S Serota, BSc(Eng), FICE, MASCE and G Lowther, MSc(Eng), MICE, AMASCE. This paper was first published in GE’s January 1973 edition.
A series of experiments has been performed to compare the practice of penetration testing, particularly the usual American way of employing a slip rope and cat head rig with the usual British method of employing an automatic free fall trip monkey. Under controlled conditions, the trip monkey gave the same SPT result as the slip rope with one turn on the cat head. With two turns or more, the SPT was about 40 per cent higher. It appears that Americans usually employ two turns or more, and are, therefore, impeding the fall of the monkey. This throws serious doubt on the accuracy of various published works on the SPT.