Piers under pool before construction? Expansive soil.

[Twolabs]

We're still in the planning stages, and one thing that was worried me is our expansive clay soil. I've done some searches on the forum here, and there doesn't seem to be a clear consensus on it. We built our house 4 years ago and had a soil test done then (trying to get the test results from our builder). The slab engineer recommended a post tension cable slab, and for the most part it has worked. We've had a few small surface cracks here and there in the garage and back porch, but no doors/windows sticking. I've spoken with three PB's so far and their opinions are as wildly varied as their opinions on SWG (a whole other topic).

PB #1, located about 70 miles away: "We always put in piers in your area. It just standard practice for us around here with your type of soil."

PB #2, located same city as PB #1: "We do piers sometimes if you have really bad soil. Get me a copy of your soil test. If the expansion is above X.XX then we'll most likely do a few piers. The gunite shell is fairly solid and will be tied into the pool deck via rebar."

PB #3, located about 60 miles different direction: "We put in 1/2" rebar, tie it in to the pool deck, etc etc etc. We guarantee our gunite shells for life. That thing isn't moving."

So far PB#3 has probably built the most pools and I would say has the most experience, and he's most likely the PB we're going to go with. However, I've seen the nasty effects of moving soil in my parent's two houses they've built. The first one was on expansive clay soil (worse than ours) and they had to spend tens of thousands of dollars with a slab foundation repair company to fix it all because the builder skipped town. Driveway sinking 4 inches from the garage...doors sticking...leaks in slab..etc. So, I'm especially sensitive to these issues and for the amount of money we're about to dump into our "backyard oasis" I want to make sure all bases are covered.

Anyone have any experience with these types of issues? For the most part, I can't find a ton of info about it...so that makes me thing it is much less of a problem than expansive soils with house foundations. I'm thinking it might be worth it to try to find a engineering professional to give their independent professional opinion.

 

[Twolabs]

Also mods if you want to combine this with my other thread I understand. I just didn't know if this warranted a different thread because it had to do with a very specific question that others may have. Thanks!

 

[Afluv]

I have the same questions. Go to http://websoilsurvey.sc.egov.usda.gov. It may have some soil expansion numbers for your neighborhood/lot. I listened to an engineer on the phone while sitting in my PB interview. He saw my data and said piers were recommended but at about $10k extra that would have killed the project for me. Then he recommended chemical soil injection ($3200) and #4 rebar. Problem is that nobody in my neighborhood (all the same soil for the most part) has done that nor have their PBs recommended it. Most just recommend a gravel pack under the pool, more steel or nothing at all.

 

[Spaniard]

I did chemical injection. The builder said he'd back it no matter if I did piers, injection, or nothing. But I've had a lot of movement with the house and wanted a little piece of mind. It hasn't been long enough for me to tell anything, but fingers crossed...

 

[epro05]

I live in Keller, TX. Built our pool in 1995 and over the next few years the pool shifted. Now there is a 2+ inch difference in waterline from one side of the pool to the other side due to the pool shifting. Not much I can do about it so I live with it. Fortunately the deck around the pool shifted with it, so the deck still aligns with the pool coping. I've always had my theory of why it shifted. After pool build completion, they dug a 4 to 5 foot deep trench around one side of the pool to reroute the electric for the house. After the shifting, that's the low side of the pool. I think the trench allowed the earth to move towards the trench.

 

[poolcritic]

Twolabs, You are very wise in giving consideration to how the soil at the pool site will impact the performance of the structure. First of all, Site specific soil testing by a licensed geotechnical engineer is recommended in conjunction with the construction of any in-ground concrete swimming pool and/or spa, especially where expansive clay soils are known to exist (Association of Pool & Spa Professionals). The findings of that soil testing should be provided to a licensed structural engineer for his/her recommendations on the structural design for the specific type of soil at the pool site. You should expect that if you have highly expansive soil or differences in soil properties at the pool site, additional costs for the construction of the structure as much as 15-25K+. That may seem like alot, but if you are building a 100K+ pool, then you may want to consider this level of expertise on your project.

I realize that the expense may not be tolerable for your pool project as it is for many of the thousands of pool built in Texas every year. You should also know that screw in helical piers (dbl helix piers) do not perform well in areas where highly expansive clay soils exist. Oftentimes on projects with highly expansive soils (PVR >5 inches), concrete piles are recommended but cost quite a bit more than screw in helical piers. My soil engineer has documented failure of these screw in helical piers on numerous cases. These helical piers (screw in piles) will rust despite what builders think and oftentimes are not placed correctly either improper torque specifications and/or inadequate surcharge placement for the structure. The problem is that without soil testing, there is no way to know if the helical pier will ever get torqued high enough, more importantly whether rock formations are present within 20 Feet to be screwed into. You see, the helical pier has a maximum depth limit. What some builders do is they simply stop drilling the pier at 20-25 FT even though the torque pressure is low. The other problem is that you have a non-qualified professional specifying the torque in the absence of all the critical data necessary to specify torque.

So, before you make your decision consider these things. 1.) Do you have the budget for engineering and the additional costs for construction? If not, then accept the risk that anything you put in or on the ground will be subject to movement, stress and potentially structural failure in the presence of highly expansive soils. 2.) In the absence of engineering, do you really want to pay for procedures or "ground packages" that are specified by non-qualified professionals on the hope that they may prevent failure? 3.) Chemical soil injections in areas where highly expansive soils exist are at best only marginally successful (to a depth of 3 Feet) and not supported as a solution to prevent failure in in-ground concrete swimming pool & spa structures where such soils exist. 4.) Many thousands of swimming pool are built every year without engineering and last >15 years without failure, it's only significant when its your pool and you decided not to follow the recommendations provided herein and echoed by all pool & spa related industry association regarding in-ground concrete swimming pool construction.

 

[bmoreswim]

I have nothing of value to add but just to comment that these expansive soil discussions scare me. Seems like a
properly qualified engineer would be able to give a person the proper way to go. But I'm sure it's not that simple.

 

[Twolabs]

Forgot to reply to this. Thanks all for the info. Poolcritic you certainly seem to know your stuff. I am trying to get our soil analysis from the engineer who designed my slab 4 years ago (post tension cable, had held up well so far). I have asked him for my soil analysis along with how much he might charge to give his opinion to review the plans. I am very surprised there isn't more out there about pool substructure. Googling "Do I need piers for my swimming pool?" yields two relevant results... And neither one of them are that helpful. I have found some more info but had to do a little more digging. I am surprised I don't run across more problems like what epro05 described on here. Searching for piers on this forum didn't yield much either.

I will be very interested to what he has to say and what my soil analysis says.

Thanks again for all the advice.

 

[CusePool]

What specifically is the definition of an "expansive" clay soil? The web soil survey linked by Afluv (thanks you by the way) indicates I am on Lake Charles clay loam. I know there is a high clay content (~50%), but I don't know if it is expansive or not. Is all clay expansive and it is just a throw-away word or are some clay soils not expansive?

 

[Twolabs]

I don't know if there is a specific definition but a soil test would tell you. The engineer would be interested in the PVR (potential vertical rise). This is a measurement of how much the soil will expand when wet. Expansive soil with a high PVR can exert tremendous pressure... Several thousand lb per sq inch.

 

[CusePool]

Thanks twolabs, I'll see if that data is available on the soil survey. (of course for me it is just academic, because my pool shell is already in the ground!)

 

[rhawke]

Twolabs, I am in the same situation and was just curious if you ended up installing Piers or not and if not, whether you ended up having any issues? I do have a soils report but most pool builders aren't interested in it and said piers are not needed. Only one high-end builder that told me he has piers in about 25 % of pools told me his engineer reviewed it and highly recommended piers.

The PVR noted in our soils report is 2" ...

 

[Twolabs]

I just happened to log in today and forgot about this thread.

Well in the end, we opted out of doing any type of reinforcement, besides throwing down some crushed white rock which my PB did as a last minute decision (which I've since learned might not actually be a good idea anyway). We wound up leaning on their experience plus I did wind up getting the opinion of the engineer that designed our foundation when we built our house in 2010 that included a core test and soil results. He also had testified in court cases against pool builders. Based upon our Plasticity Index (PI) he thought were we fine. It's still in the moderate range, higher than I would like, but that is just the nature of our soil around here.

As far as the movement since then, overall it hasn't been too bad. I knew it would be impossible not to have some movement, and we have had some. Some small hairline cracks in the pool deck and around one area of the pool we've had some cracks form around the skimmer and have had some small surface spiderweb like cracks in the plaster. I've had the PB out for various things and he wasn't worried about any of them at all and so therefore I'm not worried. This is throughout periods of extreme drought and major rain (i.e. This summer in Texas, now back to a drought).