Help with a foundation issue...

[e36jon]

Greetings all

I am trying to pass a water line and an electrical conduit under my perimeter foundation (Below grey breaker box in photo.). In the not too distant future I also want to replace the slab as well. My issue is that the house is built on sand (Good ol' beach sand. Really.) and doing any excavation is frustrating as the sand just flows back into the hole, and in the current case potentially harmful as the foundation may become unsupported.

In the pic below you can see my attempt to drive in some perimeter shoring (?) to keep the sand from flowing out from beneath the slab, the foundation, and a large post footer.(Bottom of pic. I poured those myself buy building a 4-sided form that I drove down and excavated at the same time. The dimensions and depth were both less than the current effort.) When it's time to do the whole slab driving in sheets of material all the way around seems like a non-starter.

The 1/2" ply is a bear to drive in (It needs to be 18" deep). The corrugated (Foamed polycarb, from Home Depot) is too flexible to work well. Drinking beer and staring at the problem has also proven ineffective, but I'm not giving up on it yet.

So, how do others deal with this situation? Is there something I can spray the sand with to stabilize it? I looked but didn't find anything consumer grade. Is there a tool or equipment I should be renting?

Any and all help appreciated.

Cheers,

Jon

 

[Red 17]

I think you need an engineer to assess the soil and suggest some kind of augmentation to firm it up. I am thinking something like mortar mix, fence post concrete, or something similar. Also, driving pipe, augering out some sand, drop some gravel in the pipe, pull the pipe. Repeat until you have a more stable mix.

You could also use a larger diameter pipe as shoring under the box and for a bit of the distance. A big diameter round concrete form might work.
https://www.sonotube.com/sonotubeconcreteforms/round.aspx

 

[TommyK]

Given the space limitations I don't think there is an easy answer. Where effective shoring is impractical you dig what you have to dig and then deal with with the extent of the undermining by cutting back the slab to solid ground and/or buy filling voids with concrete or flowable fill.

The only thing you might try is using some 3/8' plate steel that you can wail on with a sledge hammer as you dig down to bottom of footing but if you have to buy it won't be cheap. You aren't going to be able to drive plywood without destroying it.

 

[Red 17]

Steel or well braced wooden box/rectangle can also be used as shoring. Just as with the tube suggestion above, and just as digging a well, you excavate inside the box undermining the walls of the box. It will sink and give you room to work inside.

If that sand is dry you could even excavate with a shop vac if that would make it easier.

 

[e36jon]

Thanks for the quick feedback! And here I was hoping it was just a matter of increasing my beer intake...

TommyK: In the photo you can see my attempt at making some blocks to go on top of my sheet goods so that I could wail on it with my 10lb sledge. I even tried my rotary hammer. Without being able to excavate as I go it's just too tough to do...

Red17: The actual digging is easy money since it's just loose sand. I don't have the nightmare of rocks or heavy clay... Your 'remove-and-replace' trick may work for the slab replacement as I could chip away at that myself before I had a crew come in to do the bulk of the project. I want to do a 5" slab over 6" of compacted base which puts me below the bottom of the perimeter foundation...

 

[JimR1998]

If you tunnel in from the outside do you have more room to work? If you can dig a deeper hole outside you may be able to push through a hard conduit. Use a shopvac to **** out the sand in the conduit, use a long drill to loosen it up if it gets too hard.

I think that wet sand may be easier to stabilize than dry sand. Just thinking of the kids making sand castles on the beach. I've also seen some youtube videos where they tunnel under a sidewalk or driveway using water pressure. If nothing else, something to watch when drinking beer...

 

[TommyK]

Thanks for the quick feedback! And here I was hoping it was just a matter of increasing my beer intake...

TommyK: In the photo you can see my attempt at making some blocks to go on top of my sheet goods so that I could wail on it with my 10lb sledge. I even tried my rotary hammer. Without being able to excavate as I go it's just too tough to do...

Red17: The actual digging is easy money since it's just loose sand. I don't have the nightmare of rocks or heavy clay... Your 'remove-and-replace' trick may work for the slab replacement as I could chip away at that myself before I had a crew come in to do the bulk of the project. I want to do a 5" slab over 6" of compacted base which puts me below the bottom of the perimeter foundation...

You have to pound it down a couple of inches and excavate a couple of inches and repeat. This is the same principle as using a trench box. I've done it down to 20 feet deep. It takes time.