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Help: Cave underneath concrete steps

When we moved in, the home inspector missed this (now) obvious sinking concrete pad atop the stairs to our basement. One day last fall when raking leaves, I stuck the handle into the crack at the base to see how deep the hole was and it went all the way up to the hilt. Same for a shovel, swallows the whole thing.

Looking at the arrangement a bit, the stones beneath the heat pump let rain drain through and the plastic tarp probably didn't capture all the water, instead funneling it toward the stairs, eventually eroding a huge cavity beneath them. The HVAC is about on its last legs, and when I replace it, I'll regrade the area and build a new pad. In the meantime, I dug up the edge and found the foundation piece was loose and no longer attached.

My question is: can I simply shove in a bunch of watery portland cement & sand to fill this hole enough to keep things from getting worse, rather than replacing the steps? A more intrusive DIY project might be to saw off most of the upper platform to get better access to the cavity, but even then can I simply fill the hole with concrete, rebuild the pad, and feel good about the repair?

I know another option would be to hire a mudjack or some such concrete lifting tool. I like doing things myself, so I'm hoping to avoid hiring someone unless that's what you all recommend...

Thanks very much for any advice! I'm in northern VA if that makes a difference to you.

Originally had a wooden platform which probably helped pool the water that fell on the HVAC ... maybe previous owners didn't have gutters at this corner??

Removed the wooden platform, getting a better look at the crack.

Despite being heavy, the lower portion of the foundation pulled right out.

Pretty decent cavity

Can see the back of the steps going down even. There is maybe 6" between bottom of the concrete and the stones/dirt.

Re: Help: Cave underneath concrete steps

Shoving concrete under a pad is not going to work real well because you cannot see what you are doing and if the concrete is truely supporting the pad. Dig out a work area adjacent to the pad. Shovel and alternately tamp #57 gravel under the pad until you have filled the void. When you replace your heat pump you can make a decision whether or not to relace that pad. It wont be easy because you will have to saw cut the concete, jack hammer it out, form it up and replace the pad

Re: Help: Cave underneath concrete steps

A "fix" will have to include treating the cause for the soil erosion. Therefore your first step will have to be the repair of the cause. The photos don't show the whole picture, but it could be that the gutter over the a/c compressor is malfunctioning, allowing all the rain runoff into this little area.

Second step: remove the pad anyway you can without damaging the wall or the stairs. It should come out fairly easily.

Third step: backfill the eroded soil and regrade, then compact.

Fourth step: form for a new pad and pour new concrete.

Fifth step: consider pouring a slab in the entire area. Slope it away from the steps as well.

The key is to divert water away from this area. Living in N. VA tells me that you get tons of rain.

Re: Help: Cave underneath concrete steps

CaptTCB: Okay, you've confirmed for me that simply shoving in concrete isn't going to work. I was barely hanging onto hope it would work. For digging alongside, this area is filled with all sorts of crappy fill-dirt grossness; clay, rocks, and interspersed utility cables. This somewhat makes me suspect now settling being the problem moreso than erosion. Is 4-6" of settling normal? I'm leaning toward the harder work of removing the pad...

dj1: I spent some time out in heavy rain checking out the gutters and the roof runoff appears to be well controlled. There was a broken HVAC condensate tube (pumped up from the basement condenser) that was probably flooding the area with gallons over the summer A/C months, now fixed and moved away from the house further. I'm definitely planning on some regrading even prior to replacing the heat-pump to ensure any runoff goes toward the grass.

Okay, the pad removal step... I put a groove in the concrete that looks like an expansion joint in depth and width. I was thinking of dropping a sledge on the landing and trying to get a crack right at that expansion joint. Or I could get a diamond blade for the grinder and try to make this groove deeper, but I definitely can't get 4" without renting a real tool. Good idea/bad idea?

Roger on steps 3-5. That's now my official plan once I can get the landing slab off.

Thanks very much for your help guys!

Re: Help: Cave underneath concrete steps


A skill saw with a 7-1/4" wet/dry diamond blade is good enough to create a nice groove (your concrete doesn't seem to be 3-1/2" deep), from the end of the railing curb to the wall in a straight line.

Then a 10 lbs sledge hammer with some elbow grease will knock this landing out in a minute. Just be careful around the wall, the top stair and the railing curb.

Send new photos once done...

Re: Help: Cave underneath concrete steps

I would agree with dj (Step 5) and clear everything out & pour a concrete slab that would cover that entire corner, butt up against the house foundation (isolation joint needed on any part of the new slab that touches the existing house foundation); a new slab would connect the two entryways, be slightly pitched toward the grass area so that the walkways in rainy & snowy weather would be high & dry with good footing, & would eliminate any water seepage now being experienced from the crushed stone.

Are you getting any water accumulation when it's rainy at the base of the cellar entryway?? Is there a drain there??

Excellent photos!

Re: Help: Cave underneath concrete steps

dj1, you are my hero for an excellent recommendation! $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!

Re: Help: Cave underneath concrete steps
Dobbs wrote:

(isolation joint needed on any part of the new slab that touches the existing house foundation)

Dobbs, can you clarify what type of isolation joint material you would recommend? Found some info on this page that possible materials are tar paper, closed-cell neoprene, and cork. Need a piece of tar paper between the old & new slab?

Do I need to do anything to tie the new landing to the old stairs? I left the rebar protruding from the stairs and could add another piece into the new concrete that goes beneath the steps.


Re: Help: Cave underneath concrete steps

Nice job, Dan!

The rebar can stay, you don't need additional rebars into the top of the stairs. Just build up the soil, gravel and compact.

The isolation joint is actually a fiber expansion joint - sold at Homedepot and Lowes, it measures 3-1/2"x6' and you place it along the foundation, before your pour.

As mentioned, give the new slab a tiny slop away from the house, for drainage.

Re: Help: Cave underneath concrete steps


Isolation joint material is a low-cost item sold in the big box stores either right next to the Quikrete concrete bags or in the outside Landscape area; for residential work it's usually 3/4" or 1/2" thick, tar-impregnated fiber board in 4" or 6" widths & approx 10' long; I've even used the 3/4" styrofoam sheets, since they easily cut with a utility knife & often cost less; tar paper can be used, but it usually looks disheveled & is tough to apply to get to a 1/2" or 3/4" thickness when used along a slab---it is more likely used around 4" piping/ducts, etc., before a concrete pour to prevent the 4" pipe from cracking the new slab; a short 1/2" thick isolation joint is also used if you have a long slab to pour & can only do 1/2 the slab in one day; in such a case a short piece of isolation joint is placed before the nest day's work is continued.

Yes, I would install at least one more piece of rebar under the stairs; it looks in one of the photos like there's another cut short piece of rebar; if you just take a hammer or hand sledge hammer & strike it several times, you should be able to knock it (bend it) downwards so that it will also catch the fresh concrete when it is poured & also act as a tie-in between the old work & the new; a standard slab thickness for this kind of work would be 4".

Quikrete has a quantity calculator at the site below, if you intend to use mix your own concrete sold in 60 lb or 80 lb bags at the big box stores; this usually means mixing one or two bags with water at a time in a wheelbarrow, or a self-constructed wooden trough.


Re: Help: Cave underneath concrete steps

It might be a good idea to dig and pour a couple peers where the slab will end. Sauna tube with rebar bent 90 degs heading towards the step.....It will help some to reduce the new slab from sagging over time......


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