Pipe and H-Pile cross sections

[MotorCity]

In a text by Braja Das, I came across a discussion of calculating areas and perimeters for pipe and H-piles. He states that the soil plugs formed within these sections during driving should be included when calculating the areas and perimeters for these types of piles. My question is, "WHY?".

I would expect to use the actual area and perimeter of the steel cross section. It does not seem logical that the soil plugs would contribute in any way to the structural capacity of the pile. Is my interpretation incorrect? Any thoughts?

 

[cap4000]

I know its hard to understand but it is true. Everyone uses the soil plug method for load carring capacity. The main points that I think you should consider is: 1)type of soil you are driving into 2) reduce your plug area by 50 %if necessary 3)Stiff clays with high skin friction are probably ideal for a full soil plug and 4)LOAD TEST your H-Piles to confirm and validate your design and assumptions in order for you to get the proper level of "COMFORT" with these issues. Good Luck.

 

[SlideRuleEra]

Consider an HP 12 as an example. The actual perimeter is about 6 ft. The recommended perimeter for calculations is say 4 ft. The portion of the actual perimeter that is bounded by the inside of the flanges and web is ignored for the following reason:
Think of the soil in this area to be a peninsula, flange on two sides, web on one side. If the pile moves, the soil at the base of this peninsula will most likely shear instead of breaking the friction contact with the web and flange areas. Years ago Bethlehem Steel published a photo that shows the soil (clay in this case) still in place between the flanges of an H pile that was extracted after 14 years of service.

 

[Focht3]

Soil plugs in pipe piles are a well established phenomenon; the formation of plugs in H-pile sections is not. Personally, I am reluctant to "count" on the soil plug for capacity unless the pile is driven several feet into a dense sand, or has a closure plate welded into the lower section.

Das' comments reflect a regional perspective - not all parts of the U.S. (not to mention international practice) follow the approach he outlined.

The use of a soil plug can be a real problem if the same factor of safety is used for both skin friction and end bearing. Skin friction mobilizes at small movements (0.2 to 0.25 inch, or 5 to 6 mm) while the full end bearing may not mobilize until the bearing area has deflected 10% of the pile width/diameter. This can lead to serious settlement issues. I customarily use a FOS of 2 for friction and 3 for end bearing for this reason. And I also look very hard at the working load with respect to the calculated allowable capacity -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[tincan]

Motorcity,

Have done salvage & demolition work in these areas. The plug does exist in the pipe section, the stiffer the soil/clay the more extensive the plug. The H sections vary with the material of embedment, for stiff clays yes, for loose sands, no. These were observec on salvaged members and some of the sections may have been lost.

Also, on the pipe and on timber, I have noted a build-up of between 1/2" to 2" of clay on the exterior of the piles. This clay build-up could not be readilly disloged with a 5# maul and a heavy swing. Have not noted this on the H piles.

I think the main thing is to be consistant in your decision. Best, Tincan.