Stiffness and Elastic Shortening re: Micropiles
Stiffness and Elastic Shortening re: Micropiles
(OP)
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.
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.





RE: Stiffness and Elastic Shortening re: Micropiles
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.
RE: Stiffness and Elastic Shortening re: Micropiles