Questions on Assumptions of Wall Friction Angle for Kp

[NoWittyHandle]

I am having trouble figuring out how to calculate Kp when designing heel blocks that resist raker loads (a heel block is a concrete block that supports rakers, primarily through passive resistance) . What assumptions are usually made about the wall friction angle when computing Kp? Is the assumed wall friction angle effected by the angle of the force acting on the heel block? Assuming that the earth is on the left of the heel block, is the passive force acting on the back face of the heel block rotated clockwise or counterclockwise to the normal to the back face of the heel block? How does the inclination of the back face of the heel block effect the wall friction angle?

I am using Figure 5 on page 7.2-66 of the DM 7.2 US Navy Design Manual to find Kp. This graph appears to be more conservative than Figure 8 on page 7.2-69 of DM 7.2. There is a note on Figure 8 that Kp values are unconservative for wall friction angle/phi angle > 1/3, so Figure 5 may have been adjusted to account for this fact. The first edition of the Foundation Engineering Handbook on page 426 states that it is common to take wall friction angle/phi angle = 2/3 when computing the passive pressure in front of sheet pile walls.

Thank you for your responses.

 

[jdonville]

EricinNJ,

Don't make your life complicated. Use Figure 2 instead (Rankine) which assumes delta=0. I usually assume phi=28 deg for a cohesive soil, unless it's unusually moist (closer to the LL than the PL). Note that you should also probably take credit for any sliding friction in addition to passive pressure. See page 7.2-63 for friction factors.

Notwithstanding the above, you should probably be consistent with the design approach mandated (if any) for your project.

Jeff

 

[NoWittyHandle]

The Rankine Method would be applicable to a cantilevered retaining wall, but for mass concrete, such as a gravity retaining wall or a heel block, the Coulomb Method is generally recommended. Also, wall friction considerably increases the passive resistance, hence my interest in properly estimating the angle of wall friction.