Caisson Backfill for Soldier Pile and Lagging

[EngMan40]

Contractor wants to backfill the caissons for the soldier piles with stone or sand instead of 100 or 200 psi flowable fill. He thinks this is the only way he can retrieve the steel piles. The original design of the 12 ft cantilever system was with 2 ft diameter, 13 ft long caisson backfilled with flowable fill. The material around the caisson is dense to very dense sand and gravel. His design is 2 ft diameter, 5 ft long caisson filled with AASHTO No. 57. or No. 10 with a raker at top. My initial thought that the backfill material in the caisson will not be as compacted as the natural soil and it will reduce the width of the embedded part to width of pile with reduced friction angle, . Anyone analyzed such system or saw it constructed before? your thoughts.

 

[jayrod12]

I agree, you won't get appropriate compaction so you'll end up with a fair amount of rotation taking place prior to getting the resistance level you need. I'd hold firm on the lean mix fill.

Around here the shoring contractors budget for losing the soldiers and if they end up being able to recover them it's considered a bonus.

 

[BridgeSmith]

I'd require them to provide a design showing that their proposed solution works, with deflections that are within the project requirements, if they want to deviate from what was contracted. Presumably, they bid the job based on the lean concrete backfill, so you're not obligated to let them deviate. Likely, they'll just drop it and use what was specified.

They probably can't do it with the specified depth, but if they want to go deeper, it might work. If they can show that it will work, and they're willing to split the savings, or at least pay for the time it takes to review the revised design, you could consider it. Depends on whether you care to have a friendly relationship with the contractor, or not.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Can't the contractor withdraw the steel piles right after the flowable fill has set?

 

[BridgeSmith]

Can't the contractor withdraw the steel piles right after the flowable fill has set?

I don't see how you can have a soldier pile retaining wall if you remove the soldier piles during construction.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BridgeSmith]

Generally, the progression of construction for this situation would be to drill the holes, set the piles in the holes, grout the holes up to planned dredge line, let the grout gain the required strength, and then excavate in front of the wall and drive the lagging progressively down to the dredge line. If it's a temporary wall, they remove the lagging and backfill progressively as well (or excavate behind the wall, if the new finished grade is at the dredge line. Regardless of the construction progression, the grout has to set up and get fairly strong before excavation can even begin, and will be fully cured before the wall could be removed.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Enable]

Agree with BridgeSmith: soldiers are to be embedded in flowable, solid concrete from the depth of the caisson to the excavation floor; with the top portion being U-fill so it can be dug out for placement of the lagging. Though, it sounds like the OP is specifying U-fill for both above/below the excavation floor. Depends on how concerned you are with deflection I suppose. In either case, do not let them use gravel.

If the contractor wants to retrieve the piles, they can drive them instead of using the caisson method. Note: driving causes vibrations which may or may not be a concern depending on adjacent structures. Also, still may not be economical to retrieve.

 

[GeoPaveTraffic]

Short answer is that you can't retrieve the soldier piles in a soldier pile and lagging wall. Just that simple, it can't happen. The piles should be cut off several feet below grade once most of the excavation is backfilled.

Mike Lambert