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Large Dia Pile - End bearing mobilisation
2

Large Dia Pile - End bearing mobilisation

Large Dia Pile - End bearing mobilisation

(OP)
Hello Everyone,

I am currently working on a river bridge project. The location is a delta region. Scour depth is 40m. Pile dia adopted is 2m & 2.5m.
So to make the pile as a long pile, the critical length is used as the minimum embedment below scour level.
So 30m min. embedment is provided. The pile length is more than 70m.
In geotechnical capacity of pile the contribution of skin friction is 40% and end bearing is 60%.
The strata is fine sand below scour level. (above scour 5m clay is there)
Due to limit in overburden pressure after certain length the increase in length does not contribute much in capacity.
As the pile is terminating in Sand the end bearing is having high contribution in capacity.

but during construction the base cleaning may not be proper and the end bearing contribution may not result.
i don't want to rely much on end bearing as similar strata nearby (5km approx. far) pile load tests results on 70% contribution on skin friction & due to difficulty in cleaning end bearing is very less.

So, any solution for this situation is available please provide.

Thank you.


RE: Large Dia Pile - End bearing mobilisation

We haven't found an adequate solution for this. We have abandoned trying to combine side friction and end bearing and just design for one or the other. We bump up the diameter or drill deeper.

I surprised you can make that work under scoured conditions, with a 40m unbraced length for a 2m or even a 2.5m diameter shaft. When we get that slender, the P-delta moments get too large and the buckling always bites us.

RE: Large Dia Pile - End bearing mobilisation

I also am in the same boat as Hotrod10 either end bearing or friction fit, not both.

RE: Large Dia Pile - End bearing mobilisation

I think the issue is strain compatibility. It takes more strain to mobilize the ultimate value for end bearing - less for side shear.

But, we don't mobilize all of that, eh? We mobilize a portion of the ultimate load.

I think when in the working range of stress, the issue of strain compatibility is of less consequence.

I'd consider the ultimate of both. I'd look at the LRFD for both. I'd then look to see how much of the ultimate is mobilized by the LRFD and make my judgements accordingly.

It'll all be demonstrated in the load test.

just so folks don't think I'm being cavalier, I'll offer the following: I back-checked pretty aggressive design using the N-value (N-bar) correlations to shear and end-bearing. I got pretty close to the ultimate found in the load test and I got pretty close to the safe load, based on a safety factor of 2.5. I think such back-checks useful, even though LRFD has transcended such an approach.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Large Dia Pile - End bearing mobilisation

Not sure if your similar site had the same pile lengths but it sounds like these are going to be relatively long. The deadload of the pile itself may help reduce settlement to give you a higher capacity from a not-so-clean shaft bottom. Tough to put a number to that though.

That's one hell of a scour depth. I would of guessed that you'd be looking for a much bigger shaft or some other alternative. But then again, it would depend on how many piles are in your group. Wouldn't the river widen substantially if you saw 40m of scour? I would have whoever is in charge of those calculations also look into if the roadway leading up to the bridge is going to end up in the river.

RE: Large Dia Pile - End bearing mobilisation

I forgot to ask. . . is this the maximum theoretical scour depth? Was it based on scour rate or was it based on the D50 of the clay, which I think they let you default to 0.2mm?

We have a bridge in redesign, originally constructed in the '50s. Calculated a deep scour hole, but reason prevails. Can it really happen? Well, the calculations, idealized flood, and the simple D50 approach says it will! Dead reckoning says it will not!

We got the FHWA to use the site for a demonstration of their ISTD (in-situ test device), which is in development at Turner-Fairbanks. Well, they did their study. Not sure if it'll give us a better design. Working on that still!

Are you familiar with the work of Briaud at UT Austin? He actually has a protocol for using a squirt gun to index an undisturbed sample. No joke, get a squirt gun and see how big a dimple is made. It's sort of like the pocket penetrometer for scour, eh?

Have fun, but. . . This is an area of engineering that needs better control!

f-d

ípapß gordo ainÆt no madre flaca!

RE: Large Dia Pile - End bearing mobilisation

(OP)
Yes the scour depth is design scour depth (2 times the scour depth obtained by Lacy's method). The scour calculation is based on silt factors and discharge. Silt factors for cohesionless soil is taken from mean dia of particle (from grain size distribution) and for cohesive the shear strength is used for deriving silt factors.

The length of bridge is 5.7km. the Spans are 242m. it's a cable stayed bridge.

To reduce the time of construction the client want to go for pile foundations, as in well foundation time of construction for sinking of well will take too much time. (though sand will not create much settlement problem).

I think BASE GROUTING will be a good option for the end bearing improvement.
Is any one have experience on load tests have base grouted ??


RE: Large Dia Pile - End bearing mobilisation

Base grouting should help but I don’t have any experience with it for shafts. Maybe reach out to a local shaft contractor. They usually will give out free advice for a foot in the door on a big project like this.

RE: Large Dia Pile - End bearing mobilisation

I think that I may have seen some case history discussion of preloading with Osterberg cell for improving base resistance of drilled shafts someplace recently (or I may be misremembering)? Would it be more cost effective than base grouting?

RE: Large Dia Pile - End bearing mobilisation

Refer to Fellenius 1995 paper, critical depth does not exist.

RE: Large Dia Pile - End bearing mobilisation

Given that this appears to be a critical project, I would try to do some pile loading tests at the site (in actual working piles) to verify the pile capacities and then, once capacities are verified and the design is validated, I would move to installing all the actual piles. BTW, pile dynamic testing can give you the contribution of friction vs. end bearing.

RE: Large Dia Pile - End bearing mobilisation

Quote (HENRYZAU)

Refer to Fellenius 1995 paper, critical depth does not exist.
Never heard of critical depth before. To be honest I can't believe it was ever believed to be a 'thing'.

RE: Large Dia Pile - End bearing mobilisation

I have considered this several times, but never tried it: If you are concerned about loose or weak material left on the bearing surface, consider placing some grout in the bottom and stirring it into the slough with the auger, being sure the auger touches the bottom of the excavated hole. If you add about twice as much grout as the volume of the slough, perhaps the mix will not be compressible at the design bearing pressure. Let the mixture set overnight. If you want to explore the hardened mixture before placing concrete, you can cut some samples with a small core barrel, say 6 inches in diameter. Then airlift out the sediment that has collected overnight and place the concrete.

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