Ultimate vs. Allowable Passive Pressure - which to use with a safety factor

[skimboard20]

Hi all,

I recently received corrections comments from a building official regarding a small shoring wall design. The wall consists of driven wide flange piles and 4x12 timber lagging. It retains about 5 ft of soil.

We have a soils report for the project where the soils engineer gives us an ultimate passive pressure of 400 pcf to use for the design. I used this value to design the piles, along with a safety factor of 1.5, and a safety factor of 1.2 when I included a seismic surcharge.

The building official is saying that we need to use an allowable passive pressure in our design, and points to IBC 1802.1. This has a few people in my firm confused. Should we be using an allowable passive pressure + a safety factor of 1.5? Is the safety factor of 1.5 meant to be used with an ultimate passive pressure? The soils engineer, who is experienced in this type of project, says that using an allowable passive and a safety factor is overkill and is disagreeing with the building official.

Looking back at soils reports from other firms on different projects, we were definitely given an allowable passive pressure in the report, which was then used in our design, with the corresponding safety factor.

I'm wondering if anyone has insight into how to think about this. Thanks!

 

[HTURKAK]

We have a soils report for the project where the soils engineer gives us an ultimate passive pressure of 400 pcf to use for the design. I used this value to design the piles,

Did you use working stress combinations?
Section 1802 Design Basis states
(Bearing pressures, stresses, and lateral pressures used in this chapter are allowable pressures
or stresses, not strength level values. )
If you are checking the overall stability , apply active soil pressure one side and passive soil resistance at opposite side and check for OT and look for a FS say 1.5
In your case if the shoring system supporting an existing building , facility etc , eventually your calculation will work but with a cost of lateral displacement and tilting .
I just want also remind that , the full passive pressure will develop with an around 50 mm displacement.