Pile Design Using Dynamic Probes

[kieran1]

We have been asked by a contractor to design a precast concrete pile (450KN safe working load) However the only information available are dynamic probes. 10 probes were carried out and the N100 values plotted. The average depth is 9 metres for the 25blows/100mm values. The hammer weight is 50Kg, drop height 500mm, Cone diameter 45mm, cone type DCP.

Is there a way of relating these values to SPT's or designing the piles directly from the dynamic probes?. I have asked for a proper geotechnical investigation to be carried out but unfortunately the client is adamant he won't pay for it. I've been told the soil is mainly peat overlaying a bounder clay but as no investigation has been carried out we can't be sure.

 

[garrettk]

Some projects just arent' worth doing - this appears to be one of them.

I'd suggest walking away. If they client won't pay for a proper geotech investigation what else won't they pay for?

 

[Patgeotech]

I agree with garrettk, some times the risk is too great to get involved and the consequences when it goes wrong, always end up back to the Engineer involved. I would argue with your client that the cost of a proper geotech will be much cheaper than when the project goes wrong. Pity most clients never understand that at the time, but come running back when it does goes horribly wrong. Goes back to the saying "one gets what one pays for!"

We option use dynamic probing for establishing pile depth, but I always back it up with one or two boreholes. The boreholes enable me to inspect recovered samples and I can compare SPT to dynamic probe results. I have found that they are usually not too far off the mark of each other. Try get your client to do at least one borehole, next to one of your probing locations, then you can compare SPT/DP results and possibly adjust them. You must establish the peat depth/thickness - depending on the peat type, it could be influencing your probe results quite a bit.

I think they were some German codes of practice (DINS) that enabled one to convert dynamic probing to equivalent SPT values.

Just one more thing, should you decide to continue with this project, warn the client of the problems and what your thoughts are, and most importantly get it minuted in writing! Could prove valuable to you one day.

Hope this helps.

 

[BigH]

kieran1 - agree with the others. But there may be circumstances (who the client is and your relationship) that might weigh "politically." Make sure you are up front with all involved and do put it in writing. I would suggest, though, that you can scour around the area and see what geotechnical reports might be available at nearby sites. I've found success in the past - finding a site with geo info immediately next door to one that I found myself in a position like you are in. If you can find more than one, then all the better - it may show consistency in the general area which can also be "gleaned" from the local government engineers, the contractors in the area and the like. Then you might only have to worry about what might be different such as fill and perhaps buried stream channels. Not that this is the best way to go about it, but knowing the general area, the overall strata/geology, you probably can put a pretty good handle on what you would expect. Then make allowances for other conditions.

 

[kieran1]

Thanks all

I will outline potention problems with site to client and provide set calculations for piles and insisty that all piles are tested.

Kieran

 

[BigH]

All piles tested how? Don't walk into a trap that way.