Horizontal Modulus of Subgrade Reaction

[hemiv]

I'm working on a problem where a retaining wall will be loaded toward the backfill, and I'm trying to come up with some deflections. I've created a model of this structure with a mesh of spring supports to simulate the backfill. I'm looking at the Das text Principles of Foundation Engineering, chapter 11 section 16, for horizontal modulus of subgrade reaction. This whole section is about piles, but I'm wondering about applying his Nh values (in pci) toward my spring mesh. Basically, K (lb/in) = Nh (pci) * depth of spring (in) * tributary width of spring (in).

 

[oldestguy]

A rare circumstance. I'd just use the usual geotechnical methods evaluating lateral earth pressure, active and in this case passive.

 

[EireChch]

to my knowledge, retaining walls are not designed using horizontal modulus that is intended to be applied to piles. A pile is anywhere between 0.3m to 2.8m+ in diameter. Retaining walls are typically designed using Plane Strain methodology. There is a strain incompatibility issue.

I do know that WALLAP using a subgrade reaction model, but i doubt it is the same methodology as piles.

 

[fattdad]

movement to mobilize passive pressure is about 3x the movement to mobilize active pressure.

Does that help?

f-d

ípapß gordo ainÆt no madre flaca!

 

[hemiv]

Thanks y'all.

fatdad, so say I have a 20' cantilever wall.

1) Is this movement measured at the same location of the developing pressures? E.g. Let's say it's 1"/10' to develop passive pressures. At 15' up the wall, I'll need 1.5" of deflection towards the backfill at that location for passive pressures to develop? What about at the very bottom of the wall (z=0)? I theoretically need 0" of deflection for passives? Or is it that the top of the wall needs to maximize deflection (2" in this example) before passive pressures can be considered anywhere?

2) In my particular case, where I'm designing for a loading scenario towards the backfill, I will never have active pressures. Active pressures only develop when a wall moves away from the soil, not towards. So unless I'm willing to allow the passive deflections, I should use at-rest pressures?

3) If I model this thing with at-rest pressures return deflection values in excess of the 1"/10' rule, that means my model is no good and I need to reiterate using passive pressures? Conversely, that model using passive pressures should show at least 1"/10' deflections to be considered valid, right?

4) I don't think I really have a cantilever wall - It's a got a significant curvature to it, which going to stiffen it up and make it act in some ways like a braced wall. Now, for purposes of analysis, if I can prove the wall without counting on this self-stiffness, all good! But is there anything additional I should keep in mind in this situation, from a geotech perspective, as opposed to a typical flat cantilever retaining wall?