Sloped backfill for Passive Pressure

[eric1037]

Hi!

I'm designing a cantilever retaining wall for a bridge abutment. The backfill on both sides of the retaining wall are sloped. They both have negative beta angles with respect to vertical.

My questions are:

How do you calculate the passive pressure with a negative slope? Is it as simple as using a negative beta angle in either the Rankine or Coulomb equations? If so, is there an equation or rule-of-thumb that tells you how far down the wall you can count on the passive pressure to resist rotation?

We are using clean, granular backfill.

Thanks and I look forward to your responses!

 

[GeoPaveTraffic]

eric1037;

I'm not sure I follow the geometry of the situation you are describing. However, under most circumstances I would not use passive pressure on a retaining wall as part of the sliding or overturning resistance calcs. One of the questions you need to answer for yourself is "Will the soil on the passive side of the wall ALWAYS be there?" Unless you can answer YES, then you should not use it in the analysis.

If you are going to use passive pressure, then you need to determine what the appropriate pressure is, which of course leads to your question. Assuming that the soil on the passive side slopes down from the wall, then there will be a reduction in the passive pressure available to the wall. You have options to determine what that reduction would be. One option would be to do a stability calculation from the bottom of the wall to the face of the backfill. You do this by calculating the sliding resistance on several surfaces and comparing them to the amount of passive force above the slide plane. The lower of the two amounts is available to the wall as a passive force. Another option is to draw up the wall and pressure envelopes, and just chop off the upper portion of the passive pressure envelope based on past experiences. Another option, as suggested previously would be to look up a cookbook method and do what it suggests.

My advice is to do the stability calculation. That way you will get a better handle or "feeling" for the situation and will be better able to use professional judgment in the future.

 

[DRC1]

A table of curves based on log spiral failure surfaces derived by Caquot & Kerisel exist for passive pressure for negative slopes. I use the "pile Buck " manual, but it published in other texts. The loading is triangular and extends from the ground surface. The passive pressure coeficent however is considerably less than that for level ground. Be sure to probely correct for soil/wall friction.
Geopavetaffic has a good point I have used passive pressure for temporary retainig walls. Pemenant walls usulally do not relay on passive pressure except for permenant sheet piling

 

[eric1037]

Thanks for the input. I used the NAVFAC method. The project involves a bridge span that has a constant horizontal force into the soil. That's why the passive pressure can be used.