Driven Pile Downward Capacity 4

[pelelo]

Hello,

It is my understanding that the driven pile downward capacity is the Ultimate pile capacity in Compression (please correct if I am wrong).

By following NAVFAC DM 7.02 guidance, I noticed they provide formulas for computing the allowable pile capacity (see attached).

NAVFAC does not specify a Factor of Safety when computing Qall.

Anyone knows what would be the FS in case I need to compute Q ultimate based on NAVFAC Q allowable formulas?

Thanks.

 

[oldestguy]

The simple ENR formula has a theoretical factor of safety of 6. According to Chellis "Pile Foundations" apparently most other formulas have a SF of at least 1.5

 

[BridgeSmith]

The AASHTO LRFD spec applies a resistance factor of 0.5 to the the yield capacity (Fy*A). That applies so long as the pile is fully braced (embedded in soil). If the soil around the pile can scour out, or is otherwise partially exposed, then the unbraced portion is designed as a column. Combined with the use of load increase factors on the loads, the resulting safety factor for structural capacity is around 3.

The older AASHTO Standard spec used nominal loads and a capacity of 0.25Fy*A (SF=4)

Rod Smith, P.E., The artist formerly known as HotRod10

 

[pelelo]

Thanks a lot Engineers,

2 more questions:

1 - Do you understand that the downward pile capacity is the ultimate capacity?,

2 - From the file attached, the ENR formula Qall = 2 WH / (S + 1). NAVFAC specifies S (average net penetration) in inches and H (effective height of fall) in feet. Do you think that makes sense?, it is my understanding that H needs to be inches.

Please let me know,

 

[BridgeSmith]

Ultimate capacity for a pile can be reached in several different ways that I can think of, and probably one or 2 I'm not thinking of:

1) There's the structural limit of the pile in compression, which we discussed above. That's typical for steel piles driven to refusal.

2)There's the structural limit for a pile in tension. Typically that's only reached by concrete drilled shafts in uplift, which some categorize as piles.

3) There's the geotechnical limits in bearing (resisting downward forces in end bearing and/or side friction) and uplift (side friction resisting upward loads).

For #2, I suspect the formula given is an approximation, not an actual equation, per se, so the units aren't necessarily consistent. I suppose they could have written it with a constant of 12 in the denominator and specified H in inches, but then the user would have to multiply H (which would typically be in feet) by 12 and then divide the answer by 12.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[oldestguy]

The formulas have a built in safety factor, so that is the design load with the safety factor included. While the units don't seem right that is the way that formula works.

 

[jdonville]

The NAVFAC or ENR formulas may not be the ones you want, as they do not AFAIK account for the LRFD approach, and the ENR formula is deprecated. FHWA has better guidance available here

https://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm

.

 

[pelelo]

Thanks a lot engineers,

Oldestguy, if you are required to compute the Q ultimate (not Qall) using the ENR formula, what would you do?,

Qall = 2 WH / (S + 1), would you remove 2 from the numerator and use H in feet and S in inches?

 

[jdonville]

The ENR formula is deprecated. Recommend that you look at the Modified Gates Formula if you must use a dynamic formula.

Regarding factors of safety, this reference may be helpful for non-LRFD approach, and has references to a number of informative papers:

https://www.pile.com/wp-content/uploads/2017/03/14-Likins.pdf

 

[HENRYZAU]

None of the dynamic formula is reliable unless it's calibrated via pile load test (static or high-strain dynamic)on a site.

 

[Bobby46]

You do not need to correct the units in the ENR formula. The formula has been simplified over the years. The top of the equation originally had a 12 in it to convert from feet to inches. There was a FOS of 6 on the bottom of the equation. These have been combined into a 2 (12 divided by 6).

At one point the original article for the ENR formula was available on

http://www.vulcanhammer.info.

It is titled Piles and Pile Driving

Edit: Here is the article

https://vulcanhammerinfo.files.wordpress.com/2017/08/wellington-formula.pdf