Base Shear Impact on Foundation Micropiles

[rb2017]

Hello All,

I'm a project manager for a mid size construction firm in Canada that performs various shoring and ground improvement types of work. My current job involves the installation of about 40 each T52 micropiles at two benches along the side of a steep slope. The piles get embedded within the foundation footings at each bench and eventually a residence is built above.

The job is a bit unconventional as it has two different geo-technical engineers from two different firms involved; one engineer (A) is responsible for the slope stability assessment while the other engineer (B) is responsible for the micropile design. Engineer B was originally responsible for the slope stability assessment, however chose to opt out of this responsibility for various reasons.

Recently, the structural engineer for the job increased the base shear shown on the drawings by nearly 4 times the original amount (42 kips to 160 kips) due to a significant increase in the weight and layout of the building. My initial thought was that increase base shear equals increased lateral loading at the top of the anchor and that both engineers would be telling me that it would affect the design. Engineer A, however, is telling me that it won't affect their slope stability assessment while Engineer B is telling me that there will certainly be an impact to the micropile design.

In short, does an adjustment in base shear impact the design of a foundation micropile? Can it have an impact on the design of the micropile while not impacting the slope stability assessment? I've perused the web for hours and came across conflicting studies, hoping for some clarity on the topic and a more thorough understanding of how seismic forces affect micropiles.

Thanks!

 

[Shu Jiang PEng SE]

 

[MotorCity]

If there is an increase on the lateral forces on the piles, there will be an increase in the lateral force of the soil. The lateral force has to go somewhere, it will follow the load path into the soil. If the 160k (which by the way seems incredibly high for a residential structure) is spread over several piles, I would think it would have negligible impact on slope stability as opposed to if the 160k lateral force was only resisted by a few closely spaced piles. To me, it seems like it would depend on the number and spacing of the piles.