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

 

[SlideRuleEra]

Is there any guidance/reference to determine when end bearing of H-Pile should be ignored?

Yes, Sections 3 and 4 of "Bethlehem Steel H Piles", which can be found on this page of my website.

H pile are "non-displacement" piling with "large" surface area (for skin friction) and "small" cross sectional area (for point bearing).

Another time to ignore point bearing is when piles are used in soil that has underground voids (such as solution voids in sedimentary limestone). It's possible that pile driving stopped just inches above an undetected void. If significant loading is applied to the pile point, the void could collapse causing the pile to settle. We had to apply this principle for design/construction of an electric generating station.

 

[dik]

Unless I'm missing something here, I've always considered the driving resistance as a measure of pile capacity and have never separated the friction from the bearing. My experience has been that it's the foundation type selected that determines the economy and that once selected, the foundation cost is relatively insensitive to slight variations in loading... my $.02. I don't design foundations too tightly, either.

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

-Dik

 

[Ron]

How about "always"!

 

[dik]

'always' to what?

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

-Dik

 

[HTURKAK]

The perimeter calculation and assumptions are always a question for H-Piles. When you calculate the perimeter with the assumption of plugged , the perimeter would be less ( gross perimeter of H-pile , so one will get low skin friction, high end bearing) . When you calculate the perimeter with the assumption of unplugged , the perimeter would be more ( interior web and flange surfaces H-pile also considered ) , so one will get high skin friction, low end bearing ...After static test , still the contribution of end bearing to the total resistance is unknown..

IMO , the best practice, do both analyses (unplugged and plugged) and use the lower value for design.

EDIT: I have a problem with with this forum.. I have added the print screen. The letter size is less than 5 and hardly readable.This problem is only at ENG-TIPS forum.. Any suggestions?

 

[HTURKAK]

MY LAB TOP SCREEN IMAGE..

 

[geotechguy1]

If you like wasting other peoples money, ignore it.

 

[MTNClimber]

I was always taught to include end bearing for H-piles. I don’t see the point of excluding it. What would be the concern?

 

[EireChch]

Also "alwasys" to what


I am with MTN on this, I always consider end bearing in pile capacity. For H piles, the soil plugs in the flanges. I consider the area of steel section plus the plug area.

One thing to think about though is mobilisaiton rates. Full end bearing is mobilised at approx 10% of diameter. Full shaft is mobilised at approx 1%.

10% of diameter would lead to excessive settlement, hence why many people (not everyone) only consider a portion of end bearing capacity. I was thought to consider 20% and have generally stuck with the approach.

The flip side of this is that we are generally so conservative in our factors of safety, parameters etc that when you do a load test you almost always have well above the required capacity. Based on this reason my boss often forces me to consider full end bearing capacity. He is reducing some of the conservatism by considering full end bearing capacity. Funny way to do it but he gets a lot of repeat clients. He tends to live on edge but contractors love him for it...

 

[Settingsun]

EireChch, what sort of site verification do you do when forced to take the full end bearing? Australia has a varying capacity reduction factor that depends on whether you do eg static test, PDA, or just theoretical calc. There's quite a benefit to doing a few PDAs.

Under eurocode, wouldn't you be doing limit state, so reduced end bearing for serviceability but ~full for ultimate (taking account of capacity factors)?

 

[EireChch]

We do static tests which just confirm capacity, no separation between skin and end bearing capacity. CAPWAP and Bi-directional load tests can be used to separate the two. We work with some large diamter piles greater than 1m in dia so static tests are impractical and too expensive so we do a lot of BDLT.

We typically do a pile design and specify that we consider full skin and end bearing capacity. Consultants/clients dont challenge on the amount of end bearing we consider.

Then we do a load test find we have adequate capacity. But we dont see fully mobilised end bearing for the main reason that piles arent failed, even using a load 2.5 times allowable.

We find that contractors dont care what end bearing or skin friction we get, just as long as the sum of the two is above what they need. Our pile capacities are less conservative than other consultants in the market which is good for our business.

We often find that we are way too conservative in our skin friction calculations. For example, the critical depth recommendation limits skin friction, however we often get skin frictions that are 2-3 times above those calculated using critical depth assumption. This cant purely be down to our parameters being too conservative as if we increased those it couldn't lead to such an increase in capacity. See below, we did 2 no BDLTs on large diameter piles. The 1.5m dia test didnt come near failure however we pushed the 2.5m diameter futher a got higher skin frictions.

 

[MTNClimber]

10% of diameter would lead to excessive settlement, hence why many people (not everyone) only consider a portion of end bearing capacity. I was thought to consider 20% and have generally stuck with the approach.

Sure, if you're applying a load near the ultimate capacity of the pile. Usually my steel piles are either long or end bearing on rock. The skin friction on the long piles bearing in sands are typically close to half of the pile capacity. The allowable pile capacity is at best 1/2 of the ultimate. So if the pile only sees 1/2 of the ultimate then it's not using much of the tip resistance or possibly any at all.

I feel like ignoring end bearing is too conservative.

 

[EireChch]

MTN - I dont disagree with you

(edit, I removed the ! to make it sound more like "yes I agree" and not sound like "I AM NOT DISAGREEING WITH YOU"

 

[Ron]

@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

 

[dik]

Thanks, Ron... that was my understanding, too. I can't see the inclusion of this having a real financial impact on the cost for foundations. To reiterate, I've found that foundation costs are relatively insensitive to minor changes in loading... unless you end up with site issues.

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

 

[Settingsun]

EireChch, do you get to increase your design capacity by doing load tests? Eg we could increase the capacity reduction factor by ~25% (0.60 approx to 0.75 approx) by doing static load tests on 2% of piles. This can be taken at the design stage without doing test piles, if you're confident enough in the ultimate capacity calculation.

 

[SlideRuleEra]

Since the OP's question is about "dense sand", I agree with ignoring H-pile tip bearing for that soil (so does the Bethlehem Steel reference I provided).

However there are other soil conditions (such as deep layer of hard rock) where H-pile tip bearing can be essentially "infinite"...

"What Goes Down Comes Up - Sometimes" Bengt H. Fellenius

 

[MTNClimber]

SRE,

I’m missing your reasoning for ignoring tip resistance in dense sand. I haven’t thoroughly read the whole manual you posted but under Section 4 - Friction piles - sub section - h-piles in sand, gravel, sand and gravel it states:

“H-pile is sufficient to develop a high intensity of compressive stress both at the point and along the sides of the pile, with correspondingly high values for point resistance and skin friction.”

I didn’t see anything in section 3 either. Can you clarify what your reasoning is?

 

[dik]

I prefer the term 'really big', myself.

About 40 years back, my wife and I were looking for a new stove. We were approached by a salesman who informed us that the stove we were looking at had infinite temperature control. He was puzzled when I informed him that if I lived forever, I might not be able to appreciate that feature.

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