When to Ignore End Bearing for driven H pile 2

[Shakta]

Is there any guidance/reference to determine when end bearing of H-Pile should be ignored? Recently I found some driven H-pile design that ignored end bearing at all though the soil is dense sand. As a result the pile length becomes higher. Appreciate if you share your experience. Thanks

 

[geotechguy1]

>@dik....always as in end bearing is typically irrelevant in H-piles...they don't displace much so little or no accumulation of anything below that helps bearing

The zero length pile designer:

[ol 1]
[li]Don't include end bearing because under working load conditions little or no end bearing is mobilised[/li]
[li]The pile is now designed based on skin friction only[/li]
[li]under working load conditions for skin friction only, some portion of the skin friction will not be mobilised[/li]
[li]According to the 'ignore end bearing' or as I like to call them 'zero length pile designer' we should now eliminate the portion of the shaft where skin friction isnt being mobilised from our calculation according to teh same logic as 1.[/li]
[li]Repeat ad naseum, you now have a 0m long pile. Or an infinite length pile? Congrats. Maybe we should call it Shroedinger's pile: is it 0m long, or does it reach to the center of the earth?[/li]
[/ol]

 

[EireChch]

Get off the fence Geotechguy1, let us know what you really think.......

 

[MTNClimber]

Haha Eirechch

 

[SlideRuleEra]

MTNClimber - I'm looking at factors that could influence point bearing in dense sand:

Cross sectional area of an HP is really small, say, 15.5 in[sup]2[/sup] (0.108 ft[sup]2[/sup]) for an HP 12x53.

Not only is the area small, the geometry of the pile tip makes it a poor "footing"... for HP 12x53, essentially 36" x 0.435" ≈ 15.5 in[sup]2[/sup].

In dense sand, a properly driven HP is going to have a lot of surface area to develop skin friction. Relatively little load is going to even reach the pile tip. Section 3 of the Bethlehem book talks about how even end-bearing HP driven to rock fail before the tip is fully loaded (unintended skin friction at work).

As a bridge contractor, we would extract temporary HP when finished. To do this (in dense sand), the best way was to drive the the pile a few inches deeper to break skin friction. Then pulling the pile was easier (but not always a piece-of-cake). To me, this just confirms that (in dense sand) point bearing, while probably not zero, is minimal compared to skin friction.

In total, I consider the pile tip bearing of an HP in dense sand to be just "noise".

This is based only on my experience. I will defer to those who do include pile tip resistance since:

Design of piling is better defined now than it was when I was in the business.​

Since I never worked for an engineering firm, never had a client who pushed for every ounce of pile capacity. For our in-house work at generating stations, other factors were more important than first cost.​

 

[geotechguy1]

>As a bridge contractor, we would extract temporary HP when finished. To do this (in dense sand), the best way was to drive the the pile a few inches deeper to break skin friction. Then pulling the pile was easier (but not always a piece-of-cake). To me, this just confirms that (in dense sand) point bearing, while probably not zero, is minimal compared to skin friction.

We install the things by smashing them into the ground with a big hammer so I suppose that logic is consistent with never relying on end bearing

All we're really trying to do is end up at point A (or make sure if we are wrong we end up at B and not C).

 

[SlideRuleEra]

We install the things (temporary HP) by smashing them into the ground with a big hammer...

An example of the difference between a contractor and an engineer's viewpoint of the same project.
To a contractor, any temporary HP (or other steel shape) is worth recovering for (temporary) reuse on another job.

 

[dik]

A really bad way to save money, IMHO...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[MTNClimber]

Thanks for the clarification SRE.

PDA and static load tests with strain gauges data that I’ve witnessed and reviewed have shown tip resistance in sands can provide with substantial addition to the capacity. Even if the end bearing is worth 15% of the total capacity, that reduction in pile length can save a lot of money on projects with 100’s piles.