Basement wall design?

[Redacted]

Hi there,

I am trying to determine how to design the basement walls for a new downstairs apartment in a house that was built over 100 years ago. I am checking an architects detailing.

The new basement walls will be inner masonry reinforced walls. The area behind the wall is not loose soil, it is very stiff compacted sand, that feels like a rock cut. The new wall will be built a few inches offset from the rock cut and backfilled with concrete. Do I design this inner basement wall like a normal retaining wall? As in assume that the stiff rock cut acts horizontally on the wall even if in reality this perhaps wouldn’t happen unless the rock cut somehow shifts in position? The new wall will be built up to the ceiling but how can I adequately connect the new basement wall to the upper floor slab to ensure that the new basement wall is also stabilized by the vertical upper floor level loads? Any help will be appreciated.

See the attached images that shows the architects detailing and a photo of the area.

 

[oldestguy]

A few comments. Why backfill with concrete? Looks more like a problem than a benefit. Assuming a vertical load will eventually be carried and the blocks can then carry some backfill load from outside, why not build up in sections to reach the upper limits and also apply some load, as with wedges up there, or with jacks on lower blocks to lift a little and allow dry packing of mortar. Then a backfill, if necessary,can be dry sand, much lighter than concrete and likely will hang up with the "silo effect" and then exert only minimal "backfill loads". If that nearby compact sand can take it, consider drilling in some re bars bedded in mortar, to then lay with "L" ends in the bedding of the blocks. That might significantly give better assurance that the wall can take lateral loads.

 

[Redacted]

Thanks for the response @oldestguy

That's true dry sand would be better from a lateral load point of view.

So are you saying to only consider the backfill load acting on the wall and not to consider the rocky sand material as retained?

I did consider drilling in some bars to act like tie back anchors but they would not be able to hold in the sandy material. They can be drilled into the concrete house wall above the sandy material though.

 

[oldestguy]

My thought is that this is a typical basement block wall and likely the backfill and existing material are giving less push than the usual situation, but that the wall needs a load on it to resist these horizontal loads anyhow. Thus the dry packing or wedging on top is essential assuming some load is there to be utilized. Apparently there is no need to be concerned with possible saturation and footing drains, etc. If there is, the sand backfill would be great and act as a filter also.