Case history: Influence of ground water level on vibrations induced by heavy forging hammers

Heavy forging hammers in an industrial complex in Switzerland produce vibrations which are felt in surrounding residential areas as far away as 800 m. The current case history documents several campaigns of recording vibrations at the source as well as received in a 3-storey house at a distance of 410 m from the forging hammers. The vibrations recorded on the third floor of the house clearly exceeded the acceptable values at certain times, whereas they did not exceed the limits at other times. The relevant subsoil conditions and properties are known from seismic and geotechnical investigations. The subsoil consists of gravelly material with a highly fluctuating groundwater table. The groundwater table is influenced by a nearby river. It exhibits fluctuations of several meters, continuously recorded by a piezometer. Because the intensity felt by the inhabitants and also measured at the receivers varied considerably over time, it was suspected that the groundwater level in relation to the foundation base of the forging hammers could cause this variability even during constant forging operations. Detailed dynamic Finite Element Analyses (FEA) have been carried out to investigate the influence of the fluctuating groundwater level on the reduction of the vibrations travelling through the subsoil. The FEA clearly showed the groundwater table fluctuations to have a considerable influence of up to 35% on the vibrations felt and recorded at the reference house. The active control of the groundwater table along with other vibration reduction measures are shown to reduce the vibrations to acceptable values.