Basement Extension Beam Under Existing House Wall?

[psychedomination]

Hi there,

I recently got called to a job site where excavation has already occurred for a basement extension project.

The excavator has excavated very close to the existing house wall. It appears that the house wall was sitting on a limestone chunk that was resting on a soft sandstone type material. See image below :

I question whether the sandstone can take the weight of the wall/roof with the excavated material removed in front of it? The main wall is about 8" away from the basement wall. I recommended that the owner stabilise the wall as a matter of urgency. Am I being overly cautious or is there some contribution from the existing basement wall that can be utilised?

As part of the permanent basement extension works they will be looking to install a beam under the existing main house wall to stabilize it (see architects intent in sketch above), which they have asked me to design. However, I am wondering how they will be able to get the beam installed? I know for basement underpinning they typically excavate the length in about 1m sections but I'm not sure how the contractor will go about doing this when installing a full length beam (~8' in length). They don't have any specialist equipment only a few prop jacks. If you can provide some guidance on how walls are propped in situations like this to facilitate a beam install that would be very helpful.

Any other advice would be greatly appreciated.

 

[WinelandV]

I would recommend replacing the soft sandstone with concrete, using underpinning-style procedures. Bolting a beam to the side of the existing wall is 1) a giant pain and 2) not a great load path.

Is there any chance that the floor is cantilevered over the existing basement wall and holding up the CMU block wall? If so, it may be stable in its current condition.

Please note that is a "v" (as in Violin) not a "y".

 

[psychedomination]

@WinelandV

Thanks for your recommendation. Filling like a normal underpin would perhaps be an easier solution. However, I believe the reasoning for the beam is to maintain a certain room size.

For clarity - If I go the beam route and design one, it won't be bolted to the side of the existing basement wall. It will bear on new build walls at the support locations(apologies, I should have made that clear in the OP) see floor plan for this location below :

 

[SWComposites]

How/why did the original wall get offset from the basement wall?
The beam thing looks like a horrible idea.

 

[psychedomination]

@SWComposites

The walls are existing - I'm not sure why the original builders wouldn't have lined them up.

As for the beam, I did a quick check just now on the loads and the beam would be somewhat lightly loaded, even with the walls above. The beam in the storage area would have a UDL of ~53kN/m and resulting bending moment of ~44kNm. A steel beam or concrete beam should be able to take those loads quite comfortably. The beam in the stairwell area will have a slightly higher moment but from a beam capacity stand point, it shouldn't be an issue. Is there another concern I'm not capturing regarding the use of a beam (deflection, shear etc would be checked as well)?

The main issue that I see is the logistics of a beam installation with regard to stabilising the main wall during the placement of the steel beam? I guess one potential way may be as shown below (orange lines are prop jacks with a timber or steel needle through the existing main wall to prop it up):

Thoughts? Is there a simpler approach?

I'd like to propose an option to the client for consideration. I'm sure if the temporary support works end up being too extensive, the client would reconsider underpinning the house with CMU blocks or concrete.

 

[Tomfh]

You should at least propose filling the void with masonry/concrete. The beam seems a lot of work just to achieve a slightly wider space.