## 4 Self-Sensing Geosynthetics
### Metal-Based Self-Sensing Geosynthetics  
Most fillers in this category are metal oxide and metal nano- and micro-particles such as iron, gold, and silver. The characteristics of these nano-materials vary drastically and are vastly different from their bulk counterparts. These nano-particles are semiconductors, making them suitable candidates as sensitive components for applications to different self-sensing geosynthetics.  
[PERSON] et al. [105] developed a metal nanoparticle-based polymeric rod sensor from 400 nm gold and 20 nm chromium thin films. Because a polyimide substrate and an  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**OF Sensor Type**} & \multicolumn{4}{c}{**Measurement Precision**} & \multirow{2}{*}{**Detection Area**} & \multirow{2}{*}{**References**} \\ \cline{2-2} \cline{4-6}  & & & **Spatial** & & **Temperature** & **C** \\ \hline BOTDR * & 30 & \textless{}1 & - & Landslide & [78] \\ BOTDR & 30 & 0.5 & - & Secant-piled wall & [79; 80] \\ BOTDA ** & 100 & 2 & - & Dike & [81] \\ BOTDA & 2 & 1 & - & Landslide & [82] \\ BOTDA & 10 & 2 & 0.5 & movement and & [83] \\  & & & & pipeline & \\ BOTDR & 30 & 1 & - & Landslide & [84] \\ BOTDR & 20 & 1 & 1 & Soil deformation & [85] \\ BOTDA & 2 & 1 & 0.1 & Pipeline & [86] \\  & & & & Energy of pipe & \\ BOTDA & \textless{}2 & 0.5 & \textless{}0.1 & cast-in-place & [87] \\  & & & & & concrete pile \\ BOTDA & 20 & 1 & 1 & - & [88] \\  & & & & & Rock & [89] \\  & & & & deformation & [89] \\ \hline \hline \end{tabular}
* Brillouin optical time domain reflectometry, ** Brillouin optical time domain analysis.  
\end{table}
Table 2: Summary of OF-sensor-based geosynthetics applied to geotechnical monitoring.  
easy production method (i.e., magnetron sputtering) were used, the sensor performance metrics--including GF and sensitivity--were also reasonable (4.4-6.9 and 0.0086 \(\Omega\)/ppm, respectively).  
[PERSON] et al. [107] described a straightforward method of incorporating non-carbonaceous fillers into a polymer matrix. Different weight fractions of \(\sim\)100 nm silver nano-particles were dispersed in ethylene glycol solvent and were stabilised by incorporating a polyvinylpyrrolidone (PVP) surfactant. The silver nano-particle suspensions were drop-cast in moulds composed of polyimide tape on a glass slide. The silver films were then annealed for 20 min at 160 \({}^{\circ}\)C. Subsequently, a two-part poly-dimethyl siloxane (PDMS) mixture was poured over the annealed silver films and cured for 2 h at 70 \({}^{\circ}\)C. The PDMS film thickness was maintained at approximately 0.5 mm. The composite films were then peeled from the glass slides, and the electromechanical activity and morphology were analysed using a universal testing bench and scanning electron microscopy, respectively. Owing to the fabrication approach, PDMS penetrated the voids in the silver film, resulting in strong interfacial locking and void elimination. This also increased the resistance of the deposited silver film. The component sensitivity and GF were affected by both the silver nano-particle concentration and the applied strain. GF also increased owing to increased inter-particle spacing between percolating nano-particles with increasing strain. The authors reported a GF of approximately 109.4 for 130% strain and 0.3 wt.% silver nano-particles. The specimens containing 0.1 wt.% silver nano-particles exhibited an even greater GF, i.e., 268.4 for the maximum strain of 110%. Although the sensor behaviour stabilised after the initial \"perturbations\" during the normalised resistance cyclic response, the early cycle tests resulted in the emergence of micro-cracks in the silver films, which negatively impacted experimental repeatability.