Bending Stress in Battered Micropiles

[peelwvu]

I'm working on a project using vertical and battered micropiles to support a bridge pier. The lateral load of the pier is designed to be taken by the batter of the piles. In the most recent design runs the worst case ratio is roughly 15 K load vs 70 K resistance factored using the horizontal component per battered micropile. The design program was set use 0% of vertical piles for lateral resistance. My question is: Will there be a bending moment on the battered piles? Do I need to check the bending stress capacity of the micropile itself? The max. axial load is around 115 K. The micropiles are 9 5/8" casing with a #20 bar (2.5" OD)

Thanks

 

[PEinc]

You should not have to design for bending in battered micropiles that are fully surrounded by soil and/or rock. If there were lateral load causing bending in battered micropiles, you could make all of the battered piles plumb piles with lower axial loads and design them for combined axial and bending. The battered piles need extra axial capacity to provide the horizontal component to resist the footing's lateral load. For example, when you design an inclined raker brace, there is not any bending to design for, except for the weight of the brace where there in no soil around the brace. A micropile is similar except that there is soil around the pile to support the pile weight.

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[peelwvu]

Thanks PEinc. You don't happen to have a reference for this do you?

 

[PEinc]

No. I also don't have a reference calling for combined stress analysis of a battered micropile but I'll will take another fast look. M.J. Tomlinson addresses this in Sec. 6.4 Lateral loads on raking piles, in his book, Pile Design and Construction Practice. I'll get you a quote in a while.

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[PEinc]

Here is the quote from Tomlinson:

"For economy, the raking piles should be installed at the largest possible angle from the vertical. This depends on the type of pile used. (see 3.4.10). Where raking piles are embedded in fill which is settling under its own weight (Fig. 6.55a) or in compressible clay subjected to a surcharge load or to superimposed fill (Fig. 6.55b) the vertical loading on the upper surface of the rakers may induce high bending moments in the pile shaft. Because of this, raking piles may not be an appropriate form of construction in deep fill or compressible layers."

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[peelwvu]

Thanks. I appreciate your help PEinc.

 

[PEinc]

You can also go to the FHWA geotechnical publications web site and look for their micropile manuals. There, you will see discussion on battered micropiles and bending.

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[PEinc]

Federal Highway Administration Publication No. FHWA NHI-05-039 December 2005


5.19.4 Battered Micropiles
The lateral load-carrying capacity of individual vertical micropiles is limited compared to
more conventional drilled shafts and driven piles. As for other deep foundation systems,
micropiles can be battered as a means to provide additional resistance to lateral loading.
Note that since micropile drilling equipment is designed to drill both vertically and
subvertically, there is not a significant increase in cost (on a per unit length basis) or quality
of finished pile for battered micropiles as compared to vertical micropiles.
The calculation of lateral capacity of a single battered micropile is demonstrated in Sample
Problem No. 1. The total lateral capacity of the battered micropiles is the sum of the lateral
capacities of each battered micropile. This lateral capacity is compared to the lateral load
applied to the foundation from the superstructure. If this sum is less than the lateral load
applied to the foundation, the remaining load is carried by the micropiles in bending (i.e., this
remaining load is used in a lateral load (i.e., LPILE analysis) for the structure).

Moreover, battered micropiles should not be used where the potential for ground
settlement around the battered micropile is a possibility. These settlements will induce
additional bending moments in the micropiles and some loss of support. Additional
discussion on batter piles for seismic regions is discussed in Section 5.21.

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