Can sheet piles be Fixed AND free earth support? Wallap vs manual calculations

[Fynch]

Hello – I was wondering if anyone could help me with the following problem:

I am using Wallap for the first time and have come across something strange. The design is a propped retaining sheet pile wall.

The bending moment and displacement diagrams indicate that it is free earth support. However, the net pressure diagram shows the passive pressure changing to active right at the bottom of the pile – indicating fixed earth support.

When I manual calculate the design using free earth method, I also get a prop force 27% LESS than the Wallap prop force.

The only difference I can see is the Wallap reports a drop in passive pressure for some reason at the bottom of the pile.

Can anyone please explain why/how:
1) A manual calc can be less than the wallap report in terms of resistance required (i.e. I thought Wallap would have optimised the force)
2) Why the net pressure changes from passive to active right at the bottom of the pile (even though the bending and deflection diagram shows free end)

 

[PEinc]

SSP walls can be designed for either the free-earth or fixed-earth method. Free-earth method gives a shorter pile length (add a safety factor), more bending moment, and a higher anchor load. It assumes that the wall is supported by the anchors and the soil below the dredge line. Calculate the embedment required for moment equilibrium about the anchor level and then increase the embedment by at least 20% as a safety factor. The calculations are simpler with the free-earth method. The fixed-earth method assumes that the wall is supported by the anchors and by a "fixed" reaction from the soil at the lower portion of the embedded length of the SSP. This fixity reduces the bending moment in the SSP. The fixed-earth method will give longer piles and a bit less anchor load. The calculations are more complicated. I use the free-earth method. It gives a more, but not overly, conservative design result.

When I do hand calcs for the free-earth method, I do not "net out" the active and passive pressures before calculating the required embedment for moment equilibrium. Doing that is busy work. Keep the active pressure on the back side of the pile and the passive pressure on the front side.

A good reference for this topic is Foundation Engineering Handbook by Winterkorn and Fang, Section 13.3, Pages 421-426. The discussion may also be in the updated edition by Fang.

http://www.PeirceEngineering.com

 

[Retrograde]

Maybe at the toe of the sheet pile there has not been enough movement to generate full passive pressure.

 

[PEinc]

I have designed a few thousand temporary and permanent retaining walls and have built many. I have never had a problem with significant soil movement in developing passive resistance for my walls. If you read many geotech references, they will tell you that a lot of movement is needed to develop "full passive resistance" often several inches In my experience with non-gravity retaining walls (both SSP and soldier beam walls in good and bad soils), movement of passive soils is not a problem for relatively competent soils and properly designed walls. This is probably because rarely is development of "full passive pressure" required. Driving earth pressures are usually conservatively high which causes the perceived "need" for high passive resistance. If significant movement is needed to develop full passive pressure, then I would expect to see much greater and unacceptable wall deflections than actually occur - at least on my projects.

http://www.PeirceEngineering.com

 

[Retrograde]

PEinc: I don't disagree with you, but the WALLAP software probably assumes a certain amount of movement is required to develop passive resistance.

 

[Fynch]

Hmm so is Wallap over engineering compared to hand calculations?

I always thought they optimised results compared to hand?

Can hand calcs account for additional movement? Because I already added on 20% to the length.

Based off both posts - do I take the lesser of the anchor loads (from hand calcs) or do I use the Wallap results which say the anchor takes higher loading by 27%?

 

[PEinc]

Retrograde, you said, "...the WALLAP software probably assumes..." I don't like to use any sheeting program that I don't know how it runs the calcs. I use CivilTech Shoring Suite for my sheeting walls because it calculates the design just as I would by hand calcs. Therefore, I can verify the program's results if necessary.

Fynch, T&P used hand calcs. Who will argue with them? If hand calcs were good enough for them, hand calcs (or a program like Civiltech) are good enough for me. Remember, there are lots of sophisticated, expensive programs available that could be deemed overkill. If there is a very important, critically risky project; maybe sophisticated sheeting programs or finite element analyses are required. But that's not usually the case. You don't need a sledge hammer to drive a finishing nail.

EDIT: I do not work for or sell Civiltech software.

http://www.PeirceEngineering.com

 

[charliealphabravo]

I had a professor at UK, Dr. Bobby Hardin. He was modeling soil behavior using finite elements in Excel. I learned more about excel that year than any other. I think we were also using the SlopeW suite as a comparison which is also FEA. I will check into that CivilTech suite if I ever have a need. Much more reasonably priced than the SlopeW suite.

 

[Retrograde]

I don't like to use any sheeting program that I don't know how it runs the calcs

That's fine, but the OP's question was specifically about WALLAP.

 

[Fynch]

So has any one else come across cases where the hand calcs are saying less resistance is required than the software?

I always thought hand calcs over engineered? But I guess if they don't take into account movement like Retro said earlier....

The sheet pile was to reinstate a river embankment. The river was eroding away the foreshore to the point it was encroaching onto a nearby reservoir bank (not filled unless flooded). This is why the software was used (albeit it was a very small job). However, when I did hand calcs to check the results, I was very surprised that the hand calcs said that less force was going into the anchor compared to the software. As such, I'm a little confused as to which to take and what may be causing the result from Wallap where the active pressure is increased at the bottom of the pile.

 

[MTNClimber]

Sometimes you have to reach out to the software company to get their explanation. My wife had to do it for CivilTech Shoring Suite and SupportIT. Both programs produced the same results across the board except for deflection. She called the software companies and found out that CT Shoring applies a 0.5 factor to the deflection results. "Based on experience" they don't see the unfactored deflection in the field or something like that (this was probably 6 or 7 years ago).

 

[Retrograde]

Both the free earth support and fixed earth support method make assumptions about the deflected shape of the sheet pile and the pressure distributions. WALLAP makes no such assumptions. WALLAP uses a subgrade reaction model where the sheet pile is modeled as a vertical "beam" and the soil is modeled as a series of non-linear springs (WALLAP makes assumptions about the shape of the stiffness curve of the springs). Given the different approaches it is not surprising you get different answers. The fact WALLAP is giving you a "worse" answer is likely due to the soil parameters you have selected.