Drilled Pier modification

[SLMPicker]

Does anyone know or have experience in modifying existing drilled pier foundations to accomodate loading above the design loading. I have a pier (6' dia. x 17' deep) that is designed for a particular moment, and would like to know what to recommend for modifications to increase the foundation capacity. Space is limited at the location. Thanks in advance.

 

[ishvaaag]

In my view I no doubt will deal most surely with this kind of problem with the help of a rigid sleeper beam on the ground, most surely connected to other piles, preferably in alignment. Then the pile I wouldn't touch except if no other solution there is.

 

[JAE]

The first thing I would do is re-visit the original design and design parameters. Perhaps a geotechnical engineer could re-study the surrounding soil and verify, or increase, your soil parameters which would possibly give you additional capacity.

 

[emmgjld]

Reviewing the original design recommendations, combined with the actual construction records may reveal some latitude for your increased loading.

Original geotechnical recommendations usually have to begin with limited subsurface holes, limited sampling, possibly less than desirable sample quality. These initial limitations are interpreted in a manner which assumes the slightly below average subsurface conditions and account for some problems with construction. Each 'Geotech' has a private safety/uncertainty allowance.

The actual construction may prove the wisdom of the 'Geotech's' unwillingness to 'go to the line' or may provide reason to allow a greater actual working capacity of the foundation element.

Now assuming the as-built working capacity cannot be sufficiently increased, it may be possible to improve the foundation soils with remedial techniques, such as Compaction Grouting. Additional elements, such as pin piles or helical piers or Ishvaaaq's additional surface beams or connectors may be used.

 

[Ron]

In addition to emmgjld's comments, you might consider a "collar" on the shaft. This will increase the upper moment capacity, the shear capacity,provide additional compressive load capacity and provide additional "ballast" for overturning or uplift. The collar would have to be dowelled into the existing shaft to affect interaction.

I have used this technique for tower shafts. Have also used helical anchors with reasonable success.

Be sure you do some investigation to determine the geotechnical parameters as JAE suggests and some investigation to find the actual physical dimensions/characteristics of the shaft. There are several nondestructive techniques useful in determining these things.

 

[SLMPicker]

Thanks, Ron, a collar is exactly what I was considering. One thing I am curious about is how to dowel the collar into the existing pier. Can you qauntify the additional capacity, or explain the behavior of the system?

 

[DRC1]

I had to support a temporary bridge off a drilled pier foundation. The bearings could not be located over the center of the drilled piers, and induced a significant moment in the piers. I installed tie backs with walers in front of the pier. Prior to tensioning the tie backs, I installed some dead man sheeting behind the piers and braced the pier to the sheeting. This prevented inducing moment in the opposite direction. By having a brace, the pier went from a cantilevered condition to a anchored condition with a higher moment capacity. It did not require space in front of the pier (except for the waler) but did require room behind the pier. Also this only works if the moment will be only ever be in the same direction.
Good Luck!

 

[Dfcmark]

Scot The best possible option would be to install mini piles eith beside or through the existing drilled pier. Mini (or micro or pin ) piles can be installed around the perimeter. they are capable of taking loads in the order of serveral hundred kips in both tension and compression. You can look fro literature by the Deep Foundations institute or the association of drilled shafts contractors.

Good luck

Mark Montgomery
deep foundations contractors inc.
Thornhill On Canada
mmontgomery@deepfoundations.net