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.

 

[oldestguy]

PE inc:

I've done it below ground level, no problem. A prison wall no less.

 

[PEinc]

oldestguy, residential rubble stone foundation walls are usually about 15 inches thick, plus or minus a couple of inches. Was your prison wall that thin? A residential crawl space foundation wall has soil on one side only. If there is any pressure involved with the injection, I would be concerned. If there is no pressure involved, I would question the effectiveness of the injection process. The amount and quality of mortar can vary greatly. I seen good mortar, no mortar, and even mortar made of lime and seashells.

I am not as much concerned about injecting the rubble stone as I am about the amount of unshored and unsafe excavation that an inexperiwnced contractor may perform. Construction procredures are probably more critical than the design of the concrete underpinning.

http://www.PeirceEngineering.com

 

[Mitso]

This is extremely helpful.

Questions for PEinc and anybody else who wishes to reply:

1. Can you explain the use of louvers? Why not just omit spaces between rows of sheeting?

From Ratay: “It is advisable to use louvered boards at all times to in order to enable backfilling behind the boards where necessary. (Louvered boards are boards placed with spaces left between them which can be packed with sand and salt hay in case the sand starts running after it dries up”

From the online FHWA geotechnical 3-volume manual: “During the placement of the sheeting spaces are often left to permit packing of the soil behind the boards. These spaces are called louvers and are formed by nailing short pieces of wood between the sheeting boards.”

2. From Ratay: “Inserts are installed for an inclined bracing or tieback system”. I am assuming that this is the temporary and perhaps permanent lateral support for the underpinning?? Pls confirm.

I am also assuming that since the top of my existing rubble foundation wall is braced by ground floor framing and the bottom of the new concrete underpinning will be braced by the new slab than it is the joint between these two walls that needs to be braced?

3. And, if I understand correctly, only the sheeting behind the pier will be left behind once the pier is completed – please confirm.

Thanks everybody for reading and contributing.

 

[PEinc]

1. Louvers are installed so that the worker in the pit can be certain that there is soil up against the rear face of the shoring boards. If some soil has been lost, the 1.5" space is big enough for the worker to replace soil behind the boards with a flat shovel. If there is ground water seeping in through the shoring boards, the louvers can be packed with hay or filter fabric to allow the water to drain without losing soil.

2. The inserts being referred to are embedded steel, usually a piece of steel beam, a t-section, or plate with shear studs. The embedded insert is placed flush with the front face of the underpinning pier. Then, after the piers are finished, you can excavate to the insert and then excavate to subgrade on an angle of about 30 degrees so that raker braces can be attached to the concrete underpinning piers for lateral support. Sometimes the brace is connected to the pier with a plate and expansion anchors. However, if the shear force is too great for a reasonable number and size of expansion anchors, then the embedded insert works nicely.

3. For continuous underpinning piers, the shoring boards between piers may be, and usually are, stripped. A little left-in-place lumber isn't a big problem unless the underpinning is to be the finished wall. In that case, the boards between piers should be stripped. The rear shoring boards always remain in place. The front boards are stripped if the underpinning is the finished wall face or if you need to remove the boards to install tieback anchors or attach raker braces. The front boards may not need to be removed if there will be backfill placed between the underpinning and the new structure in front.

http://www.PeirceEngineering.com

 

[PEinc]

Mitso, you wrote, "I am also assuming that since the top of my existing rubble foundation wall is braced by ground floor framing and the bottom of the new concrete underpinning will be braced by the new slab than it is the joint between these two walls that needs to be braced?"

If the rubble stone wall is not very high, it may not need to be braced for sliding and overturning, but the pier probably will need bracing BEFORE you excavate to subgrade and install the new slab. Until the slab is poured, it cannot provide any support to the underpinning. You don't want the underpinning to fall over when you are excavating to the slab grade.

http://www.PeirceEngineering.com

 

[dicksewerrat]

Have you looked at the invert of the sewer system for the bldg.? And in the street. Water service should not be an issue.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Mitso]

Dicksewerrat,

Are you referring to the potential need for a pump, either for a french drain leading to a pit or for a potential bathroom in the new basement?

I did a preliminary check with the city and I think I will not need a pump for a bathroom and may need one for a sump pit.

I also learned from the city that rock is about 11’-0’’ below the level of the crawl space and the water level is around there too – both positive news for this potential project.

Best regards.

 

[dicksewerrat]

I am thinking about your existing sewer line and water line. If they are hanging from the ceiling in your new basement they may cause problems.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Mitso]

Hello All,
Does anybody have any practical tips and/or solutions on how to support a rubble stone foundation wall during the pit underpinning process? I am assuming that the 100 year old mortar will not (completely) keep the wall together under gravity.
I cannot see how this support can be removed before the pouring and subsequent drypacking???
Does one leave this support to be cast into the concrete pier and the drypack?
Appreciated,

 

[Mitso]

Continuation of previous post:
I was thinking that one could use, for each pit, a horizontal metal plate with 2’’x4’’ studs wedged vertically to support the rubble foundation wall above. The plate and the studs would be left to become part of the new foundation.
If so, then one could also use a 4 foot pit instead of the recommended 3 foot pit, reducing the number of required piers by 25% - a significant labour savings. In this case, I would use a plate approximately 2’-6’’ long (parallel to rubble foundation wall) by approximately 18’’ wide.
Please let me know if this makes any sense.

 

[PEinc]

Check my web site for a presentation on collapse of buildings in Philadelphia. There are photos and details of underpinning and rubble stone walls. This may help answer your questions.

http://www.PeirceEngineering.com

 

[Mitso]

I see one slide with two images of rubble stone foundation walls. The pits are dug and the walls are being held up by what appears to be wood (plywood and/or 2''x stock). My question is what next? Does one leave this support to be cast into the concrete pier and the drypack? Thank-you.

 

[PEinc]

The wood you see under the rubble stone party wall is temporary support for the wall stones while the pit is being excavated and shored. The wood is supported on top of the upper timber pit shoring boards. If the wall stones are unstable, you will need to quickly install at least one set of ring boards so that you can support the stones. After the pit is excavated and concreted to within an inch or two of the support boards, the concrete must set overnight and then the temporary supports can be removed as the gap between the top of pier and bottom of wall is drypacked. The wood support for the wall stones must be removed. From none to three sides of the pit shoring ring boards can or must be removed depending on the amount of underpinning piers being installed and on the project requirements.

http://www.PeirceEngineering.com

 

[chicopee]

You may want to look at some of the pictures shown in this presentation: Underpinning Stone Foundation Walls - Image Results. Instead of diagonal braces, I imagined that I beam rings to support sheets against the stone foundation could work out as you remove soil and increase the depth. As you increase the depth, add more rings and more short length piles.

 

[Mitso]

Chicopee

Where can I find the presentation: Underpinning Stone Foundation Walls - Image Results ?

Thanks

 

[Mitso]

I am attaching a drawing with 3 different underpinning options for a common shared rubble stone wall of a three story home
Option A is preferred by most engineers, most labor intensive

Option B was suggested by several experienced underpinning contractor and at least one engineer endorses this method.
[ul]
[li]Pro option B: since half the shared wall is always supported much less risk of stones falling into the underpinning pit, faster / cheaper than option A[/li]
[li]Con option B: shared wall is supported by two different conditions and may be subject to differential settlement; eats up more space than option A[/li]
[/ul]
Option C was suggested by an experienced underpinning contractor
[ul]
[li]Pro option C: faster / cheaper than option B???[/li]
[li]Con option C: eats up more space than option B[/li]
[/ul]
Assuming the new wall is designed and sized properly, do you have any more comments and/or opinions on these options?

 

[PEinc]

I would not ever attempt Option C or Option B.
The rubble stone wall could fall apart or shear off in Option B.
The surcharge from the existing party wall could overturn the concrete wall in Option C.
Use Option A.

http://www.PeirceEngineering.com

 

[PEinc]

I also don't see how B and C are any cheaper than A.
They all should be hand dug and constructed in narrow sections along the existing wall.
They all have about the same amount of excavation and concrete.
There may be a little less drypacking with B.
B and C give smaller basements space which is usually not desired. The row houses already have narrow basements.
Option C has minimal dead load weight (no building load) for resisting overturning and sliding.

http://www.PeirceEngineering.com

 

[RFreund]

They all need to consider lateral stability. And with that in mind, option c does not seem realistic

EIT

http://www.HowToEngineer.com