aashto-lrfd box culvert loading

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aashto-lrfd box culvert loading
i am trying to determine how the aashto-lrfd code wants me to determine the lateral earth pressures(eh)against the outside walls of an reinforced concrete box culvert due to earth loads and live loads(hl93).
i have a box culvert that has complicated and unconventional transitions from a single "horseshoe" box to a double and a double to a triple with varying heights etc.
1.)  for this type of structure i am wondering if rankine or coulomb method should be used for lateral earth pressures.(i've read some people who believe that one should use "at-rest" earth pressures for rcb.) do i need to apply the soil-structure interaction factor that aashto requires for vertical pressures?
2.)  how does the code want the live loads to be distributed on the walls.  the governing live load pressure on one of the transitions is 690 psf on the top slab.  should i assume a surcharge height of 690 psf/120 pcf = 5.75 ft and apply it as a uniform horizontal load?
3.)  the soil borings indicate water will be present for a significant length of the 1700 ft structure.  should i assume a ko of, say 0.5, and find the soil pressure with pe=0.5*(gamma soil - 62.4 pcf) + 62.4 pcf?
thank you for your time.  any information the forum can offer would be greatly appreciated.
1. most rcb's are considered as rigid when compared to the adjacent soils.  the non movement of the walls as opposed to that of a retaining wall results in the use of the "at-rest" ko = 1-sin(f) ==> 0.50.  the soil interaction factor, 12.11.2.1 is less than 15% for embankment installations, which most rcb's fall under because of the work area outside of the box, unless you're under a high fill, its usually neglected.
2. if you have a uniform loading on the top slab distribute the loading moments and shears thru the cell(s).  the stand truck load and lane load should be distributed depending upon the depth of fill over the top slab.  see 12.11.2  the ll surcharge 3.11.6.1 is an other loading.
3. i usually check for both wet and dry conditions.  wet conditions should give you around 90 psf acting on the walls.
i usually spreadsheet the different loading and then combine them with their load factors for the different load conditions. it makes a nice little program.
the above is based on aashtto 1994
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