Stone foundation repair question

[Zachary Hoyt]

I have run into an unpleasant surprise when I started repairing the collapsed foundation wall at the house I am working on, and I'm hoping that someone here may have some advice about what to do now. The wall I am working on is about 12 feet long and 6 feet high, and close to 2 feet thick. In the middle near the bottom of the wall there is a huge stone which has a pretty smooth top set at a slope of about 30 degrees. I dug under the stone and around it and tried to roll it forward so the sloping face would become vertical, but it would not move with a 4-1/2 foot crowbar or an 8 ton jack. I am thinking about several possible ways to repair this wall, but they may all be bad ideas for all I know.
1. Make a buttress of stone in front of the sloping stone
2. Drill into the stone (with a hammer drill?) and put some pins in, sticking up.
3. Lay some rebar across the stone, with the ends buried in the new wall on either side
4. Make a header over the sloping stone with some heavy steel, supported on either side on the new wall.

I'll be grateful for any ideas or guidance. I had thought it would just be a matter of removing the rubble from the collapse and cleaning up and re-laying the stone, but I wasn't expecting this extra wrinkle.

[Maurice Mcmurry]

I would try option 2 combined with option 3. Concrete bonding adhesive and type S mortar should allow you to make a flat top on that rock. You could make a small form to pack mortar or concrete against. Your project is quite an undertaking!

[Kev Williams]

I honestly haven't a clue on how to help, but I had a hard time figuring out what I was looking at, so I added some light to the pics, maybe it'll help others help out?

r1.jpg r2.jpg r3.jpg

[Tom M King]

I would cut a flat spot on top of that stone with a hammer drill, and wedges and shims, and relay the wall. When laying stone, it's best to leave your best flat spot on top so the next layer has a better chance of finding somewhere to rest. The reason is obvious here. Drilling the hole for that pipe took out a good section of that wall just because they put the sloped side up.

[Zachary Hoyt]

Thank you all very much for your advice. I have ordered 7 sets of wedges and shims and will give that method a try. If I can knock off the front or make a ledge I will be very pleased, and splitting stone will be a useful skill for later. Can I use a regular masonry drill bit or do I need a special bit to drill stone? I meant to ask that before.

[Michael Schuch]

An SDS Max type hammer drill should drill through the rock with no problems. Like said above if you drill several holes you should be able to knock the top of the stone off. A horizontal top on the rock with the remainder of the drilled holes should make a very high friction surface to hold what ever is put on top of it.

I have drilled through several rocks with a hammer drill and it usually goes pretty easily. I have always drilled vertically and the weight of the drill is usually about right to supply the needed down force. I like to use a trickle of water the keep the tip of the drill cool and wet the dust to keep it from blowing up into a rock dust cloud. You should wear a mask while drilling rock! The dust from drilling is quite fine, pretty easy to inhale and the drilling will kick it up into a cloud. The Bosch SDS Max drill bits are of good quality and last quite long as well as cutting quite well. The 4 cutting surface bits (two straight lines of cutting surface across the bit face perpendicular to each other) seem to cut faster and last longer. Cheap HF hammer bits aren't worth the hassle in my experience.

Regular masonry / concrete bits go through the basalt rocks in my local quite easily.

I hope you aren't thinking of using a regular hand drill that claims to be a "hammer drill". Rent a SDS Max hammer drill and it will take care of the job quite easily and save you a lot of effort. I picked up an old Bosch spline drive hammer drill from a second hand store for $40. It lasted about 5 minutes before it refused to drill no more. I put in a pair of new motor brushes and have gotten more use out of it than I had ever imagined I would. Cleaving the face off a rock with several holes drilled in it it has been quite easy for me (like I said previously; local basalt rocks) By the time you get to the 4th hole the chunk will usually fall off by itself. You will probably discover pretty quickly that you will need to start drilling perpendicular to the surface of the rock. Once you have a pocket formed you can rotate the drill and bit to a horizontal position.

I would be concerned about the vibrations from a hammer drill dislodging the rocks above the one you are drilling... please be careful!

https://www.ebay.com/itm/29499777597...IAAOSwc8BigZWw

[Tom M King]

Here's a video from my favorite stone tool manufacturer on wedges and shims. I'm sure there are many youtube videos too. You can buy Chinese wedges and shims off Amazon much cheaper than Trow and Holden. I spray paint them with flourescent colors to make it easy to find them after a stone splits.

https://www.youtube.com/watch?v=NueQ2Lx6eKo

The stone in that foundation looks like river rocks. They drill pretty easily, so you may not even need a hammer drill.

[Jim Becker]

Lots of walls are built with stones that are not flat/level, including angled as you show. If that stone is stable, leave it be. With or without reinforcement by pins, etc., when you mortar in new stones and rubble, the cement will make things strong. Working some rebar into the repair isn't a horrible idea, IMHO.

[Maurice Mcmurry]

Stone masons around here work just as Jim has described. Even using wire to hold wonky rocks in place until the mortar sets. The concrete bonding adhesive painted on clean stone prior to type S mortar is very strong and very sticky. It is also rated for thick mortar joints. I have packed type S into joints up to 4 inches thick with good results.

[Bill Dufour]

FYI My Hilti hammer drill is rated for a 1/2" diameter hole at 18 inches a minute in hard concrete. So much faster then a home store version.. Stone will have silica so wear a dust mask. Use a hose to flush out the hole from time to time or you waste time recutting the dust. A drinking straw and a mouth full of water can be enough to get a face full of muck.
Bill D

[Lawrence Duckworth]

...years ago Dow Chemical had a product called Sarabond and could be used to lay an 8' jackarch w/o a steel lintel for support. ifn the stones are cleaned well I would mix a gallon of bonding agent from home depot with every batch of type S and forgo all the rebar and hole drilling.

....opinions


bonding agent is a polymer (Elmers glue)

[Thomas McCurnin]

As the owner of a wilderness cabin with a rustic stone foundation, I agree with Jim that trying to fix that single stone is probably a fool's errand. A better approach is to try to stabilize the foundation with Type M Mortar, chicken wire, No. 4 rebar above and below the miscreant stone. I do like the idea of a header of sorts which could be as easy as Number 4 rebar covered with Mortar.

[Zachary Hoyt]

I figure it must be time for a followup on this thread, since I finally completed the work on the foundation yesterday. I drilled 4 holes in the sloping rock and tried to break it with wedges and shims. I hit the wedges with a 40 oz hammer for a while, and then a 16 pound sledge (not that I could get a good swing, but still, I hit them pretty hard) and nothing happened. It took about an hour to drill each 3/4" hole to 4" depth with my little hammer drill.

I finally got the wedges back out of the holes and put in rebar in the holes, and also laying across the top of the sloped part and built the wall on that. I'm not proud of the result, but it's not much worse looking than the other walls of the basement, and they have held up the house for over 100 years and through being flooded, so I hope it will be okay for a while. Eventually the rebar will rust through and it will probably collapse again, but I hope that won't happen for 30 or 50 years. There were two of the biggest stones that fell from the wall that I was not able to break with the sledgehammer, though I did make one of them smaller by chipping off bits. Some of the other big stones broke into nice chunks after only 4 or 5 hits. These rocks seemed to have a grain along which they would break, and the ones that wouldn't break seemed to be going all over the place, at least the bits I broke off seemed quite random.

Thank you all for your advice. I wish I had been able to do a better job, but time was running short. I put more than 3/4 of a dry ton of mortar into the wall, and tried to follow the rules of putting rocks where they would stay without the mortar and leaving a flat surface on top of each rock.

[Frank Pratt]

It took about an hour to drill each 3/4" hole to 4" depth with my little hammer drill.

If you'd have rented a rotary hammer, it would have taken only a few minutes per hole. The little 'hammer drills' are not the real deal. They are more of a noise maker than a hammer drill.

[Zachary Hoyt]

Yes, I know, but I would have had to drive 30 miles each way to rent and return the drill, plus paying for the rental, so it didn't seem worth it. I also wondered if the more aggressive hammering would shake more stones loose from overhead, and if I squashed their expensive drill I would have to pay, besides any ensuing medical expenses.