Battered Pile Retaining Wall Design

[craigmcg]

I have a question regarding the design of a batter pile that braces a sheet pile retaining wall through compression. In reviewing a couple designs performed by other engineers, I am tending to disagree with their analysis methodology, which is similar to how a truss is analyzed. This is as follows:

1. Obtain lateral reaction force from soil loads on sheet pile wall.
2. Load transfers to top of batter pile through sheet pile cap as a horizontal and vertical force component.
3. Length of batter pile is then designed to the axial capacity of the soil. This is idealized as a pinned connection at the pile tip, and therefore a truss member is theoretically possible.

Like a truss, the loads are applied at the ends and the pile becomes a simple tension/compression member. This would be fine, except the embedded portion of the pile will experience midspan loading from the soil restraint, and a moment will therefore occur. Does this sound about right?

 

[PEinc]

For inclined raker braces supporting a sheeting wall, I calculate the horizontal wale load per LF of wall. I chose my brace spacing. I multiply the horizontal wale load per LF by the horizontal brace spacing and divide it by the cosine of the brace angles in the vertical and horizontal directions to get the axial brace load. I then calculate the unbraced length of the raker brace. I pick a trial brace size from the AISC column tables based on the axial load and the unbraced length. Then, if the brace is long or if the trial brace is near its maximum tabulated load, I may increase the trial brace by one size. Then, I check the brace for combined stress from the axial load and bending from its own weight and any surcharge that might be applied or built up on the brace. There is no truss analysis. Then, I design a concrete heel block for the bottom end of the brace. I try not to use driven piles as raker braces.

The most important thing is to design a roll chock for the wale at each brace location to prevent the wale from rolling over due to the upward pushing action of the brace.

 

[kslee1000]

Craig:

If I read your post correctly, your concern is the moment at the pile-pile head interface. Traditionally, a pinned connection is assumed true for pile extends only a limit (small) amount into the pile head, thus the moment, if any, is negligible. It allows the pile to be treated as axially loaded member to simplify the design.

In the other hand, you will need to provide a lot more to achieve a fixed head pile. And then you would have to deal with soil-pile interaction to complete the design.

Pinned, or fixed, really depends on the joint details.

 

[DaveAtkins]

No one seems to be answering craigmcg's question.

craigmcg, I would say soil pressure is equal and opposite on all sides of the batter pile. Therefore no moment. Does that make sense?

DaveAtkins

 

[kslee1000]

From 2nd read, Dave you are right.

 

[BAretired]

Like a truss, the loads are applied at the ends and the pile becomes a simple tension/compression member. This would be fine, except the embedded portion of the pile will experience midspan loading from the soil restraint, and a moment will therefore occur. Does this sound about right?

You have not indicated what type of pile you are using. It could be drilled and belled, drilled straight shaft or driven pile. The soil restraint will be skin friction and/or end bearing. In either case these forces will be directed parallel to the pile and centered on the pile center. The pile, if drilled, will completely fill the void so that soil pressure will be transferred through the pile to the soil below.

The only way you could get bending from midspan loading would be for the soil mass to settle within the pile length and move the mid section of the pile.

Best regards,

BA

 

[kslee1000]

Further thoughts on this:

Practically - no bending in the battered pile.
Theoretically - possible, but negligibly small.

The battered pile would have internal bending moment due to selfweight (similar to stairway). However, it is offset somewhat by the differential soil pressure along the pile (cut a virtical plane through the battered pile, the soil pressure at the lower interception point is always r(Z2-Z1) higher than the upper interception point.

r = soil density
Z2 = elevation of the lower interception point
Z1 = elevation of the upper interception point

Making sense or not? Please comment.

 

[PEinc]

craigmcg needs to provide better information. My guess is that we are all somewhat confused. craigmcg said, "... a batter pile that braces a sheet pile retaining wall through compression." If the raker brace were driven into soil behind the sheeting wall, it would provide wall support by tension. Therefore, I assume that the raker brace is on the excavation side of the sheeting wall. Therefore, it is in compression. Since the brace is on the excavation side of the sheeting wall, there is no soil surrounding the brace except if the brace is driven below the excavation subgrade. In my experience, driving a raker brace is not typically done. The lower end of a raker brace is usually connected to a concrete heel block or is attached to some other structure, such as a new or existing footing.

 

[BAretired]

craigmcg,

A sketch would help us understand the problem a little better.

Best regards,

BA

 

[beton1]

Agree with PEinc, usually concrete heel. I too am confused.