Steel piles driven with Follower in Glacial Till and Chalk

Offshore, pile driving monitoring (PDM) is a well recognised practice which can be employed to 
control driving behaviour and determine pile hearing capacity. The use of an instrumented 
followerto perform pile driving monitoring and pile testing activities is less common due to the 
higher complexity of signal interpretation. However, PDM presents advantages in terms of cost 
optimisation during construction and testing activity, particularly in cases where the piles are 
driven below water and validation is needed for a large amount of piles.
This paper aims to present the testing procedure, the main field observations during pile driving 
activities and some examples of the interpretation of signals recorded during driving, of a vast 
pile driving campaign carried out on hollow cylindric piles of 0.61m OD and length variable between 
14m and 22m. These piles were mainly driven in glacial till deposits overcoming chalk.
A follower was used to drive the piles to target penetration with the aim of avoiding the use of an 
under-water hammer and sensors. This mobile add-on pile section was designed to allow operations in 
a water depth of approximately 20m. On a pre-defined number of piles, strain gauges and 
accelerometers were installed on the external shaft of the "test piles", and placed at two 
different levels respectively, at the top of the follower and the pile. This allowed the strains 
and accelerations to be measured at different levels, assessing energy losses between the pile and 
follower system.
Signal matching analysis procedure at both instrumentation levels was used to define the follower 
and pile behaviour while driving. This allowed the use of the instrumentation at only the follower 
top for the majority of the other tested piles.
Most of the piles were driven in glacial till deposits, and signal matching analyses performed at 
the end of continuous driving evidenced a high variability in space of the estimated statie soil 
resistance to driving. Short and long-temt restrikes were performed to estimate the long-term pile 
capacity, evidencing a relevant gain in terms of back-analysed statie resistance only after a 
significant waiting period.
In some locations, the pile tip was entering by 1/3 of the pile shaft within chalk deposit. On 
these piles, the long-temt capacity was also estimated. Recorded signals after a short term 
restrike or driving interruption evidenced a fast evolution of the interface behaviour. An increase 
of pile hearing capacity is proven with long-term restrikes as well. This paper discusses the main 
findings of this pile driving testing campaign, with focus on the functionality of an instrumented 
follower to drive piles in shallow water.
Understanding the hammer/follower/pile/soil system in detail enables to instrument the follower 
rather than the pile top and allows the validation of a large number of piles in both a cost and 
time effective way.
Soil plug measurement was performed on all tested piles to check the global behaviour of the pile 
and soil while driving, and supporting hypotheses used for the signal matching analyses.