Reduction in Lateral & Axial Capacity of Closely Spaced Piers 1

[GTOFAN]

Can anyone provide general capacity reduction guidlines for closely spaced piers, for both axial and lateral resistance??
Thanks!!

 

[JAE]

For drilled piers, many geotechnical engineers have offered me this:

For piers spaced 3 x pier dia. or more apart - use 100% of the given capacity.

For piers spaced between 1.5 x dia. apart - use 50% of given capacity.

For piers spaced between 1.5 and 3 times dia. apart - straight line interpolation.

I don't have any knowledge of how lateral capacity is affected by spacing.

 

[eric1037]

This is a fairly complicated subject and there are differing views on group effects.

It depends on soil type, spacing, time between driving (for driven piles) and several other factors. There hasn't been satifactory investigation into pile group capacities to establish hard equations. The Coduto Foundation Design book has a very good discussion on the topic. Also, Michael O'Neill authored a paper in 1983 entitled "Group Action in Offshore Piles", from Geotechnical Practice in Offshore Engineering, ASCE.

 

[SlideRuleEra]

Here is Professor Broms' summary of pile group efficiency is both clay and sand. Some of his charts give a nice overview

http://www.geoforum.com/knowledge/texts/broms/listchap.asp?Chapnr=n

http://www.SlideRuleEra.net

 

[sundale]

I have seen the same reductions for vertical capacity that JAE lists. If they are closer than 3 diameters, then ask your geotechnical engineer. Sometimes the geotechnical engineer will give you different reductions for end bearing and side shear values.

In soils reports I have read, I have typically seen 8 diameters as the threshold for no group action effects. Laterally loaded piers are fairly voodoo as a single element. Group action seems like it would be quite complicated.

 

[vulcanhammer]

Some of this can be found in the FHWA's book on driven pile foundations, available at

http://www.vulcanhammer.net/download/driven-piles.php