## 2 Retaining Wall Displacement Measurement Review  
### Problems with Retaining Wall Displacement Measuring Instruments  
The stability of retaining walls is assessed complexly. This is done by examining the water level, ground subsidence, and the displacement of the retaining wall using different measuring instruments, such as inclinometers, water-level meters, strain gauges, load cells, and tiltmeters. The lateral displacement of the retaining walls is an important factor for identifying the behavior of the retaining wall [4; 12]. Consequently, in several countries, including the United States, China, and Korea, the lateral displacement threshold is set, while carefully managing the lateral displacement of the retaining wall [22; 23; 24].  
Generally, inclinometers are used at construction sites to determine the lateral displacement of retaining walls. They are installed before underground excavation to examine the lateral displacement of the retaining wall during the excavation stage and the retaining wall retention period. To measure the lateral displacement of the retaining wall using an inclinometer, a hole with a depth above the excavation depth of 2-3 m is bored at a specific distance from the retaining wall, and the inclinometer casing is installed in the hole. This is followed by grouting in the inclination casing and the installation of a protective cover, as shown in Figure 1. After the inclinometer casing is installed, a worker inserts an inclinometer into the casing to measure the lateral displacement of the retaining wall according to the measurement cycle. However, measuring the lateral displacement of the retaining wall using an inclinometer has three problems:  
1. Difficulty installing inclinometer casing  
Since the inclinometer casing must be installed around the retaining wall after boring to measure the displacement of the retaining wall using an inclinometer [5; 25], heavy equipment (boring equipment) and manpower are required. Additionally, if the inclinometer casing is twisted while being inserted into the bored hole, the inclinometer will get stuck, which will cause errors when measuring the retaining wall displacement.  
2. Measuring 2D local lateral displacement of a retaining wall  
Owing to the nature of inclinometers wherein only the 2D cross-section of the retaining wall can be measured [5; 25; 26], it is quite challenging to determine the overall lateral displacement of the retaining wall. Further, it is difficult for measurement experts to identify areas where maximum deformation may occur and select the best optimal inclinometer installation position based on the results of the ground survey at the construction site and the design of the retaining wall [1].  
3. Measurement by manpower  
The lateral displacement data of the retaining wall are collected at each measurement point after the inclinometer is moved to measurement depth when a measurement expert visits the construction site once or twice a week (measurement cycle). However, as this measurement method that is based on manpower, errors may be detected in the collected data, since the measurement point changes depending on the expertise of the measurement expert [27]. Moreover, considerable manpower is required because measurements must be done at all measurement points near the retaining wall. Additionally, since it takes approximately one week for the measurement expert to prepare an analysis report using the collected data, it is difficult for a construction site manager to evaluate the stability of the retaining wall in real time.