Liquefaction...

[KevinChez]

I am designing the foundation for a pre-fab building. The soils report recommends caisson piles because the site is undocumented fill on sand and clay. The sand is in the water table about 5 feet below the base of the proposed foundation. The geotechnical report says liquefaction is a concern.

The site is an empty lot in Manhattan NY. The adjacent buildings are brownstone type construction. I doubt they are on piles. My problem is the project is woefully over budget with the pile foundation. The client hoped a mat slab would suffice. I am not comfortable with a mat slab given the geotechnical report. However if a seismic event causes liquefaction the whole block of buildings would come down?

I am sticking to my original design but I am battling the client over budget and is exhausting. My reason for writing is some reassurance that I am not being overly conservative. Thank you.

 

[skeletron]

Did the geotech report give the recommendation for caisson piles?


...but I can't recall if I have ever solved that problem yet.

 

[oldestguy]

I'd get another geotech more familiar with the area. Besides adjacent buildings will wreck yours even if you are on piles.

 

[GeoEnvGuy]

So the old brownstone geotechnical reports didn't consider liquefaction and the one for your project does because it's called the current state of the practice.

You can ask the municipal building department if you can build this project without considering liquefaction induced settlements. Another option is you can try to reinforce the slab to handle large settlements and differential settlements.

If your interested in seeing some examples of the effects I recommend watching Dr. Bray's liquefaction induced settlements lecture.

https://m.youtube.com/watch?v=mO7LAZgtdoI

 

[SlideRuleEra]

My reason for writing is some reassurance that I am not being overly conservative.

Read 2019 New York City Hazard Mitigation - Earthquakes to make your own assessment of the risk.

http://www.SlideRuleEra.net

 

[KevinChez]

@skeletron...yes mini piles or caisson piles. Caisson piles in rock made the most sense on this site.

Thank you @SlideRuleGuy, @GeoEnvGuy, & @oldestguy.

 

[fattdad]

If there was a boring with N-values (refer to Standard Penetration Test), then you'd have data to provide. If you don't have these data, then there'd be no real basis, but fear, to consider the risk of liquefaction.

There is some code-referenced design acceleration, I'd suspect. If you know that value, then the N-values can inform to what extent the risk is real. Seed and Idris have nomographs to address this topic. Most geotechnical engineers know about Seed and Idriss nomographs. Not all states may allow such simplification; however.

I live in Virginia, we have code references. I also work in the highway business, so Federal Highways have their design requirements to get funding.

Not sure what to advise? I'd want to know the regulatory setting and see some data - more data than the water table is at 5 ft and includes sand and clay.

f-d

ípapß gordo ainÆt no madre flaca!

 

[MTNClimber]

Interesting that someone wants to build in Manhattan and is complaining about the cost.

 

[KevinChez]

We do have borings. N-value varies from 12 @ 15 ft, 8 @ 20 ft, 13 @ 25 ft, 14 @ 30 ft & 13 @ 35 ft. This is 140lb Hammer with 30" drop.

 

[MTNClimber]

Did the geotech run the liquefaction analysis? They should be able to tell you how much settlement to anticipate and you can pass that info onto the owner. If it’s a small amount maybe you can get away shallow foundations with the owner accepting the risk of settlement and distress.

Edit: Before going that route, what is the pre-fab building for?

 

[KevinChez]

The building is six story residential, steel frame.

The geotech did the liquefaction analysis. I am going by the recommendations made in their report.

 

[MTNClimber]

Sounds like your hands are tied. Did they say how much liquefaction settlement is anticipated?