block would only last long enough to drive one or two piles, which posed quite an i s sue when combined wi th high production costs. In the light of the above, it had been apparent that neither of the materials would be sufficiently good on its own. The collaborating suppliers, who are experts in the fields of plastic material manufacturing and modifying, confirmed that it wasn’t helpful to use additives or to increase the carbon content (aiming to enhance thermal conductivity). Aluminum blocks shaped like thick disk springs would assume a cylindrical shape over time, while nylon blocks with extra cooling holes would still melt in the core. In pursuit of a better solution, three targets were established for the development of a new cushion block: 1. Improve pile driving performance 2. Enhance durability and longevity 3. Reduce noise generated during driving What We Know and What We Want to Know — Methodology There is no ideal material for cushioning the impact from the drop weight. Construction is not the only industry in need of a damping material capable of efficient heat transfer (i.e., cooling). If it were, it would be widely used; so, it could be assumed that nihil novi sub sole (that is, there is nothing new under the sun). Nonetheless, Dawson Construction Plant decided to create a laminate component, consisting of alter- nating aluminum and nylon layers. To create a complete image of the potential benefits and risks, it was decided that three prototypes would be created. The layers would vary in thickness, shape and number of aluminum plates. To achieve these goals, we’d retain, as much as possible, the nylon’s damping properties and the aluminum’s durability and heat transfer efficiency. The preliminary research consisted of simulations and numerical approximations of the effects the chosen variables would have on the cushion block, so we could try and achieve the optimal results for each prototype and then gauge expectations accordingly. The first insights were gathered using AllWave-PDP, pile driving prediction software for impact pile drivers that is based on numerical stress wave analysis. Three cushion block models were made to represent soft nylon (modulus of elasticity, E ~3 GPa or 435 ksi), hard nylon (E ~6 GPa or 870 ksi) ) and aluminum (E S~69 GPa or S S 10,000 ksi). Similar for all of the modelled components of the pile driver, the dimensions of the cushion blocks were based on an actual HPH2400 hydraulic piling hammer. During the simulation, the setup modeled driving a AZ 18-700 pile into two arbitrarily prepared strata models — one representing a generic granular soil and the other a cohesive soil. plastic variant performed better, where a 13% and 5% decrease in the maximum the blow count was observed compared to the aluminum and sof t nylon blocks, respectively. Likewise, nylon performed better protecting the pile head, where there was a decrease of more than 15% in the maximum compression when using this softer material. It was speculated that the improvement in driving performance when the nylon block was used was due to the longer rise time and decay of the impact. Having prolonged impulse means that the force acting downwards on the pile won’t be as affected by the returning wave. However, this improvement comes at a cost of attenuating the peak force, which means that mobilizing large piles might prove difficult in some situations. In contrast, when using aluminum cushions, the impact imparted tends to be harder but shorter in duration and with more high frequency harmonics. As a result, the force acts downwards very quickly (relatively) and then the recoil is almost instantaneous (depending on the pile’s length). Thermal and explicit dynamics Finite Stress wave model setup In both soils, the nylon cushion blocks performed better. The soft nylon cushion block seemed to be better suited for the granular soil, as a 21% decrease in the maximum blow count was observed compared to aluminum block and a 5% decrease when compared to the hard nylon block. In the cohesive soil, the harder Three prototype Nylon-Al cushion blocks Element Analyses (FEA) were performed next. The thermal FEA investigated the heat transfer capabilities of the two materials used. From the analyses, we could assess how much cooling of the nylon could be achieved with various shapes, sizes and numbers of aluminum plates. It came as no surprise that even the thinnest plates had a significant effect, given the thermal conductivity of 6061 aluminum alloy is 600 to 800 times greater than that of Nylon6. A design using two thin aluminum plates efficiently cooled the nylon and decreased the maximum temperature in the nylon plate by nearly 68% compared to the solid nylon cushion block in a steady state regime. In addition, each variant facilitated the dissipation of heat from the previously melting core, shifting the central plane of the nylon plates closer to the outer faces. The series of explicit dynamics FEA simulations revealed the stress-strain patterns in conventional and the various test designs, as well as the extent of elastic deformation and stress concentration. Interestingly, the equivalent stresses DEEP FOUNDATIONS • JAN/FEB 2020 • 73
