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P-Y Curves

P-Y Curves

P-Y Curves

Does anyone know where i can obtain a spreadsheet that computes p-y for various layers such as clay, sand ?

I realize that programs such as COM624 and LPile compute these but sometimes you have to input based on modfied soi layers.


RE: P-Y Curves

Be careful of freeware analysis/design software from the internet.  The shear values with depth are wrong.

RE: P-Y Curves


Be careful of freeware analysis/design software from the internet.

Excellent advice, which should be extended to all analysis/design software from anywhere.  As the statistician George Box said, "essentially, all models are wrong, but some are useful".

But assuming FixedEarth is referring to my LatPilePY spreadsheet, is it true that the shear values are wrong with depth?  That depends on what sort of "wrong" we are talking about.  As far as I know, for given loading, pile section properties, and spring stiffness values, the output shears, bending moments and deflections are in static equilibrium, and in accordance with the standard theory of bending, although users should of course check that.

Where the spreadsheet could be said to be "wrong" is that for a specified set of soil properties it will generate a different set of py curves to that generated by COM624.  The main reason is that COM624 uses an initial stiffness value for sands and stiff clays that is proportional to depth and independent of pile diameter:
P = kxy
P = soil force/m
k = initial stiffness constant
x = depth below ground level
y = pile deflection.

  In the current implementation of the spreadsheet I have multiplied the k value by the pile diameter, rather than the depth below ground level, so the stiffness is proportional to pile diameter, and independent of depth.

Both approaches are of course gross approximations, but either can produce useful results, if the results are reveiwed by an experienced engineer and checked against alternative analyses and testing.

The COM624 documentation gives very little justification for the approach used, and where the assumptions used have been examined by others they recommend caution in use of the results.  For instance:


402 Caution must be exercised when the recommended nonlinear p-y curves are used
in other contexts than for evaluation of lateral pile capacity in the ULS. Such contexts
include, but are not limited to, SLS analysis of the pile, fatigue analysis of the pile, determination
of equivalent spring stiffnesses to represent the stiffness of the pile-soil
system as boundary condition in analyses of the structure that the pile-soil system
supports, and in general all cases where the initial slope of the p-y curves may have an
Design of offshore wind turbine structures
October 2010

Nontheless, the PY curves generated by the spreadsheet were intended to follow the method used in COM624, which at present they don't do.  A revised version of the spreadsheet will be posted in the next day or two that will provide the two options outlined above for calculation of the initial stiffness, with the COM624 method being the default.

Doug Jenkins
Interactive Design Services

RE: P-Y Curves

IDS- I have tried earlier version about a year ago with several soil properties. Towards the bottom of the pile depth, I was not able to match the shear values. My main point is about downloading programs  from the internet and using it for design without any verification.

RE: P-Y Curves


My main point is about downloading programs  from the internet and using it for design without any verification.

We are in total agreement on that!

Doug Jenkins
Interactive Design Services

RE: P-Y Curves


I totally agree, noone should ever use spreadsheets without a thorough review.  We as professionals should take every precaution when dealing with information provided by others or the web.  Good comments.


Ryan R. Smith
Geotechnical Engineer

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