Driven Piles in Intermediate Sand Layer

[Curious_Sam]

The site I am working on has layered strata with stiff to very stiff clay in the upper say 35 ft followed by medium dense to dense sand between 35 ft to 45 ft followed by very stiff clay to a depth of 65 ft followed by medium to dense sand to 100 ft. The client wants to utilize driven square concrete pile and tip the pile at 40 ft depth below grade in to the intermediate sand layer and utilize capacity in skin friction as well as end bearing.

My question is what should be the minimum thickness of sand layer below the pile tip to count on the end bearing capacity in sand?

 

[oldestguy]

Not enough info for a dependable answer. Kind of job? pile spacing? Why 40 feet? Any load testing? How installed (DRIVEN? JETTING?)

 

[dik]

Be careful using end bearing and skin friction, often doesn't work due to engagement of the loading. Often concrete driven piles are hex shaped and pre-tensioned. Capacity determined by the energy required to move the pile a short distance during driving.

Added: Stressing has to accommodate the reflected tensile wave to keep the pile from cracking across the section and 'dusting'.

Dik

 

[SlideRuleEra]

...what should be the minimum thickness of sand layer below the pile tip to count on the end bearing capacity in sand?

IMHO, it does not take much. The way I read your description the pile would be embedded 5' into the sand with another 5' below the pile tip. That should be more than enough for reasonable loads. The big challenge may be obtaining 5' of embeddment in the sand. The pile may not even have much, if any point loading. A clay layer (upper 35') will often be deceptively easy to drive thorough but will establish significant skin friction a day or so after driving is terminated. Friction from the 5' embeddment in sand will contribute to reduce point loading, too.

If the clay layer will provide any needed lateral support, I would terminate driving shortly after reasonable confirmation that the pile has reached the sand. Otherwise, the Contractor may have to switch to a more powerful hammer for the 5' embeddment in sand.

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[Retrograde]

I rule of thumb I have come across in the past is that the thickness of the layer below the toe of the pile must be greater than 4 times the pile diameter. This for single piles, you need to be careful with group effects.

 

[LRJ]

API RP2 GEO (2011) and ISO 19902 (2007) - the standards typically used for open-ended, cylindrical steel piles installed offshore - states to use 3 pile diameters. The suggestion by Retrograde of 4 pile diameters seems reasonable to me for the pile you describe. However, it very much depends on the relevant strength parameters. Only analysis will be able to adequately inform you of the right action to take.

 

[Okiryu]

Same as Retrograde and LRJ, I think it will depend on the pile diameter. We use 2 to 4 pile diameters.

 

[dik]

3 dia's is common in many jurisdictions.

Dik

 

[BigH]

I've done a job where we had about 25 ft of soft to firm clayey silt overlying 10 ft of compact to dense sand overlying extensive soft somewhat sensitive clay. We founded our piles in the 10 ft sand layer but did not use pile groups. We had grade beams and set the piles something like 7 to 10 diameters apart along the beam so that there would be no overlapping of the pressure bulbs. We used timber piles and things worked out well. We kept the piles within the "middle" of the sand layer (checked on site with Gates Formula).

What I'd like to know is why do you need piles?? What kind of structure?? If you have 35 ft of stiff to very stiff clay which implies overconsolidated soil . . . It would be interesting to know why you can't use spread foundations.

 

[PEinc]

BigH asked a good question. The pile design load has not been indicated. Driving concrete piles through 35 feet of stiff to very stiff clay may take some effort and probably develop more capacity than needed, especially considering set-up. In addition, care must be taken when selecting the pile hammer and driving criteria so that the concrete piles are not damaged. What does the WEAP analysis say?

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