Sheet pile for scour protection

[Carnegie]

We’re trying to resolve the forces acting on a sheet pile wall. If you look at the attached pdf we believe the effective width of the concrete arch and the soil wedge of select structural fill on top of the arch can be analyzed as a line load distributed through the 8’ footing. We’re not so sure the 16’ of indigenous fill should be analyzed as surcharge acting on the wall. If anyone has any suggestions it’ll be welcome. Thanks.

 

[PEinc]

Although there are a few different ways to model this design, I would design the sheeting for an 8' wide strip load surcharge from the arch's footing (unless pile supported) plus an area surcharge load behind the strip surcharge. The area surcharge would be the weight of the 16' overburden soil plus the expected live load traffic surcharge. You may want to check the surcharge using both a rectangular qKa analysis and a Boussinesq analysis. You also need to use the after-scour height of the sheeting wall. Also, consider if there is any lagged water head (hydrostatic pressure) to be added to the sheeting design. I would also assume that the sheeting will need to be anchored back to prevent excessive deflection and settlement of the arch footings, especially if the design height of the sheeting exceeds 8'.

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[Carnegie]

We had thought about using the 16' of soil as a surcharge but then figured it would act beyond the shear plane of the soil. If anything an inclined wedge of soil behind the frame (acting on the footing) chould be added into the line load. We'd also include the NY State standard 250psf surcharge. Any thoughts? Thanks.

 

[cvg]

As PEinc suggested, you should check this with full saturation behind the sheet up to the original invert of the channel (elev=229.0) or perhaps higher.

 

[Carnegie]

Thank you for your inputs. Just to let you know the top of the sheet pile wall is "composite" with the side of the footing using shear studs. The 8' depth is the calculated scour depth.

When you say that the 16' should be looked at as a block do you mean a 16'x16' block of soil? Some people at the office didn't feel the 16' surcharge acts on the sheeting directly but on the frame itself and puts a resultant force above the sheet pile (transferred from the side of the frame to the footing to the sheet pile)

 

[PEinc]

The location of the failure plane (with respect to the 16' earth surcharge) depends on the height of the sheeting wall and whether the plane starts at the scoured dredge line or at the tip of the sheets. The traffic surcharge should be applied to the top of the 16' earth surcharge and not to the strip load from the arch footing. The maximum footing bearing pressure should already include the traffic surcharge.

If the earth surcharge and the traffic surcharge are beyond the failure plane, then a qKa rectangular surcharge should not apply. However, this could be a hard sell to the highway department. If you use a Boussinesq surcharge analysis, the effect may not be too bad because of the distance behind the sheeting.

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