# Mobile LiDAR for Scalable Monitoring of Mechanically Stabilized Earth Walls with Smooth Panels
## 1 Introduction  
Mechanically stabilized earth (MSE) walls are low-cost and have easy-to-install precast concrete panels. This type of infrastructure is used extensively as it resists destabilizing earth forces acting onreinforced soil areas [1]. MSE walls are often found along transportation corridors. Just as an example, there are roughly more than 1500 MSE walls in the state of Indiana, USA, apart from those on local public agency routes [2]. An MSE wall has multiple components to ensure its efficacy including a precast paneled facade and backfill layers that have been compacted and strengthened through a metallic or geosynthetic reinforcement material [3]. Modular facing blocks, which make up the facade, can be either smooth or textured--with the former constituting the majority of existing MSE walls [4; 5]. The primary purpose of these panels is to prevent the backfill material from spilling out yet allowing excess water to seep and escape through the joints. Collapse of an MSE wall will have a high repair price tag in addition to endangerment to corridor users. Therefore, an accurate yet scalable inspection methodology is important. Governing bodies--e.g., the U.S. Department of Transportation (DOT)--relies heavily on the long-term efficacy of the MSE walls and therefore looks to organizations such as the American Association of State Highway and Transportation Officials (AASHTO) for a standard set of serviceability measures. AASHTO utilizes longitudinal angular distortion (\(\alpha_{L}\)) and transversal angular distortion (\(\alpha_{T}\)) as part of their standard serviceability measures (AASHTO 2014). The \(\alpha_{L}\) is described as the differential settlement between two points along an MSE wall row of panels divided by the lateral distance between these points. The \(\alpha_{T}\) is the differential normal-to-plane displacement of the MSE wall along a vertical profile divided by its height. The acceptable \(\alpha_{L}\) values for an MSE wall with precast concrete panels depend on the panel area and joint width. For example, for a 19 mm joint width and panel area in the range from 2.8 m\({}^{2}\) to 7 m\({}^{2}\), the tolerable \(\alpha_{L}\) is 1/200 [6]. During construction, the tolerable \(\alpha_{T}\) values for an MSE wall must be less than 1/160 while using a 3.048 m straight edge. Post construction, \(\alpha_{T}\) should be less than 1/240 when measured using a vertical line [6]. The U.S. Federal Highway Administration (FHWA) has additional specifications for tolerable out-of-plane displacements between neighboring panels [7]. Such specification dictates that out-of-plane displacement at a joint must not exceed 9.53 mm (3/8 in.) throughout the construction stage. These standard serviceability measures, excluding the out-of-plane displacement, are considered global measures that concentrate on the vertical settlement and lateral deflection of an MSE wall. In spite of the common acceptance of these standard measures, most MSE wall failures start with local deformations. Just as an example, Figure 1 shows a local deformation where failure resulted from an excessive angular tilt between neighboring panels. Therefore, any type of effective monitoring should deliver measures that assesses the deformation pattern for each panel individually within an MSE wall. [PERSON] et al. [5] provide examples of proposed measures that fit these criteria. Figure 1: An illustration of (**a**) Mechanically stabilized earth (MSE) wall failure that could have started with (**b**) an excessive bulging at the joint between neighboring panels (images sourced from [PERSON] [8]). A monitoring approach needs to take into consideration the ease of its implementation and scalability. Traditional surveying instruments, e.g., a total station; geotechnical field instrumentation, e.g., an inclinometer; and terrestrial laser scanning (TLS) are commonly used data acquisition modalities. These data acquisition tools are time-consuming and require direct access to the MSE wall site.