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Re: Retaining Wall Question

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Bill Allen, S.E. wrote:

> While specifying gravel backfill is noble (the most common reason for
> retaining wall failures is the inability to relieve hydrostatic
> pressure),
> the vertical component I referred to in my previous post occurs at a
> vertical plane at the heel of the footing. This vertical plan
> basically
> divides the soil that will rotate with the retaining wall footing and
> the
> soil that remains behind. Within this plane, there is soil to soil
> frictional resistance equal to the tangent of the angle of internal
> friction times the horizontal component. This condition exists unless
> your
> gravel backfill extends beyond the footing heel.
> Regards,
> Bill Allen

Not sure how your statement about specifying backfill as "noble"
minimizes the valid comment about the gravel/soil interface. If it isn't
there, then someone hasn't followed thru, if it is, then the use of a
vertical component in design is too liberal. If the heel weren't there
(property line wall) would the vertical friction still apply at the back
of the wall? Another consideration about actual construction procedures
is that the excavation rarely provides a vertical cut and often is
somewaht wedge shaped. This is commonly filled either partially or
totally with "backfill" consisting either entirely of drain rock or in
some combination with compacted soil.

My vote stays with careful consideration of backfill and determination
of whether to use a vertical component based on that conclusion. As for
concerns about how large the footing becomes without considering this
"added" component, we certainly should care that we are designing a
reasonable solution. Often other effects such as return walls and
corners are ignored because it is simpler to design a cantilever element
as opposed to a wall plate system. This points toward judgement and yes,
fee amounts. (I knew I wanted to talk about fees!)

Anyway, as they say ... just my 2 sence

Barry H. Welliver