Alternatives to Underpinning for Interior Basement Drainage 3

[MechE2020]

I have an older home that requires an internal french drain in the basement around the perimeter to provide drainage. The walls are field stone ~ 7 feet tall that are parged on the inside. The base of the walls is ~ 3 inches from the top of the floor. So my concern with the french drain is undermining the soil foundation. I was going to underpin the wall with short ~ 14 inch tall x 18 inch wide piers ~ 3 feet long and got spooled up to do it, but I keep thinking that there may be a simpler solution to providing drainage and preventing undermining of the foundation walls since I am not lowering the basement floor and still providing the lateral restraint at of the floor. The attachments show the current condition and a couple ideas/options I was thinking about. I'd like to get any ideas/thoughts you may have.

I'm also curious to know how to calculate the active lateral soil pressure that would be seen by such an adjacent structure, that would govern the design.

 

[oldestguy]

All these ideas are Pretty damn elaborate and difficult to do. One initial comment, be sure any drain is properly filtered. The easiest is a simple perforated pipe, with 1/8" holes backfilled with ASTM C33 concrete sand. Forget the gravel unless it is wrapped with a filter cloth (takes time and effort).

With that stiff clay soil I'd look at segments of underpinning. Then place the drains inside trench and no special forming, etc. You can tamp that concrete sand some to give it some resistance to settling, etc. The black plastic wrinkled drain pipe is pretty good under drain pipe with slots suitable for concrete sand backfill of the pipe.

Finally with that form of basement wall, I'd not get too excited if it settles some. Just jack the floor up and dry pack mortar on top of old wall.

 

[SlideRuleEra]

MechE2020 - I read your other thread about this project and looked at the calcs for this tread in some detail. Am reluctant to tell you this, if you want to proceed call a geotechnical engineer... your calculations are an absolute mess:

Active earth pressure on the existing wall is much higher than shown - the math is wrong.

If the existing wall is considered a free-standing gravity retaining wall (ignore restraint at top or bottom), resistance to overturning will cause eccentric loading of supporting soil. This will result in a surcharge load (to a proposed wall below the existing wall) that is not uniformly distributed. That can be compensated for, but it does not matter, math for the horizontal force from the surcharge is wrong, too.

I don't agree with the way dead and live loads are being applied to the existing wall.

Sliding resistance to the existing wall has not been accounted for. A concrete floor 1 1/2" thick is of negligible worth for sliding resistance.

IMHO, all of the proposed options, one through six, uses concrete in a way that provides only trivial restraint to the existing wall.

Obviously, the wall works, as-is, but there is something happening that is not being considered... I don't know what that is.

Bottom line: Don't "mess" with the wall without qualified professional guidance."

http://www.SlideRuleEra.net

 

[MechE2020]

Thanks Oldestguy. After sketching all those ideas out, the underpinning sure is the least complex. My hesitancy has been around digging the dirt out from under the wall, potential loose stones, and settling. That got me thinking about other viable solutions that would acted more like a bench footing.

I appreciate the insight on the drainage sand too. Someone had suggested putting ~3 inches at the base of the underpinning pier to allow filtered ground water to come across from the exterior into the interior drain and even further de-water after storms. Any thoughts on that?

 

[MechE2020]

SlideRuleEra,
Thanks for taking the time to look at my post and respond. I'm certainly not a Geotechnical Engineer (I'm sure that was/is obvious). I have found this forum to be very helpful. My question about the lateral soil pressure is more for my curiosity as a Mechanical/Civil Engineer on what loading "would" size something like this as it is different than a standard retaining wall because of the existing wall surcharge (DL & LL). I was hoping someone on here might help me understand that or maybe point me to a resource to better understand it.

My main objective for this post was to get some subjective feedback on alternate designs for this application. Or maybe there is something I'm not thinking about. As it stands I have an Engineer stamped plan for the underpinning (though it is ignorant to the many invaluable previous insights from OldestGuy, PEinc, and others on this forum). So I'm good to move forward on that plan and make sure the finer details I've learned here are also incorporated. But before I do that, I just "needed" to see if an alternative solution was viable.

If anyone on here has an alternative suggestion or thinks that any I sketched up are advantageous over the underpinning for my unique situation, then I would certainly reach out to a professional Geotechnical Engineer to get an approved plan that corrects my math and officially sizes the structure.

I think I mentioned this on my other thread, but my 1950's cape is like the 25 others in the Philadelphia suburb where I live. They all get interior basement french drains and as you can see from the sketches, since the bottom of the wall is only 3 inches from the top of the thin floor, the drainage ditch with a 4" pipe and stones compromises the footing. I'm trying to keep that footing soil intact or shore it and avoid some of the issues my neighbors may see in the years to come from these drainage systems.

Thanks again.

 

[PEinc]

Option 7 is the only one of your options that I would consider. Make sure you check overturning and sliding for the temporary condition when the wall is underpinned but the drain is not yet installed. I don't like the sand under the underpinning. Maybe use some narrow, widely-spaced, horizontal strips of drain board under some of the underpinning piers? Or, place some PVC weep hole pipes in the drypack and forget about the sand. The weep holes would feed the top of the stone drain.

It is better to lose a few wall stone when underpinning than to collapse the entire wall by digging deeper and along the entire wall.

http://www.PeirceEngineering.com

 

[SlideRuleEra]

I was hoping someone on here might help me understand that or maybe point me to a resource to better understand it.

As it stands I have an Engineer stamped plan for the underpinning...

I understand your situation better than you know... I started as an ME, 50+ years ago (Life Member ASME), but much of my work in heavy industry was a mix of ME, Structural & Geotech.
See: "Earth Pressure & Retaining Wall Basics for Non-Geotechnical Engineers"

You have a sealed plans from a structural engineer... for a project with significant geotechnical loading. Why don't you have a detailed technical discussion with that engineer about these two notes on their drawing:

I don't have experience with the proposed type underpinning (it does not work where I am... poor soil to deep depth), but my understanding is that this type underpinning is primarily for vertical loads. To me, looks like the existing wall has high lateral loading, and very little way to resist it.

http://www.SlideRuleEra.net

 

[MechE2020]

PEinc,
Much appreciated. I had not thought about adding weep holes. Great idea that makes a lot of sense. I'd rather stay away from disturbing the new pier footing soil. What are your thoughts on just adding 1" pvc to the say top 1/3 of the pier pour and not messing with the dry pack?

With the caveat that I'm not a Geotechnical Engineer and this is for my own interest/understanding in the subject, I've attached what I believe would be the loading that would govern this and the checks you suggested. Had to dig out more books as my last attempt was way off (thanks SlideRuleEra). If my figures are correct (even if they are not), seams wise to make sure I provide temporary lateral support during the project and only to be removed when the floor is fully cured. Would be interested in any/all thoughts you might have on the subject.

 

[PEinc]

Do not eliminate the drypack. Properly placed and rammed drypack is whet properly transfers the building load to the underpinning
The lower the weep hole, the less water buildup behind the piers and the less that could possibly seep up between the slab and the piers. I would probably place the weep holes in the underpinning concrete but at the bottom of the pier so that they drain to your stone drain. Before setting the weep pipes in the concrete, cover the ends of the pipes with filter fabric to prevent loss of soil.

http://www.PeirceEngineering.com

 

[MechE2020]

Or, place some PVC weep hole pipes in the drypack and forget about the sand.

PEinc,
Thank you. Apologize for the wording, I "meant" to not mess with placing drypack around a pvc pipe. No intention to leave it out. On the contrary, I wanted to maximize it's presence. By placing them in the pier, it adds the benefit of increased drainage behind the pier as you mentioned. Thanks for the added insight into the fabric.