Presumptive Soil Values per IBC table 1806.2

[Martino8]

In our area (Western WA), it's very common to work without a Geotechnical report for residential construction.
The acceptable standard by most jurisdicttions is to use 1,500 psf for allowable bearing pressures.
I presume this is intended to cover a "worst possible case" based on the lowest pressure listed in IBC table 1806.2.
However that approach falls apart for other soil properties.

Bearing Pressure
In our region ground surface soils can be anywhere from uncontrolled fills to very dense glacial till.
90% of geotech reports recommend anywhere from 1,500 psf to 7,000 with 2,000 - 3,000 being the most common.
So using 1,500 seems like a fair and conservative value for bearing pressure.

Passive Resistance
Using the same "worst possible case" approach, the lowest passive value from table 1806.2 = 100pcf.
In over 25 years of reviewing Geotechnical reports, I've never seen a passive pressure that low.
The most common value from reports is usually about 300-350pcf, with MAYBE as low as 200pcf once in a blue moon.

Friction
Same problem, but an EVEN WORSE dissparity.
"Worst possible case" from Table 1806.2 = 0.25 (or even 0.00 for clay)
Geotechs seem to use allowable friction = 0.35 almost universally (already including 1.5 SF)

Active Pressure
IBC Table 1806.2 doesn't provide any presumptive values for lateral soil pressures.
Geotech seem to use active pressure = 35pcf almost universally (occassionally 40pcf)

Seismic Surcharge
Code says lateral seismic earth pressures "shall be considered"
No guidance is given in IBC Table 1806.2
Geotechs are seriously all over the map on this design consideration.
I've seen everything from 5H all the way up to 16H!!!

Thoughts for soil parameters without a Geotech report??

 

[phamENG]

My thought: don't do it.

I know it's common, but I also know that I'm not a geotechnical engineer and I also know that there is specific language in my insurance policy barring me from performing it. Well if I select a soil parameter and use it in my design without it coming from a geotechnical engineer or specific code guidance, I'm practicing geotechnical engineering. And if there's a problem because of that...well my insurance is going to run for the hills.

So if the owner refuses to hire one, I'll design based on code minimums (if the owner actively chose to save $5k up front in exchange for a conservative design that will cost an extra $50k-$1100k in construction...not my fault) and then require the contractor to hire a geotechnical engineer to come out and verify my 'basis of design values'.

 

[JAE]

At least for the 1,500 psf bearing pressure assumption allowed in the IBC - if we use that we usually require in our notes that the contractor must hire a soil/testing lab to come out and inspect the soils prior to foundation construction (and after foundation excavation) and verify that the soils are capable of providing 1,500 psf allowable bearing pressure.

 

[TheDW]

Table 1610.1 has presumptive active/at rest pressure values, ranging from 30 to 60 pcf

Ive seen other firms work where they assume a reasonable (to them) friction angle, and calculate an active pressure coefficient, giving them values quite a bit less than that table. I usually assume one of the 45pcf types of backfill

 

[haynewp]

We use this assumption for smaller projects and complain a lot if we have to use it on larger ones. We always put the note about inspecting the soil locally after excavation whether the value is assumed by IBC or if there was a geotech report, since this inspection is required by IBC Ch 17 anyway. I believe this inspection should be done by an inspector under the owner or owner's rep per IBC Ch 17.

 

[lexpatrie]

I don't think you've worded this very well, for bearing capacity the lowest value is likely conservative and less likely to cause issues, when it comes to a retaining wall, using the lightest soil for pressure on a retaining wall on the high soil side, will make it less safe. Quite a few designers will ignore passive pressure on the toe because it's disturbed during excavation and not explicitly retreated/compacted. A higher sliding friction coefficient makes it less safe.

You are making contradictory presumptions about the soil in these situations but the objective is to provide a safe(er) design.

When you say residential, do you mean 2,000 ft2 or 20,000 ft2? In the smaller case I think "standard of care" around you would tend to waive soils reports, larger projects, less so.