## 1 Introduction  
Most countries have established restricted-access and controlled areas for critical facilities. Such areas include not only critical structures such as power generation facilities but also the necessary infrastructure for securing the area. The infrastructure must be both economical and reliable, necessitating the continuous monitoring of its long-term behavior. Due to its national situation, South Korea has many areas with controlled or restricted-access. This makes the monitoring of the enclosed infrastructures by experts challenging due to time constraints and concerns related to monitoring accuracy. Therefore, it is necessary to find a way for non-experts to have free access to utilize the restricted-access or controlled area to monitor the infrastructure. Mechanically stabilized earth walls (MSE walls) are representative structures in restricted-access and controlled areas. MSE walls are based on the principle of reinforced earth; i.e., reinforced earth increases the shear strength of the ground because the interaction between the soil and highly tensile reinforcement inhibits ground deformation due to an increase in the internal friction angle, the development of adhesion forces, and an increase in confining stresses [1; 2]. MSE walls are preferred for securing the stability of various structures, such as slopes, bridges, roads, etc., due to their economic construction and high field applicability [3; 4; 5; 6; 7; 8; 9]. Hence, MSE walls are commonly found worldwide [10; 11; 12; 13]. In the past, South Korea primarily used reinforced concrete walls to secure important facilities. However, due to their superior applicability, the construction of MSE walls has increased rapidly since the 1990s. In particular, the scale of MSE walls has increased due to the securing of more effective sites and the enlargement of facilities, and they are increasingly constructed using facing blocks. An MSE wall consists of wall facings, reinforcements, and backfill soil. The wall facing not only forms the appearance of the MSE wall but also prevents the loss of backfill soil. Generally, wall facing is applied to panels or blocks. Reinforcements are divided into non-extensible and extensible reinforcements according to the material properties. Non-extensible reinforcements include smooth steel strips, ribbed steel strips, and steel grids. Elastic reinforcements include geosynthetics, such as geogrids and geotextiles. The reinforcement interacts with the backfill soil to form the reinforced earth zone of the MSE wall [12; 13]. The design of the MSE walls is evaluated for both external and internal stability. The external stability assessment considers the reinforced earth as an integral structure and applies the same concepts as those applied in the stability of a gravity-retaining wall, i.e., base sliding, overturning, and bearing capacity. Internal stability is evaluated in terms of slip failure, pullout of reinforcement, and tensile overstress. In addition, global stability, including that of reinforced earth, is considered if the MSE wall is applied to a slope or if there is a high embankment slope behind the MSE wall [1; 2; 14; 15; 16; 17; 18]. When problems arise with the external, internal, and global stability of a constructed MSE wall, obvious symptoms emerge, such as cracking and the settlement of the wall facing. However, as with all infrastructure, following stabilization, MSE walls can develop structural instability over time, necessitating ongoing maintenance. In particular, damage to the MSE walls is very difficult to repair due to their structural characteristics, which require fast, economical, and reliable detection and monitoring [12; 13]. The maintenance of structures is dominated by traditional and passive surface monitoring instrumentation methods, such as inclinometers, displacement meters, crack gauges, and total stations.