Active Pressure on Cantilever Retaining Wall

[Shanman_]

For a cantilever retaining wall supported on drilled piers, a Geotechnical Engineer on a previous project stated that the active pressure should be considered over 1x the pier diameter below the depth of excavation - continuing all the way to the bottom of the pier.

On a current project of ours, the Geotech states that the passive pressure given to us is "net" and thus active pressure can be discontinued through the depth of embedment. We are skeptical of this verbiage. The report also lists passive pressure as an ultimate value and recommends that the Structural Engineer determine an appropriate safety factor - atypical from what we've seen as well.

The experienced folk here are explaining that active pressure doesn't make sense below grade for X number of reasons.

I did some research for CALTRANS & NYDOT retaining wall examples, sources I hope I can rely on, and found that both agencies consider active pressure taken throughout the length of the pile.

If I were to have it my way, I would consider active pressure a tributary width equal to the pier spacing above grade with a tributary of 1x pier diameter below grade. However, this has a dramatic impact on the required depth.

Any thoughts on the topic would be appreciated. Whether or not active should be considered in this region or about the concept of "net" passive or anything else.

Thanks!

 

[Tomfh]

Below grade there can still be active pressure behind the pier or wall. There is active/passive-/at-rest pressure on both side of the wall for the full depth of the retaining element. Above the centre of wall/pier rotation it is active pressure behind the wall. The active pressure doesn’t just stop because you’re below grade. The soil behind the wall doesn’t care what’s in front of the wall. It’s soil and will push regardless.

 

[Settingsun]

Net passive is just passive minus active, but I'd prefer to receive both values from the geotech. It doesn't save you any effort to receive only net but it may leave you without required numbers in some circumstances.

In terms of soil pressure on your structure, imagine the structure just disappears leaving a void. What pressure would need to be applied to the now-exposed soil faces so they don't move? That's an indication of the pressure on the structure and why there's still active below ground.

For piles, arching may reduce or increase the pressures on the structure.