Live Load Surcharge Application to Retaining Wall Shear Key

[Trouser]

I have been asked to evaluate the feasibility of the attached retaining wall section for a DOT highway project. The wall will be retaining earth above a highway and will be supporting traffic (i.e. business parking lots).

Sliding is the controlling scenario. Frictional resistance is minimal along the sidewalk due to the relatively small normal force (no soil). An equivalent live load surcharge of 2 feet of soil is applied to these walls per AASHTO. The shear key provides additional passive resistance, but needs to be excessively large due to the fact that the key is also subject to the live load surchage. A factor of 0.5 is applied to the passive pressure resitance on the key and the live load surcharge is multiplied by the active earth pressure coefficient of ~0.3.

Does the live load surcharge apply to the shear key if it is moved to the center of the side walk?

 

[ishvaaag]

Irrespective of AASHTO specs, if you don't effectively prevent the vehicles approaching the edge, they will, eventually. Hence, except effective barriers are set in this intent, one would likely want to keep the vehicle loads extending to any point they can reach.

 

[Trouser]

Thank you for your general input ishvaaag.

Does anyone have any input based on standard practice? Are live load surcharges typically applied to shear keys?

I am convinced that a live load surcharge should be applied to the shear key in its current location, however, what if the shear key was located beneath the sidewalk, which is not subject to vehicle loading?

 

[Trouser]

See this sketch.

 

[ztengguy]

i would not apply it. what is on the other side of the sidewalk, is anything help stop sliding there?

 

[Trouser]

The roadway is located on the other side of the sidewalk. No passive resistance is being mobilized here since there is a possibility of any roadway work (paving, etc.).

 

[msquared48]

Personally, I would not apply it.

However, if the shear key is moved as you suggest, the wall will behave differently in my view. If the shear key is placed, say halfway down the slab length, say 2 feet or so, instead of the wall rotating about the intersection of the slab and wall, it seems to me that it will tend to rotate about the shear key. This could affect the reinforcing size and placement.

Mike McCann
MMC Engineering

 

[jeff91]

If I understand the question correctly there is potential for a live load on the retained side of the wall. This live load will in turn produce horizontal active forces in the soil that will load the retaining wall. To account for this live load a surcharge is assumed to act behind the wall. Given that the shear key will be structurally connected to the rest of the wall, no matter where it is located why would you not include the horizontal forces due to the surcharge on the shear key?

Typically shear keys are aligned with the stem of the wall to allow the vertical bars to be used in both the stem and the shear key without having to provide standard hooks.

Question - is there a reason that a heel cannot be constructed for this retaining wall to improve the sliding resistance?

 

[Trouser]

The unique wall layout is due to right-of-way issues on the project. This layout minimizes impacts to businesses and allows them to keep the majority of their existing property and property rights.

I actually found the answer to my own question in the Structural Engineering Reference Manual Figure 2.11. They apply live load surcharge and active pressure from top of wall to bottom of footing and passive pressure from top of footing to bottom of key. See for yourselves:

http://books.google.com/books?id=nD...ineering reference manual 5th edition&f=false

Thank you all for your discussion. I think that I will be purchasing this book.