End Bearing Values for cast-in-place concrete piles in dense gravel?? 1

[claytill]

I am working on a project where we have heavy vertical loads (1200-2500 KN)to be supported by 760 mm diameter drilled cast-in-place concrete piles. The water table is below the maximum depth of the pile, which is ~15 m. My research on maximum end bearing values for piles in dense gravel yielded an ultimate value of 12 MPa (by Poulos).

Doe anyone have any other references for maximum values in gravel? Or any literature with load tests completed for a similar scenario?

Using a Factor of Safety of 2.5 results in an allowable end bearing value of 4,800 kPa. Does this seem high?

Your comments would be greatly appreciated.

 

[fattdad]

If "dense" means N=30, then qp would be (about) 4*30 [tsf] or 120 tsf. Using F=3 that'd give you 40 tsf (3,800 kPa). Considering a 30-in diameter drilled shaft, you'd then have 4.9 sf for an allowable capacity of 196 tons (392,000 pounds).

Sorry, I'm not that good in metric, but I tried. . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[hokie66]

4.8 MPa end bearing in gravel, no matter how dense, sounds very high to me. To me that is the end bearing you would use in moderately weathered rock. I am not a geotechnical engineer, but have done a lot of bored pier designs based on geotechnical recommendations. You would also add the skin friction component.

 

[oldestguy]

Another question. How clean is the bottom of the excavation, before pumping the concrete. I assume this is what we call a grout pile. That condition will control what load you can place there.

If this is a form of caisson, installed inside a casing, the same question. If the gravel is disturbed and not left in its original condition, you are asking for settlement from that loosening. Any water there may also affect things by washing out the cement from the mix.

Take a look at the current ENR web site and the article about Clyde Baker. He has written articles about how you can have settlement of caissons due to these bottom of excavation situations.

Try this link:

http://enr.construction.com/

There is a nice article about him.

Then look up some of his papers at ASCE.

I'd much prefer to load test some typical piles to be sure I had the bearing that was predicted.

 

[emmgjld]

Before I use relatively high end bearing pressures for any Deep Foundation system (Drilled Piers, Driven Piling, Auger Cast Piles, etc) within alluvial soils, I would need either load tests or a good, LONG history of local use. The same criteria would apply for some formational materials and interesting geology.

 

[BigH]

One thing that I have seen done to counter, to a degree, the disturbance of a bored pile in sand (or gravel) is the use of a plate on the bottom of the cage. Below the plate is a geomembrane which has grout tubes extending up through the plate to the surface. After the pile is concreted and "set", they then pressure grout the geomembrane beneath the plate until the pile just starts to uplift thereby giving some compaction of the base that might have been disturbed.