Remedial Sheet Pile Reinforcing 2

[CoryChamberlain]

I am dealing with sheet pile problem and would appreciate any advice you may have.

My previous boss (who is now deceased) designed a sheet pile wall system adjacent to a railroad. The system is cantilever PZ-27 sheets for the majority of the wall where the retained height is 15' with a 2:1 sloping backfill up to the closest track which is located approximatly 20' (min)from the wall. A some locations the wall is tied back due to an increase in wall height. Most of the back fill is ballast or sand. The first 5' to 7.5' of embedment is silty clay, black mottled soft clay with a qu averaging about 1 tsf wall. The next next 13 feet or so is a hard clayey silt with a moisture content of around 10%, qu exceeding 4.5 tsf and spt exceeding 90 blows/ft. Rock occurs at about 20'. The piles were fitted with shoes and driven to about 15' embedment.

During driving, another structural engineer was contacted to elviate the ballast rolling from the driving operations. Their solution was to add about 3' of ballast before the wall was driven to the specified tip elevation.

Also at this location, the piles hit possible obstructions and were cut short of specified lengths (ranging from 1 to 4').This area has been monitored since excavation and some of the deflections are nearing 6" near the top.

My boss had designed for 800 psf strip loading for cooper E68, with bousinesq and no hydrostatic pressures.

I have considered supplemental reinforcing for the wall such as tiebacks or cantilever soldier piles (drilled to rock) placed in front of the walls, but I am not exacly sure how to handle the residual stresses in the pile and earth. Removal of the backfill and using a deadman pile for straiting/anchoring doesn't appear to be an option.

What are the stability checks to perform for the toe if tie backs at the top are added? What forces are the tiebacks, soldier piles and wales designed for since deflection has loading has already occured? When adding tiebacks, are the soil pressures still active behind the wall? What are the ways to analyze the long term delfection for creep and seasonal variations in the soil the cantilver section with and without remedial reinforcing to ensure deflections are tolerable?

Any other experiences and solutions with a similar situations would be useful.

Thanks

 

[cap4000]

I have never been involved with your situation but have some comments and suggestions. Is the PZ-27 undersized?? Most railroad companies have "Embankment Zones and Excavation Restrictions" as guidelines which you must have. Soil Anchors or Nails may require special permission as to distances and lengths allowed toward the tracks. My guess is that 20 feet back sounds ok, but, the sloping backfill and water getting into the soil might be where the 6 inch deflection comes from. The rock @ 20 ft and obstructions has dictated the depth or else the sheets should of been driven deeper. You are caught between a "rock and a hard place" so to speak. My suggestion is get to the the "California Trenching and Shoring Manual" off the web and search the Soldier Pile Section. It has design procedures for Soldier Beams in Soil with Wood Lagging and print all 40 pages. It is a great reference just to have.
Good Luck!!

 

[GeoPaveTraffic]

CoryChamberlain,

6 inches of deflection for a cantilevered sheet pile wall does not sound that bad to me. Is the wall currently moving or has the movement stopped? Is the railroad requiring some remedial work or is your client? If you don't know if the wall is still moving, my first option would be to monitor the wall and see what is happening.

If the wall is still moving, I would probably install two levels of tie backs in the area where the wall is moving. I would design and space the tie backs to take 100 percent of the at rest earth pressure. The tie backs should also be checked for the apparent pressure diagram for a tied back wall as shown on page 461 of Peck, Hanson, and Thornburn (among many other sources). I would preload the anchors to 80 percent of the apparent pressure diagram or 100 percent of the contributory area of the at rest pressure, whichever is higher and leave it alone.

The wales should be designed for 100 percent of the largest possible tie back load. No soldier piles would be required.

There are other ways of getting around this and the above is probably over kill. However, my experience with the railroad is that if someone else is paying, they want it all and the in end they usually get it. If your client is the railroad then start with one level of tiebacks about 5 feet from the top of the wall, monitor it and see what happens. Just be sure to tell them that additional tie backs may be needed.

Good luck, let us know what happens.

 

[PSlem]

cap4000 Thanks for the tip on the Calif Shoring Manual. I also printed the Foundation manual, although for the patient you can get them mailed for $50 each.

 

[PEinc]

CoryChamberlain,

Sorry, but I can't agree with GeoPaveTraffic that 6 inches of deflection does not sound that bad. A rough rule of thumb is that the settlement behind the wall can be twice the lateral deflection. I don't think that 12 inches of settlement (or even 2 inches of settlement) is very desirable or even normal. It seems excessive to me. If I had settlement that large, I would consider the wall to be a failure even though it may not have fallen over.

Also, a sheet pile wall only 15 feet high should be able to be stabilized with a single tier of tieback anchors - especially if it had been designed originally as a cantilevered wall. A 15' high tiedback wall should not need as strong a sheet pile section as a 15' high cantilevered wall. If you have sufficient sheet pile toe embedment below subgrade, a single tier of tiebacks should suffice unless you are using very low capacity anchors. In which case, you may want to use two tiers in order to prevent cutting too many holes through the sheet piling at the same elevation.

Tiedback walls are usually designed for active earth pressures. These walls are usually flexible walls which develop enough movement to justify the use of active pressures. You should check the requirements of both AREMA and the particular railroad for their design requirements for a tiedback sheet pile wall. They may specify a particular earth pressure diagram and other requirements.

You mentioned that the wall deflected 6 inches. I assume that only the cantilevered wall deflected excessively and that the tiedback portion is performing satisfactorily. If so, can't you just tieback the cantilevered portion of wall the same as the originally tiedback wall?

 

[VAD]

Hello CoryChamberlain

From the information provided the length of sheetpiling embedded below original ground varies from 18 to 20.5 feet. therefore the total length of sheet pile to sloping top varies from 33 to 45 feet if I interpret your depth of embedment as I have done. If incorrect please let me know. The sloping backfilll and addition of 3 ft of ballast would result in the depth of embedment being in my opinion a bit on the low side

In addition, I do not personally like the hard clayey silt material for terminating the sheetpiles as its behaviour during driving could result in possible loss of original characteristics.If I am visualizing correctly this material type is less likely to close back after driving. Fixity conditions may be compromized requiring deeper depth of embedment. I would have perhaps opted for a different system to retain the fill as you are thinking at this time.

The 6 inch deflection is a bit large for the free height of wall and suggests that if it is continuing the wall is loosing integrity and could eventually fail. I guess you are looking at this aspect.

As PEinc stated it is assumed that the tieback areas are performing well. If not, then I would begin to get worried about my overall geotechnical and structural behaviour.

I would look at the design of additional anchors as well and would want to take them into rock.

Good Luck

 

[CoryChamberlain]

We have been continually monitoring the wall for over a year now. The wall is only moving very small amounts each month (.02 to .03 feet). The tied back portion and other cantilever portions of the wall are performing satisfactorily. The actual deflection would be hard to calculate since there is a initial forward batter due to construction that cannot be verified. So the 6" or so is a guess.

Even if a tieback is added now, wouldn't it be like a sequencing of a tieback wall if constructed properly (although the cantilever portion would be a lot greater for this case).

With this much deflection already, would it even be possible to develop the apparent pressure diagram suggested for tieback walls. I think the active case would be more appropriate. And how would you handle the sloping backfill for the apparent earth pressure diagram.

I am still wondering how to handle any stress redistribution with the tiebacks installed to a certain "pre" load. That is why I suggested maybe soldier piles be placed at a certain space in front of the wall.

 

[VAD]

Thank you for the added information. By rule of thumb movements in excess of 2mm per day are generally considered to lead to critical events. Your situation is well below this and hence changes the picture.

Your reaction of putting a wall outside and designing same for lateral pressure seems to be reasonable. I would not try to get too concerned about redistribution of stresses etc, but to consider whether this wall is needed.

A few more questions. Is the railroad adding ballast to their tracks in this area more frequently?. What is the obligation to jump in and do something immediately at this site?. What are the risks of continued small movements?.

Unless there is a concern that there would be dramatic movements of the tracks railroads have lived with minor movements for a number of years. I have dealt with lanslides failure planes going below tracks from adjacent highways. Railroads have no issues as they tend to inspect and realign as necessary. I am not saying that this is the way to look at things by any means.

I would continue to monitor the movements of the wall a bit longer. Perhaps tiltmeters could be installed as survey monitoring may not be satisfactory. The Slope Indicator Company etc can provide some information on this.

Another aspect is to look at permeation grouting of the backfill material. This could have the effect of also preventing infiltration of water. Hoewever, you need to solicit the advise of speciality contractor on this type of work. Grouting the material could assist to reduce the lateral earth pressures and alleviate the need for the soldier pile wall.

The overall costs and long term performance are things to consider. If you have to put the new wall then I tend to agree with your approach based on the information provided.

and good luck

 

[Focht3]

It certainly doesn't sound like the wall is about to fail. But you do need to proceed with caution -

I agree that the cantilevered wall sections don't seem to have enough embedment. I'm not terribly concerned about the silt at this point, although I would have monitored its behavior during construction.

I don't like the idea of proceeding with a fix without high quality data, however. I strongly urge you to install two or more inclinometers behind the cantilever sections of wall and begin monitoring them ASAP. You need to be sure that the toe of the wall is also stable; "stability" at the top of the wall can be very deceiving...

Installing a new wall will be terribly inefficient, although the railroad may force you to use that option. Tiebacks and walers will result in a better solution - technically and financially. Use at rest, not active, pressures for design. After all, you will be pushing into the soil - and the wall won't be able to move after the tiebacks are installed.

If you have room for a new wall, do you have room to install bracing? You could install large drilled piers, walers and braces to pick up the lateral forces. This isn't as cheap as tiebacks, but it can be done. If you do install a new wall, you should assume that the existing wall isn't carrying any loads, and use at rest pressures for design. If you use active pressures, you will probably end up with another mess in a few years - after all, the existing wall was designed using active pressures, right?



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

 

[CoryChamberlain]

The wall was originally designed for active pressure and based on the lateral movement one can say the soil has mobolized and is currently in an active state. The soldier piles idea would not be a new wall, but basically stiffners for the existing wall. The would be placed in front of the sheets and connected (somehow)to reduce any addtional deflection and would also carry live load from the trains.

Although the soldier piles are feasible, I believe I am leaning towards the tie backs. At rest pressures seem to be reasonable for design.

 

[DRC1]

The biggest concerns would be the 90 blow material, and the 2:1 slope. The 90 blow material would make it very difficult to advance the sheets with out damage, which may be part of the problem. The 2:1 slope will increase the earth pressure coefficient considerably and many times is not properly accounted for in the design. A 15 foot high cantilever wall with a 2:1 slope is probably pushing the limit for the cantilever distance. At 20 feet offset, the E-68 load on the sheets should be minmimal. You do say that no hydrostatic pressure was included in the design. You may want to revisit that assumption, as the sheets would cut off any water draining down the slope. The soils below the excavation line appear to be relatively impermiable, so the sheets may be dammng water. You may want to look at installing drains with graded stone at the botom of the sheets.
As far as reinforcing the the sheets, either the soldier pile option or the tie backs can work, But I think you will find that the tie backs will work and be signifcantly less money.
To estimate the amount of stress in the sheets, anayize the current design and determine the moments. The original defelection was probably less than one inch and therefore the deflection was not consided in the design stress. The additional stress due to the excessive deflections can be approximated by assuming the sheets are fixed a few feet below the excavation and applying a triangular load to the system. Back calculate to get the load that causes a 6 inch deflection. then you you can compute your stresses. You can also look at the load to see how it compares to the load in the orginal design assumption.
I would design for at rest loads at least equal to the back calculated load. To obtain active loads, the wall must continue to move, which is what we are trying to prevent. If you use anchors, the anchors may actually move the sheets back a litle when they are locked off. The anchors will also reduce the load on the sheets at depth (if designed on the free earth method).
A .02 to .03 movement is too much for a wall that has been built for a while. At that rate you will pick up an inch every 5 years. However, what happens is that as the wall starts to fail, the movenments accelerate. Although I would continue to moniter the wall, I would remediate the wall now and continue the monitering after construction to ensure the action was succesful.
A good reference (although some in this forum disagree with me) is the Pile Buck Sheet Pile Design Manual, available from Pile Buck at:

http://www.pilebuck.com.

Another reference is the Army Sheet Pile Manual, available at :

http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-2-2504/toc.htm

Good Luck

 

[GeoPaveTraffic]

With respect to earth pressures: Even though the wall currently has active earth pressures, if the wall is stiffened, such as with tie backs, then the pressures are going to increase above active. This is the reason that I would recommend using the maximum of at rest and the apparent pressure envelope as discussed in my previous post.

With respect to the wide range of posts: I believe that this illustrates the professions and practitioners wide range of tolerance for risk. I would recommend that you talk with the client/owner present some options and let them make an informed decision.

 

[PEinc]

Long term load cell monitoring of tiedback walls shows that rarely does the tieback load increase significantly above its original lock-off load, which is usually 70 to 100% of its design load, which more often than not is based on active earth pressures.

 

[Focht3]

And those studies are on new construction, not repairs of distressed structures....

And the studies don't evaluate load transfer to the embedded portion of the retention system, which could significantly alter our view of the total resisted force....

And inclinometers were not used, which would permit a direct assessment of the pressure magnitudes and distribution on the backs of the walls....



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

 

[CoryChamberlain]

A few more questions:

Since the piles are embedded in a cohesive material, one should consider both the immediate (undrained) and the long-term (drained) conditions. Is there a recommended value to use (without further testing) for the drained soil friction angle based on blow counts or cohesion of the cohesive material?

Also since it is embedded in a cohesive material, I am assuming that head can build on the back side of the sheets (assuming the drains clog)without any seepage under the sheets that would reduce the passive force in front of the wall. I there any other way for this example the passive soil in front can be reduced due to seepage?

How can one handle the sloping backfill for either an at-rest state or for the apparent earth pressure? Can one still assume a failure wedge behind the wall and calculate the soil mass from the sloping backfill within those limits as a surcharge? I usually modify Ka for a sloping backfill based on log-spiral charts, but not sure for at-rest or apparent earth pressures.

Thanks for all your input!