drilled pier spacing

huangyhg

drilled pier spacing
does anyone have a recommendation on drilled pier spacing? i've read in a foundations book from college, "3 diameters center to center". does this make sense for belled piers also?
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i agree with your 3 diameter spacing (minimum). the spacing has more to do with how the soils carry the load than the pile / drilled pier type. put the piles / drilled piers too close together and you tend to overload the soils (whether they are point bearing or skin friction supported).
belled piers use end bearing.  spacing generally determined by bell dia.
dik
it depends on soil conditions, generally.  i've seen recommendations for 3 diameters clear, sometimes 3 diameters center to center, even some that allowed piers to be adjacent to one another.  the spacing recommendations should be in the same geotech report that gave you the pier recommendations in the first place, but absent that info, 3 diameters is a pretty good rule of thumb.  it also applies to belled piers.  use 3 times the larger diameter.
there are two issues.
1)  make sure they aren't close enough together that you get interconnection of concrete between the drilled shafts.  this is especially important when constructing nearby drilled shafts on the same day.
2)  if drilled shafts are too close together, then group effects come into play and the group tends to act as a single foundation.
in the first case, we normally recommend a minimum of 4 diameter from edge to edge.  in the second case, it depends on the number of piers in the group and the soil conditions.
to answer your question, the diameter would be the belled diameter.