Liquefaction Potential Analysis - Who Decides It's Required?? 1

[UTvoler]

Hi all,

First project on the NC coast coming up; all my projects have been inland thus far. Project is in flood Zone AE with finish floor above the flood elevation, so at the beach but not underwater. Geotech asked if we needed a liquefaction potential analysis, to which I responded "huh??" I did a little googling and reading, and since we're 300-yds from the water and presumably on sand responded yes. Proposal came in, included a 100-ft boring and CPT and was reasonably expensive. Discussed with the geotech, and he indicated in his long career he had only done the CPT/liquefaction analysis a few times for projects in the same types of locations. I asked who decides it's required, and he said it was up to the SER. He also volunteered that it was his understanding that the analysis was not required because wind controls the structural design rather than seismic. We settled on excluding the 100-ft boring and CPT, and he would review the standard 20-ft borings with me to help me decide whether there are any soil/water table conditions that might point to needing to the liquefaction analysis. So my questions:

-What experience do you guys have; who decides whether the liquefaction analysis is required; SER or Geotech? Is the approach described above to get a "standard" investigation with the results to guide whether further investigation is warranted reasonable?

-Whether or not seismic controls the structural design (I'm certain it won't with 134mph windspeed), if there is risk for liquefaction wouldn't it need to be accounted for in foundation design even if wind controls the overall structural design?

-Not clear on ASCE 7-16 20.3. I checked the period of our building, and it's less than 0.5s so we meet the exception. Geotech is expecting site class D based on his experience, but if it is determined that there is liquefiable soils than that drops us down to a Site Class F but a site response analysis is not required due to meeting the exception? Is that the gist?

Any thoughts/experience is appreciated!

 

[dik]

I don't do any significant seismic stuff, but I would think this would squarely be in the realm of a good geotechnical consultant.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[MTNClimber]

The building code decides. What type of structure are you building? IBC lists it as a requirement. IRC does not mention the word liquefaction.

You can perform a liquefaction analysis using corrected SPT blow counts. You don't need a CPT rig, but it will provide better data. To induce liquefaction in deep loose soils, you need a large PGAm. I would ask an experienced geotech if a 100' boring is actually required. I did a quick search of the seismic maps, and it looks like the coastal NC has a pretty low PGAm, assuming a site class D. A 40'-50' boring may be more appropriate, but talk to a geotech that has a better understanding of NC soils and liquefaction.

 

[JedClampett]

dik is correct. And they should recommend the mitigation and hopefully design it. Not to disrespect our Geotechnical brethren, but they seem to be very good at dropping a liquefaction bomb and disappearing. Same with a lot of foundation improvements. I spend too much time calling Geotechnical Engineers to get their specifications on soil improvements (that they recommended).

 

[MTNClimber]

That would be required if they get PAID to do those things. A liquefaction analysis doesn't take long and is standard for my firm if something questionable is encountered. Design work and writing a specification for a project can take a while. Often, clients balk at the price of just drilling and writing a geotech report. Helping past that point is, unfortunately, not something they are always interested in. That is why we need some allies (cough cough structurals cough cough) to recommend engaging the geotech after the geotech report is issued.

 

[phamENG]

First project on the NC coast coming up

Whereabouts, and why type of project?

I've done a few big beach houses along OBX...the geotechs out there that do residential scratch their heads when you ask for lateral capacities of the piles.

NC building code only requires a liquefaction analysis for seismic design category C and higher structures.

 

[UTvoler]

Thanks all!

@MTNClimber: IBC lists it as a requirement for SDC C-F. So if we're in SDC A/B (which we will be) no evaluation for liquefaction is required? Period? (@phamENG says so). It's seems that as the SDC gets worse the risk would for liquefaction would increase. But no risk at all at lower ground accelerations (PGAm = 0.047 btw, which I would assume is not "large")? Or maybe not enough risk to warrant the assessment, which is exactly the definite answer I like.

@phamENG: Nags Head, 4-story hotel, SDC A/B, typical shallow reinforced concrete footings (fingers crossed).

 

[phamENG]

Don't count on shallow footings. There are flood design requirements all over the place. So be careful. I live/practice in Hampton Roads and Nags Head/Kitty Hawk is our go-to beach, so I'm familiar with the area. But for a project of that size they may be using some other mitigation methods.

What geotech are you using? They got gobbled up by one of the 'pac-man' firms discussed on here recently, but GET is a great geotech firm in the area. They have an office in Elizabeth City. (Haven't worked with them in a few years - they stopped doing residential after the buy out and I haven't gotten back to commercial since opening my own shop.)

 

[driftLimiter]

I had a project recently with a potential local foundation settlement of 5" due to liquifiable soils. I was able to get shallow spread footings to work, but we tied them all together with grade beams for the lateral spreading concern, and we designed the superstructure connections to tolerate the local displacement.

You can attempt to make the foundation rigid (mat foundation) or design the structure to move with the soil. You don't have a whole lot of other choices.

 

[MTNClimber]

UTvolver - A 4-story hotel likely means the IBC is the controlling code with local amendments. IBC references ASCE 7 for Site Class Definition. A geotechnical engineer needs to determine if a soil layer is liquefiable to determine if the Site Class is A through F. I would talk to a different geotechnical firm. To me, it sounds like the person you spoke to isn't well-versed in the building code or liquefiable soils.

 

[UTvoler]

@phamENG: We'll see; if they're not typical/shallow foundations as our proposal included a change order will be shipped....Not going to name names, but ding ding their office is in Elizabeth City (both firms that have proposed, actually).

@driftLimiter: Did you direct the geotech to provide a liquification analysis, or did they get there on their own?

@MTNClimber: IBC for sure, as amended/adopted by NC. The Owner has solicited the geotechs, and asked for my review/comment on the proposals. Not much choice on my part, but I could certainly voice concerns. Guy I spoke with seemed fairly knowledgeable, but I'm a newbie in this arena for sure.

 

[driftLimiter]

@driftLimiter: Did you direct the geotech to provide a liquification analysis, or did they get there on their own?

The Geotech told me that it was a liquifiable soil. In my area this is a required part of all geotechnical investigations.

IBC 2018 1803.5.11 and 12 require this in all SDC C and above.

 

[dik]

In a seismic zone, I would suggest that a good geotekkie would insist on this. Cost and payment wouldn't be up to debate.

I'm still curious to see how Millennial Tower with fare.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik