-of-service drainage system passing through this area. This may allow water to access this area under the pool now, or at some point in the past.
The presence if this pipe might also be an indication of previous attempts to deal with ground water or naturally occurring water systems. While it appears this line is not in service, it must be considered as a potential source / cause for the pool failure. If this pipe channels drainage water from the property then it could have been leaking for decades at some point in the past. This could account for, or at least contribute to, why the sinkhole under this pool is so large.
One particular concern that all inspecting parties had was the potential for rotting wood in the ground due to a once very large tree that is now only a stump. This tree was very large, and the stump measures just under four feet in diameter. From the closest side the stump sits 12 feet from the edge of the pool and must surely have had an extensive root system in the ground which is now dying.
The tree has been cut down for around two years time at the point where the pool failed. One of the main reasons for my return visit was to inspect the hole for rotting wood. If this theory were true there should be significant evidence of rotting wood and roots in the hole.
During the inspection I was able to find the tip of a piece of wood exposed at the bottom of the hole. After some pulling I was able to dislodge a piece of dead and rotting tree root over two feet long. Further probing of the ground revealed multiple other rotting roots although they were all much smaller than this one large section I removed. It is entirely possible that an extensive root system is decaying away underground, or possibly being eaten by insects, and this created a void under the floor of the pool.
The wood that I removed with my hands was large, and appreciable, and rotted enough to be able to tear it free with my bare hands. Anyone who has dug through roots knows that they do not give easily, and this one was huge by that standard, and it was extensively rotted for me to be able to tear it straight from the ground like this.
A very thorough examination of the area did not reveal any burrowing animal nests or evidence of any kind. The swarming wasps that were active in the sinkhole on my first visit were completely gone on my return visit. While the giant sinkhole is clearly connected to the sidewall of the pool, it is reasonable to assume that this is from the cascading torrent of water, and not necessarily from an animal or insect burrow. The upright where the pool drained is the one that is not installed inline with the rest of the uprights in this side of the pool. The pool owner states it was like this since the pool was first installed. While not particularly attractive in the finished product, there is no direct evidence that this upright contributed to the failure of the pool as far as I can see.
My initial impression was that a leak in the floor of the liner allowed the water to channel out an area under the pool floor. Now that the full extent of the size of the sinkhole in known, I am entirely less confident about my initial impressions. I think it is more likely that the wood rotting in the ground is a contributing factor, and I still can see no evidence of burrowing animals or insects that I could point to as the cause.
The pipes buried below the pool I feel are inconsequential despite being an obvious red flag - a red herring more from my point of view than anything else.
The ground is porous in the area where this pool was built, and I believe there was extensive root structure below the floor of the pool which has since rotted away leaving an extensive system of cavities in the ground. The immense weight of the pool was enough to compact these cavities and eventually cause the ripped liner and subsequent draining of the pool.
If you have not read it yet please check out part one of this article. Also be sure to leave us your opinion in the comments below (or through social media) as to why you think this pool failed. The pool owner is eager for answers and as of right now all we have are theories.
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