FEATURE ARTICLE Screening Tool for Impact Hammer Selection When selecting an impact hammer to drive a particular pile, the decision is often based on local experience. In those cases, the contractor falls back on previous experience or draws on the experience of the hammer manufacturer to select the hammer that will be able to do the job. Obviously, a more scientific approach would be to perform a pile driving simulation using one of several commercially available software packages (e.g., GRLWEAP, AllWavePDP or PDPWave). However, this type of analysis requires quite a bit more effort. First, an accurate model of the pile driving process has to be developed, which includes a model of the soil profile at the job site and input into the analysis program. Then, the model must be analyzed to assess whether the combination of hammer, pile and soil model work. Over the years, numerous individuals and organizations have tried to develop some- thing that would fit nicely in between these approaches: something more advanced than a gut feeling yet something that could be done on the back of an envelope; in other words, a “Goldilocks” approach — not too simple, not too complex, but just right. Screening Tool Criteria The goals of the approach are twofold: (1) select the appropriate impact hammer to drive an open toe steel pipe or H-pile, and (2) then use the model for the subsequent high strain dynamic testing (HSDT). The Goldilocks approach centers around five different criteria. 1. Maximum Driving Stress To minimize the risk of damage, the maximum driving stresses are generally limited to 80 to 90% of the yield stress (fy) of the steel section. This approach does not consider issues such as hammer alignment, initial imperfections, denting and/or elastic buckling, and pile material fatigue history. However, even when these issues are not Recommended driving energy, momentum and ram weight limits Source API AREMA AREMA ASCE ASCE CFEM Crapps Hunt Phan et al. Prakash Basis Energy Energy Energy Energy Energy Energy Limit Maximum Minimum Preferred Range Pa ≤ 534 kN Pa ≥ 534 kN Maximum Momentum Minimum Energy Energy Energy ASTM D4945 Weight Maximum Minimum Maximum Original Formula Wr·He/As ≤ 4200 Wr·He/Pa > 0.026 0.038 < Wr·He/Pa > 0.052 Wr·He/Pa > 0.038 Wr·He/Pa > 0.048 Wr·He/As ≤ 6000 Wr·√He/As ≥ 1575 Wr·He/As ≤ 4200 Wr·He/Pa ≥ 0.043 Wr·He/As ≤ 6300 Preferred Range 0.01 ≤ Wr/R ≤ 0.02 Units kJ/m2 kJ/kN kJ/kN kJ/kN kJ/kN kJ/m2 Derivation Wr·He/As ≤ 4200 Wr·He/As > 2250 Units kJ/m2 kJ/m2 3280 ≤ Wr·He/As ≤ 4460 kJ/m2 Wr·He/As ≥ 3270 Wr·He/As ≥ 4100 Wr·He/As ≤ 6000 kN/m3/2 Wr·√He/As ≥ 1575 kJ/m2 kJ/kN kJ/m2 kN/kN Wr·He/As ≤ 4200 Wr·He/As ≥ 3760 Wr·He/As ≤ 6300 2580 ≤ Wr/As ≤ 5160 Note: For the conversion it is assumed that Pa = 86000·As where As is the steel area of the pile in sq m. For the ram weight criteria, we assumed that R = 3·86000·As AUTHORS David Tara, Thurber Engineering, Ltd. and Gerald Verbeek, Allnamics USA and Verbeek Management Services DEEP FOUNDATIONS • NOV/DEC 2017 • 101 kJ/m2 kJ/m2 kJ/m2 kJ/m5/2 kJ/m2 kJ/m2 kJ/m2 kJ/m3 applicable, limiting average stresses to 0.9fy or even 0.8fy does not necessarily mitigate the risk of pile damage (Mostafa, 2011). 2. Maximum Impact Velocity and Critical Height of Fall The theoretical maximum induced stress in a pile due to impact driving can be determined analytically. For a very stiff pile cushion, this stress is not a function of the ram mass but of the pile impedance and ram velocity. For steel with yield stresses between 43,500 and 50,760 psi (300 and 350 MPa), the theoretical critical height of fall or drop height (Hcr) varies then between 9.2 and 12.5 ft (2.8 and 3.8 m), assuming friction losses are disregarded, which corresponds to impact velocities of about 24.3 and 28.2 ft/s (7.4 and 8.6 m/s), respectively. These drop heights seem reasonable given that most diesel hammers operate successfully with equivalent strokes of up to about 11.5 ft (3.5 m).
