retaining walls for basement walls 1

[jec67]

Occasionally one encounters a situation in designing a basement wall system where a retaining wall is required. For example, when there is an opening in the floor structure above that is located along the edge of the basement wall, such as a stair opening, elevator shaft, etc. In these cases, I have typically designed the wall in the region as a retaining wall.

Recently, I was discussing such a situation with my coworkers, and we had the following issues that we could not resolve: Should the wall be designed using the active pressure coefficient or "at rest" coefficient of friction? It seems there are several factors to be considered, such as: will the backfill for the wall be placed before or after the floor above is constructed, and if the wall is designed using the "at rest" soil pressure (Ko) is the wall stiff enough to realize the state of stress in the wall?

I write this post in hopes of gaining some insight into other engineers' approach to designs of this nature.

Thank you.

 

[WillisV]

jec67,

If it is designed using your approach, I would use the at rest coefficient mainly because it will result in a more conservative design than the lower active coefficient and in "grey area" cases such as this conservatism is warranted.

That being said I normally design these cases for smaller openings so that the portion of wall adjacent to the opening spans horizontally between the two full-height areas on either side of the openings. The horizontal rebar in the wall at this location might increase a bit and then the vertical portions to either side of the wall at the opening are checked with the additional force spread out over a reasonable width (normally I use half the width of the portion of wall at the opening on either side).

For very long openings adjacent to the top of the wall, I have placed a horizontal outboard beam at the top of the wall adjacent to the opening (or as high as it can be placed and still be below final grade), designed the basement wall just like normal, and then designed this beam to handle the reaction at the top.

 

[bjb]

When a basement wall has to be designed as a retaining wall, I always use the at rest pressure. It's my understanding that the deflection necessary to get the active pressure in a retaining wall includes rotation of the wall section, and I don't think that will happen with a basement wall. Also, the Army Corps. of Engineers recommends the use of at rest pressures in an Engineering Manual (EM 1110-2-2502). The reasons they give include that the walls are usually fairly stiff because of safety factors, and that horizontal pressures may in time return to at rest conditions after the initial development of active pressures. I usually only design retaining walls only for the active case when they are only "site walls" that can tollerate some rotation.