Lateral Resistance of Cast in Place Concrete Piers

[longhorn1]

I'm currently working on a project that involves a tall retaining wall that is resting on top of cast in place concrete piers. The lateral pressure of the the soil applied to the retaining wall is creating a large (75 kip+) lateral load at the top of the pier which must be resisted by the soil pressure surrounding the pier. In order to determine the depth of the pier required to resist the lateral loads, I have calculated a depth based upon a soil resistive pressure of R=1000psf. The depth required to resist the lateral load is +/- 35 feet with a 30" diamter pier. This depth seems unreasonable to assume that the pier will remain vertical while soils at this depth are required to provide lateral resistance. Has anyone encountered a similar problem with forces of this magnitude? If so, is there a way to analyze this without having to create a pier with an extremely large diameter in order to reduce the depth required to resist the lateral loads?


Thanks In Advance,

 

[austim]

Longhorn1,

See if you can unearth a copy of Foundation Engineering, by Peck Hansen and Thorburn. Article 22.6 "Pile supported retaining walls should help you along. page 371 in the 1953 edition.

Simply, you will get nowhere trying to take the horizontal load on vertical piles/piers of any reasonable size. Instead, take the load in axial compression and tension on raking (or 'battered' if that is your more common usage) piles.

 

[Ginger]

Analysing laterally loaded piles can be done using Broms itterative method, but I agree with Austim about using raking piles plus the tension capacity of the vertical piles.

The British Steel Piling Handbook 7th Edn offers outline guidance on what we call relieving platforms.

Regards Andy Machon

 

[JAE]

I would urge you to seek out further references as per austim above.

You didn't mention whether the piers extend to the top of the retaining wall, thus also taking the bending in the wall. Keep in mind that the lateral force in the wall is not centered at the top of the pier, but higher on the wall (1/3 wall ht. above the pier) so you not only have lateral force, but moment at the bottom of the wall as well.

You're correct that the pier will not remain vertical - what happens is at some depth there is a point of rotation where the pier will actually develop compressive forces in the soil on the back side of the pier below that point.

The lateral force will be partially taken by the pier, but only near the top of the pier. The remaining (majority) of the lateral load should be resisted by dropping the bottom of the wall into the soil (below the "dredge" line) to develop adequate passive resistance in the soil. Short of this, the only other way is to use tie-backs into the earth behind the wall.

 

[ayfarm]

I would use lateral base anchors to resist shear demand of wall.