Estimating Settlement for Driven Piles

[qhtony]

Hello, everyone... This is my first time posting a thread and my question may look a bit silly.

I'm a graduate engineer and recently I have been assigned to a bridge foundation (driven piles) design work. Basically what my colleagues suggested was to use the SPT-N values to calculate the side friction and end bearing. I don't have any problem with this approach (although I was a bit surprised because it seems too easy and too rough for me) and through the calcs, the piles will be founding on very stiff clay with SPT N value of 40 at around the depth of 25 metres (below surface). The capacity will be via side friction as well as end bearing (50-50). However, when it comes to the settlement estimate, I wanted to run a consolidation test on the underlying clay material (below pile toe) but my colleagues were saying consol is a waste of money for deep foundations. And they simply use a chart with some simply calculations (I haven't got a chance to look at what method they are using) to come up with a settlement estimate.

So my question is, what methods do you guys usually use for estimating settlement? And is it really not worth it to run a consol in this situation??

Thanks.

 

[Melzar]

Check out the AASHTO LRFD Bridge Specification for the stress distribution in the clay - 10.7.2.3.1-1. Your situation sounds like you might consider an end-bearing pile distribution. The equivalent footing for this would be the plan area of the piles located at the pile toe, and the stress would decrease at a 1H:2V angle. Use some empirical correlations to get your recompression index and conventional consolidation equations to estimate the group settlement. I think you will find that it will be low enough that you will be comfortable skipping the lab testing.

 

[civilperson]

The piles you design will only be loaded to 33% of the ultimate capacity. The consolidation will come from end bearing loads in the stiff clay, (16.6% of the ultimate capacity). The increase in loading of the soil 75 feet below the bridge will be nominal and the amount of consolidation will also be nominal. (Of the same order of magnitude of elastic shortening of the piles.)

 

[qhtony]

Thank a lot guys...It makes perfect sense to me now...

 

[oldestguy]

It would seem highly unusual for one to find a case where the material below an end-bearing, or near that situation to have settlement amount to anything of importance.

However, I know of only one case where this has occurred in my many years of this business. That was in Florida where a peat layer was present below friction piles in sand.

Otherwise, unless soils are really very soft for deep depths, there is no reason to be concerned.

Finally, for a bridge there usually are other factors, other than pile induced pressures that can cause settlement. One is the adding of high approach fills over an otherwise competent soil situation for piles alone. In that case, the added pressure of fill is far more important than pile induced pressures.

 

[fattdad]

Consider an equivalent areal load at a depth of 1/3rd into the skin friction stratum. Take the anticipated design load (skin and tip) and distribute it over the plan dimension of the pile group. Then using the equivalent pressure (i.e., load/area) calculated settlement.

For stiff clay, you may want to consider either recompression index (i.e., 1-d consolidation paramenters) or you may want to consider elastic modulus.

Not sure if words convey what I want to say.

Example: Let's say you have 9, 100 t piles and the pile spacing is 5 ft (i.e., a pile group footprint of 100 sf) Let's say you drive to a depth of 40 ft through some organic slop and then get into some stiff clay where you stop driving at a depth of 58 ft (i.e., 18 ft into the stiff clay). What I'd do is take the 900 tons and divide it by 100 and get 9 tsf. I'd then look at the settlement that would be realized by a 100 sf area loaded to 9 tsf at the depth of 46 ft.

Hope this helps. . . .

f-d

¡papá gordo ain’t no madre flaca!

 

[qhtony]

Thanks f-d...The method/theory you suggest looks pretty good to me. Just wondering if it's in any textbooks that I can refer to?? Or what's this method called?

 

[BigH]

Tomlinson's Pile Design and Construction.

 

[fattdad]

What BigH said. . . .

Tomlinson has a few variations that are worth reviewing. I pretty much just gave an overview. I recommend the Tomlinson book for anybody working with piles.

f-d

¡papá gordo ain’t no madre flaca!