passive pressure for shear key

[mamebraso]

A foundation has been designed for a very high containment barrier on the depature of a bridge. Because the size of foundation is too small to resist sliding, a shear key has been provided. The question is, since the foundation is on the edge of the embankment and the only soil retained by the key is 3m of soil beneath the rest of the foundation and then the embankment falls off at 45 degrees, can you really consider the passive pressure provided by the key? If no at what distance can the passive pressure be mobilised. If it was ground level then you could consider it infinitely long. help out

 

[GeoPaveTraffic]

Passive pressure is only one check that should be performed on a shear key. The sliding resistance on a shear plane passing below the shear key should also be checked. Depending on the soil conditions and geometry the will often produce a lower resisting value than the passive pressure on the key. In any case the resistance assigned to the key should be the lesser of the passive and sliding value.

Also, multiple shear planes should be checked. It is much like doing a slope stability analysis. In fact, I have often used slope stability software to do additional checks.

 

[mamebraso]

Apart from the horizontal force (shear force) acting on the foundation there is a corresponding moment acting in the directon of shear making failure along the plane passing below the shear key not probable. The resisting forces i have in the horizontal are therefore the frictional force at the base of the foundation and whether to use the total passive on the key. I take the point that i should be doing slope stability analysis.

 

[DRC1]

There are values for passive pressure in negative slopes. These can be found in log spiral charts. Note that these values are substainally smaller than the same soil with level backfili. Also since you are close to the edge, you should use a suitable factor of safety of safety on the passive presure.

 

[BigH]

Along the line of GeoPaveTraffic - perhaps you will have problems with bearing pressures (high pressures near slope). Need to check out via Meyerhoff method (or other methods). With respect to DRC1's comments, you will need to be sure that the passive pressure spiral (or wedge if you are using normal Rankine (Colomb) style) will be "full" or will be truncated by the slope. If that is the case, you will not be able to use the full of the passive pressure. Consideration could be given to using a graphical (oh dear me!) method such as Culmann to check passive force available.

 

[GeoPaveTraffic]

mamebraso said "Apart from the horizontal force (shear force) acting on the foundation there is a corresponding moment acting in the directon of shear making failure along the plane passing below the shear key not probable."

I don't understand this comment. If you generate a free body diagram of the soil between the shear key, the base of the wall, and the slope, the shear on the bottom of the surface is a limiting factor as to what can be generated by the shear key. If I'm missing something, let me know.

 

[PEStructural]

The log spiral chart for decreased passive pressure due to a sloped backfill can be found at the following website (page 12):

http://www.dot.ca.gov/hq/esc/construction/Manuals/TrenchingandShoring/Steel_Sheet_Piling.pdf

 

[mamebraso]

GeopaveTraffic,
If i understood what you said. I thought you were referring to the shear plane that should be checked as indicated in EC hambly Bridge foundations and substructures. If that is the one, i have checked that since the free body diagram for that also includes the weight of the foundation acting at an angle and also the soil capacity not to shear against the impact force at angle. If i got you right then that has been checked and the free body diagram indicates foundation will not pull out in that fashion. Thanks for all the comments and those received from the others. i have used the log spiral chart for the decreased passive pressure