## 1 Introduction  
A subgrade is an important part of a road structure. Settlement is a prevalent issue often encountered in subgrade use that causes adverse effects on the stability and initial strength of the subgrade. Consequently, the strength and stability of the pavement are compromised, completely degrading the subgrade structure and minimizing driving safety and comfort. Unfavorable physical characteristics of the underlying soil layer and external load conditions make many special soil subgrades prone to large-scale continuous settlement and deformation, resulting in excessive long-term settlement after the completion of the embankment [1, 2]. Temperature changes cause uneven lateral settlement of permafrost subgrades [3]. Long-term monitoring of subgrade settlement and related early warning strategies play a crucial role in preventing and controlling major disasters. Conventional monitoring technologies for subgrade settlement, such as inclinometers, extensometers, total stations, global positioning systems, and static leveling systems, have been in use for a long time and are generally classified as \"traditional point-based monitoring technologies\" [4]. Emerging monitoring technologies include interferometricsynthetic aperture radar (INSAR), digital image correlation (DIC), and terrestrial laser scanning (TLS). For wide-area monitoring, INSAR [5; 6], TLS [7], and DIC [8] can achieve high-precision measurement over a wide range, reaching centimeter-level or even higher precision. However, these technologies are mainly used to measure the displacement of the outer surface of a subgrade or pavement structure. Because INSAR monitoring is limited by weather and blocked objects, ultrahigh resolution data are rarely obtained. TLS and DIC measurements have the advantages of a wide range, high precision, and a high acquisition speed, but continuous long-term uninterrupted observation is difficult to realize. The aforementioned traditional point-based monitoring techniques or quasidistributed fiber Bragg grating can monitor the dense internal deformation of a subgrade. However, a limited number of subgrade monitoring sections can be selected in advance, and monitoring blind spots are inevitable. Increasing the number of sensors results in a sharp increase in the complexity and cost of the system, which is not feasible for long-distance and large-scale monitoring [9]. The aforementioned problems have affected the ability to identify disaster risks in road engineering. Therefore, the development of automated technology for the real-time monitoring of distributed internal deformation in a subgrade would constitute a substantial advancement in the study of the early induction process and evolution mechanism of large-scale uneven settlement in a subgrade. The distributed optical fiber sensor (DOFS) is such an alternative monitoring technology [10]. This novel monitoring technology has been developed over the last 20 years. Compared with electromechanical measurement, optical fiber sensing has the advantages of being safe, explosion-proof, resistant to corrosion and high temperatures, small, lightweight, flexible, and convenient to use; it is also able to prevent electromagnetic interference and it can realize remote signal transmission and measurement control. Since the first report on using Brillouin scattering in optical fibers to measure distributed temperature and strain [11], various distributed optical fiber monitoring technologies have been developed, such as the optical time domain reflectometry (OTDR), Brillouin optical time domain reflectometry (BOTDR), Brillouin optical time domain analysis (BOTDA), and optical frequency domain reflectometry (OFDR), resulting in considerable improvements in the sensing range and spatial resolution. OTDR is primarily used to measure ambient temperature, and its spatial resolution can reach about 1 cm [12]. BOTDR can be used in long-distance distributed sensing; however, it achieves a rough spatial resolution of 1 m [13].