Lowering a basement floor

[fenderbassman]

I am a structural engineer designing a remodel and enlargement to an existing house in Denver. It is a single-story ranch with 8' poured concrete walls on 10" piers at an assumed (to be confirmed) spacing of 16'. No water. Soil is non-expansive clay with claystone at 18'. The client wants to lower the basement floor 14" for 1/2 the basement area. New piers and walls will be adjacent to most of the lowered floor and present no problems. There is approximately 20' of existing piers and wall adjacent to the lowered slab. These walls will have to be extended 18" to be below the new slab elevation. I have considered a concrete sister wall extended to the lower elevation but this would be intrusive architecturally. Also considered are sequentially poured concrete extension walls that could be dowelled into the bottom of the existing wall with horizontal bars connecting all. Both the sister wall and the extension walls require void forms underneath all and a new interior perimeter drain. Lateral helical pier tie-backs will be added to resist soil pressure with temporary steel bracing in-place during construction.

I have extended walls on footings, this one is on piers. Does anyone have experience with this kind of problem? Thanks

 

[JedClampett]

I've never done, nor considered, anything like this. But it sounds risky and expensive. I'm sure your client has good reasons to lower their basement, but I can't imagine it passes a cost/benefit analysis.

 

[msquared48]

For the fourteen inches, even considering utility connections, it seems like it might be simpler and cheaper to raise the house instead.

Mike McCann
MMC Engineering
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