Bearing Capacity calculation from Dutch Cone Penetrometer tests 1

[PRIVY]

Can anyone give me the formulae for calculating bearing capacity from Dutch Cone Penetrometer readings (qc values? And how to calcultae safe working loads for Pile installation from qc values as well?

Thanks

MPA

 

[ishvaaag]

1. Get qc average in the depth of influence in kgf/cm2, the depth of influence being 8 diameters above base of the footing to 3 diameters under such base.
2. Divide the value by 4, and take its integer value as N, i.e. N=qc/4 (integer, when qc in kgf/cm2)
3. Establish a permissible settlement DeltaAdm
4. Then qallowable=permissible working stress equals to

qallowable=(DeltaAdm/25 mm)·(N/0.052) kPa

For example for a permissible settlement of 20 mm and qc=100 kgf/cm2 in the depth of influence,

qallowable=(20/25)·(25/0.052)= 384.61 kPa = 4.99 kgf/cm2


allowable working streess at the tip for the pile to forestall higher ELASTIC settlement than allowed

see that allowing twice elastic settlement you may get twice allowable working stress to the limit of the settlement at the point permissible by the code and superstructure.

long term settlement effects if any can't be derived from the simplistic formulation (or shouldn't be used for clayey soils)

I once did a Mathcad Worksheet covering this annd must be freely available at Mathsoft's site, Collaboratory for Mathcad 2000, seach Pile, SPT, Meyerhof and should appear (the correlation with the SPT N as above is therein as well)

It seems to me this is a quite stringent correlation, since the weaker values of qc above the tip are easily taken whilst those under, 3 diameters below, are less data and "refusal" readings use to be quite ... almost forfeited for precision when charted. Must not if you are going to use this method since they are what are to get up the allowable working stress and represent truly more the resistance at the tip. Accuracy in those 3 diameters below tip is in this case paramount.

 

[Mccoy]

ishvaag,
sorry I show up a bit late, I wanted to ask you this but forgot it in a way.

The above formula is good in static conditions, what about seismic conditions. In the 'usual' terzaghi-like formulas you just add corrective coefficients for structural and ground inertia, whereas in the dutch cone correlation you should have some correction on the bearing capacity value itself.

The only one I'm aware of, Paolucci and Pecker 1997, yieldes obscenely conservative amounts. If you're going to use it, you are out of a job.

 

[ishvaaag]

McCoy, certainly someone more qualified than me is needed to provide proper answer to your question. If I find something of interest on the issue (tan I will be looking at) will post it at this thread.

Also to note again how to state a safe bearing value geotech experts seem be needing to be overly conservative. I understand this certainly may be the case to get the same reliabilities than at the structures, but still, as classical approach of designing foundations proves, I feel we are for most cases more interested in average or probabilistic behaviours, and so more realist predictions of the same, and less conservative (safe! reliable!) ones.

This maybe perhaps rationalized adding a multiplying factor weighing the likelihood of some unwanted effect happening; say, you want 99% of settlement bad effects covered, you should account you may have such settlement and so on, the same for bearing capacity. A lookup table, better than no guide. And a difficult lookup table to build.

 

[mamoz]

while is that people dose not want to teach other how to calculate DCPT.

 

[mamoz]

we need a grate man to put us true on Dutch Cone Penetromete calculation with well explanation.