Pile Working Load 1

[ONENGINEER]

There are many formulea on pile capacity and one becomes uncertain which one to use.

The site is characterised by 4 m of granular random fill overlying the native sand and gravel (till-like) materials. If I drive an open steel pipe pile (12" OD), how much wouldthe nominal working load be? Thank you.

 

[kieran1]

are you serious?

Kieran

 

[ONENGINEER]

By serious, does it mean that it is such a simple question asked? (maybe I am missing some obvious matters)
(Assuming SPT = 33 for the soil under the fill and that the pile could reach to refusal after penetrating 4 m into the bearing gravel strata, using the lower and upper limit constants the working load ranged from about 350 to 850 kN. Meantime a contractor's rough prediction is 90 kN).

The available formulations yielded diverse results. I have not done any post installation performance monitoring of open steel pipes driven into sand and gravel but for some other types, to be able to rate the suitability of a specific formulation. I know that the actual working load is only determined by PLT, however, would like to see if the comments by experienced engineer could help.

(Assuming SPT = 33 for the soil under the fill and that the pile could reach to refusal after penetrating 4 m into the bearing gravel strata, using the lower and upper limit constants the working load ranged from about 350 to 850 kN. Meantime a contractor's rough prediction is 90 kN - a little wide range).

 

[BigH]

Personally I wouldn't use an non-displacement open-ended pile in granular soils - but would use a displacement pile such as a closed ended steel pipe pile - or better yet a Franki expanded base pile.

One of the biggest problems when considering the "geotechnical design" for allowable bearing pressure in granular soils is the choice of Nq. Depending on the method chosen (Meyerhoff, Vesic, Berezantev, De Beer, etc), for your "N=33" which implies a friction angle of say 36deg, Nq can range from 70 (Vesic) to 300 (Meyerhoff driven). This is a dilemma for sure - many have determined that Berezantev is the most "reliable" and hence Nq ~ 100. The working load would be the safe load based on shear and permissible settlement. With a closed-ended steel tube pipe pile driven with a suitable hammer and to a suitable set (I would typically use the Gates formula modified by Flaate), you should be able to reach 100 tons (US) as a working load - but you need to determine if this is reasonable for your case. Tapered piles would typically have a greater working load (see Nordlund). I know of one very experienced geotechnical engineer in Vancouver (passed away many years ago) who basically said - I don't know what the "formulas" tell me but its good for 200! (US tons). In your case, the "random" fill would be expected to offer little frictional capacity and you might also have to consider any downdrag forces that might result over time.

Your investigation should have revealed if there are any "soft" layers within short distances of the pile tip - this could lead to reductions in load.

Anyway - these are my quick thoughts on the issue.

 

[ONENGINEER]

BigH: Could you please explain how the N=33 was related to friction angle of 36? Thought it would have been 40-46.

 

[BigH]

Just went to a chart that appears in many texts (Peck Hanson and Thornburn for example - with props). Is it an "absolute"; no, but then, again, with all the other inaccuracies, it is good for preliminary check. I would never use 40-46 degrees for a friction value in design . . . One point, too, to be made is that if your N value is in gravel, it might actually be overstating your value due to the gravel sizes.