Stiffness and Elastic Shortening re: Micropiles

[tocoadog]

Everyone
Long story short, City of Albuquerque, NM, US, is looking to add 2 lanes of bikepath between 2 opposing lanes of a 2 span bridge. Area is tight (less than about 20 feet) and utilities are very near. Pipe piles due to vibrations are out, imo.

The structural has asked for micropile recommendations. Makes good sense to me. Anyway, I've never been exposed to the foundation. Before going on vacation, an engineer in the company emailed me saying that micropiles are more elastic (lower stiffness) than shafts and large piles and that at higher working stress, the elastic shortening should be evaluated.

Now my questions are:

Stiffness is EI. So, with a smaller I (smaller diameter pile), the pile has lower stiffness? And, this is really bad, what role does the stiffness play in the development of the pile? I should know this, but for the life of me I can't seem to remember what role flexural rigidity plays in deep foundation design. I've even consulted my books.

Also, elastic shortening is the reduction of the length of the pile. Now, what causes elastic shortening? And is this a worry for other pile foundations? I assume this is determined through a pile load test with a tell-tale...?

Thanks for the input.

 

[Panars]

The flexural stiffness of the piles is determined from EI, but the vertical "stiffness" is determined from EA. With a smaller diameter, the micropile will be less stiff both in bending and elastic compression. Since the structural is asking that elastic shortening be evaluated, I think he/she is concerned about the vertical stiffness.

Flexural stiffness of a pile can be used to resist lateral loads. A finite difference program like LPILE or COM624P is usually used to analyze laterally loaded piles, but there are simpler analysis methods that don't require a computer. One source is the Naval Facilities Design Manuals 7.1,7.2, and 7.3.

Elastic shortening of the pile is one component of pile settlement. It is usually quite small at working loads and not significant when compared to settlement of the pile. However, micropiles can support very large loads for their size, and so elastic shortening might be a concern.

For your application, have you considered augered cast-in-place piles? These are much cheaper than micropiles, although they do not have as high a capacity.

 

[DRC1]

If you have a beam with several columns, and some of the columns are less stiff than others, they will deflect more. Thus if a uniform load is placed on the beam, the more elastic beams will deflect more under a given load tha the stiff ones. Thus the elastic beams will carry less load and the stiff ones more than what would be computed by a standard beam equation. (because the assumption is made the supports are equally rigid). The actual load in the columns can be computed, although it is not as straight forward. Also expansion joins and other details can be used to control load transfer.