lateral forces on retaining wall from wheel loads

huangyhg

lateral forces on retaining wall from wheel loads
i am designing a retaining wall which could potentially have hs20-44 wheel loads surcharging the earth on the back of the wall.
using a variation of the boussinesq equation and for a wheel load of 32 kips, i am getting a maximum pressure of 830 pounds at 1 ft below grade decreasing to zero at about 6.5 ft when the wheel load is 2 ft from the wall.
when the wheel load is 5ft from the wall, the maximum pressure is 135 pounds at around 3ft below grade decreasing to zero at around 12ft.
as the distance of the wheel load from the wall will vary, i propose to obtain a load envelope for the possible variations.
has anyone done a similar design and do the pressures above seem reasonable?
thanks
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i've never ran any numbers, but i always have been happy using the traditional two foot of additional surcharge to simulate a wheel load.  apply the resulting lateral component over the entire depth of the wall.
when you start placing wheels very close to a wall, you do get some very high local loads.  but in real life, it is not normal for someone to pull a fully loaded 18 wheeler so close to a retaining wall.  
do you need to include a barrier impact load also? this can have major implications.
csd
what i use is contained in em 1110-1-2502, 29 sep 89, figure 3-27, p. 3-50.  this document can be downloaded at the following url:
aashto stipulates a two foot additional soil load to account for this.  it makes it easier but always works.  impact forces must be accounted for, however.