Increase capacity of existing drilled friction piles

[canwesteng]

Has anyone heard of a method to increase the capacity of existing piles? We have some piles under a building that are currently overloaded. Options are to either do a ton of work to add adjacent piles, put in a dummy pile and load test, or this mystery method that eng tips will come up with.

 

[PEinc]

This won't be cheap or fast, but maybe you can drill beside each pile, one at time in a pile cap, and grout in a regrout tube similar to those used for regroutable tieback anchors. Then, regrout through the tube to increase the pile confinement and adhesion to the pile. This would required a lot of jumping around so that you did not disturb too many piles at one time in any pile cap. Not sure how you can prove the added capacity. There are two capacities to consider - geotechnical and structural. The pile itself structurally has to be able to hold the increased load.

http://www.PeirceEngineering.com

 

[oldestguy]

I'd drive a grout tube on an angle to get under each pile or pile groups, assuming there is better support somewhat below the pile tips. I'd then do compaction grouting under the pile tips. It may take several "layers" of compacted ground and grout zones to "do the job". I'm assuming this can be done from outside the structure, threading the injection tubes between pile groups also.

 

[canwesteng]

Would compaction grout below the tips of the piles not potentially lift up the piles? I like the idea of grouting around the piles, but it's going to hard to quantify the extra capacity.

 

[PEinc]

If the piles are friction piles as indicated in the thread's title, I'm not sure that compaction grouting below the pile tip would increase the friction capacity. It may help the end bearing. To increase the capacity of each friction pile, I think you would, unfortunately, need to drill through the pile cap and down along the side of each pile. Angling from outside the cap may not be as effective as you would like.

http://www.PeirceEngineering.com

 

[MTNClimber]

What's the subsoil profile look like and what type and length are the piles?

 

[canwesteng]

Sub soil is poorly compacted fill (5'), medium hard clay (20'), then hard stiff clay (20'+), with a later of weaker sand. Piles are anywhere from 40' to 60' feet deep.

 

[MTNClimber]

You may have trouble getting increased capacity in the clay using compaction grouting methods. I would say a load test would be your best bet. Maybe everyone is underestimating the capacity? How do you know they are overloaded?

 

[BridgeSmith]

MTNClimber beat me to it - that was to be my question: Are they actually overloaded (actually sinking) or are they theoretically overloaded, according to assumed friction values?

If they're actually overloaded, compaction grouting will likely only be effective in the sand layers. If they're far enough apart, adding shafts in between the existing and pouring a cap over all of them may add capacity.

 

[canwesteng]

Only theoretically overloaded. I have no doubt a load test will put us in the clear.

 

[HENRYZAU]

Any on-going settlement/cracking observed? You need evidence to confirm the piles are overloaded. If yes, installing additional micropiles appear a workable solution, with cost/time implications.

 

[aeoliantexan]

Load testing seems attractive, because after you use one of the remedial approaches, someone is likely to say, "yes, but how do you know you gained the required additional capacity?".

You haven't told us what kind of a building it is, but perhaps you can load test with dead load such as pallets of material or water tanks clustered around a column. If not, perhaps you can test with tension piles (screw piles, maybe?) and a pair of beams clamped to the column or running across the pile cap. You might want to shore the first-floor beams in case the foundation wants to plunge, but that seems unlikely. How much load would you need? Would you have to prove every pile group, or just one or two representative ones?

 

[oldestguy]

Depending on pile spacing, a load test on one may not mean anything useful for the overall foundation load capacity.

 

[maxim22]

I have been involved with a project looking to put an extra storey on an existing building.

After the client spent on investigations, the potential for construction variations combined with the ground variations exceeded what anyone was comfortable with.

We excavated some pile caps beneath the ground slab in confined spaces to 3m, did horizontal cores through the piles below the pile caps and specialist geophysics using a close parallel new borehole fitted with geophones. The as-built detail was different to the construction drawings we had, and new SPTs were slightly different to old ones.

Multiple tests and other checks will be required for someone to insure it/sign it off I would have thought. I think measuring the depth of mobilisation of shaft friction can tell what factor of safety you have to play with on existing piles. Taking extra load then is at the cost of reducing overall system robustness and at least some new settlement should be expected.