Simulating Offset Backslope as Strip Footing Surcharge

[Dirt Dept.]

Designing some MSE walls with reinforced soil slopes above. The toe of the above slope is to be setback from the wall face by 6-ft. I am looking for a method/approximation to apply a surcharge as a cont. strip footing offset behind the wall. There isn't an AASHTO or LRFD procedure for this geometry so trying to think a little outside the box on this one.


Blair Rushing, PE
Nelson Engineering
Jackson, Wyoming

 

[Doctormo]

Two issues: Internal Stability and External stability.

Any time you have an odd backslope situation, you have to draw figures and look at what is happening. The fact that you have reinforced slopes on top may cause some calculation issues if the slopes are steeper than the phi angle.

Internal Stability - AASHTO use an equivalent surcharge to represent the slope behind a MSE wall applied on a level backslope (Ka for the level condition). The slope surcharge weight is calculated over the distance 0.70H back from the wall so you would take the weight of the slope after 6 ft. and divide that by the 0.7H distance to get the q-ave. The q would multiplied by Ka-level and applied to the reinforcement in addition to the normal soil load.

External Stability. - You would look at the retained height at the end of the reinforcement and see how that looks with the reinforced slope on top and the length of slope. You may be able to calculated the thrust based on a equation for Ka-slope from the end of the wall reinforcement. If the slope is too steep due to the reinforcement, you may have to use a global stability or trial wedge analysis to obtain the thrust on the wall and check stability.

Indirectly, this is covered in AASHTO but there are no easy equations or figures to help with. You have to think out side of the box as you mentioned.