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Re: Load combinations for retaining wall

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Jeff Smith wrote:

> I am working on a ski house with a 30'x45' basement with an 18 foot
> retaining wall. The ground snow load is 430psf and the potenial water table
> is 4 feet below finish grade during spring run off, so we have quite a bit
> of hydrostatic uplift as well. I am waiting for a response from the
> geotechinical engineer for design criteria. I asked him about snow load
> surcharge and it seemed that he had not been asked that question before. To
> make matters worse the sub surface wall drainage is acheived by pumps since
> there is no gravity discharge, so it would be possible for a mechanical
> failure of the back drains. Even though the floor above the wall is wood
> framed,  it seems best to stiffen the floor and design this as a restrained
> wall. Any suggestions on load combinations?(snow surcharge+active earth
> pressure+water table force+the mystical seismic increment)
> Jeff Smith

Here in Utah there are some prescriptive measures for "basement" wall design
with wood floor diaphragms. While the provisions are rather light (in my
opinion), it serves to remind me that earth retaining structures bounded with
concrete walls behave a bit more like flat plates than cantilever retaining
walls. I'd advise that you model the design along the lines of a concrete tank
or other such structure if possible. I use charts developed by the Dept. of the
Interior (Moments and Reactions for Rectangular Plates) but obviously there are
numerous ways to address design.

The ground snow load surcharge has intrigued me also but I think I've
rationalized it with the frozen ground concept. While frost depths may not
reach the 18ft + depth you mention, it can fix a substantial portion of the
upper soils.

I'd be interested in your soils consultants recommendations also. The mystical
seismic increment will further drive you nuts when deciding how to combine it
with the "other" design loads.

Barry H. Welliver