Seasonal changes in eigenfrequencies of structures supported on pile foundations

The paper is concerned with studying the dynamic characteristics of constructions and their response to vibrations initiated by weak natural environmental actions (microseismic, wind and other impacts). Such investigations require special equipment capable of recording the development of dynamic processes. Since the structures are constantly exposed to natural impacts of winds and seismic noises, it is reasonable to conduct continued observations or monitoring. The study focuses on the dynamic behavior of structures supported on pile foundations under natural outside impacts, among which the wind action and microseismic actions of different mechanisms, for example, cars are most common to urban environment. The investigation is based on the analysis of eigenfrequency, which is one of the most informative dynamic parameters for estimating the current state of constructions. Thus, a variation in the spectrum of eigenvalues is indicative of a change in the stiffness properties of the structure elements due to damage accumulation and also suggests the appearance of changes in the soil and contact pressure between the soil and foundation. In this paper, a continuous monitoring technique is used to analyze the frequency spectrum of the structure, to determine its eigenfrequencies and their seasonal changes and to perform numerical simulations allowing an adequate description of seasonal impacts on the structure frequency. The lowest eigenfreqiencies of the structure have been determined from the numerical experiment on natural frequencies of the structure-pile foundation-soil system, in which coupled springs are used to model the interactions between the pile foundation and soil. It has been found that seasonal changes in eigenfrequencies of structures supported on pile foundations are caused by frost penetration into the soil, which affects the stiffness properties of the soil-pile foundation-structure system.