Passive Resistance in front of Cantilever Ret Wall

[LicensedToPEe]

Referring to the usual references (Bowles, Coduto, Das), it is customary to neglect the passive pressure developed on the front face (toe side) of the cantilever wall’s footing. The explanation given is that “ The bottom of the footing may move slightly rearward if any overturning failure were to occur. Therefore, the resistance offered by the passive resistance acting on the front of the footing is not reliable, and should be neglected in the overturning analysis.”

The condition that I have is as follow:
-> retained height is 10 feet on the heel side
-> soil height over the toe height is 4 ft. this is 4 ft above the footing, against the front of the WALL (not just the footing).

I am using the weight of the soil (vertical component) over the toe as a resisting moment. However, considering the height “D” of this soil, the potential additional resisting moment is Kp * gamma * (D^2) * (1/2) * (D/3). I believe that since the height of the soil is quite high (i.e., extends against the wall, not just the footing), it would be quite U-N-C-O-N-S-E-R-V-A-T-I-V-E not to include it in the overturning analysis.

Any feedback on this approach is appreciated…

By the way, I am working with tight spatial constraints so the size of the footing is critical and thus the above dilemma…

Thanks!

 

[hokie66]

I agree with you. In that condition, I would include passive resistance. Perhaps ignore the top foot or so, but from there down I would use it.

 

[kslee1000]

The only time I would consider any soil effect (OT, sliding)in front of the toe is I am 1000% sure the soil condition would not be changed in any time during its service life. It is not because of conservatism, cause failure occurs time after times.

 

[miecz]

It makes sense to me to use the passive pressure, but I never have. The rational given to me for not using it was "the soil may be excavated in the future." I keep thinking that anyone who excavates that soil deserves to have the wall fall on him. The real reason I still don't use the passive pressure is that I thought it was standard practice in the industry not to. Not the maverick I used to be. I'd sure like to read posts from others that use it.

 

[kslee1000]

I also do not believe the full passive pressure, in front of toe, could be reached (developed) without significant lateral force, or movement of the wall base. In utilizing such unknown, you are playing with double jeopardy.

 

[LicensedToPEe]

kslee1000,

true, but at the same time, isn't the procedure for cantilvered walls utilizing active pressure on the heel side of the wall? If active pressure develops (and the associated movement with it), then the other (toe) side of the wall is experiencing a reaction in the form of passive pressure...

 

[kslee1000]

I don't think it is correct to say when one side of the wall has rotated with active pressure developed (soil wedge moved within the active zone), then it automatically triggers force required to cause passive soil failure (move of soil wedge within passive zone) on the other side of wall, especially when there is an elevation difference.

The use of passive pressure is more easily justified for deep pile, and deadman in anchorage system, which usually possess the essentials (intensity of load & large/deep soil mass) to allow passive state to occur.

 

[BAretired]

I tend to agree with you and Hokie66, but I am wondering why you are burying the retaining wall so far into the soil. Is it for frost protection? If so, you could raise the footing three feet and place rigid insulation to prevent heave.

As a further thought, if the wall is not tied into some unyielding object, why not let it heave? Then it will behave like the soil it is supporting.



BA

 

[vtgeotech]

kslee1000 is correct. In order to realize passive resistance you must fully mobilize the soil's shear strength, i.e. fail the soil mass in front of the toe. The amount of strain needed to realize fully passive resistance is much greater than that needed to realize active.

 

[fattdad]

Thankfully we use a safety factor on the passive resistance. I'd anticipate strain compatibility between the full driving active earth pressure and the portion of the passive that you'd allow considering your safety factor. That said, I don't use the upper 2 ft of the passive wedge owing to future uncertainty. Then again, I used to work with a very good engineer that would just use at-rest conditions for both the passive and active wedges (i.e., end up designing a gravity mass with sliding resistance).

f-d

¡papá gordo ain’t no madre flaca!

 

[GeoDocinFL]

Good thread. Disagree that neglecting passive resistance leads to "UNCONSERVATIVE" design as stated by the original poster. If anything, the opposite is true.

And, as others have pointed out, it takes substantially greater soil strains to mobilize full passive as compared to full active lateral soil stress conditions. That's a primary reason why passive resistance is either ignored, or greatly reduced with a factor of safety.