Concrete slabs - what to do...

[scorrpio]

I am facing a brain bender. I am equipping a workshop in my recently bought house. It is a detached piece, sitting off southwest corner of the house. The electric service/meter are southeast corner, main panel also there inside basement. 200 amp service, have free space to spare. A subpanel is installed in the shop, and I have 6/3+ground UF-rated cable connecting it. The idea was to run the cable along basement's ceiling, have it exit in southwest corner, and there run underground to the shop.

Problem: east, south, and west sides of my house have a walkway going all the way around. It is about 4ft wide and is made up of concrete slabs a good 7 - 8 inches thick. For the time being, cable runs above ground, but I somehow want to eventially get it underground, which means going under the concrete. Without royally messing up my walkway. Anywone got any ideas on how best to tackle it? Try lifting a slab with jacks, making a channel underneath routing cable and putting the slab back down? or try to burrow a channel from side without disturbing the slab? Any other ideas? Thanks!

[Jim Boyd]

Look up hydraulic tunneling. It can be done with pvc pipe and a garden hose for sidewalks and driveways.

[boblon]

I have seen folks dig a hole on one side of the walkway. Attack a garden hose to a length of PVC pipe. Turn on the hose and work the PVC under the slab until it exits the other side.

The running water works the dirt/sand out of the way, like a water drill. Now, this is in Florida, don't know if it will work up there in the north country.

BobL.

[LarryG]

I would tunnel under it, by some means. I've never heard of the PVC method described above but that sounds perfect. I wouldn't try to lift the slab because of the hassle and the difficulty of getting it reseated as it was and thus ending up with a tripping hazard at the joints.

You may know for certain the walks are that thick, or it could be they are just thickened at the edge, a common practice. However even a 4" slab will easily span way more than the distance you'll need to hollow out to pull your wire.

[LCHIEN]

I've heard of that PVC and hose method, too.
Should work for tunneling under s narrow sidewalk.

[Hellrazor]

It works, if you like getting dirty, you can use a pressure washer too.

[Tom Miller]

I assume that the "tunneling under the walk" approach eventually means drilling through the foundation? This may require a larger tunnel than what you'd get with the pvc/hose method, unless you drill from the inside with some degree of accuracy. I guess it could be done.

Regards,
Tom

[scorrpio]

Nope, I will be drilling a vertical hole next to the wall, and in the rim joist, the cable will go through a right angle box and some 1.1/4" rigid conduit. That water and pipe method might work, though if I try to work directly under the slab I will primarily have to deal with the gravel. Guess I'll have to wait till summer to do it though. Thanks for the advice!

[jlm]

I've seen it done this way:

Dig a trench up to the walkway on either side, then cut a piece of conduit about a foot or so longer than the width of the walkway. Flatten about an inch of one end of the conduit so it doesn't get packed with dirt, then just hammer it through the soil under the walkway, flat end first. The sharp, flat section should cut through the dirt like butter. Once it's through, hacksaw off the flattened bit (and probably the bunged up bit on the other end from where you hammered - use a block of wood to soften the blows, too), attach fittings to the ends, and hook it up to the rest of your conduit.

This method works best if you have a couple of feet on one side of the walkway where you can run your trench at 90 degrees to the slabs, so you have room to hammer the conduit through straight and level.

[gjat]

scorpio,
Speaking from multiple personal experiences, this is how I've done it. 4' is not very far, and a water hose is unneeded mess and can wash large voids under your sidewalk. I'm not sure abour your soil conditions either. I dig a trench up to the sidewalk on both sides and under the sidewalk about 6-12", leaving only 2-3'. Take a 2" piece of pipe (pvc is sandy, metal if rock/clay) about 5' long. Using a sledge hammer, pound it across. About every 6", wiggle it loose, pull it out, and knock the dirt out of it. When you get across, you can use a smaller pipe to clean the rest of the dirt out. Badda-bing, you now have a sleeve to run a smaller conduit or cable in, or attach your new conduit to it. The first one you do will take an hour or so, then you'll learn how far to drive it without getting it stuck and good pounding techniques. Oh yeah, if you use metal conduit and you want to attach to it with a threaded coupling, thread a coupling on it to protect the threads.

[Tom.Trout]

You can get yourself an 2" auger with and extension and dig down under the walk and auger under it. I've done this.

quote:Originally posted by scorrpio

I am facing a brain bender. I am equipping a workshop in my recently bought house. It is a detached piece, sitting off southwest corner of the house. The electric service/meter are southeast corner, main panel also there inside basement. 200 amp service, have free space to spare. A subpanel is installed in the shop, and I have 6/3+ground UF-rated cable connecting it. The idea was to run the cable along basement's ceiling, have it exit in southwest corner, and there run underground to the shop.

Problem: east, south, and west sides of my house have a walkway going all the way around. It is about 4ft wide and is made up of concrete slabs a good 7 - 8 inches thick. For the time being, cable runs above ground, but I somehow want to eventially get it underground, which means going under the concrete. Without royally messing up my walkway. Anywone got any ideas on how best to tackle it? Try lifting a slab with jacks, making a channel underneath routing cable and putting the slab back down? or try to burrow a channel from side without disturbing the slab? Any other ideas? Thanks!

[scorrpio]

Ok, looks like boring under walkway it is.
Another question: I will need to lower the cable to needed depth, and I am using 1.1/4 (1.5" OD) conduit for a riser. Since walkway slabs are tight to the basement, I'll have to bore a 1.5 - 2" hole. I can get a set of tungsten carbide masonry hole saws for about $30 - does anyone have any experience about drilling concrete with those? Another option is renting a Hilti drill from BORG, but I understand it is a heavy artillery for drilling many holes at once and fast. I don't care if it'll take some real slow going (like 2-3 hours per inch) to cut the hole.
Yet another option is to chuck a thinner bit into a hammer drill and drill several holes in a circle...

[gjat]

If you're drilling through the sidewalk, go with the hammerdrill and drill several holes to take out a 'plug'. I also use a cold chisel. If you're going through cinderblock wall, I chisel a circle and wack it with a hammer on each side. Of course, patch with grout mix.

[onedash]

my uncle did that at my parents house when I was a kid. I think he drilled through the wood sill, dont think he used water to make the hole under the sidewalk. He didnt use any conduit. It was the gray cable outdoor? cable and when he went into the garage he came up along the foundation and drilled through the wood. So you can see about two inches of cable going into the garage. Not 100 percent sure about the house side. Probably not up to code but it works.

[LJR]

If I understand the description correctly. You've got the UF in the house at basement ceiling height and wish to esit the house through the rim joist. If you run UF all the way out to the shop it needs to be buried a minimum of 24 inches deep. Any chance you can drop down inside the basement to that level, drill a hole through the basement wall and then continue "drilling" the width of the sidewalk, then you can start your trench.

If the UF exits through the rim joist and then you install an LB or junction box to make the turn down you need to extend your conduit to at least 24" deep to protect the UF. I don't think you're going to be able to do that by drilling a hole through the sidewalk from the top side and then tunneling under the sidewalk. If I had to go that route I'd cut a section of sidewalk out wide enough to allow hand digging of a trench, run the wire and then pour the sidewalk back. You really wouldn't need to cut a piece very wide, I think you could work a drain spade in an 8" wide sidewalk cut.