Closure to “A Parametric Study on Factors Affecting Ground Vibrations during Pile Driving through Finite Element Simulations” by Mo Zhang and Mingjiang Tao

The authors applied the finite-element method (FEM) to improve calculations of the peak particle velocity of ground vibrations generated by pile driving. This method has been successfully used to resolve various static problems in geotechnical engineering. However, the results are substantially different in the application of FEM to dynamic problems for which only general tendencies can be determined (for example, Broers and Dieterman 1992). Analyzing the accuracy of several prediction methods based on FEM simulation, Holscher and Waarts (2003) found disappointing low reliability in predicting vibration levels from dynamic sources. Therefore, the authors’ attempt to use FEM for assessing ground vibrations from pile installation should be welcomed. In practice, Eq. (1) of the original paper is usually used for approximate calculations of the expected peak particle velocity (PPV) of ground vibrations at various distances from driven piles (Wiss 1981). This equation renders assessment of PPV attenuation between two points on the ground surface. Eq. (1) provides a very rough assessment of ground vibrations as a function of the source energy and a distance from the source. Also, Eq. (1) does not take into account soil conditions, pile penetration depth, soil resistance to pile penetration, soil heterogeneity and uncertainty, and soil-structure interaction and has nothing to do with structural vibrations, dynamic settlements, and vibration effects on sensitive equipment. Of course, this equation has no connection with prediction of ground vibrations. On the one hand, approximate calculation of expected ground vibrations and even vibration monitoring yield relative information on vibration effects on structures, and these results could be inconclusive. On the other hand, condition surveys of structures before and during pile installation provide complete information on structural responses to vibration excitations, and this information can be much beneficial than vibration assessment and measurements. It is obvious that rough calculations of expected ground vibrations and comparison of those with the vibration limits is not vibration risk assessment because risk management requires analysis of overall factors affecting structures from pile driving. The authors could apply FEMto develop amethod for calculations of expected ground vibrations from pile driving, and they calculated PPV of ground vibrations on the basis of the model chosen but without analysis, parametric study, and further development of the application of FEM for prediction of ground vibrations generated by pile installation. The prediction method was not developed. The authors mostly utilized FEM simulation for assessment of the effects of the soil damping ratio and Young’s modulus of soil on coefficients k and n in Eq. (1). Also, the authors studied the influence of the number of loading pulses on ground vibrations due to pile driving and they received negative results. This could be expected because it is well known that ground vibrations attenuate between two consecutive hammer ram impacts. In other words, powerful FEM was employed for evaluation of the coefficients in the empirical equation. The authors compared the results of the FEM simulations with two equations derived byWoods (1997) and received a similar trend of them with a suitable fit to one of two lines in Fig. 2(b) of the original paper. Because comparisons of the obtained results with scaled-distance equations for Soil Classes II and III from Woods (1997) were shown in the paper several times, additional information on soil classification is needed. A classification of earth materials for four classes by attenuation coefficient indicating the material damping was proposed by Woods and Jedele (1985), and they performed a study of attenuation of ground vibrations in Soil Classes II and III. From Woods (1997), Soil Class II is competent soils—most sands, sandy clays, silty clays, gravel, silts, weathered rock (can dig with shovel and 5 , N , 15); Soil Class III is hard soils—dense compacted sand, dry consolidated clay, consolidated glacial till, some exposed rock (cannot dig with shovel, must use pick to break up and 15 , N , 50). For a spatial task of wave propagation in the layered soil medium from dynamic sources, the authors used the basic two-dimensional (2D) FEMmodel for which soil was presented as a single layer with the thickness of 50 m. Such an approximation is nonadequate to the real soil conditions and pile installation because the model does not take into account a number of the previously mentioned factors. In particular, the model used cannot reflect well-known facts that ground vibrations at the same distances from the same source at various directions are different and soil stratification strongly affects vibrations at the ground surface. However, the authors received good agreement with the scaled-distance equation for Soil Class II [Fig. 2(b) of the original paper]. It seems that 2D FEM model is acceptable for various soil conditions with values from a standard penetration test (SPT) between 5 and 15. The authors did not comment on these results. Also, it is necessary to keep in mind that pile installation in Soil Class III with values from SPT between 15 and 50 generates more intensive ground vibrations than pile driving in soil class II. This problem was not recognized. Before analysis of the parametric study, it is necessary to recall two known facts. First, as mentioned previously, Eq. (1) provides assessment of PPV attenuation between two points on the ground surface at any distance from the source, and according to a definition, k is PPV of ground vibrations at one unit of distance from driven piles. Therefore, for known PPV at Point 1, Eq. (1) provides calculation of PPV at Point 2 or vice versa for PPV measured at Point 2, PPV at Point 1 can be determined with back analysis of Eq. (1). Woods and Jedele (1985) made numerous measurements of ground vibrations at various distances from diverse dynamic sources, and they studied ground attenuation between numerous pairs of measured PPV on the ground surface. The coefficient k as PPV of ground vibrations depends on the source energy and a scaled-distance from the source. Second, experimental studies have confirmed that the coefficient n changes in the 1–2 narrow range and, in general, it appears that the n range is independent of soil type, energy source, and energy level,