Scour around bridge foundations is regarded as one of the predominant causes of bridge failures. Traditional methods primarily employ underwater instruments to detect bridge scour depths, which thus have difficulties in instrument installations and operations. The concept of scour detection derived from vibration-based damage detection has been explored in recent years to address such difficulties by investigating the natural frequency spectrum of a bridge or a bridge component. This paper presents a comprehensive review of existing studies on scour detection using the natural frequency spectrum of a bridge or a bridge component. Underlying mechanisms, laboratory and field tests, numerical studies, and data processing schemes are reviewed to summarize the state of the art, which are absent but urgently needed. Updates on recently developed scour monitoring sensors are also provided to complement the introduction. Based on the review, in-depth discussions in existing studies are made regarding a few controversial and unsolved issues to shed light on future research, highlighting issues such as the soil–structure interaction, locations of the sensor installation, and the influence of shapes of scour holes.

The relationship between unfrozen water content (or saturation in freezing and thawing) and temperature, which is referred to as the phase composition curve in frozen soils, is a fundamental relationship in cold regions engineering. Because of the lack of a physical basis, there have been only empirical equations for this relationship. This study investigated the mechanisms underlying the phase composition curve. A detailed physical basis was established on the basis of the soil freezing characteristic curve, the Clapeyron equation, and the bundle of cylindrical capillary model. From this physical basis, a closed-form equation was developed for the formulation and prediction of this curve. This physically based equation quantifies the unique correlation between saturation and temperature in frozen soils and takes various factors into account, such as the soil saturation under unfrozen conditions and the lowest temperature used in experiments. The equation was validated with phase composition data measured with a thermal–time domain reflectometry sensor. In addition, the flexibility of the equation and its excellent applicability in various soils with a wide range of properties, in large temperature ranges, and in both freezing and thawing processes (hysteresis) were proved with reported data.