Drilled piers adjacent to belled piers

[geodr]

I'm reviewing a design for an addition to an existing structure. The original structure is supported on belled piers, about 12 feet deep and 10 feet in diameter, bearing on med dense to dense alluvium. The column loads of the addition are such that deep foundations are required, and 36-inch diameter drilled piers on the order of 40 feet long are being recommended. These piers are designed for skin friction through the med dense to dense and stiff to very stiff alluvial deposits. Groundwater is at about 25 feet. The addition column lines put the new piers in very close proximity to the existing belled piers, less than 3 feet from edge to edge.

I am concerned about 1) hole stability of the new piers and inducing settlements of the belled piers during construction, and 2) interaction between load transfer of the adjacent skin friction and end-bearing foundations.

Any thoughts or recommendations ?

 

[GeoPaveTraffic]

I don't see how you can install the new piers within the zone of influence of the existing piers. Your two concerns are very well founded.

The new foundation elements should be installed at the same elevation as the existing belled piers. The new elements may still cause some settlement induced movement of the existing foundations, however, the effect can be predicted and the owner advised of the risk, etc. If the loads are too great for piers that will fit, you may need to consider a different foundation system.

Overall it sounds like a challenge. If you could provide some more information, someone may have an innovative solution for you. Either way, let us know how it turns out.

 

[VAD]

Hello geodr:

With recognition of the concerns by yourself and GeoPaveTraffic. These would be common for most of us. In terms of hole stability,it is possible to case the pile hole to below the depth of the existing foundation and then remove the casing or leave the casing in place after the concrete pour. This process would prevent problems with the existing foundation.

 

[GeoPaveTraffic]

I agree with VAD that casing the pier reduces the potential to cause damage to the existing foundations during installation of the new piers. However, it does not reduce the load induced settlements that will occur in the sands below the existing piers.

The expected settlements could be calculated and a determination made as to whether that amount of settlement of the existing structure is acceptable.

 

[Focht3]

I see a lot of agreement here.

Any thought given to replacing the affected belled piers with new deep drilled shafts? The structural details are a bear, but I have had a few projects that took this approach.

You might also consider chemically grouting the sands to reduce the risk of loss of ground. This is a good option for movement-sensitive structures, when the sands aren't too deep, groundwater isn't an issue, and the number of footings involved are small. The grouting would be limited to the area immediately around the new drilled shaft. And the chemical grouting operation would involve a minimum of risk to the existing structure. Anyway, it's a thought.



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[geodr]

Thanks for your comments all. Unfortunately there is not room to make the new foundations as belled piers at the same tip as the existing bells, although I agree that this would be the preferable option. Neither is it practical to replace the existing piers.

The two options I was considering were casing to approximately 20 feet, or using large diameter auger-cast piles. As you know, the casing option protects the hole from collapse during excavation, as long as the casing is pushed ahead of the excavation inside. If overdrilled to a depth above the adjacent belled pier tip and left as permanent casing, then load transfer should occur mainly below the casing, which addresses my concern of the belled pier and new pier transferring load via end bearing and skin friction to the same soil mass.

This may also be drawing way too far a line on it, but if temporary casing were used, I was also thinking about the stability of the hole filled with wet concrete just after the casing is pulled. Better than an empty hole or a hole filled with slurry, but not as good as soil with shear strength. Of course that is only a transient condition until the concrete sets (but geotechnical failures often occur during transient conditions !) On the positive side, the weight of the concrete resists hole collapse, and perhaps some arching could be expected around the 3-ft diameter hole 3-ft away from the 10-ft diameter loaded area (belled pier).

The auger cast pile option keeps the hole filled at all times (with an auger or concrete), but has the same issue as the temporary casing method, ie., the stability of the hole filled with fresh concrete until its sets.

So the drawback to the permanent casing option is that it extends the required pier length considerably, and leaves steel in the ground, which both cost $. The drawback to the temporary casing and the auger cast piles are that they load the same soil mass as the end bearing belled piers, and the question of the wet hole stability.

Any other ideas or comments ?

 

[Focht3]

I'm concerned about the lower 15 ft of shaft that will be below the water table. Given the other construction concerns, I'd suggest limiting the drilled shafts that will be close to the underreamed piers to 25 ft deep.

And I would prefer sacrificial casing to auger cast piles. I like to be able to see what's going on when dealing with old/new foundation issues -



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[Focht3]

One other comment: you should suggest that the contractor be required to "unload" the existing drilled piers until the adjacent shafts have cured for at least 48 hours. This will protect against a foundation failure as the work is progressing...and give you the luxury of time if a problem develops.



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[geodr]

Focht,

I'm not sure I understand your concern about piers below the water table - is it a general concern, or is it due to the adjacent piers ? Generally speaking, I don't have any concerns about drilled piers below the water table - we routinely construct those very successfully. In this case there are no loose or running or sugar sands or silts to worry about. I too prefer the permanent casing option, but the cost is quite high. To address your other comment, the existing adjacent belled piers are supporting a building, so they can't be unloaded.

 

[Focht3]

The concern is not the water per se, it's the combination of risk factors (limited work space, nearby footing, drilling in sands, drilling below the GWT.) Just suggesting that you eliminate risk factors where possible, that's all. (I've put plenty of piers in below the water table, including 120 long 30 in. dia. drilled shafts - only cased to about 40 feet. The bottom 25 or 30 feet of the shafts were drilled sans casing in a very dense waterbearing sand. We used fresh water as our drilling fluid to "balance" the piezometric levels.)

Since you can't take all the load off I'd suggest that, as the work progresses, you use temporary shoring beneath the beams that are transferring dead load to each underreamed pier at risk. It's another effort to reduce the possible consequences of a construction problem during drilled shaft construction. It's not a requirement to do so, just a prudent thing to discuss with the owner and design team. The owner may not want to do it because of cost; but if you don't raise the trial balloon and something bad does happen...



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.