## 4 Self-Sensing Geosynthetics
### Carbon-Nano-Tube-Based Self-Sensing Geosynthetics  
Due to the continuous and considerable decline in the carbon nano-tube (CNT) price from approximately 1,500,000 USD/kg in 1999 to the current retail price in the range 50-300 USD/kg [122], the research and development of self-sensing geosynthetics has recently found new potential and prospects. Owing to their highly desirable properties such as high flexibility, high aspect (i.e., length-to-diameter) ratio, high specific surface area, low density, and remarkable mechanical and electrical characteristics, CNTs are increasingly utilised in nano-composites. [PERSON] et al. [8] incorporated a specific CNT concentration into HDPE and PP to prepare piezoresistive and UV-protected geosynthetic prototypes and compared their performances with those of geosynthetic-containing CB. Their results indicated that the CB-filled specimens exhibited a marked piezoresistivity response compared with the CNT-filled PP composites. Table 4 lists the mean GFs of the composites tested by [PERSON] et al. [8]. The greater piezoresistivity response and sensitivity of the CB-containing specimens were attributed to the grape-bunch-like CB fillers, which formed a less entangled network and, consequently, resulted in higher piezoresistivity than the needle-shaped CNTs under tensile strain. [PERSON] et al. [8] demonstrated that self-sensing geosynthetic technology could lead to a more cost-effective and dependable alternative to currently available strain sensors (i.e., gauges). However, the application of CNTs to self-sensing geosynthetics remains problematic because CNTs do not suitably disperse in polymers. Although \"dispersion\" usually refers to uniformly distributed individual fillers in the host matrix, the term must be re-defined for fillers such as CNTs, which intrinsically thermodynamically physically agglomerate with neighbouring tubules to form aggregated bundle morphologies. Each bundle contains hundreds of closely packed CNTs held together by van der Waals attraction energies of approximately 500 eV/m per CNT-CNT [123]. This entanglement and agglomeration is particularly important in CNTs owing to their flexibility and high aspect ratio [124]. Indeed, enhancing CNT dispersion in a polymer composite would necessitate a trade-off between the composite mechanical and electrical properties. Usually, complete dispersion leads to smaller bundles and, as a result, smaller stress concentration zones, which could affect the mechanical performance, weight, and manufacturability of the polymeric matrix [125]. However, bundling and agglomeration both reduce the percolation threshold [126]. Because of the effectiveness and importance of both the electrical and mechanical properties of self-sensing geosynthetics (particularly utilised self-sensing geosynthetics), an optimal mixing and fabrication technique must be devised to produce the desired level of CNT bundling. Hence, determining effective dispersion strategies and evaluating the resulting dispersion quality are critical steps in designing CNT-polymer composites. The effects of processing conditions and dispersion quality on the mechanical and electrical performances of PVC composites containing multi-walled CNTs were evaluated by [PERSON] et al. [127; 128] for application to self-sensing geosynthetics and other applications involving electrically conductive polymer composites. The authors investigated the mechanical properties and electrical conductivity of specimens prepared using various mixing methods and quantified the specimen subsurface dispersion using an approach devised by [PERSON] et al. [129]. Their results revealed that specimens exhibiting superior dispersion also exhibited higher ultimate failure strain and strength, whereas specimens exhibiting poor dispersion also exhibited higher elastic moduli. The mechanical properties and piezoresistivity of tensile-loaded MWCNT-reinforced PVC composites were also investigated by [PERSON] et al. [130].