Soldier pile retaining wall using a waler with off-centered tieback

[designrider]

Consider a soldier pile retaining wall that uses a waler with a single tieback to support 2 adjacent piles. Traditionally such a tieback would be centered between the piles. However, due to project constraints the tieback needs to be installed off-centered. For example, with a 6ft soldier pile spacing the tie back is installed 2ft from the left pile and thus 4ft from the right pile. How will this change the loading to the tieback. To maintain static equilibrium in the design case described, I calculate that the tieback force will increase by 50% as compared to a tieback centered between piles. Can someone confirm or correct me?

 

[PEinc]

That is correct. However, the tieback force to the other soldier beam, closer to the tieback anchor, will be greater than originally designed.
Example: If you need 100 kips per each soldier beam, that would be a 200 kip tieback centered on the wale, 3' off each soldier beam. If you move the tieback over 1 foot to be 2' off one beam and 4' off the other, you need a 300 kip tieback design load (300 / 200 = 150%). Then, 100 kips will go to one soldier beam but 200 kips will go to the other.

http://www.PeirceEngineering.com

 

[EngineeringEric]

PEinc, If the total load of the system (2 piles@ 100k) is 200 kips... then why does the system need to resist 300 kips? Where does the extra 100 come from? I am curious and am looking at this from a structural view so sorry if the answer is obvious :\

 

[designrider]

EngineeringEric, your comments were my initial confusion too. However, for static equilibrium you the sum moments about any point must equal zero. This is an odd case. For the described conditions to be a balanced system the left soldier pile becomes a restraint instead of an applied load. The direction of soil loading on this pile is essentially reversed as it engages the soil in passive resistance to restrain waler rotation. Therefore, not only do the tieback forces increase by 50%, the bending forces on this soldier pile double!

 

[PEinc]

The bending moment on the pile with the higher tieback force will depend on the stiffness (bearing capacity, passive resistance) of the soil behind the beam. If the soil is weak, yes, the pile will bend more. If the soil is still, the increased load may not affect the bending of the beam. We often offset tieback anchors without damage to the soldier beam.
See the attached calc for the tieback load and forces applied to each soldier beam.

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[PEinc]

Correction: I meant, "If the soil is soft...." (not still)

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[designrider]

PEInc, thanks for your added comments. I agree, the effects on the soldier pile is not a direct multiple. In the off-centered case, the waler is now pushing the pile inward and resisted by passive soil pressures. The shape of the soil pressure distribution diagram will depend on soil parameters and will not be a direct scaled shape of the active pressure distribution diagram. In soft soils this design condition should not be ignored.