| Popis: |
Pile Driving Analyzer (PDA) measurements have been used since the 1970’s to monitor the pile driving process, driving stresses in the pile, pile integrity, as well as the soil resistance magnitude and resistance distribution and to judge the hammer performance in relation to pile driveability predictions carried out by 1-D simulation programs. Pile strains and axial pile accelerations are in general measured at 1 or 2 location along the pile 180° divided over the circumference, with each location composed of 2 or 4 pairs of sensors, or at 4 locations, 90° divided over the circumference. In general, for each hammer blow, these strain and acceleration measurements are recorded by special devices mounted to the outside of the pile wall. The results of the measured strains and accelerations are averaged and the final result gives indications about the behaviour of the whole pile. Although the assumption of a 1D system may in the past have been more or less accurate for small hammers and small diameter piles, for the present hammers with higher rated energies, and for current monopiles for wind energy farms with top pile diameters ranging from 6 m to 8 m this is certainly not the case anymore. In these conditions, an attempt to determine the behaviour of these piles during pile driving by measuring just at 2 or 4 local points over the circumference can lead to errors in the interpretation of the results. PDA measurements can be affected by the bending of the pile wall introduced by the deformation of the pile driving equipment, the unevenness of the pile top resulting in insufficient contact between pile top and anvil, not perfectly aligned impact of the ram onto the pile due to eccentricity offsets or inclinations, etc., as well as by assumptions made during the post-processing of the PDA measurement results. In this paper, an analysis of PDA measurements executed for a recently installed offshore windfarm is performed and a comparison with finite element results is made to determine the effect of the parameters mentioned above on the measurement accuracy. It will be shown that these effects can be quite considerable and can lead to for example a large spreading in the ENTHRU energy following from the PDA measurements. |
