# Mobile LiDAR for Scalable Monitoring of Mechanically Stabilized Earth Walls with Smooth Panels
## 2 Literature Review  
The far majority of MSE wall monitoring techniques are based on manual and visual inspection in the field with the help of basic tools such as measuring tapes, profile/contour gauges, inclinometers, and plumb lines. These strategies can lead to irregularities arising from human subjectivity [10]. Moreover, they require physical contact with the MSE wall, which might not always be possible and/or safe to field inspectors. [PERSON] and [PERSON] [11] proposed an approach for monitoring retaining walls with precast concrete panels using TLS, whereby multiple temporal scans were collected and analyzed to discern movements within a column of panels along the MSE wall. In their research, three or more spherical static targets in each scan are used as tie points for establishing the coarse registration between the scans. The authors suggest that the targets remain on site throughout the monitoring process. [PERSON] et al. [1] employed TLS for point cloud acquisition and derivation of performance measures for MSE walls. Unwanted objects, such as brackets and scaffolds, were removed from point cloud data and then a random sample consensus (RANSAC) algorithm [12] was used to fit a planar model through the wall. The points representing joints between the panels were isolated as outliers when checking the normal distances between the point cloud and the fitted wall-plane. The joint distances are then used as the serviceability measure. [PERSON] et al. [13] used a mobile mapping system (MMS) for a scalable inspection of earth walls along Austrian highways. In their work, a practical approach was proposed using an MMS composed of two laser profilers, a differential global navigation satellite system (GNSS) receiver, an inertial measurement unit (IMU), and multiple cameras, which provided true colors for the derived point clouds. Vertical profiles at 5-cm intervals were created from the captured point clouds along the retaining wall by intersecting planes orthogonal to the vehicle's trajectory with the wall surface. A regression along the vertical profile derived the tilt angle of the MSE wall facade. Their research was able to identify tilt angles with an accuracy of \(\pm 0.1^{\circ}\). However, the inspection measure was restricted to evaluating the transversal tilt angle. [PERSON] et al. [5] employed TLS to evaluate the standard measures (i.e., \(\alpha_{L}\) and \(\alpha_{T}\)) of MSE walls with smooth panels. [PERSON] et al. [5] also defined measures that represent in-plane and out-of-plane angular tilts and movements of the individual panels relative to the encompassing MSE wall faces. In this process, [PERSON] et al. [5] introduced two coordinate systems, which are referred to as the Leveled Face (\(LF_{cs}\)) and Panel (\(P_{cs}\)) coordinate systems for the individual faces and panels, respectively. The \(LF_{cs}\) considers the local horizontal/vertical directions within the site and utilizes a fitted plane through a manually cropped MSE wall face. The \(P_{cs}\) definition starts by identifying individual panels through a segmentation process, where the local point spacing and normal distance between the points and the best fitting plane through that face are determined and used as the segmentation criteria. After panel segmentation, \(P_{cs}\) was defined using the bounding box for each panel--i.e., the minimum bounding rectangle--MBR [14]. The translational and rotational relationships between the \(LF_{cs}\) and \(P_{cs}\) were used to establish new serviceability measures. [PERSON] et al. [15] introduced a procedure for monitoring MSE walls with textured precast concrete panels to evaluate standard serviceability measures as well as those proposed by [PERSON] et al. [5]. This preliminary work has shown the possibility of using mobile LiDAR for deriving reliable serviceability measures for textured walls. However, the ability of handling smooth MSE walls still need to be addressed. Moreover, evaluating the potential of this technology in handling large MSE wall sites needs more investigation.