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Sheetpiling to isolate/reduce stress between embankment fills
3

Sheetpiling to isolate/reduce stress between embankment fills

Sheetpiling to isolate/reduce stress between embankment fills

(OP)
I have been asked to evaluate the use of sheetpiling driven to rock, possibly coated, as a means of isolating stress and settlement between two adjacent bridge abutment embankment loads built in seperate phases.  The first abutment embankment will be constructed directly adjacent to an active highway bridge abutment.  The second abutment embankment will be constructed directly adjacent to the  newly constructed embankment in the first phase.   Has this been done?  Is this done all the time?  Is sheetpile  too flexible?  What is a good way to analyze this problem without an FEM program(don't have)?  Comments would be appreciated.

RE: Sheetpiling to isolate/reduce stress between embankment fills

Hello Geonet:

It is not very clear on how many bridges are to be constructed. It would appear that there could be an additional two and that one (abutment 2) would be adjacent to an existing one (abutment1) while the other, abutment 3 would be adjacent to abutment 2. Please clarify as I may have misinterpreted your intent.

Anyhow, the concept that you are required to investigate suggests to me that someone is concerned about the influence of the approach fill loads of abutment 2 on abutment 1. I presume that there is uncertainity about the design of abutment 1, although it would seem that abutment 1 could have been founded on piles bearing in rock. If this is the case then the concern would be the adequacy of the structural capacity of the piles. If the piles are not founded in rock but in ground that could undergo further settlement then this concern and structural adequacy of the existing piles may be the reason for the isolation of the abutments by coated sheetpiles.

Before going further, I would suggest that the existing and proposed abutment schemes be put in perspective by drawing to scale the concepts along with the subsurface information. From there you can look at how the loads from abutment 2 would impact abutment 1 and the underlying ground both vertically and laterally. A knowledge of the abutment 1 design and its foundations, type, embedment depth etc are essential in this respect. From here you can determine whether the reasons for proposing the sheetpiles are valid or not.

I would not be surprised if you will find that the sheetpile
separation may not be required and that you can also think of alternate solutions such as the use of lightweight fill in critical zones.

This is an interesting problem that requires first establishing the existing and proposed conditions and using simple concepts to determine whether more elaborate or sophisticated analyses are required.  

Good Luck.

RE: Sheetpiling to isolate/reduce stress between embankment fills

I've had a similar project with bridge new bridge foundations (twinning of the existing 2-lane hwy) adjacent to existing bridge foundations - spread foundations.  Soft soils, according to the soils report, for a couple of metres below design level.  How was it played by the designer?  He ignored it.  So far, okay . . .  but, then again, no one is monitoring the settlement.  I certainly don't as it might help the contractor - I don't represent him!!!

I agree that you draw up all your footings to scale in both plan and in a number of sections.  Mark on the existing foundations - type, etc.  (spread or piled).  Mark on existing ground surfaces and FRL (final road level).  Determine stresses caused by new embankment and, also important, the bearing pressures under abutment (especially if spread foundations). See how pressures interact within/beneath your existing structure.  If you have soft compressible soils under existing, don't forget that the new stresses may induce more downdrag forces.  Also, don't forget the "other side".  In our case, the distance between the abutments is only 8m.  Instead of a box bridge, they used individual spread foundations - the toes were only 3m apart.  So - you might have two or three other factors from the other side having an influence as well.

Once you have established the above, then you can look at possible measures to alleviate - one of which may be your sheet pile wall - or, perhaps, a secant pile wall, or, if the geometry is right, maybe you might want to connect all the pieces together.

Hope this gives some additional insights.

RE: Sheetpiling to isolate/reduce stress between embankment fills

(OP)
Thank you VAD and BigH.  I really do appreciate your comments.  I should clarify the problem a little more.  The first "new bridge" abutment fill is to be built several meters from the existing bridge.  This abutment fill will be anywhere from 6m deep to 12m deep.  A temporary MSE wall is being considered for the edge of this new abutment fill.  The existing bridge foundations are H-piles but actual driving depths are unknown.  However we do know that a soft clayshale exists at approximately 18m depth.  Settlements at the "existing" bridge foundations from the first "new" bridge are on the order of several inches (51 mm) as calculated using Embank program and superposition.  Once the first new bridge is constructed the "existing" bridge will be abandoned and the 2nd "new" bridge and new bridge abutment will be built over the "existing" bridge.  This 2nd "new" abutment fill will butt up to the 1st "new" bridge abutment fill.  Calculated settlements under the first new bridge from the 2nd new bridge fill are on the order of 6 inches (152.4mm)   Concerns include:downdrag and settlement of "existing" piles and abutment fill, stability of both new abutment fills (unimproved FS<1.3 full height), lateral squeeze and its impact on settlement and on piles of "existing" bridge.  I am considering lightweight fill (foam blocks), stone columns, and sheetpiling in combination with staged construction and wickdrains. The sheetpiling was the project manager's suggestion and must be evaluated.  I have neither given the thumbs up or thumbs down.  What are your thoughts?  Is sheetpiling to flexible to provide stress barrier?  Have you heard of sheetpiling used in this way and do you have any references?  Suggestions would be greatly appreciated.  

RE: Sheetpiling to isolate/reduce stress between embankment fills

I don't think you will get much stress relief unless the sheetpiling is constructed something like a cofferdam - so the lateral thrust doesn't cause flexure as much as a "hoop tension."  You'll need heavy sheet sections for this - and forget tie-backs.  Too much can go wrong, especially for bridge piers and abutments!

BigH and VAD have given you some good comments; I'm particularly fond of the "draw it up and look at it" comment.  It always helps me!  (And it would help me to better visualize your inquiry.)

RE: Sheetpiling to isolate/reduce stress between embankment fills

Hello Geonet:

Thanks for the additional information provided. The picture is now much clearer. However, I would appreciate if you can provide the detailed subsurface information for the site.

Based on the information you have provided I would venture to say that there should be no concern with the existing bridge piles resulting from the construction of the adjacent abutment fill. The design of the foundations of the first new bridge can be undertaken to account for the dragload resulting from soil settlement.These piles should be taken to competent shale. If there are concerns about the structural adequacy then piles can be bitumen coated.

Drilled pipe piles infilled with concrete may be more desirable for this site as driven piles may not reach to the depth of embedment desired. You should also use the unified approach advocated by Dr. Bengt Fellenius to undertake the design of the piles.

Settlement of the approach fill resulting in a bump at the end of the bridge would be of concern. However, I note that you will be using wicks or lightweight fill etc.

Just going by experience,unless the ground consists of very weak soils I do not think there would be a problem and hence, in my opinion, there is no need to bother about the sheetpile scheme. However, you need to discuss the issues with the Project Manager.

The above are my personal opinions as I see the problem of this site in a different light, but I could also be wrong.


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