OT: Need a Civil Engineer - Garage may be in jeopardy...

[Jeff Elliott]

Hi Fellow NC Cruiser Heads - I'm seeking a civil engineer who might come take a look at my situation near Lake Norman (north of CLT). I'd be happy to compensate said individual in cash. Above all, I'm looking for someone who's knowledgeable of soil issues and concrete and will shoot me straight.

I recently built my dream garage as an addition on my home. We ran into soil issues at which point we got an engineer to take soil samples and come up with a remediation plan which included several very deep (about 7 feet) and very expensive caissons. I will note that, when drilling the caissons, water was struck (I heard - I wasn't on site) in a couple of places. Fast forward, I backfill a large void with 131 tons of gravel and pour the slab that was to hold a couple of lifts and all my equipment.

The garage was completed in October and a couple of strange and concerning things have happened: 1) the slab has cracked outside of the control joints in a few places - once of which is the location where the foot of my proposed lift would be located. 2) We had the final grade placed two weeks ago and, inexplicably, water is emerging from the ground less than 10 feet from the corner of the addition. There are no water lines in the area and it has become progressively more wet with each day despite the lack of rain (until this weekend). I'm not talking about damp, I'm talking about digging a 1 foot hole and it fills itself up in 8 hours.

Initial dampness noticed within a few hours of final grade:

Which expanded to this:

This is a small hole that I turned over with a shovel that filled up and continues to do so:

The aforementioned "dream" (nightmare):

Thanks all!

 

[GLTHFJ60]

I can't provide a referral, but I can wish you good luck.

Is contacting the prior engineer not an option?

 

[jynx]

I would be interested in the original geotech engineers opinion, as he is your engineer of record, but I also know those guys contracts basically say we will give you advice, but we aren't responsible for S if things go sideways...

That said, cracking outside of control joints isn't uncommon, so dont worry to much there. You said they recommended caissons, was there additional structural reinforcing of the slab? Where are the caissons located? How is the stone confined, and how was it compacted? What type of stone? Are there caissons at the post lift locations or just reinforced slab? If the foundation is right then you should be good.

The water issue makes me think that they hit the water table when drilling and now that you have placed all that stone, you may have created a French drain that is now wicking thru the stone and out into the yard. Is the area where you are seeing the water fill area around the stone fill?

I'm not close, but will happily help as much as I can.

-Matthew

 

[Jeff Elliott]

Folks, thanks a ton for all the help thus far.... I'm in contact with the original engineer and he's coming to do a site visit on Thursday. In the meantime, let me answer a few of those good questions:

That said, cracking outside of control joints isn't uncommon, so dont worry to much there. You said they recommended caissons, was there additional structural reinforcing of the slab? Where are the caissons located? How is the stone confined, and how was it compacted? What type of stone? Are there caissons at the post lift locations or just reinforced slab? If the foundation is right then you should be good.

Yes, we put mesh in the slab. The caissons were located as per the plan below. The stone wasn't compacted - it was placed in the void via slinger truck. I was told that the compaction via this method was > 99%?

The water issue makes me think that they hit the water table when drilling and now that you have placed all that stone, you may have created a French drain that is now wicking thru the stone and out into the yard. Is the area where you are seeing the water fill area around the stone fill?

I'm not close, but will happily help as much as I can.

Thanks a ton Matthew - The area where I'm seeing the water is in blue above in the plan. The area where the rock was placed in the void is denoted with grey "x's" above. The area between the former void and the water is an 8' wide storage area below the slab which is swung on pans. Here are a few photos during construction that might be useful:

Void completed with waterproofing and perimeter drains that daylight below the wet area

Void backfilled with stone - anywhere from 5 - 7.5' high. Rebar in voids in block which was backfilled and rebar then turned into slab - slab thickened in areas where lift pads might hit to depth of 8"

Slab going in with mesh - 4,000 psi concrete, polished:

 

[jynx]

How much fill is up against the back corner wall where the water is surfacing? Water will ALWAYS find the path of least resistance to the surface. Most likely all is good. Looks like you did all the right things.

2 things.

1. Graded stone of that size is like filling a box with marbles. As long as it is confined (like yours is) then it should be good. We have filled voids similar under slabs on projects the past. If the rebar is turned out of the walls into the slab, but the bars do not extend across the full with of the slab, then you may see a crack around the perimeter about where the tails of those bars end. Not a huge deal, that is just where the reinforcement stops/transitions to the wire. May not happen, but if you see it thats why.

2. Does the 'crawlspace" in the back have a slab floor? If so, then what you could be seeing is water migrating under that slab, along the footings and from the pictures, it would appear that that corner is likely the lowest point, so that is where any trapped or captured water is getting to and surfacing.

All in all, I dont think this is too bad. Sounds like the water that was hit is likely migrating up along the caissons, which I suspect those crawlspace foundations are sitting on, and the migrating along the footings to that low corner. A French drain along that back and side wall may be all that you need to fix you up.

Please let us know what the original engineer has to say.

Also, I'm not sure how your home is constructed, but you could also be seeing water migrating from along the house basement walls and just daylighting on that corner. Lots of possible causes, but the likelyhood of any being a terminal diagnosis is pretty slim.

Good luck and keep us posted.

 

[Jeff Elliott]

Thanks a ton for the encouraging words @jynx !

Here are some answers:

How much fill is up against the back corner wall where the water is surfacing? Water will ALWAYS find the path of least resistance to the surface. Most likely all is good. Looks like you did all the right things.

Well, technically there is zero stone against the back wall as you rightly pointed out the crawl space section sits between the void that contains stone and where the back wall is located and the water is appearing. That said, the area where the stone is the highest is the section where that first grey "x" is above. The stone is probably 7' deep there.

1. Graded stone of that size is like filling a box with marbles. As long as it is confined (like yours is) then it should be good. We have filled voids similar under slabs on projects the past. If the rebar is turned out of the walls into the slab, but the bars do not extend across the full with of the slab, then you may see a crack around the perimeter about where the tails of those bars end. Not a huge deal, that is just where the reinforcement stops/transitions to the wire. May not happen, but if you see it thats why.

The rebar only extends 30" into the slab. Otherwise the only reinforcement is the mesh which, I don't believe the lifted very well during the pour. That might explain the cracks for sure.

2. Does the 'crawlspace" in the back have a slab floor? If so, then what you could be seeing is water migrating under that slab, along the footings and from the pictures, it would appear that that corner is likely the lowest point, so that is where any trapped or captured water is getting to and surfacing.

yes, it has a slab floor and that corner is in fact the lowest point

All in all, I dont think this is too bad. Sounds like the water that was hit is likely migrating up along the caissons, which I suspect those crawlspace foundations are sitting on, and the migrating along the footings to that low corner. A French drain along that back and side wall may be all that you need to fix you up.

I think that's plausible for sure. That would also explain the significant water in a couple of the corrugated pipes - they were likely the ones carrying moisture out of the void from the interior perimeter drains.

Please let us know what the original engineer has to say.

Will do. No basement but bad soil for sure. I can bury the probe to the handles just about anywhere around the house (and I weigh a buck fifty). That said, its a brick veneer and had been standing for 11 years with only one small crack where a porch joins the main structure so it must be ok.

Thanks again!

 

[angerhater]

Never mind the water, I would think it's a bit early for the slab to be cracking already. its not like you were cutting corners here.. I cant imagine a gravel bed that thick would not need to be compacted

 

[jynx]

If the slab wasn't water cured (kept sprayed with water, covered with plastic, or covered in wet burlap or similar) then the top will typically cure faster then the rest and can cause some surface cracking.

And even then, sometimes it still cracks. Small hairline cracks in the surface are hard to avoid. We have heavily reinforced slabs that have small surface cracks. It sounds cliche, but when you hear folks say "concrete cracks" it's true. Large cracks are concerning, small cracks aren't usually too concerning.

 

[Jeff Elliott]

Well folks, the engineer came by today. Augered down 3' on either side of the wet area and those hole were and stayed bone dry. Based on that, he feels we've got a blocked drain or a very isolated spring vent. Either way, the cure is a french drain.

As for the cracks, he says shrink cracks. We laid a straight edge across them and they were very level and are 1mm or less.

So, he says we're good. Time will tell.

Thanks to all for the advice!!!

 

[GLTHFJ60]

Phew!!

 

[jynx]

Glad to hear it's good news. And based on what you're saying, I wouldn't give it another thought. Just glance at things once or twice a year to make sure nothing's moving (like obvious changes, not check it with a straight edge) but otherwise, enjoy that new addition.

 

[GetNRekt]

Nothing to add, just want to say that is a beautiful house with an amazing add on garage! Glad it worked out.