Drilled Shaft Group Efficiency

[moe333]

I am trying to find a reference for reduction in axial capacity of a drilled shaft due to group action. Many of the references I have looked at discuss it vaguely but none I can find provide recommendations. What I can find is that they should be spaced at least 2.5 to 3 diameters minimum, and that there is no reduction for spacing greater than 7 diameters. I am also looking for reductions in uplift alone.

My specific application is 18 to 30-inch diameter drilled shafts embedded 20 to 50 feet in weakly to moderately cemented cemneted siltstone/sandstone, no groundwater. Groups of up to 4. The capacity of the shafts will be from end bearing and shaft friction in the lower 5 to 10 feet.

Thanks for any references.

 

[civilperson]

Group capacity factors are used for driven piles that use skin friction as a component of bearing capacity. Can be greater than one in sands and less than one in cohesive soils. Drilled shafts are usually end bearing and do not need any reduction in cohesive, (no densification of sands occurs so no increase for group factor).

 

[JAE]

moe333,

I've seen numerous geotechnical reports that provided me with drilled pier capacities and recommended the 3 x diameter spacing. Most of those would then go on to say that if I had closer spacing - we could go to 1.5 x diameter and use 50% of the capacity....interpolating between 50% and 100% from 1.5x to 3.0x.

 

[BigH]

There is the FWHA Report on Drilled Shafts - it is too large for me to download by my phone line (122 MB) but you should be able to do so (third publication from top at the following URL):

http://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm

 

[moe333]

Thanks BigH but I have that publication and it does not provide specific recommendations.

 

[Panars]

The AASHTO LRFD specifications (4th ed.) state that the efficiency factor for groups of drilled shafts in choesionless soil should be 0.65 for center-to-center spacing of 2.5 diameters, and 1.0 for spacings of 4 diameters or more. You should linearly interpolate for spacings between 2.5 and 4. For clays, you consider the soil block enclosed by the drilled shafts as a single foundation element.

In the commentary it notes that these recommendations are based on theoretical considerations and on limited load test results. They refer you to O'Neill and Reese (1999) for additional details. This is the FHWA Drilled Shaft publication.

AASHTO does not specifically address drilled shafts that are bearing in rock. In general, I would not use a group efficiency factor for drilled shafts socketed into rock.

 

[minorchord2000]

Do the group factor reductions apply to shafts only under axial loads, or does it apply for shafts which also resist lateral loading?

 

[jdonville]

minorchord2000,

The recent AASHTO LRFD specifications, as well as the most recent FHWA Driven Pile Manual (not available for free download), address the latest research for pile group efficiencies resisting lateral loads.

Jeff

 

[moe333]

Panars, are the efficiencies you reference for uplift as well?

 

[Panars]

minorchord2000-

The group efficiency values I gave above are for axial compression loading of drilled shafts. The AASHTO LRFD specification gives different values for lateral loading.
These are:

Center to Center         P multiplier
Spacing            Row 1     Row 2      Row 3
  3B                0.7       0.5        0.35
  5B                1.0       0.85       0.7

moe333-

No, the values are for axial compression loading only. For uplift of pile groups and drilled shaft groups, the AASHTO LRFD specification uses the lesser of the sum of the individual piles (shafts) or the uplift resistance of the soil enclosed by the pile group. For cohesionless soils, the uplift resistance is the weight of the soil lifted by the piles. This is a block of soil bounded by a rectangle at the pile tips (assuming the group is rectangular) and with sides that are sloped 1H:4V, getting wider towards the ground surface. This makes a inverted truncated pyramid. For cohesive soils the uplift resistance is the weight of the soil block enclosed by the piles plus the shear resistance along the sides of the pile group.

The reference given for uplift of pile and shaft groups is Tomlinson, 1987, Pile Design and Construction Practice.