Row house basement dig out with underpinning 1

[Mitso]

We are looking into digging out a basement from about 3 feet of crawl space on our row house that dates from about 1910. Our 3 story building has a rubble stone foundation and is “attached” on both sides to other 3 story houses. Both neighbors have crawl spaces too. In parallel to coming to this site for preliminary technical advice, I am talking to real estate agents to gauge the value of such work.

I am an architect, no wise cracks please , and I have a lot of experience working on these types of homes. A knowledgeable and competent general contractor with whom I have worked with on a few different jobs in the past has indicated that he would like to take this on. He does not have any experience in underpinning structures, no wise cracks please

I have read on this site, on multiple occasions, that one should only use an experienced underpinning contractor. However, if I hire a structural engineer who has experience in underpinning structures and both he and I follow the contractor closely, is this feasible?

Any advice on choosing a structural engineer? Any suggestions in Montreal Quebec Canada?

I will also see what the best known underpinning turn key contractors can offer me. I will look into which structural engineering consultants these specialized contractors use - I know these companies mostly sub out this consulting work)

I would also love to see some properly executed drawings for this type of work – any offers?

And, of course, any other suggestions / comments would be welcome!

Thanks.

 

[jayrod12]

That is one hell of an undertaking. It's going to be extremely difficult (it might be economically unfeasible) to successfully shore the excavation when you have next to no room above and zero room beside. Maybe one of the more experienced guys here has a thought.

Without trying to sound rude or arrogant my thought on this would be to not waste anymore time. Barring some miracle type invention, or the inclusion of the other row houses in this construction project, i.e. shore up all buildings attached and build them all a basement, I can't see this working. too many unknowns, too much risk. The engineering fees on this alone make the basement not worth it.

Sorry but that's my opinion,

 

[Mitso]

Jayrod12 no need to apologize.

I am posting here to get independent professional advice - good and bad, pro and con.

This project is not uncommon around here, just like in Brooklyn, DC, Philly, etc.
What is normally done is a ramp is dug from the back service alley down to the back of the house and the digging begins…long and painstaking…and as you mention, not without risk

 

[jayrod12]

My biggest concern would be excavating close enough to the neighbouring crawlspaces without the ability to get shoring in place. I'm not sure what the soil is like in Montreal, if it were like here in Winnipeg where it's stiff clay for a good 20-40 ft minimum I may not be as concerned but I would hate to have the side wall of the excavation collapse and take the party wall footing with it.

I would want to build the new wall inside of the existing rubblestone wall (annoying to lose the floor space I know) but then it would act as a retaining wall for the neighbouring crawlspaces and the existing foundation. It would also make drilling the new piles significantly easier if they aren't trying to install them below existing structure.

I hope it works for you,

 

[oldestguy]

Aside from the work, what will be the arrangements for insurance? In posting here you recognize the need for an experienced geotechnical engineer before and during the work in addition to structural. Each (Engineer and contractor) may require special riders for this work. Monitoring the party walls also is another task perhaps requiring another engineer (surveyor?) specializing in that. One final requirement is that agreements with adjacent owners will get attorneys involved. City Building commission authority required there? Nothing is easy these days.

 

[mikeofBelAir]

It is possible to do it. I have seen dozens, if not hundreds of row homes in Baltimore where the crawl space was excavated to make more room. Some with 5' of head room, some with 6', some more. All the homes i have seen were built between 1905 and 1940. I have seen only the finished product, and even that varied. The general solution seems to have been to excavate 6" to 12" outside the footing, and pour concrete in short sections. Since i only see one side of the wall, I don't know how they handled the opposite side, if they did anything at all.
You are correct to involve an Engineer.
Good luck.

 

[ameyerrenke]

In my area OH/KY, I've seen basement excavations of this sort where soil was excavated, and a retaining wall was built a few feet in from the existing foundation wall. This would offer the added headroom, and if done properly would not disrupt the existing footings. Of course you would loose some floor space, which my be a deal breaker since its likely your row house is already pretty narrow.

Anyway... Best of luck! This will not be a small undertaking!

 

[PEinc]

The rubble stone foundation wall should be underpinned (under the existing wall, not out in front of it. From my experience, problems arise when someone tries to dig in front of and along the existing wall without any shoring. Hand dug and shored underpinning piers are the way to go. Check my web site for my presentation on Recent Building Collapses in Philadelphia.

http://www.PeirceEngineering.com

 

[Mitso]

Thanks all.

PEinc, is was hoping for a reply from you. I watched your presentation on Recent Building Collapses in Philadelphia. I also watched the following presenation: NYC Department of Buildings - An Underpinning Symposium.

Do you know where I can see a proper set of drawings for this type of work?

 

[Parrapit]

We have a similar situation, if I understand your discription correctly and We have done this before by the way, we need to increase the size of a cellar under the house the celler foundation act as support for the rest of the house above.

We need to increase the depth of the cellar with around 2.5 meters.

So method statement would be to excavate in sections, dependant of the type of soil between 1 to 2 meters then leave a section then again.

With the reinforcing protruding out on either side (push into soil or bend) then cast the concrete, once you excavate and box the next section the steel is continues. Yes you wil have a vertical dry joint.

With this being a massive waterproofing problem a penetron concrete must be used with a channel and dry skin built on inside of retaining wall and channel between retaining wall and dry skin, channel flows into drain and pumped up to storm water system. Also get a waterproofing specialist to recommend a product for their joints.

This is a costly excercise.

 

[Parrapit]

We have a similar situation, if I understand your discription correctly and We have done this before by the way, we need to increase the size of a cellar under the house the celler foundation act as support for the rest of the house above.

We need to increase the depth of the cellar with around 2.5 meters.

So method statement would be to excavate in sections, dependant of the type of soil between 1 to 2 meters then leave a section then again.

With the reinforcing protruding out on either side (push into soil or bend) then cast the concrete, once you excavate and box the next section the steel is continues. Yes you wil have a vertical dry joint.

With this being a massive waterproofing problem a penetron concrete must be used with a channel and dry skin built on inside of retaining wall and channel between retaining wall and dry skin, channel flows into drain and pumped up to storm water system. Also get a waterproofing specialist to recommend a product for their joints.

This is a costly excercise.

 

[Mitso]

Where are you located Parrapit?

It would be much easier to understand if you provide a section showing "channel between retaining wall and dry skin"

 

[PEinc]

The excavation for an underpinning pier needs to be shored if more than about 1.2m deep or if the soil is loose and will not stand vertical when excavated. Concrete pier underpinning, unless very shallow, is usually not reinforced. It is too difficult for a worker to install reinforcing steel inside a small, deep, timber-sheeted pit. It is also difficult for a worker to excavate inside an underpinning pit if there are reinforcing dowels protruding into the pit from the adjacent, poured piers. The mass of the concrete pier usually is sufficient that reinforcing steel is not needed. For example, if a rubble stone foundation wall does not need reinforcing steel, a solid concrete underpinning foundation should not need it either. Underpinning needs to be designed for the vertical building load and for the lateral earth pressure that wants to overturn and/or slide the underpinning piers. If underpinning needs to support about 3m of soil, it will usually need some type of lateral support, possibly for both the temporary and permanent conditions. If a basement is being deepened by 2.5m, the underpinning will most likely need permanent lateral support unless the building is heavy and the underpinning piers are massive.

Pit underpinning should not be attempted unless the site has been dewatered to below the bottom of the underpinning pits. Otherwise, you will lose soil while excavating. If you lose soil, there will probably be settlement of the building. It is dangerous to be inside a sheeted underpinning pir when soil has been lost from behind the shoring boards.

There are not many books that show good underpinning details. A couple books that do are:
1. Temporary Structures in Construction, 3rd Edition, Chapter 11, by Robert T. Ratay, Mcgraw-Hill
2. Foundation Engineering Handbook by Winterkorn and Fang, Van Norstrand Reinhold
3. Earth Retention Systems Handbook by Alan Macnab, McGraw-Hill
4. Underpinning by Prentis and White (the "bible" on underpinning) 1950, Columbia University Press

http://www.PeirceEngineering.com

 

[Mitso]

PEinc this is extremely helpful. I checked:
• details in Temporary Structures in Construction by Ratay (although 1st edition)
• details in the free FHWA geotechnical 3-volume underpinning manual online and
• your “recipe” described in this old thread:

http://www.eng-tips.com/viewthread.cfm?qid=54204

I now have an idea of how to install and fill a concrete underpinning pier.

Now, more generally, can you help shed some more light on the following:
1. How to start in a row house that has access only via front and back? How to gain entry in a confined crawl space? How to access the foundations in a crawl space?
2. Permanent and/or temporary lateral support options, as required by the unsupported new underpinning foundations

Thanks again.

 

[PEinc]

There is no easy way to access a shallow crawl space for underpinning. I would try to bulk out the middle while staying away from the 4 walls. Then, try digging into the remaining berm, shoring the excavation in the berm, in order to install the concrete piers. The excavation spoils will need to be moved out of a crawl space window. You may want to consider using a conveyor set up in the crawl space. Expect a lot of crawling to perform this work, hence, "crawl space."

For lateral support of the underpinning, you can extend the piers below the future basement slab. At the top of the underpinning, you still have the original rubble stone foundation wall. This wall and the underpinning need lateral support. You may need to have the front face of the underpinning extend into the basement so that it can eventually overlap the rubble stone wall so that this concrete can be braced by the first floor framing (which may need to be strengthened to support this new axial load from the wall). This is not an easy design because this is a significant deepening of the crawl space to become a basement.

http://www.PeirceEngineering.com

 

[Mitso]

Any suggestions on a structural engineer for this in Montreal Quebec Canada?

And with a good engineer on board, do you have any thoughts on me using a very good general contractor for this project, with no underpinning experience? (I have worked with him before on numerous occasions and he is competent)

 

[PEinc]

I would not trust an contractor that has no underpinning experience, no matter how good, with underpinning in a tight space like this without someone (an experienced design engineer?) paying attention and providing assistance and guidance. Too many buildings fall down during this type of work.

I don't know of an underpinning engineer in Canada but ADSC (the International Association of Foundation Drilling) lists InGeo Design, LTD in Toronto as doing geostructural design. Maybe they design underpinning. The Deep Foundations Institute (DFI) lists Petrifond Foundation Company LTD in Montreal as an underpinning contractor. If you call them, they may be able to help you or give you a recommendation.

http://www.PeirceEngineering.com

 

[oldestguy]

So far no one has commented on beefing up the existing foundations. In my experience, many of these rubble stone foundations have very little mortar between the stones below the current ground surface. It would seem that some injected grout may be needed if things are not well "glued" together in the part remaining in use.

 

[Parrapit]

I know this is not the exact existing foundation that you have but I dont have a your exact details like wall and foundation section.
But this is what we normally do it but I would remove the existing floor or atleast part of it.

So see my link below I dont agree to do mass concrete as you are limited on space. Concrete should be a waterproofing concrete as previously mentioned. Not sure on how exactly your wall is placed on the stone foundations but if possible should be removed and placed on new concrete retaing wall. Cause it will probbably fall as you excavate under it any way.

Also you can make the footing of the retaining wall quite large as it forms part of your floor. Channel should be connected to a pipe that goes into your storm water.

If the waterproof concrete is enough I would just go with that so that maintenance can be done on the concrete wall if dry cavity is built one would not be able to see condition of concrete wall.

 

[PEinc]

oldestguy, I would not try to inject a rubble stonne foundation wall. I would be concerned that it might blow apart.

Parrapit, are you building this cantilevered foundation wall in short sections from within a sheeted pit? The new basement will be too deep to just open cut for the new wall. Are you lap splicing or mechanically splicing the horizontal reinforcing steel? Are you drypacking between the top of the new wall and the bottom of the rubble stone wall? With this type of eccentric L-shaped foundation, I would be concerned about the resulting bearing pressure and potential settlement.

http://www.PeirceEngineering.com