Standard Penetration Values less than 10 blows/ft 4

[BearcatEngineer]

Our geotech expert has taken another position and hard to get a hold of. He recommended that a geotech expert be in contact if the soil report shows standard penetration values less than 10 blows/ft. What should we be doing on these areas of the site to work around or correct this problem. There will be a wall and column foundations as well as traffic and parking potential.

 

[BigH]

Well, I would contact a local geotechnical company - perhaps a freelancer or one-man band and get him on board. What your former geotech told you is that if you have N values less than 10 - this is worrisome to a degree. Of course, how worrisome depends on the project, details thereof, etc. Sounds like you need help. Go get it. If it costs a few C-notes, then spend it. Don't put your insurance to the risk.

 

[BearcatEngineer]

I know what you are saying BigH, but some of the recommendations provided to us by the geotech. consultants are expensive. I was told by out expert not to take their recommendations as law, there will always be cheaper ways to remedy the situation. My problem is that I don't have the experience or the references to make those decisions. Do you recommend any reference books or seminars to attend to try and gain knowledge to be able to question the consultants with some bit of back up?

 

[BigH]

It is a risky game you play. Expensive? From my experience geotechnical projects today (investigation/report) are considerably less in number value than they were 15 to 30 years ago - due to the competition (which, as a colleague of mine has said - is "prostituting" the business - but that's another story).

I firmly believe that you can get a good one-man band to work with that will be reasonable. But, rightly said, you should have some background. I would suggest, as a first step to obtain the following texts - Terzaghi and Peck (1967 version), Peck Hansen and Thornburn (geared a bit to structurals), Tang, Tschebotariof, and Tomlinson Foundation Design book (pre-Eurocode version). These are all good practical books that can give you a good basic understanding. Where are you located?

 

[BearcatEngineer]

Thanks for the references, guess it is time to start readying.

I am Located in Ohio.

 

[SlideRuleEra]

You may also want to take a look at the free geothechnical download resources available

http://www.vulcanhammer.net/download/

http://www.SlideRuleEra.net

 

[Panars]

You should consider the blow counts less than 10 as a red flag. How you remedy the problem will depend on other factors, such as is the soil cohesive or cohesionless, where is the groundwater table, is the soil organic, etc.

Where in Ohio are you located? I might be able to recommend some geotech companies depending on where you are.

Since you mention traffic and parking, the Ohio DOT will improve the soil for roadway subgrades if the blow counts are less than 10. The type of improvement will usually consist of excavation and replacement with compacted granular material, or cement or lime stabilization. Which method used will depend on how low the blow counts are and what the moisture contents are.

Regarding the wall and column foundations, I believe geopiers have been used successfully for a lot of commercial-type buildings. I have not used them myself however. You might try contacting the company at

http://www.geopier.com

and talk with one of their engineers.

 

[BigH]

As indicated in my first thread, and reinforced by Panars, the 10 "N" value can be considered his "red flag" or my "worrisome, maybe" - it is a cutoff. Actually, N = 10 for low plastic clayey silts is not that bad - something of an undrained shear strength of 60 kPa (ball park allowable bearing pressure of 120 kPa). But, again, each specific project must be seen in the light of the actual soil properties and of the specific requirements of the project. I grew up just east of Cleveland in Lake Cty - back in 60s/70s. Great place.

 

[jakin]

BearcatEngineer, I would very strongly suggest that you pay a geotech company to handle all your geotech concerns. N < 10 is actually meaningless without contextualizing the problem. For example, for clay with N < 4, I would not feel confortable depending on SPT results. Rather, I would prefer using in-situ vane shear test results. Rather than seek advice on what N-values to adopt, I suggest you let a geotech firm understand what you want to achieve and work with you on a solution.

I am really concerned at the trend of folks in structural engineering practice not fully appreciating the level of risk they take by trying to solve geotech problems without expert input. Although fees for geotech advice is relatively low, inadequate geotechnical interpretation often results in very expensive, and sometimes embarrassing, consequences. If reading those foundation references BigH suggested was meant to be a replacement to seeking professional geotech advice, I would leave leaving reading them to the experts.

As Panars has suggested, there are several good geotech firms in Ohio.

 

[BigH]

jakin - agreed - but structural engineers might want to have a background - I no where indicated that this should replace the geotechnical input - see earlier replies. Similarly, I am working on highway projects as material/geotechnical engineer. Yet, I read books/reports on the layout of the alignment, why they do things the way they do, etc. - so I am more aware of their problems too.

 

[RCEJD]

I agree with jakin. There are several factors that require the specialized knowledge of a geotech just as there are in structural. Many of these issues are not found in the books referred to above. These are excellent foundations (sorry for the pun), but in many areas of the country, the sites are more complicated and require more in depth knowledge ....

I think for the basic sites, it is great to see the structurals learning more, it helps everyone including the client, but the more complicated projects require the specialization of all parties. I would be concerned with any geotech making a blanket statement that uses N<10 as a guideline .... is it a "N", an N1(60) value, as pointed out above, in clays standard penetration is essentially meaningly except in specific applications, etc.

I am not sure why the "one-man" operation recommnedations ... find an experienced geotech that should be involved anyway ! The cost is cheaper than the attorneys fees or the contractors change orders.

Good luck.

 

[BigH]

RCEJD - perhaps you've misunderstood the "one man band". There are many geotechnical consultants out there with plenty of experience who would rather work on their own than with a "company". The concept does not mean that they are not knowledgeable. I suggested a firm first, but there are knowledgeable geotechs who work on project by project basis. Don't sell them short. Secondly, there is no question that "books" are only books. But if you look at the books mentioned, you will find there is much more "guts" to them than what is standard fare today. Tschebotarioff explains by project examples why plate load tests in clays are basically worthless, how he developed many of the braced excavation theories, etc. Similarly, Tomlinson's book(s) are excellent with real problems. They have soul. Does Bowles or Das have a "real" problem in them?? Not that I ever saw. Anyway - in the end, no structural type should do without a geotechnical on board even, as in very good soils, it is only to . . .

 

[RCEJD]

BigH: Yeah I did misunderstand your meaning. Thanks for the clarifications.

I do agree that the books you discussed are excellent, and refer to them regularly. However my point is the application of the concepts is not always direct and there are nuances ...

I refer to Das and Bowles frequently and have found aspects to be excellent. I had never thought about it from the practical problems perspective.

Thanks BigH for the feedback.

 

[fndn]

BearcatEngineer:

This is what happens when non geotechnical engineer makes soil decision. I did a bottom of foundation probe on a residential lot, and recommended undercut depth, removal/replacement and footing dimensions. The client agreed and everything worked out. I also advised him, that the adjacent lot(15 feet away) will be very similar, but more likely a little worse. The client decided it was too expensive to pay my $300 fee and built a house without a soils report. Then he called me to see if we can ok-"they forgot to call you last month ///"! I insisted on an $1100 investigation and once I reviewed the data-They had 6 foot of compressible soils. The client paid for the teardown, investigation, hauling of concrete debris, their client falling behind 2 months and yes he still paid me the $300.

Another note, soil engineering is a combination of science and art and takes many years to develop. The books will give you the fundamental knowledge, but you need years of lab, field, and senior engineer supervision and sometimes input of an engineering geologist before you can design.

A Member of

http://www.civilvillage.com

 

[DRC1]

Actually the most imortant thing to have to be a geotechnical engineer is common sense. Geotechs do not design to strict codes like structurals. (not that common sense is'nt needed there) A big problem I have with a lot of geotechs is that they make cookie cutter recomendations with out a lot of regard to he current project. There are several geotech firms I can recognize simply from the recomendation page - Don't even have to look at the cover. So can see why Bearcat engineer gets frustrated. Around New England, soils with N=10 is for soil liquifaction - probably not a worry in Ohio. As a basic text I like Bowles - Foundation Design 3rd Ed. The Canadian Geotechnical Manual is a good place to find good information. I think it is od you want to understand more about this part of enginneering.
To everyone else, before I spend next week getting zapped, let me say that the people who take the time to participate in this forum seem to be the sharp questioning people the profession needs, However, as someone who sees a lot of reports, the trend to get the report out and not spend a lot of time addressing what makes the site unique and how to deal with it is hurting the profession and I felt I should say something.

 

[CIVGEOT]

DRC1: After 20 years plus in the business of geotech in California, I agree with you.