Liquefaction question 2

[pelelo]

Hello,

I am working on a project which consists on the construction of 5 story buildings.

The Stratigraphy on this project is basically Silty sand (SM), silt (ML) and sandy silt (ML),with SPT values lower than 10 blows per ft.

This situation occurs during the first 30 feet. Going deeper, SPT values increase significantly to 40 and 50 blows per foot (and sometimes even more).

Also the water table is on average 2 feet off the surface.

No doubt this is potentially liquefiable escenario.

I performed the liquefaction analisys posted by Idriss and Seed in the 1996 and 1998 NCEER Workshops.

I computed the Liquefaction Factor of Safety every 5 feet for each boring. In some borings I got several layers with the FS less than 1.

In other borings, i got the FS < 1 in only 1 layer. (15-20 feet depth typically).

Other than excavating and removing the loose soils and filling back up with an adequate material, I know deep foundations can be another solution. Maybe the last one is better because excavating 20 feet would be kind of expensive.

So my question is, is it necessary to improve the soil conditions if only a 5 feet layer could liquefy?.

I am not sure what would be the minium loose soil thickness I need to keep in mind in order to improve the soil conditions (e.g replacing the material or recomending deep foundations).

Please let me know.

 

[rowingengineer]

I don't know enough to help with the technical side of liquefaction, Maybe Big H will help out with that. However I would expect deep foundations to be the recommendation from the geotechnical engineer with the results you have quoted.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[moe333]

Not knowing all the details, I would say yes for numerous reasons. Ground improvement with stone columns or a few other methods is an option. You would need to dewater to excavate and replace, probably a difficult task especially if there is nearby development.

 

[Geostructsparks]

You don't have potential for excessive settlement for a 5-story building on loose silts and sands, so you would need a mat/deep foundation/ground improvement anyways?

Penetration methods (SPT and CPT) generally are piss-poor at liquefaction prediction in borderline silty sands and silts, but the alternative (dynamic triaxial testing) isn't practiced except for major projects. Consider if most of the reputed liquefaction potential is in fine-grained soils (say >35% fines), check whether they are above or below the watercontent/LL criteria of 80%, in which case (above) probably liquefiable or at least no change in diagnosis and (below) apparently not liquefiable.

The last question is what the post-liquefaction settlement and lateral spread potential there is. If the site is flat with no nearby grade changes, or (according to Youd 2002) N60 blowcounts are all greater than 15, the it may not lateral spread. According to practice about a decade ago, then you only have Tokimatsu and Seed post-liquefaction consolidation, which may be 1% if your blows are 20 to 30, e.g. 1% x 5' = 3/4" However, more realistically and up-to-date, keep in mind that lateral differential shaking may be in the range of 10% to 50% of the liquefied deposit thickness even so. Consideration should be given to foundation punching, as occurred in the Turkey earthquakes about a decade ago, where it was common in silts, however there are not really any design methods for this.

Probably enough information there to fashion a noose for yourself with...

 

[fabianjm]

Replacing the material or deep foundation should be based on both economically results analysis in case of deep foundation you can do correction the Ni value at the deep level of liquefaction while pile settlement should be take care too

 

[FixedEarth]

pelelo;

This is either deep foundation project, lower the GWT and do a Mat foundation job or improve the upper 30 ft with ground improvement method.

In my opinion, this is not overexcavation project due to the high GWT. Besides, it is the upper 30 ft that is a concern and not the upper 5 ft.

It is best to get the right analysis tools. LiquefactioSPT software by Soilstructure and "Soil liquefaction during earthquakes" book by Drs. Idriss and Boulanger, 2008 available at Amazon. The 1996 methods are just outdated.

Also assuming GWT could easily rise to the ground surface, it will be hard to get an adequate bearing capacity value and control tilt and differential ststic settlement simultaneously.

There are also sructural solutions that will help but it will not solve the liquefaction issue.

Geostrucparks;

You are correct, Static settlement is not governing, It is seismic or dynamic settlement which will be the governing criteria. Static may be near the 2" magnitude but dynamic would certainly approach a 4" magnitude.

 

[pelelo]

Fixedearth thanks for your imput.

I will try to get those references.

 

[FixedEarth]

pelelo-you are welcome.

I have a 3 story exactly same soils profile as your job. I plan to give them two options: an interconnected footing with plenty of rebar or a 12 inch mat.

My seismic settlement is nearly 5 inches, but you and I know that is theoretical number. In CA, the worst "recent" liquefaction effect was the 1933 earthquake.

The only reported liquefaction problem with that was behind some bulkheads on dredged soils.

 

[moe333]

Fixed Earth,

You should have a look at the accounts of liquefaction damage to the buildings in the Marina District and the Cypress Freeway in San Francisco area from the 1989 Loma Prieta EQ.

 

[pelelo]

Fixedearth,

Thanks again for your input.

By recommending footings or a 12 in mat, wouldn't the differential settlements be a concern?. I understand it is just a 3 story building and using deep foundations would be very expensive. On the flip side, by using deep foundations differential settlement will not be a concern at all.

I got another question,

What approach do you use compute the peak ground acceleration (agmax) for your liquefaction analysis?, the one I use is by Donovan (1971 or 1973), in which agmax is function of the EQ magnitude and the epicenter. I can give you the formula but I don;t have it with me at the moment.
I asked you this because I am not sure if this is the best approach or If it is widely accepted. A co-worker recommended to use it, but I just wanted to confirm.

 

[FixedEarth]

moe333;

I will try to locate your referenced papers. It should be noted that my 3 story is a wood framed residence that is lightly loaded. Also my soils are SP, SM and SW. There is no bay mud, artificiall fill or any dredged materials. It is alluvial soils that are loose to medium dense. The existing house has been there for 50 years and seen many earthquakes withount any distress.

Pelelo;

This is only my opinion based on experience and which I have used on at least half a dozen sites. A mat foundation or interconnected footings makes the foundation semi-rigid. So it will be able to toelrate some differential settlement. But, differential settlement can only take place if the loads are of different intensity or the soils are of varying strengths and thickness. We have neither on this site.

For PGA, I take the higher of the Probabilistic value from an oline site or SDS/2.5 value. Both values are based on the site coordinates and the site geology.

Something should be said about local geology. Some sites have 50 ft of Sand for example but in a 6 block zone, there are deposits of Peat. If all you did was a 50 ft boring and liquefaction analysis, you would be at risk of being on 30 ft of peat.

Lastly, if this project was 15 miles away in a certain locality, I would put it on concrete piles, even if it was a one story residence. There are a lot of factors to weigh-It goes beyond liquefaction zone site. One is the PGA, a 0.25g is very different from a 0.85g.

 

[pelelo]

FixedEarth,

Thanks again for your reply.

I noticed in order to compute SDS, you need to find Fa (Site Coefficient) and Ss (Mapped MCE spectral aceleration reponse at short periods).

I noticed you can get Ss from IBC (1613.5(1) through 1613.5(14)). Those Figures are only for US lands. In my case my firm is currently working on a non-US land project, Do you know where can i find this information?.

Also, I noticed those figures are for Site Class B only, in my case my Site Class is "F" as my soils are potentially liquefiable, do you know why is it only for Class "B"?.

Your PGA value from an online site is from

http://earthquake.usgs.gov/earthquakes/shakemap/list.php?y=2010?

Thanks again

 

[FixedEarth]

I don't know the methods used overseas. Sorry.

Maybe if you mention the country and the region, one of our colleagues may respond.

 

[dgillette]

I don't think you really want to rely on differential settlement being minor because of the inferred lack of "...soils...of varying strengths and thickness," especially with the water table and liquefiable material so close to the surface.

 

[FixedEarth]

I forgot to answer your Site Class "B" question. There is free software, if you search for "NSHMP Hazard Java" or similar, you will get the program. However, It is mostly for US and surrounding territories.

You are correct, the default is Site B, however, on the above software, you can click on site modified and pick D or E or similar. The Ss and S1 wil be the same, but your Fa, Fv, Sms etc will change.

I was thinking about this as I drove to a distant job site this afternoon. Can you check with your engineering geologist or seismoligist and find out the characteristics of your design fault? Then find very similar fault in the US and get coordinates close to that fault and see the seismic paramters you get. This may get you in the vicinity and you could fine tune it from there.

 

[pelelo]

Fixedearth,

Sorry for the late reponse.

My project is located in the northern part of the Dominican Republic. It is about 15 miles away from the Septentrional Fault zone (SFZ) which runs from Puerto Rico all the way to Haiti.

Thanks again for your input about Site Class B question. I will get the software and see If can get a similar fault zone located in a US territory.

Thanks again.

 

[dgillette]

The Septentrional Fault is a strike-slip fault with a very high slip rate (I don't recall specifics), and could give you a darned big shake with recurrence interval of a few hundred years. (Hispaniola as a whole gets a big one every 50-70 years.)

When I was there for a conference in 2000 or 2001, we were taken to a site near Santiago where Dr. Carol Prentice of the USGS was trenching a fault that I think was la Falla Septentrional. The nearby creek, which couldn't have been very old in geologic time, was offset several meters by the movements. We were also shown a stone wall that was said to be the remains of the first church in the New World, destroyed by earthquake very shortly after construction, AD ~1500. (No theological discussions, please.)

Consider searching the USGS website for publications by Carol Prentice. She might be able to point you in the right direction if she hasn't published the information you need herself.

http://www.agu.org/pubs/crossref/2003/2001JB000442.shtml

http://specialpapers.gsapubs.org/content/326/63.abstract

DRG

 

[Mccoy]

I computed the Liquefaction Factor of Safety every 5 feet for each boring. In some borings I got several layers with the FS less than 1.In other borings, i got the FS < 1 in only 1 layer. (15-20 feet depth typically).

That's a scenario of potential significant differential settlements and tilting, I agree with dgillette

Fines content in the silty sands might mitigate the risk

With a 5 storey building I know I couldn't sleep with anything less than foundation piles (maybe stone columns or similar ground improvement).

One obvious although sometimes overlooked aspect: no lateral shaft friction can be allowed in the liquefiable layers and pile will work at least partially as a pillar if liquefaction is triggered.

I'm currently working on a project at risk of liquefaction where the foundation engineer proposed tapered, driven piles, I would find the idea pretty good for a site like yours and you'll have an estimate on the resistance of the bearing layer for every single pile.

 

[pelelo]

dgillette,

Thanks for your input. No question the link you provided deals in details about the Septentrional Fault Zone. I will continue searching for more papers by Carol Prentice.

Mccoy,

I agree with you regarding the deep foundation solutions, post-liquefaction settlements and tilting might be a problem if they are not addressed.

 

[BigH]

http://cee.engr.ucdavis.edu/faculty...Brandenberg_et_al_Load_transfer_ISEE_2005.pdf

http://cgm.engr.ucdavis.edu/publications/ucdcgm0301.pdf