# Design and Evaluation of IoT-Enabled Instrumentation for a Soil-Bentonite Slurry Trench Cutoff Wall  
[PERSON]  
1 Computer Science, Bucknell University, Lewisburg, PA 17837, USA 1  
dry bentonite. The ingredients are blended to form a flowable backfill that is pushed into the trench displacing the slurry. The process is shown schematically in Figure 1 and photos from the research site described in this paper are shown in Figure 2.  
Despite the fact that SB cutoff walls are commonly used, field measurement of performance of these walls is lacking [3]. Since hydraulic conductivity has been shown to be stress dependent [4; 5], it is necessary to understand the state of stress in the SB cutoff wall in order to accurately determine the hydraulic conductivity. As evidenced in Figures 1 and 2, a soft compressible material is placed in a narrow trench. During consolidation of this soft material, there is stress transfer from the SB backfill to the sidewalls of the trench (the less compressible formation material). As a result the stresses within the backfill are less than geostatic, that is, less than the self-weight of the material. Models developed to predict the stresses in SB cutoff walls, include \"arching\" model [2] and a \"lateral squeezing\" model [6]. The arching model assumes that the trench walls are rigid. The lateral squeezing model assumes the sidewalls move inward and there is stress and strain compatibility between the backfill and the sidewalls. The modified lateral squeezing (MLS) model which takes into account the stress-dependent nature of SB backfill compressibility [7]. None of these existing models relate the stresses in all three directions and they are all models rather than direct in situ measures of stress, therefore, knowledge of in situ stress is required to better predict the in situ hydraulic conductivity.  
In order to investigate the stress state of SB walls, a full-scale, instrumented, and internet-connected SB slurry trench cutoff wall was constructed with the support of the National Science Foundation [8]. The SB cutoff wall was outfitted with instrumentation to directly measure stresses in all three principal directions as well as vertical and lateral deformations. Data was collected  
Figure 1: Illustration of the excavation and backfill process to construct a soil-bentonite slurry trench cutoff wall [2].  
Figure 2: Excavation and Backfill at the test site.  
and transmitted from the site and stored in a database. A website dashboard was developed to display the sensor information.  
The main contribution of this work is not any one specific component but the demonstration and evaluation of technologies used in combination to bridge the technological divide between traditional scientific instrumentation and the IoT. The current state of IoT technology is sufficiently mature that we were able to construct a highly reliable solution by following established best practices and available open-source software packages. This paper presents a model that can be followed by other researchers to reap the benefits of IoT without sacrificing scientific accuracy or data integrity. The instrumentation system coupled with modern IoT data collection and visualization has proven extremely useful in understanding the changes in the stress state over time and could be deployed elsewhere as a means of on-demand slurry trench cutoff wall structural health monitoring for real-time stress detection linked to hydraulic conductivity.  
The structure of this paper begins with a discussion of the SB wall instrumentation in Section 2. Following this, the primary issues of Data Collection, Transport, Storage, Visualization are detailed in Sections 3-6 respectively. Results from the past two years of operation are discussed in Section 7. Finally, conclusions and future work are presented in Section 8.