A standard penetration test, abbreviated to SPT, is used when carrying out ground site geotechnical investigation works. It uses a device called a split-spoon sampler attached to the end of a metal rod.
This is driven into the ground by dropping a hammer weighing 63.5kg from a height of 76cm on to an anvil attached to the end of the drive rod. When the sampler has penetrated 150mm into the ground, the number of blows is recorded, and this is called the ‘seating drive’. The process is repeated until another 300mm of penetration has been achieved. The blow-count required to penetrate the last 300mm is known as the standard penetration resistance or N-value. It is important that the same weight is used for pre and post treatment investigation testing and when comparing tests carried out by different parties on the same site, as different weights will give different blow counts and a false impression of ground strength. The sample obtained during the SPT is used for lab analysis to determine the geological context and behavioural type of the material.