Liquefaction and Seismic Settlement 1

[StructuralEngGuy]

Hi all,

I'm currently working on a project where our geotechnical engineer has stated that we can expect seismically induced settlements ranging between 3.29" to 3.39" and differential seismic settlements ranging between 1.64" and 2.23". The depth of the liquefiable layer is fairly deep at around 30 feet below the ground surface. I'm wondering what kind of remedies others have seen for this situation. I realize I should be asking my geotech this question, but I haven't been getting very helpful information out of him. I'm interested in seeing if others have had similar situations and what resolutions were used.

Thanks

 

[GeoPaveTraffic]

Still recommend that you talk to the project geotechnical engineer.

That said, a couple of comments. First predicting seismically induced settlemetns to 0.1" is nonsense. The predicted settlement is about 3 inches with differential settlments of about 2 inches.

Second, one option for remedy is to do nothing and execpt the seismically induced settlement. Another option would be to place the structure on a deep foundation bearing below the problem layer. Other options are various types of ground improvement to reduce the potential settlement.

 

[msquared48]

Pile. Lots of pile.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BigH]

Try compaction piling to improve the densities of the liquefiable layer.

 

[BigHarvey]

You can use compaction grouting to improve only the deep layer involved in the liquefaction potential.
You might also have the calculations of your geotechnical engineer checked : It always bothers me to see results as accurate as 3.29" or 3.39" ! We are not dealing with steel or concrete but soil which requires some humility !

 

[fattdad]

try getting better subsurface exploration data (not that I know what you have). If you got SPT N-values and some correlations predicting 3-in settlement for the rare case of the full design earthquake, that's not too bad, eh? However, if you run a dilatometer or cone penetrometer to actually measure the soil properties in situ, you may have better control on the site conditions and refine these estimates. Maybe all the way down to 2.148 inches - ha!

I have an industrial client that allows foundation settlements up to 3 inches. We give him liberal designs and rarely experience the predicted 3 inches.

The cost for these soil improvement methods can be great, so making sure you fully understand the data limitations is important. You may not have a problem afterall.

Even if the new data confirms the 3 in value, you need to let the client know what the cost benefit ratio will be to improve the site just to limit settlements to 1 in.

sorry to ramble. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[rowingengineer]

I always like to know what they did next door to solve the problem, can help sometimes.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[msucog]

hahaha...wow talk about having to keep track of significant digits! flags went up before i made it halfway through the first sentence of your post (as others have identified).

firstly, i would perhaps take a look at the geotech...the results are a bit "odd" due to the reported results. it may be worth asking them what they are thinking or maybe even involving another geotech if these guys are the "lowball" firm. the client may have saved money on the exploration...but you typically get what you pay for with geotechs. i typically suggest talking with the project geotech first before shopping around for a second opinion...they may have an explanation for the way they reported the settlement potential. on the other hand maybe they don't have a clue as to what they are doing.

grouting, deep soil mixing, piles, etc. perhaps check with someone like hayward baker if you need to get an opinion from the folks that do this work and to handle on the associated costs. there are also more sophisticated "things" that you can do to evaluate the site related to seismic potential. the costs savings can be signficant which may help offset the costs of remediation or piles. you can perform a site specific seismic assessment with shear wave velocity assessment. the shear wave velocity assessment can help look at the liquifaction potential before and after remediation as well as help to better evaluate the seismic site class. the site specific can possibly help achieve up to a 20% reduction in the seismic design parameters as per IBC (assuming in US under IBC). also, a more sophisticated analysis can be performed as part of the site specific to look at the site's soil response. and if remediation is performed, in addition to lowering the seismic potential, it may also get you to shallow foundations depending on the loads involved and site conditions. some of these assessments can be expensive but the results could save lots more money.

feel free to email me if you need a little input off the message board. i'm currently between jobs so i have a bit of time on my hands to try and keep my wits sharpened. it is simply my user name at g mail dot com. (speaking of which, i will gladly forward my resume to anyone possibly interested in utilizing my services...simply send an email request)

good luck

 

[StructuralEngGuy]

Thanks all for the replies. I will have another conversation with my geotech before making a decision. Rowingengineer: the funny thing is, we actually designed the building next door (150 feet away) last year using a different geotech, and he reported that liquefaction was not an issue and that seismic settlements would be on the order of 0.85". So naturally, we've had conversations with both of them and have gotten nothing but "i'm right he's wrong" responses. Quite the frustrating experience.

 

[fattdad]

So, it boils down to who has the better insurance - ha.

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

10 different geotechs will give you 30 different responses depending on the site, project dynamics, level of risk that is acceptable to the owner, etc etc...just because an answer is different doesn't make it wrong

 

[rowingengineer]

I would take the two reports and get an expert opinion. I would make contact with my local struct/geo group and see if they have any recommendations on experts.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[FixedEarth]

Soils are heteregenous. You can have 2 borings within 50 ft apart, say each is 50 ft deep and you can get one boring data to liquefy and the other to not. So the 150 ft distance is not applicable.

If the liquefiable layer is 30+ ft deep, and Special Publication 117 only mandates analysis in the upper 50 ft, you can argue that 30 ft of non liquifiable is a safe protection against 20 ft of potentially liquefiable soil. But there is a but! What is the dynamic settlement and differential settlement.

Then there is the question of what type of structure are you proposing and in CA, who is reviewing the report. Some counties will allow 4" total dynamic settlement while a next door municipality will limit you to 2.5" total seismic settlement.

Whoever you hire should be using liquefaction analysis based on the latest 2008 Boulanger and Idriss liquefaction analysis method.

I would Google local geotechnical firms.

 

[DRC1]

One question I have is that 30 feet is really the edge of the effective depth of liquifaction. I am not sure how you can get 3 inches of liquifaction settlement, but not be concerned about long term settlement. Some may say the influence from the foundation does not go that deep or thatsince it is sand all settlement is imeadiate, but a deep layer like that is probably floating in ground water and any stress or other changes that cause drainage may lead to long term settlement. It just seems odd you would get so much settlement from such a deep layer from liquifaction, but not from anything else.

 

[sunny7uc]

Liquefaction is a concern and should not be taken lightly. liquefaction concerns can be as deep as 55-60 ft. I suggest stone columns or other ground improvement methods that fit your site constraints. You need to densify the soils so the potential of liquefaction is reduced.