Should Passive Earth Pressure be Used to Resist Overturning of a Cantilevered Retaining Wall?

[Jeremy Dye]

For this loading scenario, the toe side is on the left, and the heel side is on the right.

When we check overturning, we divide the resisting moments by the overturning moments to determine the factor of safety against overturning. Should the passive earth pressure be included in the resisting moments? Normally I ignore it because usually I only have a couple of feet of soil over the toe, and it doesn't make a big difference one way or the other. However, in this situation, I have 18' of soil over the toe (so my footing can go down to bedrock), and this is making my design extremely costly (5' thick wall and footing, etc.).

Here are the 3 ways I can think of to treat the passive earth pressure:
Option 1: Completely ignore the passive earth pressure.
Option 2: Use only the portion of the passive earth pressure that is "engaged" to resist sliding (often less than the full allowable passive pressure).
Option 3: Use the full allowable passive pressure.

Is there any literature out there that addresses this situation?

Much appreciated,
Jeremy

 

[cvg]

EM 1110-2-2502
29 Sep 89
Chapter 3
3-8 b

b. Estimation of Passive Resistance. A conservative and convenient de-
sign approach is to assume the resisting-side force is zero for overturning and bearing capacity
analyses and for structural design. However, in some cases, such as walls with relatively deep
foundations, it may be desirable to consider some lateral resistance for these analyses. To
justifiably assume a non-zero resisting-side force, the material must not lose its resistance char-
acteristics with any probable change in water content or environmental condi- tions and must not be
eroded or excavated during the life of the wall. If such assumptions can be justified, at-rest
conditions may be conservatively assumed on the resisting side. Resisting-side pressures and
forces generally should not be assumed to exceed at-rest conditions when calculating the base
resultant force and location and when designing structural components. How- ever, if the
driving-side earth force exceeds the sum of the resisting side
at-rest earth force (if present) and the maximum available base shear force calculated using
unfactored shear parameters, the additional required resis- tance should be assumed to be provided
by additional resisting-side pressure. In no case should the resisting-side earth pressure exceed
one-half the pas- sive pressure calculated using unfactored shear strengths for overturning and
bearing capacity analyses and structural design.

 

[oldestguy]

For at least one of your stability checks all soil should be considered as submerged and the unit weights accordingly taken as lower than not submerged..

 

[Jeremy Dye]

Thanks oldestguy. In addition to the load case you suggested, I'm also considering no water at all (which reverses the toe and heel side).

 

[fattdad]

if the wall can tolerate the movement required to mobilize active and passive earth pressures. . .

Otherwise, you can just design the wall for at-rest on both sides and obtain the lateral resistance by the base shear. It's conservative, but will also limit the potential movements required to engage the soil friction.

f-d

ípapß gordo ainÆt no madre flaca!