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Underpinning Questions
4

Underpinning Questions

Underpinning Questions

(OP)
Hello,
I live in a 1950's single family brick cape cod home (30' x 30') just outside Philadelphia. We have always dealt with a wet basement and in particular mud/clay and silt coming in at the basement floor to wall joints. The basement height and height to ground are both 7 feet. The basement walls are rubble stone ~16 inches thick. The stones appear to be ~ 1 ft long at the "footer" level. The inside of the wall is parged 1" thick and the floor is monolithic ~2" thick. An interior french drain was installed before we bought the home over 10 years ago. After about 5 years, a decent amount of mud started coming in because the french drain system clogged along one wall... Anyway, being an engineer, I am concerned about foundation undermining and thought I would mass underpin the rubble foundation so I could place the waterproofing a minimum of 6" above the new footer instead of below it which it currently is. This involves underpinning in sections of 4' between 14" to 18" below the existing foundation and leaving a 2" - 3" gap above the pour to place dry pack the next day. I have a drawing that calls 2 #5 bars fairly continuous between the 4' sections.

My questions are:
1) In a 70 year old home, what should be done while excavating to ensure the "footer" stones don't drop? I don't think that ones above them will due to the interior parging, but not too sure about the condition of the mortar inside the wall.

2) Since I'm only doing this for waterproofing and the floor is staying in the same place, do I need the horizontal rebar? The floor will laterally support the bottom of the original wall and there will be 6" or soil at the bottom of the new sections.

Thanks

RE: Underpinning Questions

Where did the drawing come from?

RE: Underpinning Questions

Could you supply a sketch?

RE: Underpinning Questions

(OP)
Thanks for the replies. I got the drawing from a local Structural Engineer. I would like to get ideas/advise here to discuss these aspects before proceeding with the job. I've attached some sketches. The first is the drawing for my house from the structural engineer (disregard my chicken scratch - It is signed and sealed, but I clipped that out). The second is "REFERENCE" from another sketch I found online that has more detailed instructions called out (however it is for a basement deepening and without waterproofing).

RE: Underpinning Questions

Because your existing french drain has clogged up in about 5 years, what makes you think that the stone-filled drain that is shown will not also clog up? You do not need high-early concrete. Your drypack does not need to be Embeco or similar. You do not need reinforcing steel or vertical key ways between piers. Also, check the site grading around your house to make sure that any surface water runs away from your foundation walls.

www.PeirceEngineering.com

RE: Underpinning Questions

(OP)
Hi PEinc - Really appreciate and respect your inputs. A couple things about the existing drain/situation...
1) Not sure how long it was installed prior to buying the home
2) Installed with very shallow pitch with not much stone - assuming they knew enough to limit amount of soil removed under foundation
3) Installed about 2" - 4" below bottom stone bottom surface (see sketch)
4) Only bottom and sides wrapped in geo-textile fabric
5) Shower stall material used to capture water from wall, but interior parging stops at floor. Therefore, gaping created an area mud could extrude through exposed bottom rock course voids

I think that with better pitch, more stones, 100% wrap of fabric, and drain pipe above the bottom of the footer it should last much longer.

The yard slope is okay (could always be more pitch). I think the problem stems from old tree root decay from two former trees ~15 ft from the house.

What drypack and concrete would you recommend?

The engineer said I might regret not placing continuous reinforcement. I've gotten extremely conflicting information about this. Is your rationale based on the lateral restraint that the drypack and ground provide in shear?

Any advise on my first question?

Much thanks again!

RE: Underpinning Questions

Add a little to PEinc. If you change that single sized drain backfill to ASTM C33 concrete sand (fine aggregate) from your ready-mix concrete supplier and wrap the pipe with filter fabric, you should get by with a long life for the drain. If the pipe has holes on the bottom 1/4, filter fabric can be omitted.


Edit: Wrapping the clear stone with fabric sounds neat, but a "bitch" to do it right. You can't goof it up with concrete sand. Leave that clear stone for another person to use elsewhere. My instructions usually are "No clear stone allowed on the job". They who use it never come back to fix the plugged drain.

RE: Underpinning Questions

Drypack can be 2 shovels of sand to 1 shovel of Type I, II, or II Portland cement. Add just enough water to make a moist (not wet) mixture. You should be able to squeeze a handful into a nice firm ball. Ram it into place using a small sledge hammer hitting on the end of a 2x4 as the ram rod. The underpinning concrete can be almost any readily available strength. 3000 psi to 4000 psi concrete is common. If you are pumping the concrete, you will need a cement-rich pump mix, possibly with smaller (about 3/8") and rounded large aggregate.

I have designed many dozens of underpinning jobs and have also constructed many, especially in the Philadelphia area. Reinforcing steel in underpinning is almost NEVER needed. Remember, unreinforced concrete will be much better than the rubble stone foundation wall that has lasted 70 years. Most block foundation walls have very little to no reinforcing steel. Unreinforced concrete will be fine and is typically used. The compression stress in relatively massive underpinning concrete is almost always very minor, almost negligible. Overturning of the underpinning should not be a big concern for your shallow underpinning. For the same reason, I expect that sliding will not be a problem either. Sliding would be a consideration only when you excavate to install the perimeter drain.

About Question #4: With a rubble stone foundation, your underpinning pier width should be about 4 feet. If it looks like any stones want to fall out, you should close the underpinning width to maybe 3 feet. If only a stone or two fall out when digging a pit, that's not usually a big deal. You can just install more drypack to fill the missing stone area. Sometimes you may need to build a temporary shelf to support the stones during excavation. Then, pull the shelf out to install drypack. Be careful when underpinning in saturated and soft soils. The excavation for each underpinning pit should be shored to prevent soil collapse and underminning of more than the theoretical pit width.

Check out my web site for a presentation on Philadelphia’s Recent Building Collapses – Causes and Prevention. There are underpinning details and photos.

www.PeirceEngineering.com

RE: Underpinning Questions

(OP)
PEinc,
Great information. Can you provide any further details on the "shelf" material, construction, and placement during the digout? The wall stones appear to be generally 1 ft in length and 6" to 9" into the wall and they are around 4" thick. Would you still recommend drypack with such large stones if one were to fall?

Excellent presentation 👍

Thanks

RE: Underpinning Questions

I'd like PEinc's thoughts on this. Suppose before this job gets going that some preparatory work is done to stabilize the stones. How about adding a temporary "dam" of plywood from the bottom of the stone foundation and up about a foot or so. Then inject into the voids a sloppy grout mix of cement and water, no sand? Allow that to set and proceed with the job of underpinning. Adding some vibration to the stones, as by a vibrator pressed against the plywood with the fresh liquid in back of it. Might help get the liquid to fill most of the lower voids. In a trial first evaluate how it does, consider washing out some dirt before injecting the grout.

RE: Underpinning Questions

The presentation I mentioned shows the exact loose rubble stone situation being questioned. If you see that the rubble stone wall looks very loose or if these is little or no mortar, you can repoint the stones, parge the wall, or even shotcrete it before underpinning.

www.PeirceEngineering.com

RE: Underpinning Questions

PEinc - you are a great resource for the underpinning questions (and various other questions) that pop up. I have not undertaken any underpinning projects but from what I have read from you on this site, it is a very specialist field and a lot of engineers are likely over engineering it!

RE: Underpinning Questions

Here is another photo of a wood shelf that was installed to protect the worker in the pit from falling stones. Zoom in to see it more clearly. It isn't pretty but it is served the purpose. Additional blocking could have been placed on the shelf if the remaining stones were still loose,

www.PeirceEngineering.com

RE: Underpinning Questions

(OP)
PEinc,
Thank you so much for the extra photos and wisdom. This helps considerably. Looking at this and your presentation, I'm wondering what type of wood you are using. Would standard untreated pine suffice or do I need something else as a minimum? Also, since I won't have access to the exterior after the pour, the exterior width boards would be left in to rot. Is that okay, or is it preferred to remove them prior to the pour? Since, my dig heights are relatively shallow (14 to 18"), I don't think I'll run into needing the exterior boards for soil shoring. Thanks again, as all your responses have given me tons of insight into this project.

RE: Underpinning Questions

The horizontal shoring boards are dressed 2x10's, softwood (Pine, Hem-Fir, or Doug Fir-Larch)), untreated. On some (very few) jobs, the project engineer may want CCA-treated wood if worrying about termites (usually for museums and other historic buildings). The vertical corner cleat boards are 2x4's. These boards work for most underpinning piers where the maximum side length may be about 6' or less (and depending on the competency of the soil). Infrequently, pits are bigger and full-cut, 2x10's or 3x10's may be used.

The only boards that get removed are those boards between adjacent underpinning piers and those on the front of underpinning that will eventually be exposed. The other boards cannot be removed.

Usually underpinning pit excavations are shored with lumber, especially when the pits are deeper than about 4 feet (OSHA requirements, confined space, etc.). If you are digging shallow pits, and if the sides stand satisfactorily, and if the excavated pits are not too wide, and if you will be placing the concrete the same day; you may be able to get away without shoring the 2 sides and back. You should form the front of the shallow pier with braced plywood to have a neat, plumb concrete face.

www.PeirceEngineering.com

RE: Underpinning Questions

(OP)
I see. So if you leave the earth side boards and cleats, do you just attach another face cleat to the previously embedded cleat when you come to the final piers between cured poured piers? Thanks.

RE: Underpinning Questions

When you dig a pit between 2 completed underpinning piers, you strip the side, horizontal, shoring boards from the adjacent, completed piers and you install the rear horizontal boards. The ends of these boards can be tucked behind the corners of the adjacent piers or you can install 2 new, vertical, 2x4 cleats to the rear cleats of the adjacent piers if their cleats are still there. The same can be done for the front, horizontal boards once you excavate below the approach pit.

www.PeirceEngineering.com

RE: Underpinning Questions

I agree with OG about the ASTM C-33 sand. It is generally considered to be a universal filter and should keep back the mud you have been seeing. It may be too fine to put directly against the perforated drain, however. Good practice would be to envelop the pipe with concrete gravel, 3/4" max size, at least 3" thick to keep the sand out of the pipe. Do not let the soil touch the gravel. Fabric can hold fine soil in place, but tends to clog if the water is carrying soil when it reaches the fabric. if you make the water pass through a few inches of sand before reaching the gravel, you don't need fabric.

Perhaps you can place 3" or 4" of the sand in the bottom of your underpinning pit to bring the water from outside to your drain, then place the concrete on the sand.

I suspect that if you get your drain low enough and run it to a sump with an automated pump, your water problem will go away whether you add waterproofing or not.

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