Pile Refusal on Very dense Sand 2

[Fayoub]

Dear Fellow Engineers,
Anyone have an idea of how to properly analyze the bearing capacity of a driven steel pile that will definitely refuse into very dense soil (N>50) before reaching rock? In other words, I am looking for the end bearing capacity on soil refusal.

Thanks.

 

[oldestguy]

Are you sure that is sand? Many a split spoon sample at high blow count looks like sand, but it really is somewhat weathered rock. So, what's the problem if your method of evaluating bearing show you have proper bearing capacity? There are many methods out there and I presume you know you may get many recommendations here, but what would you do without this site? Chance are the pile driving foreman can answer it also.

 

[oldestguy]

Would this be in Grant county, WI. If so consider over driving and/or damage.

 

[Fayoub]

There is an approx 10ft thick very dense sand with decomposed rock on top of intact bedrock.
My bearing capacity calculations are based off of experience in similar situations in similar areas. I am sure the pile will refuse on top of solid rock.

Maybe a WEAP analysis would do best.
Please do not try to advise about identifying soils through from split spoons.

Thanks

 

[oldestguy]

Fayoub:

Sorry if I offend. Only brought this up because I've seen this to be true many places. On one job (not mine)they had numerous STP borings all calling the very high blow count weathered rock "sand". When the site was opened up, the inspector on the job and the contractor agreed it was a form of rock and you can bet that extra job cost of $ per cubic rock excavation sure taught the geotech report writer a hard lesson. Other negative results from this one case cost the engineer even more. I was only trying to help with my comment question. Glad you are confident with your assessment.

The reason I mentioned now a place in Wisconsin is that there now is a problem there with piles penetrating far into the high blow count "sand" unexpected, very likely a weathered rock situation.

As with questions about how to fix some part that won't come loose on a machine, we sometimes say "Get a bigger hammer". Perhaps that may help here.

Good luck with your situation.

OG

 

[SlideRuleEra]

Fayoub - Could you have the Contractor earth auger a pilot hole (smaller than the pile) to allow easier driving so the pile tip reaches top of rock?

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BigH]

Might I ask what type of driven steel pile you are using? H-pile (which cuts like butter - perhaps refrigerated butter), open ended steel tube piles or closed ended steel tube piles (this last one is a displacement pile).

 

[Fayoub]

Oldestguy:
Thanks a lot for your feedback. I guess get a bigger hammer may help However, then there would be overstress issues just like in WI...

 

[Fayoub]

SlideRuleEra:

That would definitely work however the augering will reduce lateral & uplift capacities.

My concern isn't to reach bedrock but rather to properly recommend the correct capacity of a pile when it refuses on soil. In other words I couldn't care less if the pile doesn't reach bedrock I'm just concerned about recommending proper axial downward capacities of a pile.

 

[Fayoub]

BigH:

Thanks for your question.

I've recommended HP-piles to cut through better as well as closed-ended pipes. Open-ended isn't advisable because of presence of organics.

 

[SlideRuleEra]

The mention of a "bigger hammer" reminded me that in hard driving conditions it is possible to select a "better" hammer (of the same size class). Decreasing impact velocity of the ram, by reducing the length of stroke, will improve energy transfer to pile. Reduced stroke length will also help by increasing the hammer's operating speed.

An example would be comparing a single-acting Vulcan #1 (15,000 ft-lb, 60 strokes / minute) to a single-acting MKT DE-20 (16,000 ft-lb, 48 strokes /minute).

The Vulcan's 5000 lb. ram free-falling 3 ft., combined with higher blow count rate, will give superior performance to the MKT's 2000 lb. ram free-falling 8 ft.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldestguy]

One factor or several hat might assist us in answering yur post is "What is the use here?". For instance is this a situation where some factor other than simple static loading is involved and where a little differential settlement between pile caps of abutments might not be of concern? Otherwise as long as the method of determining capacity that usually "works" would fit here, why was the question of stopping on "rock" posted and later withdrawn?

 

[BigH]

You haven't really identified what the problem is - other than you don't think that the normal procedures for determining end bearing and side friction capacities apply and I am not sure why. I suggest you see Tominson's Founatiopn Design and Construction (I have the 6th edition) - it gives a very good discussion and has a sample problem of pile into dense sand. He has a very good figure (7.7 in my edition) that shows Limiting value of ultimate end-bearing resistance (MN/m2) vs calculated value of ultimate end-bearing resistance, qb (MN/m2). The limiting value depends on the OCR of the sand - with your very dense sand, I would think that the OCR is in the range of 3 to 4 (possibly more) - in any event, he recommends a maximum limit for cohesionless soils of 15 MN/m2.

Secondly, you haven't identified the structure or any settlement criteria - i.e., how sensitive is the structure. Nor have you identified the stratigraphy that you are driving through other than you have 10 ft of very dense sand overlying bedrock nor why you think that you have to "go to rock".

There is no question when driving the close-ended pile, you will refuse in the very dense sand, and not far into it. Is the soil above the very dense sand poor as far as support goes? Are you requiring lateral capacity? These are additional considerations. I would think that you would determine the ultimate bearing and side friction in the "traditional way" - following discussions of maximum values - and go from there. One thing to keep in mind is that even with a pile driven into dense to very dense sand, much of the actual load being carried will be in skin friction/adhesion (which is why I was wondering what the soils above the very dense sand are) with only a limited load carried down to the pile tip. This can be seen in Tomlinson's Figure 7.13 . . . as it takes less movement to mobilized skin friction and more movement to mobilize end-bearing. Unless there was very good reasons not to found in the very dense sand, I am not sure why you wouldn't.

Then, during construction, checks can be made as to confirm your design capacities. Of course, pile load tests are preferred - but that would be on a limited number of piles. As a quick check, the use of the modified Gates dynamic pile driving formula could be used. (

http://www1.wisconsindot.gov/documents2/research/07-04pilebearingcapacities-f1.pdf)

I realize that many will not like the use of a dynamic formula but as a check - it can be done on the spot at the end of each driven pile.

 

[Retrograde]

A rule of thumb is 400 x N. If you take N as 50 that gives a base bearing capacity of 20,000kPa.