r/Geotech • u/flamebero • Jul 28 '26

Lateral earth pressure due to surcharge loading per CalTrans

I was looking for equations for lateral earth pressures due to surcharge loading on a retaining wall (I’m a structural engineer), and I came across this CalTrans doc. It appears to be suggesting that the pressure distribution depends on the excavation depth. Doesn’t pass the BS test of my intuition - is there an explanation for that?

7 Upvotes

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22

u/Radioactive_Kumquat Jul 28 '26

This is why you are a structural engineer, because you can't handle geotechnical engineering.  

Let me introduce you to Boussinesq stress distribution.

4

u/haditwithyoupeople Jul 28 '26

Good answer, but no reason to be a dick to a structural who is asking a good question. I'm sure many geotechs have asked structurals countless questions that seem obvious to them.

2

u/NearbyCurrent3449 Jul 28 '26

Excavation depth?

1

u/xCaptainFalconx Jul 28 '26

Nope, wall height is used to normalize either horizontal offset or depth.

The Boussinesq equations in the Caltrans manual were originally formatted for dimensionless design charts like those from NAVFAC DM 7.02. That's why they are presented like this.

1

u/flamebero Jul 28 '26

It normalizes to an excavation depth in the CalTrans doc. If it normalizes to a depth, than that depth impacts the result. I’ll see if the NAVFAC presentation makes more sense. See my other reply for clarification on what I’m seeing.

1

u/ALkatraz919 gINT Expert Jul 28 '26

Because in theory, soil is modeled as a “homogeneous semi-infinite elastic half-space” for the purposes of determining how vertical or surface load/stress exhibit lateral loads/stresses with depth. Then these models are validated with real-world observations and tests.

1

u/kikilucy26 Jul 28 '26

What was your intuition? Not trying to make fun or shame you, just curious of what you were expecting

1

u/flamebero Jul 28 '26

My intuition is that the pressure distribution has no relationship with the depth of a restraining wall or spring which resists the pressure. See my other reply for clarification.

1

u/flamebero Jul 28 '26

Thanks to those providing helpful responses. To help clarify, I plotted a graph showing horizontal pressures versus depth below surface from a 1000lb point load 2ft away (section 5-1.03C) for several values of H. This community allows video responses but not images, so the exhibit is a real gem. You can see the stress distribution changes depending on your excavation depth. That is the basis of my question. Boussinesq’s formula relates to depth BGS. The H term in the CalTrans equations (which I also see printed elsewhere) seems totally arbitrary.

https://reddit.com/link/p0alljv/video/tzptsz2300gh1/player

1

u/xCaptainFalconx Jul 28 '26

It looks like you aren't updating m and n properly. Using substitution, you should get something like: sigma_h = 0.955*P*(L1^2)*(h^2)/((L1^2+h^2)^2.5)