Shoring (pile) design using passive pressure 1

[kkoloj]

The way in which we determine the depth of a shoring member (e.g., drilled concrete caisson) is to apply all load above fixity (e.g., shear, moment, etc.) and then use the passive pressure resistance provided in a soils report to determine the depth below fixity (an aside: right now we are using a method by E. Czerniak). Typically, we receive information such as "use a passive pressure of 400 psf/ft up to a maximum of 5000 psf". However, I recently talked with another structural engineer who mentioned that when he does his shoring design he DOES NOT use a maximum value. That is, he lets the pressure increase at a rate of say 400 psf/ft without an upper limit. He noted that the maximum value does not apply to piles/caissons. I'm not sure if this was because shoring is generally temporary or if it had more to do with the geometry of piles/caissons. Has anyone dealt with these issues?

Thank you.

 

[DRC1]

I am not familar wit Czerniak, however this sounds like this approach may be more applicable to a laterally loaded cassion than a shoring support. There is a significant difference in that most of the lateral load is applied above grade andis generally is a portion of the axial load. The primary purpose of a shoring support is to carry lateral loads above and below grade with very little axial load. Further the method appears to only apply passive pressure to one side. If the supprt is cantilevered, the support will experince passive on the back moving to the front. If the strucure is tied back, it will have a different pressure distribution. If the structure has several ties, the applied loads will be adjusted.
To answer your question, in a sand or gravel, the passive pressure generally increases linerally with out limit as a function of depth. In a clay, the passive pressure is generally constant regardless of depth. Silts areone or the other, depending on the soil. However, these pressure distributions must be modified to account for changes in soil strength, water table loactation, the effect of seepage pressures, reductions or additions due to sloped excavations or fills added pessures due to surcharges from footings or equipment. Designing a soil retention system form only a recomended pressure in a report is dangerous. The writer probably never intended it to be used that way.
I would develop your own pressure diagrams from the site conditions, overall information in the report and the boring logs.
There are methods designed specifically for shoring, I strongly suggest you consult a geotechnical engineer expierenced in shoring systems for guidance on this design.

 

[kkoloj]

DRC1:

Thanks for all the information.

<To answer your question, in a sand or gravel, the passive pressure generally increases linerally with out limit as a function of depth. In a clay, the passive pressure is generally constant regardless of depth. Silts areone or the other, depending on the soil. >

Can recommend a reference for the above. I would really like to read-up on this information. Or is this something that should be found in any geotechnical book?

Thanks again.

 

[PEinc]

AASHTO and FHWA have much written about tiedback soldier beam walls. Their publications address design of the embedded soldier beam toe and passive resistance.

Get copies of the following:
FHWA-DP-68-1R Permanent Ground Anchors
FHWA/RD-82/047 Tiebacks
FHWA-IF-99-015 Ground Anchors and Anchored Systems
FHWA-SA-96-038 Earth Retaining Systems