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Computational Procedures for Passive Earth Pressures

Computational Procedures for Passive Earth Pressures

Computational Procedures for Passive Earth Pressures

(OP)
I am trying to calculate the Passive pressure factor Kp using LRFD Figure 3.11.5.4-1 and I get high Kp about 9. This seems to be hight...any idea

RE: Computational Procedures for Passive Earth Pressures

What's the reference and the units?

RE: Computational Procedures for Passive Earth Pressures

(OP)
It is in the AASHTO Bridge Design Specifications LRFD Figure 3.11.5.4-1

RE: Computational Procedures for Passive Earth Pressures

You are on the right track. If you are using a vertical wall, your friction angle would probably be 32 degrees. You need to find the ratio of the frction angle of the soil to the wall divided by the soil friction strength. For example if you have clean sand and steel sheets, the friction angle is 17 degrees. Divide by 32 degrees you have approximately .5. Using the table in the upper left hand corner and interpolating, you end up with a factor of .7. Multiply the value on the curve by the computed factor to get ultimate passive. Here it would be 9 x .7 = 6.3. For typical (non LRFD) design I would divide by a factor of safety of 1.5 that would yield a passive pressure of 4.2. This method is an acceptable methood, and typically yields a higner passive pressure than Rankine or Coulomb. I have not done LRFD for earth retention, so you probably do not divide by the 1.5 F.S. But check with others on how to proceede from the point where you get 6.3.

RE: Computational Procedures for Passive Earth Pressures

While the numbers add up to higher passive values, I know many (me included) who wouldn't use more than 3 in analysis (when it is acting as restraining).  This may be "conservative" to many - but given that things don't always stay the same, likely prudent.

RE: Computational Procedures for Passive Earth Pressures

If I recall correctly, the curves are based on Caquot/Kerisel.

Jeff

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