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Overbearing Rockwork

A swimming pool is made of steel-reinforced concrete, right?  So the shell will be fine if you pile lots of weight on top of it, right?  Wrong, Scott Cohen says emphatically -- and here he explains why no sensible builder should ever make such a potentially disastrous mistake.
A swimming pool is made of steel-reinforced concrete, right? So the shell will be fine if you pile lots of weight on top of it, right? Wrong, Scott Cohen says emphatically -- and here he explains why no sensible builder should ever make such a potentially disastrous mistake.
By Scott Cohen

It happens more often than it should:  Even in times when trade shows and educational enterprises such as Genesis 3 all stress the importance of knowing the basic forces at work within and around pool shells, I am all too often called in to investigate cases in which a builder has made a large and careless mistake that can have disastrous consequences.

The point these contractors are overlooking is that the bond beams of many (if not most) pool shells are engineered in such a way that they will support not much more than some sort of coping.  In short, they lack the reinforcing steel required to enable them to carry any significant vertical surcharge of weight without eventually failing.  And when the beams do fail, in many cases the damage telegraphs itself down into the walls of the pool – not a good way to relieve the structural stresses.

The scenario:  In what I’m assuming was a remodel but which may have been initial work, the homeowners apparently wanted (and the builder agreed to design and install) a big rock waterfall on the corner of the pool near the spa.  No matter whether it was a late change order or a renovation job, it’s my suspicion that the original project’s engineer knew nothing about the extra burden that would be imposed by this added structure and had no opportunity to upgrade the bond beam’s cage and size accordingly.

(I won’t comment on the aesthetic offenses that resulted from the homeowners’ desire other than to say if they wanted a waterfall “in the worst way,” that’s exactly what they got.)

The decision to include the waterfall structure involved the piling up of several tons of weight on a part of the pool shell that clearly hadn’t been engineered to accommodate anything more than maybe a light ledgerstone wall.  As can be seen in the photographs, the downward pressure compromised the bond beam, and cracks were ultimately transmitted into the walls at a corner on one side and at the material transition on the other.  The vessel no longer holds water, and the required repairs will be expensive.

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Beyond not being the prettiest pool appendage I’d ever seen, this rock structure was destined to cause problems because the bond beam beneath it hadn’t been designed to accommodate the surcharge created by the weight of the material used to assemble it.

The fix:  The kindest solution here would involve removing the rock waterfall and replacing it with the lightweight wall the shell seems to have been intended to support.  There’s no other way to stop the cracking, which is significant enough that water has penetrated the wall down to the steel and has caused rust to bleed through to the plaster finish.  (It’s more than advisable to chip out material to investigate the full extent of the damage and replace the steel as needed.  If nothing is done, the rust will continue to appear at the surface.)

To make this sort of unanticipated waterfall work from an engineering standpoint, two things would need to happen.  First, the existing bond beam would need to be bolstered with additional steel and concrete, meaning this part of the wall would need to be opened and rebuilt according to an appropriate engineering plan.  Don’t guess what should be done or play engineer:  Hire an expert to make written recommendations for remediation. 

Second, the area behind the shell may need to be excavated along the full length and width (back into the landscape) of the proposed rock structure and, to avoid any hinging effect due to differential settlement, possibly down to the level of the shell’s bottom to avoid a lateral surcharge.  And again, consult with your engineer for the best solution to your site condition challenges.

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Before long, the weight of the waterfall began to crack the shell on the outside edges of the structure.  This allowed water to penetrate into the shell and cause the reinforcing steel to rust.

Once a new steel cage has been inserted (and grounded, of course) and a proper French drain system has been installed, new concrete may be applied up to the level of the existing beam.  

After the new concrete has cured, a new waterfall (and, one hopes, a nicer waterfall) can be added, with the surcharge imposed in the process presenting no problems at all.

Lesson Learned:  Just about every change made atop a pool shell after the concrete cures is something that should be reviewed by the project engineer.  Shells are great, but their bond beams are set up to support specific amounts of weight.  If a client or a designer decides that something more is needed after the shell is complete, there’s a right way to accommodate these desires and, sadly, a whole bunch of wrong ones.

And remember:  “Standard” plans are for standard projects.  Special engineering details are required whenever surcharges from slopes or weight load are introduced!

 

Scott Cohen is a construction-defect expert witness and president of The Green Scene, an outdoor design/construction firm in Chatsworth, Calif.  Past articles in the Lessons Learned series have been compiled in his book, The Candid Contractor.  He also provides consultation for clients nationwide and gives seminars on designing landscapes, swimming pools and outdoor kitchens. For more information, go to www.greenscenelandscape.com.

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