Frequency content of vertical ground vibrations caused by surface impact Wersäll, C. Department of Civil and Architectural Engineering, Royal Institute of Technology, Stockholm, Sweden Bodare, A. & Massarsch, K. R. Geo Risk & Vibration Scandinavia AB, Solna, Sweden Keywords: vibration, frequency, pile driving, impact, falling weight, shear wave, shear strain ABSTRACT: Impact pile driving is often restricted by regulations of ground vibrations that affect surrounding structures, installations and residents. While previous studies have focused on prediction of vibration amplitude, this paper studies the frequency content. Vibrations can be particularly significant when the pile penetrates stiff surface layers. This phase is essentially analogous to impact of a falling weight on a steel plate, placed on the ground surface. Results from falling weight tests are evaluated with regard to frequency content of ground vibrations. Comparison of test results with theoretical models shows that the most important factors influencing dominating frequency of vertical ground vibration are pile dimensions, pile mass, and hammer mass. An often-neglected aspect is the dynamic stiffness of the soil immediately below the plate/pile toe while the influence of deeper soil layers on frequency content appears to be negligible. Soil modulus and wave speed are strain-dependent and the strain softening effect must be taken into consideration in the analysis. Results of field tests are compared with theory. The analogy between falling weight tests and pile driving, and its limitations are discussed. 1 INTRODUCTION Although impact pile driving is a cost-effective foundation method, its efficient application in urbanized areas is often restricted by environmental regulations and, in particular, by limitations based on permissible ground vibrations. In spite of its wide application, relatively little effort has been devoted to development of practically applicable concepts to predict ground vibrations. Massarsch and Fellenius (2008) proposed a concept, which makes it possible to assess vibrations that are generated along the shaft and at the pile toe during pile penetration. However, this concept does not consider how the pile driving affects frequency content. One important aspect, which can be addressed in terms of frequency, is vibration amplification occurring in soil layers or buildings adjacent to the pile installation location. When the vibration frequency is in a certain range (commonly 10 to 20 Hz), it can coincide with resonance frequencies of building floors or structural elements. It is not uncommon that vertical and/or horizontal vibrations are amplified in buildings by many times (Athanasopoulos and Pelekis, 2000; Xia et al., 2005). This aspect is taken into account in several vibration standards, requiring that the project engineer assesses the risk of vibration amplification. The dominating frequency caused by impact pile driving depends on the characteristics of the impacting system (pile, hammer etc.) as well as on soil properties. Figure 1 illustrates that the transfer of vibrations from the hammer (A), through the pile (B) and into the surrounding soil (C) is a complex process. Experience suggests that ground vibrations can be high in particular during the initial driving phase, when the pile penetrates a stiff soil layer or fill close to the ground surface. During this phase, the distance between the vibration source and adjoining buildings is shortest. The frequency content of ground vibrations caused by surface impact is analyzed theoretically and the significance of different driving parameters is discussed. Results from falling weight tests are compared to simulation of a simplified driving system, consisting of a mass impacting on a stiff plate/pile located at the ground surface. The results of field tests are compared with theoretical predictions. The consequences of strain softening on soil stiffness are addressed. 877