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COHESION VALUE ON STANDARD PENETRATION TEST DATA

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neukcm

Structural
May 3, 2015
63
Hi,

I just want to get your advice, is it safe to use the value of COHESION determined in SPT BORELOG Report?

Please see attached file. I'm currently designing pile foundation, 9m deep. I'm using spreadsheet that utilizes cohesion to obtain the pile resistance in shaft and base.The value of cohesion, bulk density, Atterberg limits etc is given only under 12m deep but my pile penetration is only 9m. Can someone share some insights on this. Please take a look at the soil report.

Thanks,
neukcm
 
 http://files.engineering.com/getfile.aspx?folder=58ff2e79-41a3-4fe7-bbde-2da08c109c6b&file=BH-1_Model_(1).pdf
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I normally use a correlation that the un-drained shear strength (in kPa) is 6 times the STP value.

Example: if STP = 10 then Cu = 60kPa.
 
Hi Retrograde,

Thanks for your advice. Do you mean SPT value is the no. of blows needed to penetrate a 300mm thickness of soil at a certain depth? The attached borelog shows cohesion value of 91kpa only at 12m deep, but my pile will penetrate at only 9m deep, can i still use 91kpa from 0-9m below ground level? If not, do you have some idea of how to relate SPT value to pile resistance?

I've been searching the internet extensively about this topic and i just want to know if you already have some idea about this.

Thanks
Neukcm
 
Retrograde,

Please verify also, is the given shear strength in the borelog report (91kpa) can be used to determine pile shaft and bearing resistance?


I mean,
For Pile shaft/skin resistance = (shear strength * surface area of penetrated pile length)
For pile bearing = (shear strength * toe area of pile)

Thanks,
neukcm

 
neukcm said:
The attached borelog shows cohesion value of 91kpa only at 12m deep, but my pile will penetrate at only 9m deep, can i still use 91kpa from 0-9m below ground level?

No.

neukcm said:
For Pile shaft/skin resistance = (shear strength * surface area of penetrated pile length)
For pile bearing = (shear strength * toe area of pile)

No.

At 9m your pile is still in the fill. I would be skeptical about founding a pile in fill.
 
Retrograde,

If i extend the pile penetration depth to until SPT 5 from the borelog report (total depth = 13.5m), i still be needing the SPT value along the length of my pile, do you have any idea how to correlate SPT value (blows/cm) into pile shaft/skin resistance? as well as SPT at the toe of the pile to bearing resistance?

I have found some direct method (i.e. Meyerhof -please see attached file) but still want to be sure. Thank you for your time.

Thanks
Neukcm
 
 http://files.engineering.com/getfile.aspx?folder=7a2e6f16-13aa-47f1-a2ac-a1b9587ab946&file=meyerhof_screenshot.pdf
I think that you should have a face to face talk with the geotechnical engineer of record - or another geotechnical engineer. This forum is not to do the design for you, in my opinion.
 
Thanks for that reminder BigH but i understand how this forum operates. That's why on my first post i've mentioned that i am looking for "advice" and not a person who will do my design. If that so, i should have my first post as " Can someone do my design of pilecap?".
I believe that some engineers here already encountered same kind of situation and they i know they came up with the solution and that exactly the purpose why i have this post. I want the idea, the first step, the method, the advice which will be helpful on my design thus minimizing my time frame.

If they advice me to talk to geotechnical engr face to face as what you said, then i will accept that as an advice, and it will be appreciated much.

Anyway, thank you for your reply and my apology if you misinterpret my post.

Neukcm

 
This is my first time also to design a pilecap and before posting my query i have scanned all the previuos threads first, tuckling the same topic. Honestly i am very reluctant posting this query as i know there will be some who will misinterpret it.

Advice is different from getting someone to design your own project. Lucky Retrograde responded immediately and give me some ideas.

Regards,
Neukcm
 
I second Big H' s comment. My impression is that using "cook-book" numbers times the N value may OR MAY NOT apply to your case. The risk you have is far too great to skip getting local advice from an experienced geotech engineer. Are you considering that there may be down-drag from any fill penetrated, as one item apparently not given consideration? Hey, once you get sued, you take all precautions to avoid it again. Start now.
 
One more. Your title "cohesion" does not follow the usual soil mechanics usage for the word, especially with sandy materials there. That alone is sufficient for me to say get experienced help elsewhere.
 
Hi oldestguy,

Thank you for your advice. Well appreciated.

Neukcm
 
Yes I think the Meyerhof method should be suitable.

If this was my project I would just drive the pile until you achieve effective refusal in the rock. A lot of driven piles are designed/installed on the basis of measuring the penetration per blow at the end of driving. Google "Hiley Formula".
 
Retrograde,

Thanks for that Retrograde. I already adopted Meyerhof principles for my initial assessment but still will definitely look for Hiley Formula as you mentioned. Appreciated.

Neukcm
 
If you are going with this without first checking with an experienced geotech, I'll add a comment now. Assuming that the fill will give any support is a mistake that may bite you later. In the law suit the opposing lawyer will ask you why you assume that fill as worth anything, when the sampling was not continuous. You have only cursory information about that fill. What about nearby places where there was no boring? Fills cannot be depended on for anything unless you personally know it was placed and compacted right. Likewise, the test data on the one clay like sample appears goofy, with angle of friction as zero---- most unusual, considering the consistency as shown by the Atterberg tests.

Anyhow go ahead with what ever method you want, but be prepared for contractors laughing their heads off with your design and that law suit.

Do you have a mentor to guide you with these early designs? It's obvious that is needed.
 
oldestguy said:
the test data on the one clay like sample appears goofy, with angle of friction as zero---- most unusual, considering the consistency as shown by the Atterberg tests.

I thought that phi = 0 would be typical for an undrained test.
 
Oldest Guy,

Thank you for adding your comment which i know definitely is at your high expertise. Points to clear as below:

1) Consulting a Geotechnical Engineer: This is my first priority to be clear and was already done. There's no one who can understand very well the soil characteristic lying underneath other than a Geotechnical engr who conducted the soil investigation. I understand the risk here,thanks for that concern. I have spoken to them already and they told me to come up with an initial pile design first with all calculations attached into it including my considerations in the underlying strata for their review. Initial design means i need to interpret the soil on my own first and eventually they will comment based on their expertise view and revised the pile capacity calculated.

2) Pile Capacity on Fill/Backfill: Absolutely there is no strength here but what i am confused is that, what about the SPT value around this area? Is it totally negligible? My pile now was extended until 12m and using Borelog -14 (see attached) which will rest on hard clay SPT N value = 100 blows/3cm.

Currently i am working with my fellow structural engineers and every step is being monitored.

 
 http://files.engineering.com/getfile.aspx?folder=66e850ae-cf3f-4ea5-bd91-64b038967872&file=bh-14.pdf
Retrograd: OK, OK I jumped before looking.

Neukcum: A different log by far. The earlier log had deep fill, which is not shown as being well compacted, leaving it open as to possible down drag problems. At that site you can use the blow count to compute the ultimate down drag that might occur. You also will have to be concerned about the apparent support that the fill supplies during driving of piles, which may not really be there in the long run, and may actually later be an extra load to be carried.

With regard to a geotech, my experience is that they (myself included) supply all the design info to the structural engineer. Under the circumstances you have, it would appear that they only supply the boring and test data leaving others to use it as they may. That's sort of like my neighborhood auto repair facility telling me that they will furnish the lift, the tools and the parts, but I am on my own to make the automobile repair I came in for. In my case, I probably can live with that, but that is the last time I use those guys.

In your case, it would seem a different geotech should be involved. Where I live (USA) the structural engineers usually do not want to do any interpretation of site data, but leave that up to the hired geotech as his part of the overall design and build process. It's the main part of his report. There are legal and professional risks with foundations and the wise thing I recommend is leave the interpretation and design recommendations to those most qualified. At each of those sites you have, I'd give the geotech the pile load and have them recommend the best pile type and their estimate as to how deep they might go.

I'd go to your mentors and explain the difficulty you are in and the waste of time involved with a phase of the work someone else should do. If you are to do this phase of the work in the future, have them send you off to a series of geotechnical seminars and perhaps a few terms at college learning geotech engineering. You won't learn all the tricks and problems that need solving, but also hopefully a geotech mentor (not necessarily a structural) to guide you.

As an example now ask your "mentors" what their experience tells them when a very deep fill has to have piles driven. Even that seminar or soils course will have difficulty giving a dependable answer. An experienced geotech is more likely to have had such an experience. Have they ever pre-loaded such as site and used footing foundations? I have. That site you have is most unusual and beginners should not be assigned the job of doing the design.
 
Neukcm:

Looks like your site is in Singapore. There ought to be many good gotgechnical engineering firms that can do better for you than the firm used, apparently.
 
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