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RE: Drilled Pier Spacing

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Title: RE: Drilled Pier Spacing
Hello All,
Here in Colorado is a common problem to encounter swelling soil. The lenght, depth, diameter of the pier , penetration in the bed rock and the soil properties (soil bearing, skin friction, Uplift) will determine the maximum bearing capacity of the pier. which will determine the spacing between piers.
Ahcene Djebli
Project Manager I
Fort Collins, CO 80521
Tel: (970) 484-0359
-----Original Message-----
From: Adair, Joel [mailto:JAdair(--nospam--at)]
Sent: Monday, November 04, 2002 11:05 AM
To: 'seaint(--nospam--at)'
Subject: RE: Drilled Pier Spacing

I would think that you ought to be fine with your 12'-14' spacing, but I'd confirm with your geotech.  Typically, the geotech gives us guidelines regarding pier spacing that sound something like this: "With 3 pier diameters clear between piers, use the full allowable soil values (friction and end bearing).  At zero clearance (piers touching), allowable skin friction is zero.  Linearly interpolate for spacings in between."  Obviously, 12' is significantly greater than 3 pier diameters, so it's unlikely you'd have any reduction in pier capacity due to pier spacing.

-- Joel
Joel Adair, PE
Halff Associates, Inc.
E-mail: jadair(--nospam--at)

-----Original Message-----
From: richard lewis [mailto:rlewistx(--nospam--at)]
Sent: Saturday, November 02, 2002 4:33 PM
To: seaint(--nospam--at)
Subject: Drilled Pier Spacing

I am doing a drilled pier foundation with grade beams in expansive soil.
I typically don't do this so I am trying to find out some common design
practices.  Basically I want to know what might be the target spacing of
the drilled piers.  It is commercial construction.  What would be a good
first trial value?  I was thinking of trying to stay in the neighborhood
of 12 - 14 ft. o.c. and use 18" dia piers.  They need to go down about 12
feet into the soil.  Should I be looking at a larger spacing of 18 - 20
feet?  The wall construction is concrete filled foam forms.

The advantage of the shorter spacing is the system is stiffer with less
deflection, lower shear values.  The disadvantage is more drilled piers,
higher deflection and shear values.

Any other suggestions would be appreciated!




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