foundation design for settlement 4

[larce]

I am not very familiar with design of reinforced concrete yet, I really have only taken the one class in college. Now, I am given the task to design a foundation for a house based on the total settlement of up to 5" and differential settlement of approximately 3". I can't find a good example anywhere. I am thinking of a just designing a continuous footing for the loading and just see what the midspan deflection will be. Is this the normal approach? I should mention that this settlement is due to liquefaction of the soils, so it would be a sudden, dynamic settlement that is governing my design.

Thanks for your help.

 

[dbuzz]

You're kidding aren't you?

 

[SacreBleu]

We did something to that order, for a local tract homebuilder who had unwittingly located some lots on really bad soil conditions (where was the Geotech?).
We ended up designing post-tensioned foundation with compressible soil criteria, and the resulting foundation had a good share of internal post-tension beams (ribbed foundation). When the homebuilder priced the construction cost, he went ballistic...I guess they provided a park area in lieu of a few houses.
At any rate, the Post-Tension Institute has literature on this subject. There is software available (Geotechnical Tool Kit).

 

[larce]

Thank you Sacrebleu, but actually, I have contacted the post-tensioning institute with regard to liquefaction and the design of a post tension slab foundation, but I have not gotten a response from them. I did get a response from a couple of the designers of GTK software, and they do not recommend that we use the program for this soil condition. Basically, I was told that the program (compressible mode) is designed for slow gradual settlement resulting from the water being squeezed out of the soil under the vertical loading. They haven't offered me a good solution as to how I should design a pt foundation for liquefaction condition.

dbuzz, I am sorry to sound so ignorant, but to be quite frank, I have not been able to find a good source to fully understand and resolve this question. Like I said, I am really not experienced in design of reinforced concrete. Do you think you can help me? Will you?

 

[JStephen]

I think dbuzz's point was that if you don't know any more about it than that, you shouldn't be doing it on your own in the first place. Ideally, you learn from working with someone that knows it all.

Have you tried looking for a local structural engineer? Might be a lot better avenue that anonymous internet help.

I'd be curious too, about how you were "given the task". Are you working for an engineer or engineering firm? Who will have the professional responsibility for the completed job?

 

[mubash]

I think some data is missing to arrive at design of footing of house you are going to take up.In order to arrive at foundation design ,you need to know type of soil,depth of water table, load on walls and these are then corelated to Terzaghi's formula for Bearing Capacity.Have you got tested the bearing capacity of soil before going into the type of foundation and actual calculation of footing design?

 

[hasanh]

Mubash is right. Before approaching such a task you should request a soil report. Merely designing footings based on limiting the total and differential settlements without knowing the soil properties is not enough. Once you have the neccessary information such as bearing capacity, foundation levels, modulus of subgrade reaction,...,etc. only then you can addequately design footings.

 

[dbuzz]

University of Washington

http://www.ce.washington.edu/~liquefaction/html/main.html

http://www.ce.washington.edu/~liquefaction/html/how/resistantstructures.html

 

[SacreBleu]

Then use grade beams, piles, and elevated first floor (wood-farmed)

 

[UcfSE]

If you aren't that familiar with R.C. engineering you have a lot of work to do, but you should also have a senior engineer watching over you as well. To answer your other questions, you cannot just design the footing as a simply supported beam and check the midspan deflection. The whole thing settles, it does not simply deflect in the middle due to superimposed loading. It is more of a beam on an elastic foundation problem. You should be asking your seniors these questions also.

 

[larce]

Thanks all,

I do have a soils report, soil bearing info, modulus of subgrade, etc. The project is also tract homes, but it wouldn't be a few lots, it would be more like 100 lots. I don't think that our client would have a problem with using a PT foundation, I'm just not sure how I should go about designing it. I would love to use the GTK program, but and a little leary about doing so because I was told I am not supposed to. I know that other pt designers have, and I know that some soils engineers will provide values of center and edge lift that will "come close" to what I need.

I was hoping that I could find an approach to design a pt or conventional foundation "to withstand the specified settlements", as is the verbage on the soils report, since I know there is absolutely no way the client will go with pile foundation or a mat foundation.

(by the way, my boss in on vacation-I can't put this off until he gets back).

Thanks for you help.

 

[AggieYank]

In ethics, one of the things I learned was to not engineer things you have no expertise or experience in. If you do a post-tensioned foundation, and don't have a senior engineer check it at the very least, you're asking for problems. This is your boss's fault since he left you with this project, and you have no experience designing what is required.

Trouble doesn't come from the problem, but from how you try to cover up the problem.

 

[larce]

Thanks Aggie, I don't intend to be unethical about my design, that is why I came to this forum, because I was hoping to find a more reasonable approach than what some structural engineers are doing to address this issue.

I have tried other structural engineers, and my answer gets circumvented, kind of how it is here. I get sources for info regarding liquefaction, I get criticised for not knowing, I get solutions that aren't practical; I get "shortcuts" methods that I can't find in any text or code; I get it all, but don't get a formula, reference book, design example, that would satisfy me enough to say "this is how I could check my foundation for these settlements". Maybe there is no way, and the only way is by beefing up a foundation as much as feasibly possible, running some numbers pulled from lord knows where, an praying to God that the house won't fall down on anybody; because from everybody's responses (here and people that I know), that is all I am getting.

The reason why I am trying so hard is because I am trying to be ethical. Because I don't believe in my boss's (along with other structural engineers') approach to use a tool that wasn't designed to address this issue.

Sorry to vent, I am just frustrated because I feel like i am just getting the runaround because I am a question that apparantly nobody has an answer for and won't admit to it.

I'm not a structural engineer, and I am not even a registered engineer yet, maybe that is why I can admit to not knowing how to do something; even if it does make me feel dumb.

Thanks for all of your help so far, I will try to make use of your comments and sources to find a solution.

 

[SacreBleu]

You won't get much of a response from PTI because they aren't funded to answer questions. However, if you have their Design Manual, you can look for a possible solution.
Why don't you press the Geotech for more practical information ?
Example- we have used "cut-off walls"., i.e., unreinforced, low strength concrete walls that are cast at the foundation perimeter, directly below the PT foundation perimeter. They are meant to prevent water intrusion under the PT foundation. This was not a "guess"...it was the Geotech's recommendation.

Your situation at work (office culture) sounds less than optimal. Perhaps you can have a meeting with your employer to express your concerns.

 

[jike]

I do not know if this is will work with your site but you might want to discuss possibly improving the soil with "Geopiers" with your Geotech. Also you might want to contact a Geopiers rep. and discuss the soils report with him.

http://www.geopiers.com/

 

[jdonville]

larce,

The geotechnical engineer should be recommending a foundation type in the soils report. If you aren't getting what you need from the geotech, call them up and ask them questions.

If the recommendation from the geotech is for mat or deeper foundations, then listen to the geotech! If the client thinks that deep foundations (or even mat foundations) are "too expensive", try thinking about the expense of professional liability for you and your firm. The developer's first interest is cost. Your's should be good engineering.

Ground improvement might be your thing. Hayward-Baker is another specialty contractor that does ground improvement and offers a variety of technologies to do it. They have quite a few case histories on their web site (

http://www.haywardbaker.com).

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[geoffdale]

I concur that grade beams and piles are the way to go. Unfortunately, that may be way too expensive??

 

[BigH]

If I can put in a couple of baht of comments. First, larce - it would be nice to have the general stratigraphy of which you encountered at the site. Soft clay? peat? very loose sands? Stratigraphy properties (N values, undrained shear strengths, consolidation parameters).

You seem to be worried about liquefaction - I would be too if you were dealing with very loose sands or non-cohesive silts AND you are in a seismic zone - if I were in, central western Indiana, I wouldn't worry much, say. If the seismic potential is very low, why would you worry? . . . and would this cause such large settlements (use the standard approach to see if your site has a liquefaction potential).

But, given the "large settlements" you say are for design, it appears to me that you are probably in a zone of soft or organic clays. As such, you would probably put the house on a sand/gravel pad above the stratum. If thick enough, you could use a raft or thickened strip footing/raft foundation. We have done this on our current project where we had 5m of soft clay and put on 5 m of sand fill. The structures were all on raft foundations (housing and single-storey offices). This way the settlements are uniform although there might be some tilting of a few mm. We had good success - brick walls, etc. and very very minor cracking - horizontal; and you can get that on good soil too. Settlements are not all that worrisome - it is the differential that is more concern. (flexible service connectsions not withstanding).

So, please give us some more information of your site - then we can look at the "toys" you can use to found your housing project.

Just as an aside - we have put in RE walls that have undergone 800mm+ of settlement and no noticeable tilting or problems.

 

[BigH]

You might wish to check out the book by Steven L. Kramer - "Geotechnical Earthquake Engineering" - 1996 ISBN 81-297-0193-6.

 

[MotorCity]

larce,

Designing for a differential settlement of 3" is a job for the geotechnical, not a structural engineer. Unless the building is long, 3" is a large differential settlement and, is for all practical purposes,not acceptable for a house. I would discuss with the geotech ways to minimizing the amount of differential settlement instead of trying to design a structure to tolerate it.

Also, I can appreciate your enthusiasm for taking on such a project but it sounds like you have bitten off more than you can chew. Liquefaction? - this is a topic that most engineers never are faced with. Aside from the ethical issues, I don't think that one course in concrete qualifies you to take on this project without close supervision and step by step input from a senior engineer.

Good luck.