Odd Ball Sheeting Design

[bridgebuster]

We're working for a contractor and we designed a sheet piling wall that requires 35' sheet piles to support a 10' SOE. Today, the contractor said 35' sheets would be a special order. I can appreciate his dilemma because he has to drive 1/2 mile of sheeting and it's on the critical path. He's proposing to alternate 30' and 40' sheets, which are readily available. To me it sounds OK in theory but it would require some careful thought in the analysis. My question: Has anyone done this before?

 

[SlideRuleEra]

In my area, individual sheets for temporary cofferdams can vary in length because of subsurface conditions. Other than a minimum soil penetration, real-time driving results determine how deep temporary sheet pile are driven. Contractor does not stop driving a temporary sheet (or usually pairs of sheets) just because sheet pile tip has reached a certain elevation.

A 10' length difference between adjacent sheets sounds like a lot. Driving a 30' sheet next to a previously driven 40' sheet should be ok. I would not be so sure about a 40' sheet next to a driven 30' sheet.

For a half mile of sheeting, would not rely solely on calculations... let the Contractor try it, and see what happens.

If the Contractor drives the sheets in pairs, how does he propose pairing them?

http://www.SlideRuleEra.net

 

[BridgeSmith]

If the section modulus of the sheet piling is adequate for the 40' piles to carry the additional load that will be shed from the ones with lesser embedment, and the 40' sheet piles can be driven to depth without refusal, that would seem to be a workable solution.

On our projects, they would just use a 40' piece and drive it the 35'. Then they'd just leave it sticking up the extra 5', unless we made them cut them off because they were in the way of something else. Of course, usually our sheet piling is temporary, so they pull it and keep it for the next job or sell it, if they can.

 

[PEinc]

It's a little hard for me to imagine that 35' long sheets are required for a 10' cut, unless the passive soil is so bad that 25' of toe is required. If so, maybe the sheeting should not be cantilevered. It sounds like you may be applying a safety factor to the passive resistance instead of calculating the embedment required for moment equilibrium and then adding at least another 20% to the calculated embedment.

http://www.PeirceEngineering.com

 

[BridgeSmith]

The embedment seemed high to me as well, but I don't know the soil conditions, so I thought 'maybe he's dealing with really poor soils'.

What really surprises me is that the contractor is balking at 'special ordering' nearly 100,000 sq. ft. of sheet piling. For a few hundred or even 1000 sq. ft. I'd understand, but for that big of an order, it shouldn't matter.

 

[SlideRuleEra]

As a bridge contractor I was in the same boat as this contractor on more than one occasion. Job (at least the critical path work) cannot proceed without the specified piling and the piling specified are not available. The quantity of piling needed is too high, or out of the ordinary, for a steel supplier to have readily available and too small for a steel mill to give priority to in their rolling schedule. We had to wait weeks or months on some occasions for a mill to work in our order.

If the alternating pile length proposal does not work out, see if the engineer of record can substitute a different section and/or grade of steel that is available or produced by mills with more flexible rolling schedules.

Otherwise, in our case, contract provisions made unavailability of materials a legitimate cause for delaying the project.

http://www.SlideRuleEra.net

 

[bridgebuster]

Guys, I appreciate your comments. The geotechs made it work (probably to their disappointment) despite their initial rejection of the idea yesterday. They gave me a surprised look when I told them someone I know says they do this all the time down his way. . SRE, thanks for letting me know I'm not crazy, in this case.

PEinc - yes the tip is a little deeper than you would expect but the soil isn't the best and we're in a tidal zone and the DOT requires a factor of safety on the passive. We're raising a 4-lane road 6' in two stages. However, before we start raising the road, we excavate up to 4' to remove a layer of poor soil. The permanent water table is 3' below existing grade. After the first stage is built and we excavate for the second stage we wind up with 10' SOE.

 

[PEinc]

bridgebuster, sometimes you need to read carefully how the specs say to apply the passive safety factor. Some say to divide Kp by the safety factor; some say to provide a safety factor of 1.5. Remember, the passive resistance is a function of the embedment depth squared. If you calculate the embedment required for equilibrium using the full Kp, and then just increase the embedment by at least 20% (or the square root of 1.5), you will get a more economical design than by dividing Kp by the safety factor.

http://www.PeirceEngineering.com

 

[bridgebuster]

PE- I don't disagree with you on the approach. However, the spec is very clear that Kp is reduced by 1.25. We've done this before.

 

[MTNClimber]

I've seen sheets that long in bad soils for DOT projects. They may not absolutely need to be that long but when your hands are tied to a design spec you can't really skirt around it.

 

[bridgebuster]

Just got to go with the flow; but after doing DOT work for 40 years I can appreciate their aversion to risk.