Sheet Pile Soil Pressures 1

[SOFLENG]

I have been designing sheet pile walls with single level strut or tie rod supports at the top for many years using the triangular distribution for active pressures (USS, DM-7). I have encountered a reviewer who insists that I use a "one level anchor" trapazoidal distribution per FHA and the latest AASHTO. I don't agree for a 30' wall with an unstressed anchor at the top. Has the approach to this design changed?

 

[Jebisou]

AASHTO Guide Design Specifications for Bridge Temporary Works shows the trapazodial load for walls with two or more anchors (figure 4.1).

Hope this helps.

JEB

 

[SOFLENG]

Thanks, but current LRFD 3.11.5.7.1 and FHWA-IF-99-015, Page 51, indicate using a trapazoidal apparent pressure for a single anchor. I'm fine with a trap for multilevel anchors.

 

[Jebisou]

First, I am no geotech but LRFD 3.11.5.7.1 says "may be estimated". "May" being a key word. Also I would say that sheeting is temporary. I know the designs that I have reviewed design the way you have (trap loading) and I never had a problem with it since it is the standard typical way for one anchor.

I would look at the path of lease resistance if it was me. Even though all of you design calcs (summing moments and etc.)are set up to design using a trapazoidal load. What is easier? To just change the calc to please the reviewer or to fight someone who later on may be a bigger pain in the butt.

Also you will get to see what difference in the reaction and design moment really is.

My question is how do you sum moments around the anchor to calculate the embeded length with the trap loading? Do you just assume that there is only passive pressure below the dredge line? I would have to do a sketch and think about it.

That's all I have. Hope something helps.

 

[PEinc]

Sofleng,

You said you have a 30' high wall with an unstressed anchor at the top. If so, I don't see how you could have a trapezoidal pressure distribution. I would probably use a triangular pressure distribution. If the anchor is farther down the wall and is stressed and locked off against the sheeting wall, I would use a trapezoidal pressure distribution unless the soil behind the wall is very soft. Then, I would probably use a triangular distribution for the very soft soils.

If the job is not designed by AASHTO and FHWA specifications, you should not have to follow the AASHTO and FHWA design criteria for an anchored wall. Sometimes, the reviewing engineer is used to working on highway jobs and then is, by habit, looking for AASHTO and FHWA design criteria. Check the sheeting design requirements in the job specs.

If the job is a highway project using AASHTO and FHWA design criteria, you will most likely have to follow their design criteria even though the specs say that the earth pressures MAY be estimated by their unsymmetrical trapezoidal pressure distribution.

In my experience, most design-build anchored wall contractors use trapezoidal pressure distributions for one-tier anchored walls because the trapezoid gives a higher tieback or brace load and a shorter toe embedment. Also, FHWA and other research have shown one-tier anchored walls do not have triangular pressure. However, I don't think that the research was done on walls with the anchors or braces up at the top of the wall or with soft soils.

Jebisou,

For temporary anchored or braced sheeting walls with trapezoidal pressure distributions, I do not add active pressure below the dredge line. The older AASHTO Bridge Design Specifications for anchored walls said the same thing. For permanent walls, I add the active pressure below the dredge line. AASHTO does the same. I don't believe that the latest AASHTO Bridge design Specifications address no active pressure below the dredge line of temporary walls.

 

[STVU]

I agree with PEinc-Can't recall the exact title, but FHWA has recent publications online about single and multilevel tieback wall design publications. I recall reading, the researchers propose, that a cantilever flexible wall, a triangular distribution is the way, however for a single level tieback and multilevel tieback, trapezoidal distribution by Peck or Tschebatarioff is recommended. Just be mindfull, that this publication automatically multiplies active EFP by 1.3 factor.

Personally when I design single level TB flexible wall, I use the Teng method of equivalent beam analogy.(Foundation Design, by W.C. Teng, 1962) The design works, and earth pressure distribution arguments are eliminated.

There is a nice publication called "Recommendations on Excavations" found at Amazon which also show dozens of earth pressure distributions, before tieback stressing and after stressing it. I was surprised to see that even the trapezoidal shape, while keeping the same proportion, gives higher magnitude at some depths after stressing.

In your first post, you mentioned it is unstressed anchor-so your triangular distribution has to still apply.

 

[SOFLENG]

Further thought; I analyzed the wall cited in my question using both triangular and trapazoidal active distributions. The single anchor force was 50% greater with the trapazoidal. The triangular distribution has been used on deepwater sheet pile bulkheads since at least the mid-1900's. One would expect to have seen many more cases of anchor distress/failure if the pressure was actually trapazoidal.

 

[PEinc]

SOFLENG,

Triangular is more appropriate with waterfront bulkheads because of two reasons. First, the anchors are usually up high on the wall, above high tide. It's hard to foece a trapezoid down low on the wall when the tie is at the top of the wall. Second, waterfront bulkheads frequently have a lot of loose soils behind the walls. Frequently, this loose soil is granular fill. With loose granular fill, there is little or no soil arching. I would use triangular.

FHWA and AASHTO (and therefore many state DOT's) are looking for an unsymmetrical trapezoidal earth pressure distribution for one tier and multi tier anchored walls. The trapezoid shape is not the T&P trapezoid. The shape of the trapezoid varies according to the location of the tieback anchor(s). FHWA also has a formula for the magnitude of the earth pressure. They do not use EFP x 1.3 as stated above. FHWA's formulas do not provide a method for dealing with different soil layers. In short, the FHWA and AASHTO method is a PITA and is, as far as I know, not compatible with most (any?) commercially available sheeting design computer program.

We could talk all day about your problem, but unless we know what type of project you have and what the design specs are, we are just guessing about how to help you. Re-read my previous response. If you do not have a highway job and if your specs do not dictate that you have to use the FHWA method, tell the engineer no for the reasons I have stated.

 

[PEinc]

The FHWA publication is Geotechnical Engineering Circular No. 4.

http://www.fhwa.dot.gov/engineering/geotech/policymemo/gec4memo.cfm

 

[SOFLENG]

Thanks PEinc. Circular No. 4 is the vary publication my reviewer has cited in asking for the trapazoidal. By the way, it does address non-homogeneous, layered profiles in 5.2.7.

 

[PEinc]

Good, but is your project a highway project? I have had consultants who mostly do highway work review my non-highway projects and expect me to use highway design criteria just because that's what they are used to using. In thoise cases, I tell them no.

 

[SOFLENG]

Project is a launching shaft for a 66" microtunnel.

 

[PEinc]

Is it a DOT highway project??????

 

[SOFLENG]

Project is a municipal water transmission line.

 

[PEinc]

Then there is no way you should have to follow FHWA or AASHTO unless clearly specified in project documents. Tell the reviewer that you understand his position but that it does not apply to utility work where FHWA and AASHTO have no jurusdiction.

 

[DRC1]

Sorry to join so late. I understand that in some cases sheeting designers have used trapizoidal distrbutions for econmoy. I have fooled around with them on sites that I am familar. Generaly it is a very propritery apllication where the engineer had a lot of experience. Seeing some savings, FHWA has switched. That's the story I heard.
A launching shaft for a micro tunnel sounds like temporary work. My understanding of the ASHTO diagrams are that they apply only to permenant sheeting.

 

[PEinc]

I don't believe that the new AASHTO pressure diagrams for non-gravity wall mention temporary walls. The older AASHTO pressure diagrams do have slightly different diagrams for cantilevered and anchored soldier beam walls. The difference is (was?) that the temporary soldier beam walls did not have active earth pressure behind the soldier beam toe. To know this, you needed to read the fine print footnotes at the end of the section that showed the AASHTO diagrams. The FHWA design manuals for anchored walls mainly address permanent walls. There are no different pressure diagrams for temporary walls.

Economy really doesn't have anything to do with using trapezoidal pressure diagrams for single-tier braced and tiedback walls. The reasons are that the trapezoidal diagrams give higher brace and tieback loads (therefore, safer walls due to more lateral support) and monitoring and research have shown that the pressure is usually not triangular unless the single-tier tieback is very high on the wall or the soil is very loose, soft, or weak (as with many anchored waterfront bulkheads). The trapezoidal pressure diagram usually gives higher tieback and brace loads but shorter sheet or soldier beam embedments compared to triangular pressures. Often the bending moments are not very different.

It doesn't happen much lately but, when when a reviewer or agency insists on a triangular pressure distribution and the designer feels that the trapezoid is more appropriate, then the wall may be designed and submitted both ways (triangular and trapezoidal pressures) and the worst design case used. For awhile in the very early 1980's we even used to combine the triangular and trapezoidal pressure diagrams for permanent anchored walls. The upper half of the diagram was trapezoidal; the lower half was the lower portion of a triangular diagram. Eventually, instrumentation showed that for walls built in average, relatively competent soils there wasn't the higher, triangular pressure at the bottom of the anchored wall.

Many engineers today still use or insist that others use triangular earth pressures for single-tier anchored walls. Usually, this is because the engineers' only references are the old US Steel Sheet Pile (or old Pile Buck) Design Manual and/or the old AASHTO bridge design manuals.

 

[palmahouse]

Recent studies that included instrumentation show that over the long-term, loading behind anchored sheetpile walls is closer to trapezoidal-shaped. I recommend you use trapezoidal, depending on whether you think the retained soil will become fully plastic during the service time of the structure.

The total loading should be the same, independent of the distributed shape - right?

You are probably stuck with doing what your reviewers suggest anyway - I wouldn't loose sleep over it - especially if you think you are adequately conservative with your safety factor, earth pressure, and earth resistance. You probably used conservative values for earth pressures and passive resistance anyway (like we all do), and did not utilized moment reduction techniques, this may all be insignificant. Besides, this should all result in a tie with more capacity and more shallow toe depth, and may not have a big impact on cost. Will in result in a smaller sheet pile depth? Please advise.

My focus on these things is deflection and what impact that will have. After all, "failure" of retaining structures typically is associated with unacceptable deflection (and associated distress) as oppposed to oversressting structural elements.