Tension Pile Capacity in OC Clay 1

[DirtDr]

Description:
A contractor has proposed a value engineering change that will require driving H-piles that will be in tension. The load will be about 100 to 150 kips per pile. It will be driven through a 30-ft layer of slightly overconsolidated, non-plastic silt into an underlying layer of heavily overconsolidated clay. This clay has an OCR of between 10 to 20 (although some of the deeper samples had OCR about 7 based on DMT only), an undrained shear strength (based on back calculation of pile load tests, UU-triaxial, and PMT)of 8-kips/sq ft. Based on CIUC triaxial tests the effective phi-angle is 17 degrees with 700 lbs/sq ft cohesion. If the changed design is approved there will be at least one pull out test to failure.

Question:
The contractor is of the opinion that the undrained anaylsis is the critical case. We used effective stresses, ignored the cohesion since it would be disturbed by the pile driving and estimated pile lengths that were about double what the contractor estimated. It is doubtful if a pile as long as we estimated could even be driven that deep economically. At this point we have no CD triaxal tests.

Which soil parameters should be used? What other issues are involved in driving piles into OC clay?

 

[DRC1]

In general the undrained case is the critical case. However for highly overconsolidated clays, the drained case is often the critical case. This is due to low friction angles. There will be some remolding of the clay, but it should retain some cohesion. The problem is that as drainage occers, the strength of the clay will diminish over time. Unless you can accurately predict this, I think it will be hard to estimate the long term tension capacity of the piles. Ground anchors, helical anchors or belled cassions offer a flared bottom, forcing a wedge failue, which may be mor predictable.

 

[Focht3]

Hmmm,

I will conditionally disagree - for certain (and perhaps most) tension members. If the loads are sustained, then [blue]DRC1[/blue] gave you the right answer. The long term loss of negative pore pressure will result in significant strength losses over time. This will always apply to tie backs of all flavors, since they experience a consistent, long term tensile force. Tension piles/piers for transmission line angle structures also fit in this category.

But if the loads are transient and of short duration, the undrained analysis is the appropriate one to use for design. Most bridge and building piers - if they experience tension forces - will only experience that condition for a relatively brief period of time. The same is true for tangent H-frame or lattice tower transmission line structures.

Look at the loads and their duration - that should guide your design choices. And be sure to consider all of the load combinations -



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[DirtDr]

Thank you both for your quick response.

The loads will support a 35-ft tall retaining wall. On top of the wall is a park, so surcharges will be minimal - perhaps the occasional hiker or pick up truck for maintenanace. Therefore, loads should be consisdered to be sustained.

A belled drilled shaft does sound like an idea worth pursuing.

 

[Focht3]

I would guess that you have experience with underreams. A word of caution: if the OC clay has significant secondary structures (slickensides, silt seams and partings, etc.) and bell collapse happens with vertical shafts, these problems will be magnified when dealing with inclined elements. And, of course, the bigger the bell the bigger the headache -



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[DirtDr]

Yes, you are correct. The bigger the bell the bigger the problem. This clay formation is notorious for fissures and cracks - mostly near the ground surface. The size and frequency tend to decrease with depth.

Upon further reflection, I don't think there is a lot of room for bells. The piles are 5-ft apart and about 5-ft behind the row of tension piles is a row of compression piles. Immediately adjacent to the tension piles opposite the compression piles is the excavation support system consisting of steel sheet piles. It won't extend to the bottom of the piles or drilled shafts, but its existance won't make construction any easier.

A further question - we performed CIUC tests on samples from this clay. I wish now we had performed some CD triaxial tests. Is there anyway to estimate if the shear strength parameters based on CD tests will be higher or lower than the CIUC triaxial tests?

 

[Focht3]

Good question. In my experience, the [φ] angle for the effective stress results from the CIUC tests have been lower than the CD results; but I wouldn't bet on that...

A better way to look at the 'lower bound' strengths would be to see what residual [φ] values have been back-calculated from "old" landslides in the slickensided portion of the clays.

I've seen lab results as low as 12[°] in the Pleistocene-age clays north of Houston from direct shear tests run during the design of the embankments at Lake Livingston (which predated me!) (Samples were carefully trimmed so that a slickensided shear plane coincided with the failure surface mandated by the test equipment.) As I recall, the design was predicated on 17[°], coincidentally -



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[DRC1]

Focht3 was right, I was talking about long term duration. By the way what is a CIUC? is that a CUC test with pore pressure measurements?

 

[Focht3]

Consolidated Isotropically - Undrained Compression test. A 'bar' above the initials indicates 'with pore pressure measurements'.

Oh, and a CK₀UC test is consolidated anisotropically - to K₀ conditions.



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[ashjun]

Dear DirtDr,
Your question is rather confusing to me:
1. You already have undrained test parameters (from your undrained shear strength (from pile load tests, UU-triaxial, PMT and CIU test).
You can do an undrained analysis form the above, correct. But I dont understand that how you did your drained analysis using these parameters?

2. You also say that you used effective stresses analysis (perhaps you mean drained analysis ?) by ignored the cohesion; where did you get the effective stress parameters from? Dont tell me that you took phi' from your undrained test and plugged them in the drained analysis?

Please correct me or is my soil mechanics wrong?

 

[Focht3]

Setting aside the issue of stress paths, the [φ]' obtained from a series of CIUC tests "should" be the same as that derived from a series of CD tests. This comes from Mohr's envelope analyses of lab test results - a pretty classical analysis that is found in many advanced geotechnical engineering texts. This practice is well accepted and has a long and established track record.

Of course, stress paths can - and do - affect the results. But in many cases the combination of selected design parameters and factors of safety obviate the need to 'refine' the [φ]' values beyond those obtained from the CIUC tests. It's a purely practical consideration -



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[ashjun]

Hi,
Further to the above, what is CIUC test? Is it the same as CIU test but cyclic?

 

[Focht3]

The definition is in my first post of April 22, 2004 -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[DirtDr]

Thank you all for your responses.

The CIUC is the consolidated-undrained triaxial compression test. We did measure pore pressures.

There are a few well documemted ASCE and USGS publications that descibe the residual shear strengths of landslides of this clay in question. The values typically are 8 to 19 degrees with about 50-psf cohesion. Our CIUC tests are consistently at the high end of this range as related previously.

Next question:
The test results measured about 700-psf effective stress cohesion. If I don't include any cohesion the pile lengths are 125-ft long. Should some cohesion be included? Can it be justifed to use all the cohesion measured in the laboratory?

 

[ashjun]

Hi,
It is again confusing to me (to put it bluntly, you are wrong); getting the c, phi results (dash to include if you have deducted the PP as you have indicated) and then plugging only the phi prime into your equations is rather wrong when considering the drained analysis.
It seems your contractor is right when using the undrained test results and the undrained analysis. If you need drained analysis, you need to derive the drained parameters separately (which should have given you c prime close to zero in saturated soils).

Both the drained and undrained analysis DOES NOT lead to the same point on the critical state line.

Regards

 

[Focht3]

Hmmm,

I don't think anyone said that drained and undrained tests would end up at the same point on the critical state line. In fact, on April 22, 2004 I said,

Setting aside the issue of stress paths...

Let's not get too carried away here with the stress path issue; [blue]DirtDr[/blue] is dealing with a problem that is damn hard to evaluate by stress paths. This isn't a 'standard' footing settlement problem, after all. In that same April 22, 2004 post I also said,

Of course, stress paths can - and do - affect the results. But in many cases the combination of selected design parameters and factors of safety obviate the need to 'refine' the f' values beyond those obtained from the CIUC tests. It's a purely practical consideration -

And I'm likely to get some heat for this - but the use of a c'-[φ]' soil to model the behavior of an overconsolidated clay is frequently a lot more realistic than a c=0 analysis. Remember that the failure envelope isn't a straight line - it's curved. At low overburden pressures the use of a c=0 analysis can fairly reflect the likely soil behavior. But at higher confining pressures some cohesion is needed in the constitutive model to more appropriately approximate the failure envelope for a given range of overburden pressures. After all, soils are inherently non-linear.

This isn't necessarily an ideal analysis. Yes, CD tests would be nice. But we don't live in an ideal world.

Parenthetically, I've designed an awful lot of structures in overconsolidated clay soils. In almost 22 years of practice, I've only had the luxury of CD tests two or three times. Too slow, too expensive, too unreliable (leaks!), hard to evaluate - it's hard to get clients to pay for them. And I would argue that very few projects in overconsolidated clay soils are designed using site- and project-specific data from CD tests. It's really pretty rare.



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[ashjun]

Dear Focht3,
Point taken....I hate to argue with grey haired guy who has more than twice of my experience in this field..cheer up..
;-)
Would like to meet you if I am around your area anytime. Comment: You amongst others are the lifeline of this forum.
Regards

 

[Focht3]

Let me know when you'll be around! And thanks for the very kind words -

As a final comment, I do wish that more 'sophisticated' testing was undertaken on a regular basis. I think it would elevate the overall quality of the work. At least I hope that it would. Of course, that's not the way things are -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"