Strain and Acceleration Measurements at Instrumentation Distances to the Pile Head of 0.5 and 1.0 Times the Diameter—Offshore Pile-Monitoring Experience
This paper presents experience gained during offshore pile-monitoring works, namely strain and acceleration measurements, at instrumentation distances from the pile head (impact zone) of 0.5 and 1.0 times the outer pile diameter. For offshore pile installation projects in the German sector of the North Sea, regulations dictate that the axial pile-bearing capacity is confirmed by conducting dynamic pile load tests, which include capacity determination using so-called extended procedures with complete modeling. An important item to adhere to is ensuring a uniform wave front at the measurement location. Therefore, German regulations state that the distance between the transducers and top of the pile shall be at least 1.0 to 1.5 times the pile diameter. For the project discussed in this paper, allowable pile stick-up regulations at a final penetration required the use of a front follower for pile driving. This caused a high risk of shearing off the monitoring provisions at a distance of 1.0 times the diameter while driving to the final penetration. It was therefore decided to equip the piles with monitoring provisions at two levels: an upper level at a distance of only 0.5 times the outer diameter to the pile head and a lower level at a distance of 1.0 times the outer diameter to the pile head. The upper level was equipped with a transducer set consisting of one strain gage and one accelerometer at every 90°. The lower level was equipped with a transducer set at each 180°. This monitoring layout made it possible to compare the measurements of both levels and to assess the differences. The results showed that despite being closer to the pile head than allowed by the regulations, the upper measurement position gave values comparable to those at the lower location.
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