Filling gap with concrete / bottom end very deep

Hello All!
I have a porch similar to the one in the pic below. The central part of the porch is a concrete slab ( pretty sure ) and the perimeter of the porch is stone. There is a gap in between the concrete and stone area. Please look at the second picture below.
I wanna fill the gap with concrete. The problem is that the gap seems to be pretty deep. I am am afraid that any concrete I shove in there will fall through. How can I stuff concrete in the gap without the concrete falling through?
Edit: That hole is under the porch. The hole probably can take maybe 300 of those 5 gallon containers. Thats how much volume of water or anything this hole can let through.

Thank you

 

Dig or wash or blow out all that loose dirt and stuff to get a nice clean groove to patch. You do not want dirt or foreign stuff in there. Now maybe it is not all bad if stuff falls down into the void. You have to be he judge of how much material it will take to fill it if it falls through. So what if it takes a five gallon bucket full of mortar to fill it? But if you think that is too much to fill then you could try filling it with sand part way up or stuff something inorganic like fiberglass insulation down about three or four inches and hope it stops the mortar you are going to fill it with. Leave three or four inches to fill with mortar.
Mix Portland cement 1 part cement : three parts sand by volume. Dry mix this then mix the mortar with water to a pourable consistency. Oh I forgot. reserve enough Portland to mix a slurry with water to the consistency of thick cream or paint and just before you pour the mixture into the groove paint both sides of the groove with that slurry. This will help the material bond to the sides of the groove. Press some of your mixture against the sides of the grove to bond the mixture to the sides then fill it full and let it start to set. As it sets screed it flush and smooth it down. If you are concerned about wasting a lot of stuff going into the void then just remember: to a point the more material you get in there the stronger the repair will be.
You could use bagged, premixed mortar rather than buy your own cement and sand. Estimate how much it will take to fill the gap or void. The bag will tell you about how much by volume the bag will yield. You won’t have and Portland to paint the sides with but still press some of the mortar agains the sides to try to bond it. Now it is going to crack on one side or the other if there is still movement there and there probably is. Wait a few weeks or months then if it seems stable and the crack bothers you you can caulk it.

 

Fill it with sand and foam backer rod!

 

This is the OP:

That hole is under the porch. The hole probably can take maybe 300 of those 5 gallon containers. Thats how much volume of water or anything this hole can let through. I dont think I can just fill in the hole with some extra sand. The sand or whatever will keep going down and to the sides.

 

Hi Tightcoat. Should I put some aggregates into the 1 cement:3 sand mixture? Shouldn't I need to put 6 parts aggregate into that mixture?? Ty

 

Can I use the following instead of making my own 1 cement : 3 sand mixture?

https://www.homedepot.com/p/Quikrete...4020/100318532

 

Your going to have to find something to fill it up with, that much cement is expensive and a lot of work to just keep pushing it down into the void.

 

You could stuff some solid foam board in toward the bottom. Wedge it in tightly so it will support the mortar until it sets. Leave a couple of inches open above the foam for the mortar.

 

Stuff the gap with foam backer rod. It was designed for this purpose. Google "foam backer rod".

Just curious has anyone tried an expanding spray foam (Great Stuff) as a backer before filling with mortar?

 

300 of those five gallon buckets is 200 cu ft or 7 .5 cu yds. That is a big pile of sand and a lot of mortar or concrete. I think you exaggerate. If here is that much void what is holding up the slabs? And maybe you need to use mortar or concrete to give the slabs support.
At the price of the quick setting stuff for 20 # you could mix a lot of mortar. Just how wide is the gap?. The suggestions to fill the gap with something that mortar will not just push into the void are good suggestions. If you add more coarse aggregate to the mix I described it will be harder to fill the gap. And harder to finish. Use concrete mix instead of mortar mix if you buy bags of preblended material. The yield is still on the bags.
Calculate the volume of the gap after you clean the debris out of it and insert something to stop the mortar or concrete from falling into the void then decide if you want to use the quick setting stuff or mortar or concrete and do the job.
Sand is cheap but heavy if you have to carry it far. foam or backer rod or insulation or some kind of stopping material will save a lot of mortar or concrete. Leave plenty of depth. 4”. Is enter than 1”.

Yes, I am giving conflicting advice. You are there to see and decide what is best. If there is really thaT much void I think I would fill it with concrete if I could get it to flow through the gap

 

OK, lets think this one through for a second!!

If tighcoat did his math correctly were talking the equitant of a 30x40 slab, 4" thick.

Thinking a reality check is due?

 

If the stairs were closer when originally set in place then they are moving. I would want to stop this movement before addressing the gap.