HP Pile vs. Steel Pipe Pile Comparison

[mjohan]

Is their a quick, relatively accurate way to compare a steel pipe pile filled with concrete to an HP pile?

Is the relationship comparing the two I/r values a simple, somewhat accurate way to find equivalent strength comparisons?

Thanks for your time.

Mark

 

[BigH]

Is this for structural capacity or load carrying (soil) capacity? In driving into sands/silty sands, etc. I prefer the closed end steel tube pipe piles. Into shale bedrock or into till, the H-piles might be better used. I know of a case history where there was need to retap H-piles and they ended up going down quite a distance further on retapping. Really, they are two different piles each with its own best use. Don't use H-piles in sands (my recommendations) for they could drive "forever" (or would seem to - not really "forever" but for quite a long long way.)

 

[mjohan]

Thanks BigH,

I believe the piles are designed for both bearing and friction. The geotechnical report stated worst case soil seemed to be 70' of soft clay, with silty sand and better soil beneath the clay; on average 5 blows per foot in clay and 40 blows per foot in the soil below. I was asked by my client to approximate the weight of an equivalent H-pile to the 12 dia. steel pipe.

Please let me know if you feel the following method is somewhat accurate for determing an H-pile. At the steel tube institute web site I found a load chart for a 12.25" dia steel tube filled with concrete. The KL values only go up to 40', so, I determined the E value of the composite section backing out the given loads. I then recalced the allowable load for an unbraced section of 70' with k = 0.7 (pinned at top, fixed at the bottom). The allowable load was 108 kips, which coincided with the geotech report of 50 ton capacity. I then calculated several H-piles. A HP13x87 was the closest, I think I got an allowable load of 107 kips.

I was also curious as to where to find the 12" dia. pipe with 1/4" wall thickness? AISC pipes start their thickness larger than .25" at 12" dia. HSS tubes was the closest, @ 12.25" dia. with 1/4" thickness.

I know I took a shot in the dark on this one... your thoughts are very much appreciated.

Do you feel the H piles with the soils described would require a much longer length than the pipe?

Thanks again,
Mark

 

[BigH]

Mark: Will give it some thought - geotechns usually give the allowable loads and leave it up the structural designer to ensure that the structural capacity isn't exceeded. How many piles have "failed" due to structural exceedence??? I believe that the structural criteria are usually a bit more conservative than the geotech loading. Anyone else??

For pipe pile dimensions, etc., why don't you call Franki or other pile driving firm (Bermingham in Canada) and query their rep?? They are usually quite helpful (I'm assuming Franki is still Franki!!)

Another thought - one thing to keep in mind is downdrag forces. With your pile going through 70ft of soft clay - if there is any consolidation, it could impact some neg skin friction. With the pile tip in soil and not on rock, it won't be as much because of the permitted tip movement but still should be a consideration when determining the loads to be carried by the piles.

Why only steel piles?? You might want to consider an expanded base pile in the sand - I don't think that 80ft is too deep and might be able to get much larger load carrying - both from geotech and structural point of view.

 

[mjohan]

The report stated their was a downdrag force giving an equivalent allowable load. The allowable capacities were in the range of 20 to 50 tons per pile, utilizing 8" to 12" dia. piles.

I took your advice and did not include an equivalent H-pile on the pile plan I was working on. I called the geotech who issued the report and his first instinct was also that an HP pile would most likely require a longer length to develope the friction and bearing needed for the pile.

Thanks for your help.

p.s.
If you come up with an equivalent pile based on strength, I would be curious to know what that is. Assuming the clay soil had so few blow counts per foot, was I accurate in assuming that it was unbraced? and pinned - fixed?

Mark