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RE: Seismic loads on retaining walls

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A Geotechnical Engineer I know explained that he does not provide
recommendations for seismic soil forces on retaining walls unless
specifically requested.  The rational is that when you look at the seismic
loads in thecontext of the factor of safety of the system of both the
regular soil loads and the wall its self there is little likelyhood of
failure.

I have had others explain to me that when you look at the real forces on
the wall during an earthquake they are much smaller than assumed.  The
basic issue seems to be that when the ground is moving towards the wall,
the wall is moving in the same direction since both the soil and the wall
are forced to move in the same direction.

When combining soil seismic loads acting on the walls with seismic loads
from the superstructure, people seem to ignore a very basic fact.  The
seismic loads on the walls from the soil are estimates of real loads while
the siesmic loads from the superstructure are fiticious loads generated by
dividing the elastic loads by an R factor.  When the superstructure has
considerable overstrength capacity the real siesmic forces the structure
will see are significantly greater than estimated.  Thus without some
adjustments the practice of adding soil induced seismic forces to those
from the superstructure is like adding apples and oranges.

It would not supprise me if this whole issue of siesmic loads from the soil
has not been well thought out and that we are spending a lot of mis
directed energy on this issue.

I think that part of the problem is due to the fact that most geotechnical
engineers do not understand the assumptions made by structural engineers. 
In addition most structural engineers do not have a deep understanding of
how the soil  really acts.

Mark Gilligan

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