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danstrider 05-26-2012 09:24 AM

Help: Cave underneath concrete steps
My house ('74) has a basement door with accompanying concrete stairs to get up to grade. Unfortunately, there appears to be a large cavern beneath the landing and the stairs. I was raking leaves in the fall and decided to find out how large/deep the hole was, and it swallowed the whole rake. Thanks Mr Home Inspector for catching this one!

After removing the wooden walking platform and seeing that the foundation edge was no longer attached (see attached pictures), I can get a better look beneath the landing. The hole appears to be a combination of settling and erosion. I can fix the erosion problem with some re-grading in the HVAC area (it's 12 years old and getting close to replacement). My question is how best to fix the hole.

With much internet reading, mudjacking seems to be the industry standard way of fixing such problems. Two questions: with the removed foundation sections, wouldn't mud just squirt out the edges in my case? Alternately, would I be able to push watery concrete down in the hole from the side to have gravity help me fill the voids?

I had the thought I could cut off the landing and get better access to the void behind the stairs. Cutting 4" concrete is harder than I thought, eating through a grinding wheel for masonry in less than 4" and 1/2" deep. If this is the right way to go, I could rent a gas powered masonry saw.

Looking for any suggestions.

Concrete landing atop the basement stairs. Some signs of the crack.
Removing the wooden walkway and found the foundation lip beneath the crack wasn't attached.
Looking into the cavity. About 4-6" gap between the bottom of the concrete and the top of the stone foundation. There is a frog in there if you look very closely on the left (out of focus, you can see his eye).

kok328 05-26-2012 04:09 PM

Break it up with a sledge hammer, fill the hole, compact the material and then repour the slab. You may be able use the broken concrete as filler in combination with slag sand.

joecaption 05-27-2012 11:38 AM

There should have been a berm around that pore.
It's just a much thicker area on the outside edges.
Is there gutter over that area?

danstrider 05-27-2012 04:22 PM

kok328: Break up how much? Just the landing or are you suggesting the steps as well? I'm comfortable repouring the landing, hoping not to have to do the steps if I can avoid it.

joecaption: I'm not familiar with your terminology. The piece that is broken off appears to have been poured in a deeper well around the edge of the landing slab. Is this what you're referring to as the berm? If you mean some earthen berm for water run-off, no there wasn't any. I think with this area, the combination of the river stone and the wooden walkway just funneled any water toward the stairs. The gutters appear to be in good shape now, even if they weren't before. I've checked several times during heavy rains and the roof runoff appears to be well controlled.

For removing the landing slab, I added a groove that looks like an expansion joint in depth and width with my grinder and a cutoff disc (would a diamond wheel do the trick easier or must I simply give in and rent the big saw?). My thought was to whack the landing with a sledge and try to get the break to happen right where I put the groove. Good idea? Bad idea? A lot easier to stare at it and ask than destroy a couple good steps on a hunch.

Thanks for your time guys,

kok328 05-27-2012 04:46 PM

Just the landing. Hopefully, there is no erosion behind the steps. If there is you will be able to fill that once the landing is removed without removing the steps. The whole idea of removing the landing is so that you can get some good compaction.

danstrider 05-27-2012 07:15 PM

$50 at the home center for a 7" wet/dry diamond saw and a sledge hammer, 1hr of sweat, and a very lucky frog...

Success! The slab was 7" in places, since this is where it turned down for the stairs. The 3" saw cut made a great break point for the sledge work. Will have to look closer tomorrow in the sun for any movement of the stairs. The foundation wall looked untouched :-)

Now the interesting pictures...
BOOM! Very good split right at the saw cut.
The cavern runs deep.
Froggy behind on the left of the picture beneath the slug.

There is a drain at the base of the stairwell. However, looking at it on Friday, it used to simply be a hole to the dirt beneath the slab. Someone put in a PVC pipe (probably when my neighbor down the hill paid $$ to fix her/my drainage, before I owned the house) that appears to connect to the drainage system. I added hydraulic cement to seal off the metal insert to the PVC pipe and actually put water down the drain pipe.

No water pools up in the area anywhere I can see. There was a black poly beneath the river stone and HVAC, but that was folded up awkwardly at the edge of the landing slab.

I'll post another couple pictures tomorrow after the sun comes up. Busting up the concrete into smaller chunks is the first task tomorrow. Will save me a couple trips for fill material.

Thanks again guys!

kok328 05-27-2012 08:17 PM

Nice work. I was wondering how the concrete would break near the stairwell wall and the foundation wall. P.S.- A constant stream of water on the saw blade would have kept the dust down and the blade cool. How much cavity do you have under the fist step?

JoeD 05-28-2012 06:59 AM

That is just a normal circular saw. It is not made to be operated under wet conditions. NO WAY I would put water near it while cutting.

danstrider 05-28-2012 11:51 AM

I was scared of putting water on an electric saw as well, so I found a wet/dry blade and just wore a dust-mask.

Progressing well this morning. The hole beneath the steps swallowed a rake handle and started to turn the corner beneath the lower slab. I packed in gravel beneath the other slab as best I could and started building up with gravel, sand, and filler from the broken up slab. There is a deceiving amount of volume. Maybe it wouldn't be so bad if it the weather wasn't set on 92 and sauna.

Unless you folks tell me otherwise, I'm thinking of continuing the packing of material, building a form, packing a good sub-layer of gravel, then pouring the slab with the single existing long piece of rebar to tie things together. Isolation material was $2.30 at Lowes and I'm planning to put it between the house foundation and the new pour.

Still open for further suggestions. Otherwise, I'll keep posting pictures :-)

Thanks again,
Dan (2).jpg
Swallowed rake and a hoe. (2).jpg
Trying to get a view down the hole. I could see light at the bottom, from the corner of the lowest step that has a crack against the house. (2).jpg
Start of filling & tamping, using mostly the existing gravel & dirt. (2).jpg
Continued tamping, a touch more than half-way up the back of the stairs. Having to make gravel now from the broken up landing. That's some real work.

kok328 05-28-2012 05:18 PM

I hate to say it but the best approach is to pour cement in the cavity. It will be the best way to fill the void without any pockets that broken cement has. It will fill the cavity better and the steps make for an excellent form. From there, I'd build a square form and put some wire screen into the form and repour the landing. May want to use expansion material between the existing top step and the house foundation.

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