Current Issues — Problem Stated The research on cushion blocks and the internal mechanics of pile drivers, in general, is very limited. It seems to have mostly come to a halt at the handbook level. While the entire machine serves its purpose, there seems to be a lack of optimization and only a few modifications or improvements now and then. If the pile driver on the jobsite doesn’t drive your piles, a larger machine will be used and hopefully succeeds. Sometimes, the overall infeasibility of such action is considered (e.g., energy consumption and potential damage to piles/machinery) but not always. It makes sense — the job has to be done. Furthermore, if the design of the equipment is to maintain a competitive edge, each manufacturer is forced to protect the sensitive information pertaining to and obtained from the equipment. It may be that disclosing data collected from the machines’ on-board data acquisition (DAQ) system is impossible; however, manufac- turers may still be well suited to collaborate and share their respective findings to advance the overall pile driving industry. Originally, wooden blocks were used as a cushioning material during a pile driving operation. However, as the machines became larger, the cushioning material became too costly to maintain. Due to the internal friction and when the impact force and rate become high enough, the blocks catch fire quickly after commencing pile installation. As such, wooden cushioning blocks are now considered impractical, since they last for no more than a few installations of driven piles, even when using smaller pile drivers. Other materials used contemporarily for cushioning are canvas phenolic (sometimes sandwiched with aluminum), steel wire/rope, nylon, aluminum blocks and various metal springs (disk, plate, etc.). There were very few resources to build upon when the research on cushion blocks s tar ted. There was al ready some speculation about the two materials Dawson Construction Plant had been using – Aluminum 6061 and Nylon6 (PA6), which seem to represent two extreme cases. Aluminum cushions are very durable and resilient. They deliver a large portion of the imparted energy to the pile and don’t 72 • DEEP FOUNDATIONS • JAN/FEB 2020 Conventional aluminum cushion block after catastrophic failure provide damping nor attenuation. Their main failure mode is brittle fracture, which poses a danger of catastrophic failure. In larger pile drivers, it had been noticed that the aluminum blocks are even more susceptible to cracking, presumably due to 3D phenomena, such as non-unidirectional stress wave propagation. The risk isn’t great since the initial cracks are easy enough to spot during visual inspections. However, the cushions also emit high pressure levels of noise, which limits their use to non-noise restricted projects. On the other hand, nylon exhibits intrinsic damping properties. Due to its viscoelastic nature, the peak impact force is attenuated, and the blow softened. However, the driving performance of the hammer doesn’t deteriorate compared to the (elastic) aluminum counterpart. On the contrary, the anecdotal evidence has shown an improvement in efficiency, less noise generated and less damage to the machine/pile. However, the material isn’t flawless. It was reported that, in cases of more intense driving, the very center of the cushion block had melted and, sub- sequently, the component sheared through a nearly perfect conical plane. Nylon heats up due to stress-strain hysteresis — for each blow, a portion of energy is converted to heat. In the hardest soils, the cushion Conventional nylon cushion block after melting shearing catastrophic failure
