Eng-Tips is the largest engineering community on the Internet
Intelligent Work Forums for Engineering Professionals
-
Congratulations waross on being selected by the Tek-Tips community for having the most helpful posts in the forums last week. Way to Go!
Modulus of subgrade reaction 4
- Thread starter robdale
- Start date
-
1
- #2
"Foundation Analysis & Design" by Joseph E. Bowles provides a table with typical values for several types of soil.
Please be aware that these might not accurately represent the soil in the area of your design. Tests should be performed to verify the accuracy of your assumptions before construction.
Tim
Please be aware that these might not accurately represent the soil in the area of your design. Tests should be performed to verify the accuracy of your assumptions before construction.
Tim
- Thread starter
- #3
robdale -
Be very careful with calculated values of k (soil modulus.) Every problem is unique - the calculated values may not fit your site and problem. And there is no such thing as "the" k value for a particular soil that would apply to all soil/structure interaction problems. (I have commented on this subject before in another message thread. You can browse for them, or use the search feature.)
I have used calculated values of k for lots of problems; why do you want to calculate k? For pavement design? Predict foundation movements? Decide on foundation reinforcement?
Be very careful with calculated values of k (soil modulus.) Every problem is unique - the calculated values may not fit your site and problem. And there is no such thing as "the" k value for a particular soil that would apply to all soil/structure interaction problems. (I have commented on this subject before in another message thread. You can browse for them, or use the search feature.)
I have used calculated values of k for lots of problems; why do you want to calculate k? For pavement design? Predict foundation movements? Decide on foundation reinforcement?
-
1
- #6
Focht3 is correct. You should not use a calculated value in the design phase of a project. The k value depends on the soil and the movement of the structure relative to the soil. I would suggest and in-situ test, such as a flat dilatometer test (DMT) or pressuremeter test (PMT).
-
2
- #7
What about a sensitivity test in the design calculations. Use a range of credible values for the modulus. If this shows potential problems, then it may be worth carrying out further investigation. However, you may find that you can design for the conditions yielded by the worst credible values.
Robdale:
Check out this site:
This is a group of geotech engineers in Texas who have developed several foundation programs based on the use of the modulus of subgrade reaction.
Any reputable geotech lab, however, should be able to provide you with the modulus for a specific soil boring log. The "k" value, or "p-y" curve, as it is called, is an empirical relationship between the lateral load and the deflection of the soil under that load.
I attended a seminar presented by representatives of Ensoft and they seem to have a good handle on what it's all about.
Check out this site:
This is a group of geotech engineers in Texas who have developed several foundation programs based on the use of the modulus of subgrade reaction.
Any reputable geotech lab, however, should be able to provide you with the modulus for a specific soil boring log. The "k" value, or "p-y" curve, as it is called, is an empirical relationship between the lateral load and the deflection of the soil under that load.
I attended a seminar presented by representatives of Ensoft and they seem to have a good handle on what it's all about.
Polecat:
I'm glad you like Ensoft - I went to grad school with some of those guys. Their offices are located in Austin, Texas not far from The University of Texas (Hook'em!) I think highly of them, too. Lymon Reese, the founder of Ensoft, was one of my professors (and the second reader on my Master's report.) My master's report dealt with Shell's Harvey, LA laterally loaded pile group - used COM624 to do lateral analyses.
Please be careful here; p-y curves are not a "cure-all." They apply to a specific class of soil-structure interaction problems involving laterally loaded piles and piers.
Ensoft sells a variety of other computer programs that use other empirical relationships between standard soil lab test results and soil/structure interaction properties. LPILE1 uses p-y curves, and I understand that GROUP and PYWALL also use p-y curves to model soil behavior. APILE and SHAFT probably use some form of t-z curves; I'm pretty sure that APILE originated as the computer program PX4C3.
I'm glad you like Ensoft - I went to grad school with some of those guys. Their offices are located in Austin, Texas not far from The University of Texas (Hook'em!) I think highly of them, too. Lymon Reese, the founder of Ensoft, was one of my professors (and the second reader on my Master's report.) My master's report dealt with Shell's Harvey, LA laterally loaded pile group - used COM624 to do lateral analyses.
Please be careful here; p-y curves are not a "cure-all." They apply to a specific class of soil-structure interaction problems involving laterally loaded piles and piers.
Ensoft sells a variety of other computer programs that use other empirical relationships between standard soil lab test results and soil/structure interaction properties. LPILE1 uses p-y curves, and I understand that GROUP and PYWALL also use p-y curves to model soil behavior. APILE and SHAFT probably use some form of t-z curves; I'm pretty sure that APILE originated as the computer program PX4C3.
Focht3
Good point. I hope other readers will pay heed to your words of caution on this.![[bigears]](/data/assets/smilies/bigears.gif "[bigears] [bigears]")
Most of what I do is to establish loads and design poles of all kinds (being a Polecat, what else would I do?).B-) By far the majority of these are direct buried, so predicting the lateral movement and rotation is a necessary step for me to establish the P-Delta secondary moments in the pole shaft, as well as to come up with the maximum moment below grade.
Hence, I became an avid user of LPile4 and have had several discussions with Bill Isenhower and Jose Arrellaga on how, when and what K values to use. It's too bad, though, that they didn't combine lateral movement, end bearing and skin friction all in one program. Now, THAT would be real useful tool.
Dr. Isenhower gave a seminar on the use of LPile in February 2002 in Orlando at a Foundations Conference that I attended. I had lunch with him and found him to be a very knowledgeable guy indeed (as well as being extremely open and friendly).![[smarty]](/data/assets/smilies/smarty.gif "[smarty] [smarty]")
But, in spite of how good a tool that LPile is, one must, as you have pointed out, tread with caution in choosing the proper soil values to use.
Good point. I hope other readers will pay heed to your words of caution on this.
Most of what I do is to establish loads and design poles of all kinds (being a Polecat, what else would I do?).B-) By far the majority of these are direct buried, so predicting the lateral movement and rotation is a necessary step for me to establish the P-Delta secondary moments in the pole shaft, as well as to come up with the maximum moment below grade.
Hence, I became an avid user of LPile4 and have had several discussions with Bill Isenhower and Jose Arrellaga on how, when and what K values to use. It's too bad, though, that they didn't combine lateral movement, end bearing and skin friction all in one program. Now, THAT would be real useful tool.
Dr. Isenhower gave a seminar on the use of LPile in February 2002 in Orlando at a Foundations Conference that I attended. I had lunch with him and found him to be a very knowledgeable guy indeed (as well as being extremely open and friendly).
But, in spite of how good a tool that LPile is, one must, as you have pointed out, tread with caution in choosing the proper soil values to use.
Bill Eisenhower is a friend of mine - I was working on my Master's while he was working on his PhD. He's a GREAT guy. Keep in touch with him.
I was also in Orlando - it was a great conference.
I've done quite a few "direct buried" designs myself; the biggest problem in using LPILE is that it uses a finite differences approach to solving the flexure problem. When the soils are of moderate strength, DB poles frequently don't flex enough within the embedded zone to avoid numerical instability - unless the soil strengths are somehow decreased and/or the embedment is significantly increased. (I wrote my own code that treats the embedded portion of the pole as a rigid body and uses a modified p-y curve to address this problem.)
The next big problem is the use of
E[sub]S[/sub] = k * x where
k = arbutrary limiting value of the ratio p/y
x = depth below ground surface
to establish the initial slope of the p-y curve. The original research that Lymon Reese, Hudson Matlock, and others did to develop the various p-y relationships were done on relatively long, flexible piles. DB poles don't experience the same conditions. (I don't have time to elaborate on this right now.) In my opinion, the equation should be
E[sub]S[/sub] = E[sub]0[/sub] + k * x where
E[sub]0[/sub] = the initial secant modulus (also arbitrary)
and the recommended values of k increased.
In this discussion, I think it's important to understand why Reese and others chose to limit the initial slope of the p-y curves. P-y curves follow a parabolic curve equation, so as the deflection approaches zero, the ratio p/y approaches infinity. That's why they chose to limit the initial slope of the p-y curve such that
p/y <= k * x
One final note: analyses done using p-y curves are different than those for bearing capacity, beam sizing, etc. Because the analyses are primarily intended to predict pile/pier moments, the soil values are not factored in any way. Factors of safety are applied by factoring the loads.
I hope this helps -
I was also in Orlando - it was a great conference.
I've done quite a few "direct buried" designs myself; the biggest problem in using LPILE is that it uses a finite differences approach to solving the flexure problem. When the soils are of moderate strength, DB poles frequently don't flex enough within the embedded zone to avoid numerical instability - unless the soil strengths are somehow decreased and/or the embedment is significantly increased. (I wrote my own code that treats the embedded portion of the pole as a rigid body and uses a modified p-y curve to address this problem.)
The next big problem is the use of
E[sub]S[/sub] = k * x where
k = arbutrary limiting value of the ratio p/y
x = depth below ground surface
to establish the initial slope of the p-y curve. The original research that Lymon Reese, Hudson Matlock, and others did to develop the various p-y relationships were done on relatively long, flexible piles. DB poles don't experience the same conditions. (I don't have time to elaborate on this right now.) In my opinion, the equation should be
E[sub]S[/sub] = E[sub]0[/sub] + k * x where
E[sub]0[/sub] = the initial secant modulus (also arbitrary)
and the recommended values of k increased.
In this discussion, I think it's important to understand why Reese and others chose to limit the initial slope of the p-y curves. P-y curves follow a parabolic curve equation, so as the deflection approaches zero, the ratio p/y approaches infinity. That's why they chose to limit the initial slope of the p-y curve such that
p/y <= k * x
One final note: analyses done using p-y curves are different than those for bearing capacity, beam sizing, etc. Because the analyses are primarily intended to predict pile/pier moments, the soil values are not factored in any way. Factors of safety are applied by factoring the loads.
I hope this helps -
Focht3,
Is your thesis published? I am doing a lateral load test in Harvey, LA for a Sector Gate some in the next couple of months. The information in your thesis would probably be useful. If it is not published, do you know how I can see a copy (on-line at Univ of Texas perhaps?
Is your thesis published? I am doing a lateral load test in Harvey, LA for a Sector Gate some in the next couple of months. The information in your thesis would probably be useful. If it is not published, do you know how I can see a copy (on-line at Univ of Texas perhaps?
Sadly, it is not published - but I'd be happy to share the results with you.
To me, the most important result was that I could not get a match on pile behavior using the Matlock soft clay p-y criteria for cyclic loading. It was very frustrating - I got a good match on the static behavior for both moments and deflections. For cyclic behavior, the predicted behavior was way too stiff and significantly underestimated the single pile load-deflection behavior.
I talked with Dewayne Bogard (I think I have his name spelled correctly) about four or five years later; he and Hudson Matlock had studied the cyclic results - and discovered the problem. If you are familiar with the soft clay p-y criteria, the static peak lateral resistance is multiplied by 0.72 to adjust for the effects of remolding and loss of soil resistance under cyclic loading. (The factor of 0.72 was empirically derived from the results at a site that I can't recall right now; but I'm pretty sure that it was also located in Louisiana.)
It turns out that the Harvey, LA site needed a reduction factor of 0.29, not 0.72, in order to achieve a reasonable match of behavior. The Matlock, Bogard et al study was published in the 1984-1987 time frame, but I can't remember where. (It was probably an ASCE or OTC publication.) My brief narrative should help you better understand their paper; as I recall, it was a little hard to follow.
If you still need help, post another message with a web site where I can contact you.
You're working on a Sector Gate - are you with WES?
To me, the most important result was that I could not get a match on pile behavior using the Matlock soft clay p-y criteria for cyclic loading. It was very frustrating - I got a good match on the static behavior for both moments and deflections. For cyclic behavior, the predicted behavior was way too stiff and significantly underestimated the single pile load-deflection behavior.
I talked with Dewayne Bogard (I think I have his name spelled correctly) about four or five years later; he and Hudson Matlock had studied the cyclic results - and discovered the problem. If you are familiar with the soft clay p-y criteria, the static peak lateral resistance is multiplied by 0.72 to adjust for the effects of remolding and loss of soil resistance under cyclic loading. (The factor of 0.72 was empirically derived from the results at a site that I can't recall right now; but I'm pretty sure that it was also located in Louisiana.)
It turns out that the Harvey, LA site needed a reduction factor of 0.29, not 0.72, in order to achieve a reasonable match of behavior. The Matlock, Bogard et al study was published in the 1984-1987 time frame, but I can't remember where. (It was probably an ASCE or OTC publication.) My brief narrative should help you better understand their paper; as I recall, it was a little hard to follow.
If you still need help, post another message with a web site where I can contact you.
You're working on a Sector Gate - are you with WES?
I commented on the same subject under Menard's Pressuremeter .. I think its on 3 of the Tips here ...
As I stated before I am definitely against using software to analyse single pile behavior due to the many variables involved and the very need to exercise good engineering judgement which is exactly what geotechnical engineering is all about. There is a great danger in using empirical formulas just to arrive ay a mathematical solution. You must have the feel of the soil in you before you attack the problem. And always beware of analysing the pile solely as a structural element ... pile / soil interaction should most definitely be considered (not based on a wide range of values seen in Bowles for various soils 5 - 20 MPA for example ...hahaha). The Pressuremeter Theory by Baguelin, Shields, et al provides the best analysis for the calculation of subfrade reaction I have ever seen and used in my designs. And the assumptions proved fairly close to test results ..
As I stated before I am definitely against using software to analyse single pile behavior due to the many variables involved and the very need to exercise good engineering judgement which is exactly what geotechnical engineering is all about. There is a great danger in using empirical formulas just to arrive ay a mathematical solution. You must have the feel of the soil in you before you attack the problem. And always beware of analysing the pile solely as a structural element ... pile / soil interaction should most definitely be considered (not based on a wide range of values seen in Bowles for various soils 5 - 20 MPA for example ...hahaha). The Pressuremeter Theory by Baguelin, Shields, et al provides the best analysis for the calculation of subfrade reaction I have ever seen and used in my designs. And the assumptions proved fairly close to test results ..
02111933:
Welcome!
Hmmm, this is the second forum in which you have expressed this opinion. You only joined the forum the day you posted this comment; and you haven't provided any personal details. Since my experience is very different from yours, I have a few questions:
1. What program(s) have you used? Over what period of time?
2. What case studies did you evaluate to arrive at your conclusions? What specific predictions did not meet your requirements?
3. Did your comments derive from graduate studies? If so, who were your advisors? (This can strongly influence your personal bias. [It has mine.])
4. What is your geographic area of practice? (Brief) project experience?
5. Other relevant information that we should consider in evaluating your comments?
I look forward to your comments -
Welcome!
Hmmm, this is the second forum in which you have expressed this opinion. You only joined the forum the day you posted this comment; and you haven't provided any personal details. Since my experience is very different from yours, I have a few questions:
1. What program(s) have you used? Over what period of time?
2. What case studies did you evaluate to arrive at your conclusions? What specific predictions did not meet your requirements?
3. Did your comments derive from graduate studies? If so, who were your advisors? (This can strongly influence your personal bias. [It has mine.])
4. What is your geographic area of practice? (Brief) project experience?
5. Other relevant information that we should consider in evaluating your comments?
I look forward to your comments -
- Status
Similar threads
- Locked
- Question
- Locked
- Question