## 2 Retaining Wall Displacement Measurement Review
### Previous Studies and Technologies Related to Retaining Wall Displacement Measurement  
Previous studies and technologies addressing inclinometer problems were reviewed, as shown in Tables 1 and 2. The existing techniques are divided into contact and non-contact measurement methods. In contact measurement methods [6; 7; 8; 9; 10; 11], the lateral displacement data of the retaining wall are collected using different wireless communication technologies (WiFi, LoRa, 5G, and RFID) and conventional inclinometers, while the stability of the wall is evaluated using a separate analysis module. From these methods, measurement experts can determine the lateral displacement of the retaining wall without visiting the construction site, while the construction site manager can evaluate the stability of the retaining wall in real time. Owing to these benefits, different contact measurement instruments have been commercialized and applied in the field [8; 9; 10; 11]. However, using inclinometers in these methods has the following limitations: (1) they are difficult to install and (2) the measurement of the displacement of the retaining wall is limited to 2D local lateral displacement, which does not apply to 3D deformation. Figure 1: Inclinometer casing installation and measurement of the lateral displacement of the retaining wall using an inclinometer. In non-contact measurement methods, the lateral displacement of the retaining wall is determined by installing the latest measurement technologies, such as TLS, mobile laser scanning (MLS), and cameras, around the retaining wall in order to solve problems related to contact measurement methods. These methods can determine the 3D displacement of the retaining wall by installing measuring instruments at fixed measurement points (TLS [1; 2; 4; 12; 13; 14] and on cameras [3; 15; 17]) or moving objects such as vehicles, (MLS [5; 16]) with no additional heavy equipment. The MLS-based method is suitable for horizontally long retaining walls, such as those on highways. However, it was difficult to apply the MLS-based methodology to retaining walls constructed for underground excavation, which are the focus of this study, since they are vertically installed depending upon the excavation depth. The applicability of TLS-based displacement measurement was verified by several researchers using field tests on retaining walls, which are related to excavation sites [1; 2; 4], highways [12; 13], and tunnels [14]. In the TLS-based displacement measurement method, the displacement of the retaining wall is determined mainly by collecting point cloud data according to the measurement cycle and analyzing the differences in the distance between the point cloud data. This method is effective for measuring the displacement of retaining walls, because the error is in millimeters. However, the TLS equipment is expensive, and the measuring personnel must install and dismantle the equipment constantly because it is not suitable for continuous outdoor use. In vision-based displacement measurement, the displacement of the retaining wall is determined by examining the changes in the images according to the measurement cycle or estimating point cloud data using different vision methods, such as digital image correlation [15], photogrammetry [3], and KAZE [17]. This equipment can be permanently installed outdoors to measure the displacement of the retaining wall cost-effectively compared to the TLS equipment. However, a target plate is required to increase the displacement analysis accuracy and reduce the error to millimeters [3]. The error significantly increases when the target plate is damaged or missing. Further, the results of related studies are based on laboratory experiments, making it is difficult to apply the vision-based displacement measurement methodology to construction sites that are vulnerable to rain and snow, owing to the nature of the cameras. Hence,