Failure vortex: The case of the mysterious sinkhole

November 6, 2018

By Steve Goodale

The water drained along the brace visible in the hole, and out through the far wall.[1]
The water drained along the brace visible in the hole, and out through the far wall.

In late summer 2016, an above-ground oval pool was installed at a private home in a small town in rural Ontario. The project’s buttress-free design meant there would be no external A-frame supports. Instead, the walls would be connected with buried tension braces. Although the buttress-free design is more costly than traditional A-frame style wall kits, the homeowner was looking to maximize space and opted to go with the more expensive option.

The pool was the second above-ground install in this exact location; the previous one had been destroyed by a falling branch from a nearby tree that had gone down during a windstorm. The tree was ultimately cut down to a stump, which was 1.2 m (4 ft) in diameter.

In spring 2018, the homeowners hired a professional crew to open the pool. They had done the same the previous year, and had also hired professionals to close it. Upon opening the pool, however, the crew noted a slight depression in the floor near the middle of the deep end that, at first, appeared to be nothing more than a minor inconsistency in the slope.

As the pool was less than two years old, the owner contacted the original installer to alert them to the problem. They were told not to worry about the change in the floor profile, as “sometimes these things happen over the winter.”

After 30 days, the floor had become significantly worse. The depression had become more pronounced and was getting larger. The homeowner followed up with the original contractor multiple times but did not receive a service call or site visit. Rather, the contractor reiterated there was no need for concern, as the liner would “stretch” and was “impervious to damage.” The owner insisted there was a problem and continued to attempt to contact the builder about the situation—up to and including the day the pool failed.

Sinkhole

One afternoon, as the owner and their children were swimming, the floor of the deep end abruptly collapsed, ripping the liner and suddenly draining the pool. The force of the drain caused a vortex akin to a flushing toilet and water began escaping from the base of one of the installation’s long walls.

The pool drained in minutes. Water flooded onto the property and eventually into the basement of the home, causing significant damage to the finished interior.

While it drained, the owner waded over to the collapsed floor and was able to insert the handle of a rake to the hilt straight down into the ground, nearly 1.5 m (5 ft). Once emptied, two distinct sinkholes were left in the ground, which were next to each other, but not directly connected. These were not near any wall, nor connected via any visible holes or animal burrows to any of the edges of the pool. Without any clear burrow from the edge to the middle of the pool there is no direct evidence to suggest a burrowing animal or insect.

There were many other observations noted about this unique failure, however:

Further investigation

Completely unearthed, the sinkhole fills almost the entire deep end but does not connect to the walls.[3]
Completely unearthed, the sinkhole fills almost the entire deep end but does not connect to the walls.

Once the liner was removed, the extent of the damage became more evident.

Far more than the two holes originally noted, the total size of the sinkhole was around 0.45 m (18 in.) deep, 1.5 m (60 in.) long, and 1.8 m (70 in.) wide—or, slightly more than 1.22-mT (1.6 y3) of dirt, which would weigh approximately 1814 kg (4000 lbs).

So where did the dirt go?

Some would have washed away when the pool drained; however, the owner noted the torrent was mostly comprised of clean water. Additionally, when it drained, they had verified the ground was hollow when they inserted the rake handle into the sinkhole.

An inconclusive mystery

Following the failure, many unknowns remained:

Today, the pool, a project the homeowner invested more than $10,000 to install less than two years ago, is ruined. The home insurance company denied submitted claims for coverage, despite there being no clear consensus on the cause of the failure from the multitude of pool companies and engineering firms that have inspected the property.

A giant tree stump, 1.2 m (4 ft) in diameter, located adjacent to the pool.[4]
A giant tree stump, 1.2 m (4 ft) in diameter, located adjacent to the pool.

The original installer denies any accountability for the current state of the project, as the one-year warranty period has lapsed. It has submitted an inspection report, which cites burrowing animals as the most likely cause for the failure; however, the insurance company denies this determination based on a lack of evidence connecting the sides of the pool to the sinkholes in the deep end.

A direct connection from the side of the pool upright (the one not installed correctly in line with the rest of the uprights) to the sinkhole was found, but it is estimated this occurred when the pool drained, and not the reason why it happened.

An engineering firm that inspected the site noted concern for the potential of rotting roots in the ground, as well as for the mysterious polyethylene and terracotta pipes (though it was determined both lines were no longer in service and it was unlikely they were related to the failure). This inspection also raised the potential of an old/existing water system that may have been present in the location. This suspicion is plausible, but not provable—although the initial builder concurs this is a possibility.

As it stands, the property owner is left with a destroyed pool and yard, a water-damaged house, and no conclusive answers as to what went wrong.

Despite the disputes between the contractor, insurance company, and site inspectors, one thing is certain: a pool should not be built in this location until the cause of the previous failure is discovered. Once this is determined, an appropriate plan can be implemented for site preparation to prevent something like this from happening again.

Since the insurance company has denied any coverage, it is unlikely the homeowner will be able to fund further testing or replacement of the pool—which is a shame, considering they have already invested so much in the project.

 

READER SURVEY: WHAT WENT WRONG WITH THIS POOL?
  • Is the builder to blame, or did the owner simply not take enough action when it was first observed the pool was failing?
  • Should the damage to the pool and house be covered under insurance, or was the insurance company right to deny the claim?
  • Could a leak in the liner have caused this failure?

[5]Steve Goodale is a second-generation pool and spa expert from Oakville, Ont., and author of the online resource www.swimmingpoolsteve.com[6]. He can be reached via e-mail at swimmingpoolsteve@gmail.com[7].

Endnotes:
  1. [Image]: https://www.poolspamarketing.com/wp-content/uploads/2018/11/20180801_110627.jpg
  2. [Image]: https://www.poolspamarketing.com/wp-content/uploads/2018/11/20180704_100907.jpg
  3. [Image]: https://www.poolspamarketing.com/wp-content/uploads/2018/11/20180801_110600.jpg
  4. [Image]: https://www.poolspamarketing.com/wp-content/uploads/2018/11/20180801_111529.jpg
  5. [Image]: https://www.poolspamarketing.com/wp-content/uploads/2016/03/Goodale_Headshot.jpg
  6. www.swimmingpoolsteve.com: http://www.swimmingpoolsteve.com/
  7. swimmingpoolsteve@gmail.com: mailto:swimmingpoolsteve@gmail.com

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