# Design and Evaluation of IoT-Enabled Instrumentation for a Soil-Bentonite Slurry Trench Cutoff Wall
## 2 System Design  
To investigate the stress state of a full-scale SB slurry trench cutoff wall, a full-scale SB slurry trench cutoff wall was constructed with the addition of the following instrumentation as shown in Figure 3 at a test site: (1) inclinometers at eight locations immediately outside the trench to measure lateral deformations as a function of depth; (2) earth pressure cell cages to measure the three-dimensional state of stress within the backfill at three different depths; (3) paired sensors to measure moisture content and suction within the backfill; (4) settlement plates to measure vertical deformation; (5) piezometers within the wall to measure pore water pressures and in-situ hydraulic conductivity of the backfill (by slug testing); (6) monitoring wells outside the wall (adjacent to the piezometers); and (7) stainless steel electrodes for ER imaging. Power for all instrumentation at the site was provided by a 40-watt solar panel and a 24-Ah battery. The earth pressure cell cage, illustrated in Figure 4, includes three vibrating wire stress sensors mounted in three cardinal directions to measure vertical and horizontal (longitudinal and transverse) stresses. Each cage also has one vibrating wire piezometer (to measure pore pressure) and a biaxial tiltmeter and magnetic compass (to measure the as-placed orientation). All sensors are connected to a Kevlar reinforced cable that extends vertically out of the wall for data acquisition. The cages were originally designed for use in mine paste backfill and, therefore, have an open structure that is ideal for allowing the fluid SB backfill to fill the cage and cover the sensors. The cages were placed at different  
Figure 3: Location of the instrumentation at the test site. depths within the slurry-filled trench (prior to backfill placement) and were fixed in place using a rope to the surface on a guide structure consisting of four vertical legs and cross braces. The vertical legs of the guide structure were embedded into the soil below the bottom of the trench serving to anchor the bottom of the guide structure. The top of the guide structure was affixed to ground anchors at the surface. Once backfill had displaced the slurry the gauges were released from their vertical restraints and allowed to settle freely with the backfill. The stress measurements are complemented with measurements of vertical and horizontal deformations obtained from settlement plates and inclinometers, respectively. There were a number of difficulties to be overcome for the successful installation of the instrumentation. One constraint was the need to hold instrumentation in a specific vertical and longitudinal location while backfill (a thick viscous liquid) flowed around the cages during backfilling. While the guide structure of four pipe legs and cross-bracing was largely successful, there was elastic bending of the legs. Since this system can serve as a model for health monitoring of SB cutoff walls, future installation should consider additional cross-bracing and/or thick wall pipe for the legs. Despite the elastic deformation of the legs, the instrumentation was held in place during backfilling and, once constraints were released, moved downward with the backfill to produce the necessary stress-strain compatibility between the instrumentation cluster and the backfill. The instrumentation was hard-wired to the data acquisition system located above ground in secure indoor space (a modified steel shipping container). The wiring was all buried and encased in a protective PVC pipe and no damage occurred to the wiring system. In order to remotely monitor the data measured by the instrumentation and to facilitate efficient analysis of the data, a web-based interface was desired. To collect data, a datalogger was programmed to read measurements from the instrumentation on site and coupled with a transmitter to send them continuously via radio signals to a receiver on campus as shown in Figure 5. The datalogger, along with the other instrumentation, was deployed at the site in July 2016.