## 4 Self-Sensing Geosynthetics
### Graphene-Based Self-Sensing Geosynthetics  
Graphene is a two-dimensional (2D) sheet exhibiting six-member rings organised in a honeycomb pattern. Graphene is the strongest, most flexible, and thinnest substance [131]. Owing to its unique molecular structure, graphene exhibits several extraordinary mechanical, electrical, and chemical properties including elasticity, substantial flexibility, and the ability to connect with numerous surfaces [132]. Graphene is harder and stronger than other well-known materials, and mono-layer graphene exhibits an elastic modulus and a tensile strength of approximately 1.1 TPa and 125 GPa, respectively [133]. In graphene, \(\pi\) electrons move at 1/1300 the velocity of light [134]. Moreover, the graphene electrical strength is negligibly influenced by electron interference.  
In addition, the graphene electrical conductivity is 60 times higher than that of single-walled carbon nano-tubes (SWCNTs) [135]. Furthermore, graphene conductivity remains constant over a wide temperature range, which is critical for reliability in a wide range of applications. Although the wide conductivity range provides graphene with outstanding electrical characteristics, the (theoretical) specific surface area is approximately twice as large as the CNT one, that is, 2630 and 1315 m\({}^{2}\)/g for graphene and CNTs, respectively. Consequently, graphene has attracted considerable attention in different research fields particularly owing to its exceptional characteristics.  
Graphene-based self-sensing geosynthetics offer remarkable advantages such as low cost, high flexibility, light weight, and easy synthesis. Moreover, the composite sensitivity may be further enhanced by optimising the synthesis method and doping with an appropriate graphene concentration [136]. A summary of various graphene-based polymers for application to self-sensing geosynthetics is listed in Table 5.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Composite Type** & **Fabrication Method** & **GF** & **Maximum Strain (\%)** & & **Description** & **References** \\ \hline Graphene/PDMS & Chemical vapour deposition & 151 & 5 & & & **Device wrinkle relaxation** & \\  & & & & & & already presents in composite structure, resistance first & \\  & & & & & & decreased until reaching 2.47\% & \\  & & & & & & **C** Electrical resistance increased in strain range 2.4-4.5\% & \\ Graphene/PDMS & Chemical vapour deposition & 151 & 5 & & & **Resistance irreversibly shifts with more than 5\% increase in strain, indicating that nano-composite structure has been destroyed and limiting composite working range to below 5\%** & \\ Graphene/PET & Drop casting & 0.11 & 7.5 & & & **Light-Scribe DVD burner** & \\  & & & & & & produces high-quality graphene stacks & \\ Graphene/PET & Drop casting & 0.11 & 7.5 & & **C** Graphene-based strain sensor & \\  & & & & & displayed linear response during multi-cycle operation, indicating sensor longevity and precision & \\ \hline Graphene/paper & Inkjet printing & 125 & 1.25 & & **Inkjet jet manufacturing** & \\  & & & & & technology enables quick & \\  & & & & & fabrication directly on surface and larger sensing area & \\  & & & & & **Inkjet printing facilitates** & \\  & & & & & customised printing conditions & \\  & & & & & and mixing of various & \\  & & & & & 2D materials & \\  & & & & & Drop-casting 99 printed layers is & ideal for synthesising very \\  & & & & & sensitive sensors, whereas 20 \(\,\mathrm{\SIUnitSymbolMicro m}\) & \\  & & & & & is optimal for synthesising highly sensitive sensors & \\ \hline \end{tabular}
\end{table}
Table 5: Summary of different graphene-based polymer composites for application to self-sensing geosynthetics.