Load Capacity of a Driven Pile

[GeoGrouting]

Closed end Steel pipe piles were driven in soft silt/clay (14 m and followed by medium dense sand to 17 m. The 5000 Ibs hammer with a fall of 3ft was used. The blow counts changed from 5/ft to 20/ft for the last 2-3 ft of penetration. What is the best estimate of pile tension loads and compression loads. What formula would be suitable.

 

[PEinc]

Because your piles are already driven, you have limited options. You could get the required soil properties and do static analysis calculations for tension and compression capacities. You could do a few pile load tests - both tension and compression. With previously recorded hammer blow counts and and pile set measurements, you can use several dynamic pile driving formulas to calculate the compression capacity of the piles. Then, based on references, you may be able to guesstimate the tension capacity.

http://www.PeirceEngineering.com

 

[BigH]

I would check the capacity as given by the Gates Formula - Washington State has modified the original; as has several others. I have found it to be an easy dynamic formula to use - but 20 blows per foot in the last couple of feet is a bit on the light side, I would think. I drove them to about 20 blows per inch in Michigan many years ago. As for tension, this is really a mixed bag . . . several investigators have recommended using a percentage of the compression skin capacity (70%) or so - see Tomlinson. Would use this as a guide unless you actually had a tension load test done.

 

[oldestguy]

There are cook-book values for cohesion on the pile sides in several references, but I would doubt anyone here will venture to give you a number due to many variables involved.

A common thing happens in many soils is a temporary weakening of side shear strength during driving, so depending on driving resistance probably is not a good idea. Pile spacing also is involved.

For a serious determination start with pull out tests.

 

[kieran1]

I think it would be best to get the piles tested on site.

Kieran

 

[PEinc]

"What is the best estimate of pile tension loads and compression loads?"

How important is the accuracy of this "estimate?" If important, then as kieran1 mentioned, testing is the best route.

http://www.PeirceEngineering.com