Updated: May 8
The most common test method in the industry of pile testing is the pile integrity test, which is based on the Pile Echo Method (PEM). It all started with the following article from 1963, just about when I was born. It took another 30! years for real products to reach the market. Piletest, as a market pioneer, makes great efforts to create educational materials for this industry. Finding this paper took, devotion and some professional search people to find it, which made it exciting to discover.
See the following paper, on how the pile integrity test started