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Pile Lateral loading

Pile Lateral loading

Pile Lateral loading

(OP)
I would appreciate feedback and thanks very much in advance.

The pile is embedded in 2 m peat, 6 m very soft to soft silt and 12 m in medium dense sand. Ground water is almost at ground elevation. Do soft silt and peat exhibit any lateral resistance? What are the LPile parameters and the subgrade horizontal.modulus for these soils.

RE: Pile Lateral loading

"Engineering Design in Geotechnics" by F. Azizi and "Foundation Analysis" by R. F. Scott cover lateral subgrade modulus & analysis laterally loaded piles. It is vast topic, like slope stability, and is difficult to capture it in few sentences.

http://www.soilstructure.com/

RE: Pile Lateral loading

Oneengineer, I would heed Fixed earth's suggestions.
One simple aspect though is that if you have the elastic modulus you can calculate the subgrade horizontal moduli by correlations. Vesic formula, adapted to piles, is a pretty popular choice. Elastic modulus must be operational, that is degraded in dynamic conditions (and static condition if horizontal loads are static). This is for purely elastic, linear models.

www.mccoy.it

RE: Pile Lateral loading

(OP)
Mccoy, fortunately I have the E modulus for soil. Would you be kind to explain which relation between Kh and E are you referring to for sand and for clay. My search did not yield a definite answer. Thanks for all the comments.

RE: Pile Lateral loading

Oneengineer,

I am just doing some lateral analysis for piles and have information in hand.

Here in Japan, we use this equation to relate Kh and E. We normally use the secant modulus (E50):

kh = 80・E・(B・100)^(-3/4)

kh : coefficient of horizontal subgrade reaction (kN/m3)
E : modulus of deformation of the ground. The value used should be one of the following;
however, that of cohesive soil should not be estimated from the N value, but determined using either 1. or 2.:
1.modulus of deformation of the ground measured in a boring hole (pressuremeter test) (kN/m2)
2.modulus of deformation determined using the uniaxial or triaxial compression test (kN/m2)
3.modulus of deformation estimated at E0 = 7N based on the mean N value (kN/m2)
B : diameter of a pile (m)

If deflections are larger than 10mm we need to adjust (decrease) kh.

Would like to see other correlations that are used in other areas...

RE: Pile Lateral loading

Oneengineer,
The Vesic relation is used for either sand and clay. It is the same relation as that used for vertical K in shallow foundations, but a factor 2 is applied (this is explained in the Bowles textbook). Pros are that its use is usually well accepted, is the same for all soils, includes elastic modulus of concrete and moment of inertia of the foundation.



To obtain Kh, you must multiply the above formula by 2. Es is secant elastic modulus, B is pile diameter, Ef is elastic modulus of foundation, If is moment of inertia=0.1B4 for circular pile sections. mus is Poisson secant modulus

There are many simpler formulas, one example is the formula illustrated by Okyriu, which I didn't know, I wonder about its source.
Other published simple relations: kh= [0.8; 1.3] Es/B (Bowles 1982) or kh= 1.6 Es/B (Chen, 1978)

Formulas utilizing the nh factor have parameters which have been determined empirically by soiltype classes. I do not use them usually.

www.mccoy.it

RE: Pile Lateral loading

(OP)
Great answers from McCoy and Okiryu.
If I am right the multiple 2 is E/Es or is it a factor of safety if a FOS required at all. I will check how to get mus from mu. My other question is how far from the top of pile on the pile depth would this Kh be effective.

The square symbols in Okiryu should all be multiple signs if I am right.

Thanks again.

RE: Pile Lateral loading

Onengineer, actually the factor 2 is an empirical factor derived by Bowles according to whom the lateral subgrade reaction is twice the vertical subgrade reaction.

Es is a secant operational modulus, it may include degradation (dynamic) or reduction (static) schemes but it has nothing to do with the factor of 2 indicated by bowles, although very often the static reduction is taken as half the small strain E value, so I understand your reasonings.

Poisson secant: I usually use 0.15 to 0.2 for sands and unsaturated clays, 0.4 to 0.45 for saturated clayey soil.


How far from the top. Of course, the lesser the representative Kh, the farthest from the top its effect is displayed, moment of inertia of pile section remaining the same.

Now, you are going to take a single representative Kh value if your model requires it, otherwise you are going to take multiple Kh values, each every node or every homogeneous layer.

If the model requires a single value, than it should be averaged along the lenght of pile which is significantly interested by the displacement (it can be an iterative procedure). An harmonic weighted average will let the softer layers govern, which is sensible since the shallower layers are usually the softer and they govern the head displacement.

www.mccoy.it

RE: Pile Lateral loading

Onengineer, the above equation is based on data collected from pressuremeter tests conducted in Japan. The square signs are multiply signs.

Based on Broms theory, for cohesive soils, the soil resistance starts to work from 1.5B. Soil resistance distribution is linear. For granular, the soil resistance is a triangular distribution. Max. value is 3 times the soil passive pressure. See attached for your reference. Be aware that Broms theory does not consider layered soils.

RE: Pile Lateral loading

Okiryu, do you have a literature source for that equation or is that just a in-house correlation?

www.mccoy.it

RE: Pile Lateral loading

Hi McCoy, the equation is from the Architectural Institute of Japan. I found that equation more in line with field pressuremeter testing. I normally use that equation. The "nh" equations you mentioned above appear to be too conservative.

I have recently done some pressuremeter tests so I will check Bowles for the Vesic equation you have provided. The tests were conducted in a reclaimed fill area and due to the inconsistency of the fill material I do not know how to assign the proper modulus though.

RE: Pile Lateral loading

Quote (Okiryu)

The tests were conducted in a reclaimed fill area and due to the inconsistency of the fill material I do not know how to assign the proper modulus though.

Do you have results of other tests like CPTs, NSPT's... ?

www.mccoy.it

RE: Pile Lateral loading

McCoy, I do have old CPTs results for the same area. I can provide those tomorrow once am back in the office.

That is always the main issue: lots of fill (undocumented fill) and do not how to assign the proper soil properties. At the end, where thick fill areas are found; piles or soil treatment plus structural floor slabs are recommended.... however old buildings in the same area were designed on shallow foundations....

Talk again tomorrow... thanks.

RE: Pile Lateral loading

(OP)
The formulation given by Okiryu is really useful due to its simplicity. But I cannot apply to my project, not having insitu measurement of E. I have CPT data and will make it very approximate if I convert it to N and then E =7N.

Mccoy: would the notification given by Okiryu "If deflections are larger than 10mm we need to adjust (decrease) kh" be applied to Kh when we use the Vesic's formula or is there another criteria. I think that is to ensure the linearity of pile behavior(?)

RE: Pile Lateral loading

I think that you can still use E from CPT in that equation. Also the reduction of Kh due to deflections larger than 10mm is based on field test data from lateral loading tests. If you use E=7N for cohesive soils, you need to use a coefficient of 60 instead 80. This equation does not apply if you have liquefaction potential. Try to use that equation and compare the results for previous projects in your area.

McCoy, here is the CPT data we were talking about...

RE: Pile Lateral loading

(OP)
I will definitely try your formula when in my next project. Thank you for letting me know that the 80・E・(B・100)^(-3/4) equation was invalid for liquefiable soil. The soil I am considering is quite liquefiable(fos < 0.5). Not sure if liquefaction has impacts on Vesic's formula. Hope not.

RE: Pile Lateral loading

The reduction in kh if deflections>10 mm based on test data is all but technically sensible, since it reflects a non linearity in the soil behaviour, that is a modulus reduction for larger strains, which is what is well known from abundant literature.
What is specified in that case is a treshold above which a reduction in the modulus must be applied.

Vesic's formula is valid for any soils. Of course, if the layer is liquefiable, in dynamic conditions and with the cautious hypothesis that it will liquefy, then Kh=0, since the soil becomes a fluid providing no lateral resistance to deflection, there is no more any spring opposing deflection.

So the kh is valid before the onset of liquefaction, during liquefaction the pile behaves like a pillar which is not laterally confined in the liquefiable layers. Large shear stresses may occur at the boundary with non-liquefiable soil.

www.mccoy.it

RE: Pile Lateral loading

(OP)
McCoy: May you explain why we are using Es instead of E values, despite that the linearity and elasticity are paramount in the Kh solutions. Nevertheless my CPT package produces E values and wondered if one should convert it to Es values and how. Thank you again for answering my question.

RE: Pile Lateral loading

Okiryu, I saw the CPTU results, I believe an approximate conservative value for E in teh fill can be found indirectly calculating the Su values in the fill and from these using a relation such as Eundrained50= 200-600 Su (from Duncan & Buchignani, 1976).

In the soft (apparently non-plastic) silts some direct relationship might be applied such as E=qt*1.5*Cm where Cm should be about 1 at half the silts layer. You should need the raw values of qt, they cannot be guessed from the picture.

www.mccoy.it

RE: Pile Lateral loading

Quote (onengineer)

McCoy: May you explain why we are using Es instead of E values, despite that the linearity and elasticity are paramount in the Kh solutions. Nevertheless my CPT package produces E values and wondered if one should convert it to Es values and how. Thank you again for answering my question.

Oneneg, usually the CPt packages produce a secant value of E (Es) which might be an E25 or E50, not Always mentioned. If it is a tangent E it should Always bear the subscript =, like E0. E derived from geophysical tests is Always an E0. Most probably you do not need to convert yor E value, since it's already a generic operational value.

Kh should reflect the operative values we believe will exist in the design situation. Designing by an E0 value will result in very high and excessively optimistic values of kh, unless we foresee almost no displacement. Since the actual behaviour of the soil is non-linear and not linear elastic, we are just adopting the most reasonable approximation here, using the secant modulus at the design strain level. The design strain level is usually unknown but can be estimated by so called degradation schemes, like the Fahey & Carter method. This in static conditions. In dynamic conditions the strain level can be estimated by tables illustrated in regulations or more accurately by seismic soil response analyses.

www.mccoy.it

RE: Pile Lateral loading

(OP)
McCoy: Thanks for the explanation. I would further appreciate if you could enlighten me on the followings.
I went through Bowels and got a bit lost. He introduces K's and then Ks = K's/B, which is the Vesic formula that you recommended for both sand and clay. Vesic formula did not depend on depth (except what is reflected in Es indirectly).

That was understandable until I saw in the following pages of Bowels (5th edition, 16-32, 16-33...) that Ks = (Ks v) Z**n/B. I guess Ks v means Vesic's Ks. Bowel does not say if it is for sand or clay. I am deciphering what formulation to propose for Kh and whether it is dependent on depth and soil type for design purposes.

Look forward to hearing from you.

RE: Pile Lateral loading

McCoy, thanks for your comments. We do not do too much CPT so I am not so familiar with CPTs correlations with soil parameters. I heard that Robertson and Campanella provide good references for CPT. Also Professor Mayne. Need to check more on this. Thanks again McCoy !!

RE: Pile Lateral loading

Quote (Onengineer)

That was understandable until I saw in the following pages of Bowels (5th edition, 16-32, 16-33...) that Ks = (Ks v) Z**n/B. I guess Ks v means Vesic's Ks. Bowel does not say if it is for sand or clay. I am deciphering what formulation to propose for Kh and whether it is dependent on depth and soil type for design purposes.

I have the 4th edition of Bowles which is a little different but he illustrates equations with ks as a function of depth.

Fact is that: You already have the variation with depth of Es, calculated form the CPT data and from such a curve Bowles suggests that you can calculate the non linear trend (there is an n exponent on the Z, depth value). Is such a trend really useful in your case? It may be useful when dealing with wide, homogeneous areas with relatively few tests.

The laws of variation of Kh with depth (the nh method) have been developed as a generic law extracted from databases of piles load tests. The p-y curves of Matlock-Reese and Others are such an example. The use of databases has pros and cons. I have used p-y curves to check the values I calculated from CPTs or other lab and site tests.

www.mccoy.it

RE: Pile Lateral loading

(OP)
I have used p-y curves to check the values I calculated from CPTs or other lab and site tests
Did you happen to compare the lateral displacements and pressures obtained from Kh values with or without depth dependency? It would be helpful to know.

As you mentioned, the pile I am considering is at the CPT location and the trend is already reflected in Es values themselves.

RE: Pile Lateral loading

Onengineer, I just compared my elastic-plastic model along the pile vertical to the non linear model of the P-y curves. The p-y curves have depth dependency since they are a function of overburden stress. The initial tract of the p-y curves is the tangent kh. So I just made sure my secant kh would not be too much optimistic compared to the non linear trend of the p-y curves. Of course the comparison must be carried out at various depths.

It is one pretty quick way to check that your secant kh is not too far away from the published non linear literature values.

Of course, you might ask, why not to use the p-y values directly? Here in Italy structural engineers have their own geotechnical add-ins to the structural programs and such add-ins only very rarely allow for a non linear input of kh.

I read again the original post and I take it you must input the values in Lpile, which I do not know well but I'm going to have a fresh look at it, not many chances to use it here since the new building code was released.

By the way, I received a brief e-mail on the L-pile issue and which might be iffy (leading to a scam), that's why I did not reply. If anyone reading this post has written it, please write again citing this thread with your username, if you're not a registered user or if you cannot give me the details of your activity, sorry I won't reply.

www.mccoy.it

RE: Pile Lateral loading

(OP)
McCoy, you raised an interesting issue. Just wondering if the Vesic's formula above is taking care of the tangent/secant Kh values automatically, if the Es values are derived from CPT data.

RE: Pile Lateral loading

Oneng, Since the Es from CPT are almost certainly secant, the kh is going to be secant. I say 'almost' because I never happened to see a correlation from commercial softwares which explicitly carried the 'tangent' attribute or the E0 symbol, the subscripot 0 indicating zero strain.

Vesic formula is blind to the secant/tangent issue, so if E is secant kh is going to be secant and the other way around.

www.mccoy.it

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