"No brain dump underpin question" 2

[privymaster]

I would like to request PEinc's opinion on this crazy little underpin project I started. After ready some really interesting posts on this subject, I have failed to come across my question in the matter. I am lowering the floor about 14" in my 54x54 1864 2 story Italianate building in Michigan. The 2 foot thick field stone basement walls just sit on top of the virgin sand/peagravel earth right at the level of a 1930's poured concrete floor. Hence, when the concrete is removed, the bottom of the stones are right there! The foundation is in very good condition due to the marvelous soil she resides in. I have been doing a pretty Scrupulous self educated underpining job and find the work EXTREMELY tedious, but very gratifying. I have already sucessfully shored and poured all the 26 new column pads down at the finnished grade "new" slab level. Now i'm underpining 4-5 foot sections around the perimeter
and have some questions after ready all your posts. I have read about "dry packing" and have not done it this way, here is why. Am I OK??? the feild stones hold together great when I undermine them in the sections, but they are often filled with voids that run 6 inches up inside the bottom of the foundation wall in very random sequences. I don't belive with that situation, I could effectively fill those voids by cramming a mixture in there? Here is what I have been doing,. If the void goes up (in the center of the wall) say 4 inches, I make the form 5-6 inches up the outside stones, to bring the concrete level (up in the middle) high enough to fill that void. I hope you follow me here. The concrete 5.5 sack seems to really hydraulic its way up in there. I really slam those wheel barrells in. Next, I havn't been using re-bar, that I think is ok, Last, Do these segments need to be re-bared together. My blocks start flush at the outside of the wall,meeting undesturbed sand, and run 8 inches past the inside wall. I'm trying to get the building to last another 140 years. I love it like a son. Thanks!

 

[GeoPaveTraffic]

privymaster,

WOW, you have really tackled a big project for your first entry into underpinning. It sounds like you are doing everything just fine. I don't see any reason to attemp to rebar tie the different sections together, nothing else is tied in that old construction.

As to the voids in the center of the walls, that is not uncommon in very old walls like yours. It was common to build to walls, outside and inside, then to place brocken peices of stone, gravel, etc. between to the two walls. I have always heard it called "rubble wall" construction.

Good Luck and happy digging.

 

[PEinc]

privymaster,

From your description, it sounds like you are doing things reasonably well. I haven't encountered a rubble stone wall that I haven't been able to drypack, but I'm sure there can be walls like yours with voids. I've also had rubble stone walls that had to be entirely replaced during the ubderpinning operation because they just fell apart, but that's another story. To be sure that you have good bearing of the rubble stone on top of the concrete piers, you may also try to pour some grout into the wall, through the voids, just above the hardened piers. While dry packing is the preferred method of transfering the building load to the piers, there are many times when something else needs to be done. This is probably one of those times.

Your underpinning piers should not need rebars or shear keys between piers. Unreinforced concrete piers are better than the existing rubble stone. Rebars will not make any significant contribution to the end product.

If the rubble stone wall is not able to stay together with a 4 or 5 foot wide pit excavated below, you may need to close up the pier widths a little more. I also would try to keep the front face of my pits even with the inside face of the foundation wall. This will give you more room inside the basement. If necessary, I'd let the rear face of the piers extend beyond the outside face of the foundation wall unless there is some reason why this can't be done. I'm not sure why you would need to have underpinning piers wider than the 2' foundation wall unless you were adding load or will be bearing in less competent material. Usually underpinning piers are sized for bearing and for allowing a worker to fit inside the pits when going deep. Your piers are shallow so you could make them the same width (2') as the foundation wall.

You are right, underpinning is very tedious. However, if not done correctly, the consequences can be serious and expensive.

 

[privymaster]

Thank you very much for your comments, and accrued knowledge. I still am a little concerned about the drypacking. Is that to reduce the chance of uneven bearing points under the wall? I made the piers a little larger on the inside just to reduce the chance of lateral pressure pushing the piers inward, but in the sand and gravel I'm in, and the the fact of the new slab floor that will be poured in front of the piers, It wasn't probably necessary. I am also wondering now, if I shouldn't have just poured a butress type border around the hole thing. I've only got a few sections done already, could easily switch to that. But it seems putting a real foundation under her will be better? Thanks again, Scott

http://www.privymaster.org/Sidingsidewalk04.jpg

 

[PEinc]

Dry packing is intended to provide full bearing beneath the existing foundation and to provide a little pre-load to the underpinning piers. Packing-in the dry pack is similar to wedging between the piers and the footing.

Making the underpinning piers wider than the above footing or wall will not make the footing or wall more stable. However, it will make the underpinning piers more stable. I hope you did some design prior to starting the work.

Based on what little information you have provided to us, it sounds like you are OK with what you are doing.