[u]Does anyone know of any studies 1

[bridgebuster]

Does anyone know of any studies regarding the validity of the ENR pile driving formaula, Safe Load = (2WH)/(s+1) and what Factor of Safety was used? I took the formula from "Foundations of Bridges and Buildings", 1914 edition, by Henry S. Jacoby.

Here's what's going on: I got sucked into a bridge rehab project - of course no job number, but it's Lent and alms giving is good - a steel viaduct (circa 1920's) with a non-operational double leaf bascule main span, supported on timber piles approximately 70' long. The leafs (or is it leaves?) were locked in the 50's; the machinery was removed, and the pits have been flooded since.

The owner wants to fill the pits with lightweight concrete. The PM (not a bridge guy) and his design team (including geotechs) are opposed since they don't know the pile capacity. They assume it's 20, maybe 30 T (which is not unreasonable for a timber pile) and believe the piles are only loaded to 20 T or maybe 26 T. (This has been going on for 3 years now; that's why I fustrated.)

In the original drawings I found a note stating capacity is achieded with a 3000 # hammer, dropped 15 feet; 1/2" displacement for the last five blows. If I use the ENR formula and the plan data, I arrive at 30 T capacity; I would assume the ultimate would have be at least twice this number. The soil is a thick organic layer on top of sand.

Thanks

 

[Ron]

BB...looks like your formula, similar to ENR's formula, is for Q ultimate, so a safety factor would need to be applied. Usually that would be a minimum FS of 2.0, though others use higher safety factors. The TRB has lots of studies done over the years on pile capacity and predictive pile capacity equations such as Gates, Janbu and others.

 

[bridgebuster]

Thanks Ron,

The author calls (2WH)/(s+1) the ENR formula. I better look in some other ancient textbooks for a comparison.

 

[Ron]

BB...the ENR formula was modified from the original sometime in the 40's or 50's. I believe yours is the original, but the modified yields similar results.

Using the Gates formula for your application and information:

Ru=27[(Eh)(eh)(1-log s)]^0.5

Eh=hammer energy in ft-kips
Ru=ultimate bearing capacity in kips
s=set in inches
eh=hammer efficiency (0.75 for drop hammers, 0.85 all others)

Applying the recommended FS for this formula of 3.0, it yields a capacity of......30 tons!

 

[motorbiketocrank]

I'm not sure if it would meet your owner's need but have you considered very lightweight materials such as Link

Would filling with such material be a lower load than from the water currently flooding the pits? (plus you mention that machinery loads have been removed)

 

[PSlem]

Since ENR formula has a 2 on top and top is in foot-lbs and bottom is in inches the safety factor is 6

 

[PSlem]

Google search confirms 6

http://www.pile.com/reference/safteyFactors/safteyFactors.pdf

 

[bridgebuster]

Thanks everyone.

I was reading up on things and noted that a FS of 3 to 6 was generally applied to the ENR formula. I have the 1984 ed. of Das; he recommends 4 to 6 for the modified ENR.

geobdg - have you used this particular material? I don't see a structural problem with filling the pits with lightweight concrete. To me, it's a construction problem - placing 2000+ CY of concrete from a crowded city street.

What annoys me is that a group of people have been working on this for years and they can't make a decision. All they keep saying is the pit can't be filled; and the client keeps saying prove it. Whatever, I offer in terms of meeting the client's expectations will be useless.

 

[Ron]

bb...I designed a "fill" material for a "major Central Florida theme park" about 20 years ago to fill existing fake rocks, some of which were the size of a small house and the cage structures were deteriorating. We had to limit the unit weight of the fill to minimize settlement because of the added fill load.

The fill was a cellular concrete with pearlite aggregate...a hybrid of two roofing material lightweight insulating concrete fills. It worked fine.

I have also used cellular fill for pavement over a swampy area. That worked as well.

I didn't follow the link, but this is likely what geobdg is referencing or similar.

 

[motorbiketocrank]

I haven't used it personally but we've considered it along with other lightweight fills on various projects (and as Ron indicates there are various cellular concrete materials and Elastizell is just one manufacturer).

There are also various other lightweight materials (such as flowable fill with foaming agent). Of course with the material used you'd need to consider whether it would become saturated and increase the unit wt.

 

[bridgebuster]

To me, this likes the way to go, but I'm dealing with people who are only interested in finding a way not to do nothing.

 

[BigH]

There are several old papers (Flaate was one of the authors - the other Olsen) that compare various pile driving formulas with measured data for various types of piles. They have then modified the various equations based on the statistical analysis (best fit curves) between the two. See Poulos and Davis' Pile Foundation Design and Analysis Chapter 4 for discussion and details of references. (Section 4.3.2 discusses reliability of such equations).

 

[Hanks864]

Pile driving formulas can be far away from reality. During school we did some comparisons between them and the WEAP analysis and they were considerably off.

 

[bridgebuster]

Out of curiousity, how off is off?

 

[howardoark]

After 90 years, I would expect there would have been considerable set-up in the piles and that they would have a lot more capacity today than they did in 1920. Of course, that assumes they've been submerged the entire time and that critters haven't had access to them. If your piles have been in an area where they might have been exposed to air at any point (if you're in a tidal area or there was ever some huge drought) you might not have much capacity left in them.

You'd probably be better off starting from scratch with a theoretical design capacity based on CPT data than you would be relying on the ENR formula.