Liquefaction question
Liquefaction question
(OP)
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.
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.





RE: Liquefaction question
An expert is a man who has made all the mistakes which can be made in a very narrow field
RE: Liquefaction question
RE: Liquefaction question
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...
RE: Liquefaction question
RE: Liquefaction question
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.
RE: Liquefaction question
I will try to get those references.
RE: Liquefaction question
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.
RE: Liquefaction question
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.
RE: Liquefaction question
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.
RE: Liquefaction question
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.
RE: Liquefaction question
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 htt
Thanks again
RE: Liquefaction question
Maybe if you mention the country and the region, one of our colleagues may respond.
RE: Liquefaction question
RE: Liquefaction question
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.
RE: Liquefaction question
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.
RE: Liquefaction question
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.
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DRG
RE: Liquefaction question
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.
RE: Liquefaction question
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.
RE: Liquefaction question
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RE: Liquefaction question
RE: Liquefaction question
If its light wood frame residential construction of 1 or 2 story and you have some room below the foundation to the liquefaction layers, then PT or mat slabs works quite well.
I have also used a large mat 2 foot thick for a very heavy transformer foundation (300 tons). We made a bathtub out of the thing, because they were going to have an oil catchment basin anyway. We just thickened up the bottom slab. I am happy to say that these foundations passed the test of the 7.3 in Baja recently. I think the OP groundwater levels may make it to difficult to use the box foundation.
RE: Liquefaction question
RE: Liquefaction question
Furthermore, even if the differential settlement associated with liquefaction becomes high enough that is not acceptable, then you can mitigate the liquefaction by using wick-drains. In this case, you can still use mat foundation in conjunction with wick-drains. I do not think you can use conventional footing and wick-drains as the low SPT-N values does not provide high enough bearing capacity (even static condition) in order to use footing for a 5 storey building.
In respect to using piles, I have a question for my colleagues: if the entire upper 30' was liquefiable, I would not be using piles as mentioned above. However, if the liquefiable layers are only one or two confined 5' thick layers (as noted at the beginning), this should not jeopardizes the entire bearing capacity of the pile. You should still be able to use end bearing piles (good soil below 30' depth). Just make sure that everything above the liquefiable layer would be contributing to negative friction. Furthermore, the structural design of the pile should consider within 5' length of the pile, there would not be any lateral confinement and from structural point of view, this portion of the pile should be designed like a short column (pillar). What do you think?
RE: Liquefaction question
DRG
RE: Liquefaction question
Notwithstanding the above, I am still not convinced that if the post-liquefaction settlements are small (and there is no lateral spread), why can not use mat foundation (without anything else such as wick drain).
Does anybody have a comment in regard to my question for piles. I am in doubt about it.
RE: Liquefaction question
Where has this been applied? Has it been tested by an actual earthquake? What research has been done on using wick drains in silty soils? They have been used successfully at a number of sites to enhance densification by stone columns or dynamic compaction, but for "real-time" drainage in the short duration of an earthquake, I'd be very hesitant to rely on them.
RE: Liquefaction question
Actually, Towatha in his 2008 book (Geotechnical earthquake engineering) relates that in Japan some small buidings are just built like that (rigid mat foundation) and if they tilt after liquefaction they'll use jacks to straighten them up.
Sure sounds awkward, but when there is no budget, they'll fall back to remedial works if necessary.
Probably limited to 2-stories buildings max
When I told this to the structural who is building his home in a place with a little potential for liquefaction and asked me to proceed with the geotech investigations, he liked the idea !!!
This is also a place with silty soil, GW to ground level, but deep bearing layer for piles.
RE: Liquefaction question
Mccoy, Thanks for sharing the information. These buildings in Japan might have been subject to noticeable differential settlement and tilting, more than what I generally consider acceptable. I always believed that if a building undergoes a differential settlement of 2" from one end to another end, nobody cares to jack and straighten them up. For an 80' long building, 2" differential settlement from one end to another, gives L/500 that should not be noticeable. I would like to know what order of magnitude of differential settlement/tilting these buildings in Japan have experienced. However, I generally agree with your comment that the whole concept of mat foundation does only work for small buildings. If the footprint area of the building is large, mat foundation performance is questionable.
RE: Liquefaction question
RE: Liquefaction question
The Texas model was made of clean medium(?) sand, and I think the sand was placed around the drains, rather than the drains being drilled in. This was to avoid any chance for smear. Also, they were using many cycles of a low cyclic stress, so the excess PWP built up slowly in the untreated area; it wasn't like a large earthquake with 0.4g occurring within a few tenths of a second,the first few cycles.
In the Davis centrifuge tests, they used a fine clean sand, which by the scaling laws of centrifuge testing, gave them permeability equivalent to clean coarse sand. Boulanger told me the numbers once, but I don't recall them. In the first test, with smallish PHA (o.11g), the treated area did well and the untreated area had some spreading. Then they cranked up the PHA to 0.28, and the untreated area went liquid almost immediately. Its EPWP went very high and stayed high. (Can't tell you the ratio, but probably close to 100%.) The treated area showed some significant deformation and water gushed out of the drains, but the area stayed stable. Its EPWP fluctuated between 10% and 80% of the untreated area's with each load cycle, presumably more like cyclic mobility than flow liquefaction.
I draw the following conclusions:
Provided the sand is very clean and pervious and there is no smear or caking at the drain perimeters, the drains may be able to reduce the amount of damage and deformation at a liquefiable site. In the field, you might be able to achieve a clean perimeter by surging, like developing a water well.
For a dam foundation, I would never use them, for two reasons:
1) the consequences of them not working could be failure of the dam, with release of thousands of acre-feet of water onto the people downstream, and
2) stability of a dam requires good resistance to horizontal loading through the entire thickness of the foundation - a single layer of small thickness that is too impervious to drain "in real time" could be enough to allow instability. The gravity load remains on the dam slopes after the earthquake is over.
I downloaded the NAVFAC document from, of all places, Middle Eastern Technical University in Turkey. It does indeed list wick drains, but provides no guidance, references, or analysis procedures for them. I have to wonder if the authors had seen them used as part of a compaction program, e.g., stone columns, and just didn't make the distinction between a remediation method and something used to allow a remediation method to be constructed. Anyone know who wrote it?
End of rant; back to work.