Real-time monitoring of SPT donut hammer motion and SPT energy transfer ratio using digital line-scan camera and pile driving analyzer

In order to investigate the effect of standard penetration test (SPT) hammer impact on the energy transfer ratio (ETR) and sampler penetration, the behavior of hammer and anvil is monitored by using a high-speed digital line-scan camera and the stress wave is measured by using a pile driving analyzer during SPTs. By analyzing the motions of the hammer and anvil, which are digitized using the image processing algorithm, two types of secondary impact are observed to exist. Type-I secondary impact is caused by the rapid downward anvil movement at time t₁ + 2L/c and the re-contact of the following hammer on the rebounding anvil. Type-I secondary impact is dominant for N value smaller than 25 and it contributes to the ETR and additional sampler penetration. The time elapsed to type-I secondary impact decreases as N value increases. Type-II secondary impact is induced by the restrike of the pushed-up hammer on the resting anvil. Type-II secondary impact is dominant for N value greater than 50 and it has no influence on the ETR and the sampler penetration. The time elapsed to type-II secondary impact increases with the increase in N value.