Let me relate a little story if I may. I worked half a year or so in New Jersey for a Geotech firm that did a lot of work at the time in Atlantic City. Also quite a bit along the Jersey coast. One of the issues we had with the later was paludal deposits that were then covered with sand fill, settled as surcharge loading and then constructed upon. The firm “assumed” that the paludal deposits had sand layers in between and took them for faster drainage under the surcharge loading at "regular spacings". I suggested to them to use a DCP in the investigations (Canadian Pentest – 140 pound hammer dropping 30 inches) on a 60deg cone tip. The sacrificial part of the cone was truncated that fit over a tip that stayed on the A rod – but still when driving it was a cone. I figured that this would show up the locations of the sand layers, if present. “No. We do not need to do that!” . . . okay.
So I went back to Canada and a number of years later, the owner called me up and said – “Hey, do you remember that cone you were telling me about years ago?” – He wanted to get some of the cones like that – so I ended up arranging a shipment for him. Why did he need them? He pushed the pile load capacities in Atlantic City (typically Franki expanded base piles)– but confirmed with load tests – to values much larger than had been used before. Atlantic City was sand, sand, sand!. But, at one site, they did the load test and the pile punched downwards far more than typical. Investigations revealed that a soft clay deposit (unheard of at that time) existed about 1.5 m below the pile tip. He wanted the cone, when the tip location was reached, to drive down below the tip to see if there were other clayey deposits. Yes, it is always good to consider going deeper than the pile tip . . . and even when the geology is well known (20 odd jobs prior). God made sure that there are always surprises.
For what it is worth.
