?NCEES Sample #512 (Geotech PM)?

[MC_Engineer]

Problem pertains to Mechanically Stabilized (reinforced earth) wall

Wall supports an embankment with a Surcharge @ the top. Find effective Length of strip (ie past failure surface)

Solution basically breaks down to:

1) Find force acting to push out wall

2) Find strips ability to resist pullout via friction

What I don't understand is why the surcharge is included in the calculation of effective pressure for Force req'd to push out the wall, but is not included in the calculation of effective pressure for strip ability to resist pullout ... ?

Please help

 

[Blacktop]

I think often times that this sort of thing is done in the interest of a conservative design.

 

[Guest]

When designing for the tie-back for MSE walls, you are looking for the ACTIVE component of earth pressure, but only to the extent that it creates tension in the tie-back and soil interface. So, the ability for tie-back to resist a pullout is based on the friction that arises due to the properties of the soil. The active earth pressure acting on the wall is considered within a holistic approach (e.g. final design) but the LIMITING aspect is based on the specification (dimensions/spacing of tiebacks) and the properties of the soil.

If you would like to read more about design of MSE walls, I recommend:

• FL DOT Mechanically Stabilized Wall Inspector's Handbook
• Long-Term Design Aspects of Mechanically Stabilized Earth Walls (FL DOT Report)
• FHWA MECHANICALLY STABILIZED EARTH WALLS AND REINFORCED SOIL SLOPES: DESIGN & CONSTRUCTION GUIDELINES

JR