The earth pressure coefficients the WVDOH commonly uses for analyzing active and passive lateral earth pressure for common backfill types are presented as follows:
||Reinforced w/ Foam
||Gravel & Sand
||Clean Crushed Stone
Notice that in the passive case, if the wall is pushing into clean crushed stone, the Kp
value is 5.8; however, we allow Kp
= 3 to be used when Expanded Polystyrene Foam (EPS) is present to cushion the movement. This value can be further reduced as follows:
(mobilized) = (Thermal Movement/(0.05 * Wall Height)) * Kp
; Provided Kp
The minimum value of Kp
(mobilized) that we allow by the above equation is 0.5 (the at-rest value for average soil).
For most U-shaped wings that are only subjected to longitudinal movement, the use of the active case is acceptable and a Ka
=0.17 value for Class 1 Aggregate reinforced backfill can be used. If some transverse movement of the wings is anticipated, then the above passive analysis is needed based on the amount of displacement perpendicular to the wall surface.
For wings angled more than +/- 3° from straight back, the passive case is used. It is recommended that an EPS be included behind all wings other than straight back wings to avoid the higher passive loads.
When the retained soil or stone contains more than 15% of #200 sieve material, a minimum of 18 inches of select material for backfilling will be required behind the full height of the stem with a filter fabric separating it from the backfill.
The LRFD earth horizontal load factor of 1.5 will be used for the active case. Although LRFD does not provide a load factor for the passive case, we recommend using 1.35.
Active and Passive Earth Pressure Coefficients for Semi-Integral Abutments and Wingwalls
Where minimal thermal movement is transferred through the bearings to the abutment stem and wingwalls, the backwall should be analyzed separately from the stem using the passive earth pressure for the backwall and the active for the stem and wings. Where both the backwall and stem move together, they should be analyzed based on the passive case using their combined height and the thermal movement to find Kp
(mobilized) as discussed above. For reinforced backfill, a Ka
value of 0.17 can be used whether foam is present or not for the active case if the backfill consists of Class I stone.