Flexible Sheetpile Walls
Flexible Sheetpile Walls
(OP)
I have seen all the books explain Ka and Kp beneath the excavation. I would like to question this as I am computer modelling sheetpiles with springs.
I understand Kp as the limit of a soil's capacity to an applied pressure, and that the spring constant is limited by this value at any given depth. That being said, Ka and Kp should not exist together as Newtons Law says for every force there is an equal and opposite reaction.
This being said, as I am modelling my scenario, whether I use Ka pressures or not below the excavation depth, I do not get the pressures at the Kp limit, this makes sense.
When I use Ka pressure values below the bottom of excavation, I get spring reactions which give me pressures within Kp resistance allowing for a factor of safety of 2 but my sheetpile toe is kicking out. What gives? When I do not use Ka pressure values below the bottom of excavation, my toe does not kick out.
Driving flexible sheetpiles into level ground should result in a net zero pressure. Thus below the excavation there should be no net earth pressure as they cancel out? In an excavated situation should not the pressures below the bottom of excavation then originate from above the bottom of excavation through the soil structure interaction.
My geotechnical engineer has told me the active zone is drawn 45-phi/2 from vertical originating at the BASE of the excavation, and he still thinks in terms of Ka and Kp.
This has driven me crazy over the years.
VOD
I understand Kp as the limit of a soil's capacity to an applied pressure, and that the spring constant is limited by this value at any given depth. That being said, Ka and Kp should not exist together as Newtons Law says for every force there is an equal and opposite reaction.
This being said, as I am modelling my scenario, whether I use Ka pressures or not below the excavation depth, I do not get the pressures at the Kp limit, this makes sense.
When I use Ka pressure values below the bottom of excavation, I get spring reactions which give me pressures within Kp resistance allowing for a factor of safety of 2 but my sheetpile toe is kicking out. What gives? When I do not use Ka pressure values below the bottom of excavation, my toe does not kick out.
Driving flexible sheetpiles into level ground should result in a net zero pressure. Thus below the excavation there should be no net earth pressure as they cancel out? In an excavated situation should not the pressures below the bottom of excavation then originate from above the bottom of excavation through the soil structure interaction.
My geotechnical engineer has told me the active zone is drawn 45-phi/2 from vertical originating at the BASE of the excavation, and he still thinks in terms of Ka and Kp.
This has driven me crazy over the years.
VOD





RE: Flexible Sheetpile Walls
The reason that the pressure is higher on the excavation side of the sheet below the excavation is that the sheet is moving, due the unbalanced pressure above, into the excavation. As the sheet moves the springs in your model are compressed and the load increases up to the Kp value. Once the Kp value is reached the spring can continue to compress, but the load does not increase.
For your model, anytime that the sheet is moving into the soil, Kp applies to that side; when the sheet is moving away, Ka applies to that side.
RE: Flexible Sheetpile Walls
In the case of flexible walls, below the excavation level as the sheetpile flexes either way, it only sees passive resistance springs, no active in my opinion. Thinking of a diving spring board.
VOD
RE: Flexible Sheetpile Walls
VOD
RE: Flexible Sheetpile Walls
RE: Flexible Sheetpile Walls
RE: Flexible Sheetpile Walls
I am modelling the sheet piling because the conventional method gives me a depth I need to reduce. This is why I questioned this assumption of Kp & Ka pressures beneathe the excavation level. A depth I believe that can be reduced without compromising the Factor of Safety.
I am not dealing with moment arms for passive triangular limits in this particular analysis. My passive reactions are accounted for through passive springs. My results show no triangular shaped pressures on the passive side as we assume in the simplified methods to calculate depth of embedment.
VOD
RE: Flexible Sheetpile Walls
There is not one "conventional method" to design sheet piling. There are several methods, various assumptions, and a number of computer programs for designing sheet piling. All have been used successfully even though they may give different answers. What "conventional method" did you use? Is your wall cantilevered, braced, or tiedback? How much are you trying to reduce the embedment? Tell us more about your wall. It sounds like you may be trying to reinvent the wheel.
RE: Flexible Sheetpile Walls
The interesting thing is that as support condtions increase, the easier it is to model. I have never seen springs used for active/passive analysis. Location of moment arms are important for the analysis. A method of using the soil modulus is more suited to a spring model. The problem with soil modulus, is that it is hard to get accurate values. Even with marginal data, generaly fairly reasonable values can be assumed for active and passive coefficents.
Other concerns that your model needs to account for are water table conditions, seepage, lateral loads from sucharges, adj. foundations and embankments, support system, sheetpile, soldier pile, etc.
A good place to start is the Corp of Engineers manual on sheet pile design, EM 1110-2-2504. It is availale on line at http:
Earth retention is soewhat specialized field of geotech. If you are trying to reduce yor embedment from a result of a clssical analysis, it cn be done sometimes, but you really should talk to a geotech or contractor wth experience in this field. Wall collapses can and do happen.
RE: Flexible Sheetpile Walls
Thank you for your response, the conventional method is out of the USS Steel Sheet Piling Manual. The order of reduction I am getting is about 2 metres for a tie back wall about 4 metres high above dredge line. A collegue of mine did the conventional method, and then we modeled them both. Model was simple and reviewed and so the results made us question the application of loading.
DRC,
I am trying something here, your advise is somewhat patronizing. I am aware of the nuances of sheetpile design and I have narrowed it down to the application of active while questioning the passive limit from my passive spring displacements. I value your opinions in this forum and I hope my question is not a waste of time. You refer to a dowel, it is not flexible, I am modeling the stiffness of the sheetpile so it will not respond exactly the same way.
Is there certainty that flexible sheet pile walls respond to soil pressures as assumed or may we encroach in this area while ensuring a factor of safety. My comparitive analyses has made me ask this question.
Cheers
VOD
RE: Flexible Sheetpile Walls
I did not intend to be patronizing. If I offended you I appoligize, as that is surely the farthest thing from the purose of this forum. However it is difficult some times to express complicated engineering concepts in 2 or 3 written paragraphs. Sometimes it comes out well some times it does not.
Any way, my point with the dowel, is that Passive pressure develops on both sides of the sheeting and if you dont properly account for it, that could cause your analysis to show the toe kicking out. I still don't see how the active and passive pressures are modeled with springs, as that seems to indicate that the applied load may be less than full active or passive. (If this is what you are trying to do there is a good journal paper in ASCE Geotech journal a few months ago)
As for the dowel analogy, the classical method by hand first considers the stability of the sheet as a rigid body, then determines the moment capacity required of the sheet. This not to say that your method is wrong. A flexibilty approach may be amore sensible approach.I wouldnot argue that in general the classical method is conservative. My point is only that if you are using a completely different philosophical approach to the problem, it is not surprising that it does not compare exactly to the other method. I did not mean to be patronizing, nor do I mean to discourage you. reinventing a wheel is hard work, but if we end up with a better wheel that would be great.
Good Luck!
RE: Flexible Sheetpile Walls
Back to your original post, it seems to me that you can have active and passive pressures at the same elevation, with the net pressure being the difference of the two. How do you figure that they can't exist together?
RE: Flexible Sheetpile Walls
RE: Flexible Sheetpile Walls
Water under the bridge, back to the post.
jmiec,
The passive nature is not argued, it is the limit of Kp because this is the failure state of the passive soil resistance in combination with the active pressure that is questioned. A soil will respond with an increasing resistance up till Kp.
To simplify, if a soil acts, an equal an opposite reaction on the passive side is necessary. Kp is merely a soil's limit.
I think using Ka and Kp together is fundamentally flawed though it may yield a conservative result and save all the iterations.
What is the use of modeling tension only members? Tension only members do not represent the force-deflection nature of soil.
RE: Flexible Sheetpile Walls
A member can be used to mimic a spring, with k=AE/L. However, on second thought, this does not help solve your problem. Forget that I mentioned it.