## 4 Self-Sensing Geosynthetics
### Carbon-Fibre-Based Self-Sensing Geosynthetics
Indeed, low strains could only be properly detected in CF/glass fibre composites by computing the residual resistance under pre-stress conditions [159; 160]. Braided composite rods (BCRs) are another continuous carbon fibre polymer for developing smart geosynthetics and have exhibited excellent low-strain sensitivity [161; 162; 163; 164]. In BCRs, a CF and glass fibre mixture was treated with polymer resin and was axially over-braided with polyester filaments. The freshly braided structures were subsequently cured to manufacture the composite rods. By adjusting the polyester filament braiding tension and velocity, a certain degree of misalignment can be introduced to the axial CF, thereby changing the electrical contacts and, as a result, markedly changing the low-strain electrical resistance. The BCR strain-sensing capabilities were improved by reducing the composite CF content, and the maximum GF of approximately 24 was obtained at a flexural strain of 0.5%. The BCR cross-section and manufacturing process are illustrated in Figure 17.  
Recent developments in manufacturing technologies have enabled functional and complex polymer composites to be rapidly and cost-effectively produced. Fused deposition modelling (FDM) is one of the most conventional manufacturing processes, which has currently attracted the attention of many researchers owing to its potential applicability, ease of use, and low cost [165; 166]. [PERSON] et al. used FDM-based 3D printing to develop a continuous carbon fibre polymer composite and subsequently detect and recognise stress and damage locations [167]. Figure 18 clearly shows a continuous carbon fibre tow (CFT) grid and a thermoplastic (TP) matrix were automatically printed using a double-nozzle FDM printer. In this smart composite, the location of the applied stress and the resulting  
Figure 16: Configuration of CF/GF hybrid polymer composite rods. strain were deduced according to the maximum fractional change in the CF tow electrical resistance, as shown in Figure 18b. Figure 17: BCR (**a**) cross-section and braided surface and (**b**) manufacturing process. Figure 18: (**a**) Schematic of smart structure and production sequence (from top to bottom layers) and (**b**) fractional change in electrical resistance for each continuous carbon fibre tow [167]. Non-polymeric continuous carbon fibre composites are also suitable for developing self-sensing geosynthetics owing to their consolidation capabilities and increased interactions. However, the lack of a polymer reinforcement increases the effects of moisture on their sensing capability, thereby rendering this composite more prone to environmental factors. [PERSON] et al. [168] developed a non-polymeric hybrid continuous carbon and glass fibre fabric (Figure 19) for application to moisture and damage sensing. The fabric produced an electromechanical sensing GF of approximately 1 and used the Wheatstone bridge to detect a humid environment by fractional variations of approximately \(10^{-5}\) in the electrical resistance. Currently, micro- and nano-CFs have attracted attention to enhance polymer component sensitivity for application to self-sensing geosynthetics. Indeed, conductive nano- and micro-paths formed by carbon nano- and micro-fibres are sensitive to external factors including low stress, strain, deformation, humidity, and temperature. [PERSON] et al. [7] used 3D printing to develop an integrated piezoresistive geobelt wherein conductive acrylonitrile butadiene styrene (ABS) containing \(\varnothing\)7 lm \(\times\) -10 lm long CF powder was directly embedded into non-conductive polylactic acid (PLA) to fabricate a conductive wire. The geobelt stress and deformation could be determined because the conductive wire resistance increased with decreasing wire cross-sectional area. The integrated resistance geobelt eliminated sensor contact with external humidity, which could otherwise lead to sensor failure and erroneous test results.