×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Modulus K-factor in COM624p for rock

Modulus K-factor in COM624p for rock

Modulus K-factor in COM624p for rock

(OP)
We are estimating lateral deflections for a socket in yielding rock and have developed p-y curves to take into account the potential for lateral shifting at the top of rock.  I am uncertain as to what to enter for the soil modulus parameter k (pci) because I do not know how the program uses the factor.  According to the program documentation at Chapter 4, k is the slope of the modulus E versus depth.  This may be useful for soil, but the modulus is relatively insensitive to depth for rock.  Examples included with the program show higher k factors with increased soil material stiffness, e.g., 90 for medium dense sand but 2000 for weak limestone.  This would suggest a greater increase in mudulus with depth, not a lesser increase.  I have access to relevant subgrade modulus values tabulated from plate loading tests, etc. for rock, but am hesitant to specify a k-factor that may mean something in the program that does not mimic reality.  Does anyone know how to get COM624 to resolve this?

RE: Modulus K-factor in COM624p for rock

Does anyone know how to get COM624 to resolve this?

Yup.  Well, sort of.

COM624 uses the k-value to limit the initial slope of the p-y curve.  If the initial slope of the p-y curve is too steep, you can get divide by zero errors.  Theoretically.  Practically speaking, it isn't an issue.

You can handle the problem a number of ways:
  1. Divide the rock into lots of layers, and vary the k-values so the k*x = Erock (x is depth below ground surface.)
  2. Input your own p-y curves - this bypasses the k-value issue completely.
  3. Pick a reasonable k-value for the top of the rock, and then ignore the k-value unless you have a problem.

I hope this helps -



Please see FAQ731-376  by VPL for tips on how to make the best use of Eng-Tips Fora.

RE: Modulus K-factor in COM624p for rock

(OP)
I developed a p-y curve but there is nothing in the program docs that says you don't have to have a K value to start with.  I would like to dispense with K altogether -- you mean the program won't blow up if there is no K value entered?  (Example 5 with the documentation shows user input p-y values and also has a k value entered, which is why I'm confused).

We need a better rock socket program than Rocket -- which the developers acknowledge is proven only for soft rock sockets in Melbourne, I think.  That's why I am suffering with COM624p.

RE: Modulus K-factor in COM624p for rock

If the program insists on a k-value with hand crafted p-y curves, then give it a very large value so that it doesn't interfere with your manually entered ones.

I like COM624 - did my master's research on laterally loaded pile groups.  But it isn't very 'user friendly', and can be a real pain sometimes.  It does, however, give good results and has a well established track record.

How close is the rock to the ground surface?



Please see FAQ731-376  by VPL for tips on how to make the best use of Eng-Tips Fora.

RE: Modulus K-factor in COM624p for rock

(OP)
The rock is buried below about 15 ft of sand/gravel and cobble terrace deposits.  

The thing is that the bedrock upper surface was exposed to surface erosion that created steep to vertical fissures several inches to over a foot in width, that became filled with fine air-blown sand before deposition of the overlying terrace deposits.  We are trying to account for this because it holds the potential for lateral translation of the rock blocks along basal discontinuities that were detected in the borings and observed at other locations in the field.  The influence of fissures depends on the dimensions of the blocks with relation to the micropiles.  The fissures are irregular (they are not karst, by the way) but narrow to weathered joints with depth. At places it can be over 25 ft below the rock surface before the fissures pinch out entirely, but this is uncommon.  We are modeling the blocks as 5 ft high and 4 ft across, separated by fine sand filled fissures 3 in thick.  The fine sand is modeled as medium dense but stiffening as the block closes the fissure horizontally.  The P-Y curve reflects displacements under the sum of the elastic block deformation and the compression of the sand fissure resulting from the residual lateral force left over after deducting for basal shear.  

This is where the k-factor comes in.  Initially, the k-factor would be the subgrade modulus of the intact rock because the small strain deformations are entirely due to the elastic deformation of the block, with no inelastic shifting.  The P-Y curve accounts for the inelastic portion.  It's a simple enough matter to calculate the initial subgrade modulus and it is indeed high, like 10e03 pci.  

Think that will work?

RE: Modulus K-factor in COM624p for rock

It sounds like you have a good handle on the problem - it should work just fine.  If it doesn't, post another message with the details of your problem - and I'll try to help.

There is a back-up approach - use a program called BEAMCL28 that Mike O'Neill produced.  It is much "cruder" than COM624 - you have to give it all the information manually, no automatic parameter generation.  But since it is a more general routine, it does a better job of handling the odd problems.  I like to use it for Q-W analyses of marine bulkheads.

Look for it on the internet, it may be in the public domain.  Also check with FHWA - if you are interested or need it.



Please see FAQ731-376  by VPL for tips on how to make the best use of Eng-Tips Fora.

RE: Modulus K-factor in COM624p for rock

sgibob:
How did your analyses come out?



Please see FAQ731-376  by VPL for tips on how to make the best use of Eng-Tips Fora.

RE: Modulus K-factor in COM624p for rock

(OP)
I made several runs.  The deflections all turned out to occur in the sand anyway.  It made little difference whether the rock was yielding or not.

RE: Modulus K-factor in COM624p for rock

Good!  It sounds like a rotational failure is very unlikely -

Did you run the analyses using double the loads?  What were the results?



Please see FAQ731-376  by VPL for tips on how to make the best use of Eng-Tips Fora.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources