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Seismic earth pressue force increment

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We are designing a partially buried concrete box !water reservoir), and
the Soils Engineer has supplied us with the following design criteria
for the seismic earth pressure forces.

The dynamic lateral force increase due to earthquake shaking can be
estimated for drained, level backfill conditions using the following
equation:
dynamic lateral force increment, pounds/ft = 1/2 * (soil unit weight) *
(wall height) * (site acceleration).

He continues by adding the following statement after the above equation:

For purposes of estimating the resultant lateral force increment, we
recommend assuming a ground acceleration of .4g and a soil weight of 125
pcf.  Assuming a 10 ft. high wall, the dynamic lateral force increment
is estimated to be 2500 pounds per foot of wall.  The resultant can be
assumed to act at a distance of .6 of the wall height up from the bottom
of the wall and the resulting dynamic lateral earth pressure
distribution can be assumed to be an inverted triangle ( 0 pressure at
bottom, maximum pressure at top).

My question is, this suggest that we take the weight of the building
times .183 (concrete shear wall structure), plus .4 times the weight of
the soil in the profile distribution he describes.  The site is in
seismic zone 4, so a site acceleration of .4 is assumed.  However, going
through the formulas to come up with our base design shear, we only
design for .183g's of the building weight.  Should the same concept
apply to the weight of the soil?  Do we really have to use a .4g factor
(the actual ground acceleration), or when we design our shear walls and
structure for shear and overturning forces, due we use a .183 factor.

Any input would be appreciated.

Lynn