# Assessing the State of Structural Foundations in Permafrost Regions by Means of Acoustic Testing
## 1 Introduction  
In recent decades, climate change has had a serious impact in regions where the ground temperature continuously remains below 0 \({}^{\circ}\)C. These regions are called permafrost regions. According to the data in [1], the Polar Urals have been witnessing a steady increase in atmospheric temperature in both winter and summer, as well as an increase in precipitation in winter, which prevents the ground from cooling down. In central Yakutia, the depth of seasonal thawing has reached 80 cm over the past 28 years [2]. For the Russian Federation, the problem of permafrost degradation is of particular concern because a lot of the country's mineral and hydrocarbon resources are located in the Arctic zone [3, 4]. Permafrost degradation puts buildings and engineering structures under threat, as evidenced by emergencies that have occurred in recent times [5, 6, 7]. In such conditions, it is vital to timely register when frozen soils become plastic-frozen to ensure the safe usage of buildings and structures.  
Currently, there are a variety of methods and tools that have been developed for permafrost monitoring. One of the major methods is temperature control. There are two key approaches to how data is obtained on permafrost temperature: measuring soil temperature profiles, using sensors installed in boreholes [5, 8, 9], and satellite remote sensing of the earth's surface [10, 11]. There are a lot of mathematical models that have been developed to assess temperature distribution in soils [10, 12, 13, 14]. Regardless of the approach chosen, the criterion for assessing the state is the soil freezing temperature. As a result of the anthropogenic impact on frozen soils, their physical and chemical properties can change. The activation of freeze-thaw processes greatly aggravate this problem [14, 15]. It is also important to note the influence of the freeze-thaw cycle affecting the physical andmechanical properties of soils [16]. Temperature control methods are often used as part of integrated condition monitoring systems [17; 18]. Along with temperature, changes in the moisture content of frozen soils, the depth of seasonal thawing, and elevation data on foundations are monitored.  
Various methods are used to find elevation data for the foundations of structures, including levelling, differential interferometry, and global navigation systems [19; 20]. In a number of cases, deformation control does not allow for obtaining timely and reliable information about the state of the foundation. Frozen soil is a four-phase system that includes solid particles, ice, water, and gas. As a result of thawing, a phase transition of ice into water occurs. As water is almost incompressible, it is necessary that it should drain away and free the pores in order for the soil to settle. In addition, soils can move both upward (when swelling) and downward (when settling) and when such movements compensate for each other, no absolute deformation will be registered.  
Recently, geophysical methods for permafrost monitoring have been gaining in popularity. In particular, they are used to monitor the humidity and depth of the layer of seasonal thawing [21; 22]. The methods described in [21; 22] are based on the difference in the permittivity of water and ice.  
As the literature review presented here shows, none of the currently available permafrost monitoring methods make it possible to directly assess the change in the deformation and strength characteristics of soils, despite the fact that these are the key characteristics in the design of structural foundations. In our opinion, this problem can be solved by applying an acoustic method [23; 24], since the speed of propagation of an elastic wave is directly related to the elastic characteristics of the medium in which it propagates.