Mud Pumping Question

[Rich Riddle]

I removed a deck by the swimming pool because it was at the end of its life (as seen in the last picture) and skunks were starting to live under it. When I removed the ledger board abutting the concrete surrounding the pool, a surprise emerged. The ground under the concrete pad has several large gaps. It's almost as though the people who poured around the pool didn't pour the concrete directly on ground. The concrete is reinforced and has some sort of metal stakes inserted into the ground. The vertical white tube in the sixth picture is the skimmer.

Any way, it looks like mud needs pumped in the large gaps. I have no clue how this works. Any ideas you have after viewing the photos will be appreciated.

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[Ken Fitzgerald]

Rich,

A technique I would consider is purchasing some gravel, shoveling it under there and then compacting it using a bar and just tapping it.

I have raised and leveled two concrete stairways going into homes using this method. The bar I used was just a steel bar a neighbor loaned me. It works well.

[Phil Thien]

Looks like a job for a mudjacker.

[Rich Engelhardt]

Any way, it looks like mud needs pumped in the large gaps. I have no clue how this works. Any ideas you have after viewing the photos will be appreciated.

Your client may have a much larger problem than you think.
Pools can "float" if they are drained and left empty in wet ground.

Time to call in a pro to assess the situation.

If there's voids under the pool, eventually they will crack and let the water drain into the surrounding soil.

That's exactly what happened to the previous owner of one of our rentals. His pool heaved and cracked and the water drained and flooded his basement and the basements of the two houses on either side of his house.

[Stephen Musial]

You're talking about the metal stakes in photo 3? Those are wall braces for the pool and are also now supporting the deck.

The pool is most likely a vinyl liner and those braces support the walls (and deck now). Around here (St. Louis), we always put down 4" of clean rock and pour the concrete deck on top of that to facilitate drainage. There are a couple of possibility - the most likely one is that this is a vinyl liner and when it's built, there is about a 3' overdig so you can place your braces around the perimeter. There's a concrete beam poured at the bottom of the walls that locks it in place, you then place a Sonatube at the far end of the brace and fill it with concrete to anchor the brace and then you backfill. A good company will backfill with 3/4" clean rock so that water drains away from the pool walls and you don't get any bowing from freeze/thaw in the winter. 3/4 clean also goes in at something like 95% compaction so you don't have to go back and compact in lifts. Looks like these guys just used the dirt from the excavation and didn't compact it and it settled. The braces are what are supporting the deck now.

The second, less likely explanation is also that they backfilled it with the soil and compacted it but the soil is expansive clay. They may have backfilled it while wet and when it dried, the clay shrank back down. Not particularly likely but you never know. To be sure you can have a soils engineer test it and find out.

So, you could cram 3/4 clean rock under there or mud jack it. Either way, if it's expansive clay, when it does expand and can't fill the void, it will push the deck up. If it's not clay, you'll need to build a "dam" around the edges and then mud jack it to give it a permanent fix or push in the rock and leave the dam so the rock doesn't work it's way out. They do have rock shooters but I don't know how well they'd work in this application but something to look into.

The bottoms of line pools are usually a mix of cement and vermiculite so there's a little bit of give when you walk on it. I wouldn't worry about the bottom at this point in time as all signs point to a incorrectly prepared overdig.

BTW, here are my credentials: http://www.fluidsolutions.ws/projects.html

[Phil Thien]

BTW, here are my credentials: http://www.fluidsolutions.ws/projects.html

LOL, do whatever Stephen says.

(Nice work, Stephen!)

[Rich Riddle]

Without a doubt the soil in this area is clay. I don't like to ponder this, but would it be better to jackhammer up the deck currently in place and then see if it's like this all the way around the whole pool? You might be right in guessing that these folks didn't compact the soil. It seems as though none of the work done around this house happened the right way.

[Stephen Musial]

Not all clay is expansive so I wouldn't jump the gun. Most likely it was a quick shoddy job and they backfilled the overdig with spoils from the pool. It doesn't look like they bothered with the Sonatube at the end of the brace either. From the last picture it looks like there was originally an exposed aggregate deck and then at some point in time they overlayed it with texture deck and that's a plus because it's easy to repair.

If the house and pool have been there for a few years, and you've had the same kind of weather that we did (record breaking heat for weeks on end a few years ago and then lots of rain for the last couple of years), and there aren't any cracks in the house foundation, then it's highly likely that it's not expansive clay - just regular old smelly clay.

It's pretty much a given that they did the same thing around the entire pool so mudjacking will probably be the best bet. That will support the concrete and prevent any major cracks. It will also take a lot of the load off the pool panels and the braces which will lengthen the life of the pool. Once he mud jacks, you can get some of the acrylic texture, dash it on the holes, knock it down and then paint over it and the repair will be invisible. Depending on the age of the coating, you may want to paint the entire deck to get a good match.

That last picture looks like the wood deck is built directly on the pool deck and the pool deck taking the load? Or is that a post a couple of feet back sitting on a footing and they cantilevered it out to the pool? When you rebuild the wood deck, try to keep any load off the concrete deck as it's designed as a floating slab and to move a bit up and down with frost heave and thaw.

The fact that it's in such good shape for being up in the air with very little support speaks of a good strong pour and plenty of rebar/WWM/fiber reinforcement but eventually it's going to fail and when it does, it will damage the liner track, probably the liner (if it's a liner pool) and possibly the panels (if they're plastic and not metal).

[Rich Riddle]

I am removing all the decking in the next couple of weeks. Most of the deck (over 80%) is already gone. You are correct; the smaller deck received support from the pool concrete itself. It also spans much further than a 2x6 should span for the load. The deck in that area will not be replaced. They are looking at brining in materials and making a patio instead of deck. It would need very little material to get up to the height of the concrete surrounding the pool. A short retaining wall will hold the opposite end. Skunks lived under the deck and the owner doesn't want the family to return. They were friendly skunks, but .....

Plan B centers around filling the pool. It's not used often and takes valuable space, so that might be a viable option.

[Ole Anderson]

Skunks, huh? Well that stinks...

The concrete surround looks to only maybe 4 feet wide around the pool. As long as the concrete itself hasn't settled, I'm not sure it is a job for mud jacking per sea, as all you really need to do is fill the gap. Could be that the most the contractor did was to lightly tamp the backfill, and nature and time finished the job. Clay is not easy to compact with a plate vibrator, it needs more compactive force like a pogo stick or just tracking around with a bulldozer. Better yet, they needed to backfill with sand compacted in layers with a plate compactor instead of just pushing the excavated clay back in the hole. In their defense, too much compaction can push the liner walls back into the pool. As to the fix, presuming there is no slab settlement, Ken's suggestion of ramming gravel under the slab is reasonable, or you could build a dam around the edge and pour a wet grout (sand/Portland cement/water) mix that would find its way into the voids. Doing that would avoid drilling holes in the existing slab and the attendant repairs.

[steven taggart]

I used to pour concrete for a living, and have poured several decks around pools. One manufacturer I recall wouldn't let us compact around the pool for fear of bowing out the walls. So back a couple feet from the edge pieces of PVC pipe, usually 6" diameter, were set into the ground and filled with concrete to support the slab. I would guess that is what you have based on the picture. If it was at my house, I would fill the void with" flow able fill" also called "controlled density fill or Cdf". Just sand, one to two sacks of cement per yard, and lots of water. Available from any ready mix concrete supplier. Just set a form up to keep it from running all over, or berm up a bunch of dirt, make sure the cdf is very soupy, and pour it in. You can use a concrete vibrator, or a shovel handle to push it into the voids. It gets hard, about 150-200 psi, and works well. I now work for the state department of trans. And we use it all the time under voids.