What is max underpinning depth with foundation wall? (against earth pressure) 2

[Carlo01]

Hello Everyone,

This is my 1st post and would like to get an opinion on the maximum depth of residential underpinning. This is not exactly a job I'm working on, but something I've been wondering about.

So the question is, what is the maximum depth of underpinning without any reinforcing? Let's say a homeowner wants to underpin 10'-0" (an exaggerated tall depth) below existing slab on grade and let's say the existing basement wall height is 8'-0". That would make the total foundation wall depth to be 18'-0". This would definitely need to be reinforced against lateral earth pressure. I know the underpin segments are usually 4'-0" in depth (for soil stability) and 4'-0" in width, but my concern is more on the whole wall element that will now be connected with the underpin.

The 2nd question is, how do you reinforce this wall? Do you specify doweling of the top of the underpins to the bottom of exist footing and add concrete wall thickening so you're designing it as, say a 16" thick concrete wall against earth pressure?

Thank you.

Carlo G.
Structural E.I.T.
Toronto, On

 

[RPMG]

Underpins usually act as gravity retaining walls, and are stabilized by their self-weight. Tie-backs may be added. The existing foundation should interact with the underpin similar to the previous soil interaction, in bearing.

 

[jdonville]

Pretty much RPMGs post.

You can add dowels for additional shear friction at the footing to the underside of the foundation, but need to be careful that the existing foundation can take those loads. I check if the underpinning can take the combined flexure and axial load without reinforcing. If not, can either do it by a single mat centered on the section or 2 mats on the front/back faces. Flexural reinforcing (and shear friction dowels) need to be checked for development length.

The higher the axial load and wider the foundation (and underpinning), the less reinforcing you need. Generally.