Choosing Foundation Type

[massagung]

hi all,

kindly need your advise about this,
i involved in superblock project (apartment and hotel 26 storey), soil profiles accross site describe as below (average soil boring consistent) :
0-1.5 m : clay (N-SPT 60 = 5)
1.5 - 40 m : sand (N-SPT > 60)
our geotechnical team have been choosing to use driven square piles. with different pile penetration for each areas and now this project already on going.
but we have problem with pile penetration, several piles can't penetrate below Cut of Level.


M.A.

 

[BigH]

What type of hammer are you using? How deep are you actually able to drive the piles into the very dense sand? Is this a coarse sand? Medium to fine sand?
I would suppose you are using concrete piles . . .if the soil is so dense and the clay only 1.5 m thick - why are you using piles? A mat foundation might have been a better way. My personal choice in piling in sands is to use expanded base piles (Franki-piles)

 

[oldestguy]

I'd get back to the geotech firm with this. After all they should be expert in all phases including how to install them. Contractor also should be in on the discussion, since they should have recognized the problem when bidding.

Next time use another geotech.

 

[AndreBC]

Agree with oldestguy. What kind of geotechnical team would advice to install driven piles when N-SPT > 60 ? Based on this decision alone, for me it is doubtful that they are really a geotechnical engineer.

 

[BigH]

AndreBC - I have seen a job where precast concrete piles were specified and, in fact, lightly weather rock was noted within a few metres of the bottom of the pile cap AND, in one location, within the pile cap elevation. So, go figure . . . We ended up substituting 3 2 m diameter hand dug caissons (when the lightly weather rock was located about 4 m down to replace the 10 or so precast concrete piles.

 

[AndreBC]

BigH - I also had experienced that situation once, light weathered rock was found at 2 - 3 meter elevation at some location...I substitute my precast piles with mat foundation, it is too difficult to penetrate the weathered rock layer.
My point is, whether it is a franki piles or mat foundation, with that kind of soil type, I'm sure that none of us would ever consider of using driven piles, would we ?

 

[Mccoy]

Point is that, writing a generic 'sand', Nstp>60 is too little
I would like to have a look at the cores if they've been extracted. Also, I'd like to have a look at the Whole SPT dataset, intervals at which they've been carried out and so on. Any geophysical tests? Pressuremeter tests?
It sounds more like a soft rock (or maybe even a hard rock, >60 may mean everything, unaltered granites have a SPt>60.
The spt sampling interval is necessary to rule out cavities or soft layers.
And as the other posters have told, what's the sense in using piles if a thick mat would do the job? And why driven piles? Why not drilled shafts? But I've read the project is ongoing, cutting the piles seems to be the only feasible solution now.