Pile Design with Liquefiable Soils

[SoilRocks]

Does anyone know a good reference paper or text for dealing with liquefiable layers in pile design. We're trying to feel comfortable with realistic amount of downdrag or strength loss that could occur in seismic event. Thanks for any help.

 

[moe333]

SoilRocks,

You may want to do a search as this topic has been covered before. Look in "Technical Papers" on

http://www.unisoftltd.com.

This is Dr. Bengt Fellinius's software company.

 

[dgillette]

Try this:

http://cee.engr.ucdavis.edu/faculty/boulanger/

Scroll down to

Singh, P., Brandenberg, S. J., Boulanger, R. W., and Kutter, B. K. (2002). "Behavior of pile foundations in liquefied and laterally spreading ground." Proceedings ASCE/AEG/UMKC Geotechnical Conference, Kansas City, MO, April 6, 2002.

and

Brandenberg, S. J., Singh, P., Boulanger, R. W., and Kutter, B. L. (2001). "Behavior of piles in laterally spreading ground during earthquakes."The Sixth Caltrans Seismic Research Workshop, Radisson Hotel, Sacramento, CA, June 12-13, Paper 02-106.

Both of those are downloadable as .pdf files. Some of it is pure research; some of it is more easily applied.

Also, get the index in the December issues of the ASCE Jnl of Geotech Engrg and look through them.

 

[VoyageofDiscovery]

Most geotechnical engineers I know are extremely cautious with respect to liquifiable soils, even recommending very costly soil strengthening measures such as vertical drains over the entire site.

I say this to give you some comfort when you are somewhat forced to give bad news.

Regards

VOD

 

[BigH]

If you are in a liquefiable zone, you can use timber compaction piles in order to densify the loose sand/silty sand to an extent that it is not judged "liquefiable". You can then drive your production piles.

 

[dgillette]

Be wary of compaction by piles unless the fines content is small and there is some place where the displaced water can drain, such as a gravel layer or the ground surface (with no clay layer overlying the sand being treated). I've seen several cases where the permeability of the treated material was too low, and they were not able to get good densification with stone columns (conceptually similar to compaction piles), and one other where compaction by stone columns was not achieved in a test section until they added "wick" drains. You will need SPTs or CPTs to confirm adequate densification, and you should consider a test section prior to finalizing the specs if the area is large.

This permeability issue applies also to vertical drains installed to relieve pore pressure during the earthquake (mentioned by VoD). Unless the material is very clean AND the vertical drains are closely spaced, there isn't going to be a whole lot of drainage during 20 seconds of shaking. The volume of water that has to escape (in order to get just shake-down settlement and not liquefaction) could be 1 percent or more of the total volume of the soil. The water has to go somewhere, and fast.

 

[Zoomzoo]

I've only dealt with smaller projects where the foundations consisted of drilled (and driven) piles which went through sandy soils (with liquefiable zones) to the bearing elevation.

Personally, I couldn't find any references when I had to solve this problem - so, I assumed the design earthquake event, tried (using blowcount data primarily - not a lot of budget here...) to estimate what zones along the pile would liquefy, and then eliminated those zones (including a healthy F.S. multiplier) from the capacity analysis.

I suppose I would approach the problem in a similar manner if I had clay over a liquefiable sand, and I was concerned about downdrag. But if there was no clay soil in the subsurface profile, I'd just ignore the downdrag issue until someone shows me how a liquefied flowing sand can stick to the sides of a pile and pull down on it.

Zoom

 

[BigH]

ASCE has a Geotechnical Specialty Proceedings of a conference/seminar held in March 2005: See ASCE Geotechnical SP #145. Just found it out in an ASCE Geobooks e-mailing.