Deep piled footing recomendation

[Jfet]

I’m working on a low-rise apartment project where the foundation will be supported by piles. Due to poor soil conditions (sand) and a high water table, the required pile length exceeds 15 meters.

I’m considering using CFA piles, with heavier reinforcement concentrated in the upper (about 0–7 meters) to resist lateral loads, and lighter reinforcement below that depth. Is it weird/unconventional to reinforce piles this way, or is this a common approach? Would you recommend any more cost-effective alternatives?
Do you also have any advice for managing groundwater, particularly during the construction phase?

Thank you.

 

[BridgeSmith]

For bridge substructures, we wouldn't typically consider drilling holes and encasing the piles unless we were anticipating needing to drive piles more than 35 meters. Drilling is 10x more expensive per foot (or meter) than driving piles. However, the economics in your location may be different.

The exception would be for uplift resistance. If we need to resist substantial uplift, we'll drill a 1 meter diameter hole, put in a reinforcing cage similar to what we'd do for a drilled shaft (AKA caisson) and set in a pile with shear studs along the web. Unlike your situation, we actually need the flexibility in the upper portion, so we hold the top of the caisson about 5m below the abutment cap.

So, what are you trying to accomplish with encasing the pile? Is is axial or lateral resistance that the piles aren't sufficient for on their own?

 

[HTURKAK]

I’m considering using CFA piles, with heavier reinforcement concentrated in the upper (about 0–7 meters) to resist lateral loads,

This is one of the alternatives. Moreover, i have seen the reinf. cage provided only at the top 6 meters.
Type of piles in general decided acc. to local practice.
I would prefer precast RC driven piles in this case.

 

[Jfet]

For bridge substructures, we wouldn't typically consider drilling holes and encasing the piles unless we were anticipating needing to drive piles more than 35 meters. Drilling is 10x more expensive per foot (or meter) than driving piles. However, the economics in your location may be different.

The exception would be for uplift resistance. If we need to resist substantial uplift, we'll drill a 1 meter diameter hole, put in a reinforcing cage similar to what we'd do for a drilled shaft (AKA caisson) and set in a pile with shear studs along the web. Unlike your situation, we actually need the flexibility in the upper portion, so we hold the top of the caisson about 5m below the abutment cap.

So, what are you trying to accomplish with encasing the pile? Is is axial or lateral resistance that the piles aren't sufficient for on their own?

I have adjacent properties on both sides, so driven piles are not the preferred for now. (alternative installation methods other than impact hammers may still be feasible)

 

[Jfet]

This is one of the alternatives. Moreover, i have seen the reinf. cage provided only at the top 6 meters.
Type of piles in general decided acc. to local practice.
I would prefer precast RC driven piles in this case.

There isn’t significant uplift to address, so leaving the lower portion of the pile unreinforced could be a cost-saving option—though I personally haven’t used unreinforced piles on large apartment projects before.

Cost aside, do you think groundwater and sand will pose major challenges during the construction of drilling piles of this length?

 

[HTURKAK]

Cost aside, do you think groundwater will pose major challenges during the construction of drilling piles of this length ?

Ground water will not be big issue for CFA piling . If you prefer conventional CIP bored concrete pile, the concrete shall be placed with tremie method and you may need casing pile.

 

[BridgeSmith]

I have adjacent properties on both sides, so driven piles are not the preferred for now. (alternative installation methods other than impact hammers may still be feasible)

I keep forgetting that "piles" can refer to drilled or augered concrete shafts, not just driven piles.

We haven't used CFA for any of our foundations, but we may be using all larger shafts than what you're considering. 3' (1m) is about the smallest we use. For those and larger, we use exclusively drilled shafts - open hole w/ casing as necessary, rebar cage placed, backfill with concrete (with tremie in the concrete when pouring underwater), pulling casing, if used, as backfilling progresses. We've always reinforced the entire length of our drilled shafts. We have occasionally used more reinforcing in the top, extending to a depth of 2 diameters (assumed to be the limit of what can be vibrated. More about that below.

Although, whenever we have something that we're not sure how it should be done, or if it can be done, we contact one of the local drilling companies and get their input.

One word of caution from the AASHTO LRFD bridge design spec. - clear space between bars, horizontally and vertically, beyond the depth that can be vibrated, should be a minimum of 5 times the max aggregate size in the concrete mix, otherwise it may not flow into the annular space, and may leave voids around the rebar.