Discussion: Standard penetration test procedures and the effects in sands of overburden pressure, relative density, particle size, ageing and overconsolidation

Granted that good site control is exercised in carrying out the standard penetration test the energy delivered to the sampler, and therefore the blow count obtained in any given sand deposit at a particular effective overburden pressure, can still vary to a significant extent depending on the method of releasing the hammer, on the type of anvil and on the length of rods (if less than 10 m). For consistency it is essential to correct the observed blow count N to the value which would have been measured using a specified rod energy. A recommended value, which should be recognized internation- ally, is 60% of the free-fall energy of the standard hammer weight and drop. The corrected blow count is then designated as N60 and the normalized value (N1)60 at unit effective pressure (1 kg/cm2 or 100 kPa) may be regarded as a basic characteristic of the sand. Factors controlling the rod energy ratio are examined in detail and methods of deriving N60 values are developed. An examination of selected field and laborat...