## 2 Strain-Gauge-Integrated Smart Geosynthetics  
Typically, strain gauges are designed to assess the strain and stress behaviours of materials [18; 19]. However, they are often employed with rigid objects such as concrete, steel bars, and metal plates, and few studies have been conducted on their application to synthetic materials such as geogrids and soft fabrics. The application of strain gauges to geosynthetic instrumentation was first reported by [PERSON] and [PERSON] [20] in 1989 to measure the displacement of a geotextile-reinforced test embankment constructed on a soft organic clayey silt deposit in Canada. Thirty-four electrical resistors and seven electromechanical and seven mechanical strain gauges were installed on the symmetry axis perpendicular to the force applied to a high-strength 360 \(\times\) 210 cm polyester woven geotextile exhibiting an ultimate tensile strength of 216 kNm\({}^{-1}\) to measure the strain in the transverse direction. The strain in the longitudinal direction was detected using four electrical resistance strain gauges installed at different locations. The authors measured a comparatively small strain (maximum of approximately 2%) when the embankment thickness was increased to the maximum height. However, they reported that the strain gauges were easily damaged and that despite considerable care, five of the original 48 were damaged during transport from the storage shed and placement of the geotextile in the field. In addition, the output electrical resistance of some strain gauges was affected by humidity. With increasing knowledge for producing high-precision strain gauges, efforts to develop intelligent geosynthetics have gradually expanded. For example, [PERSON] et al. [21] and [PERSON] and [PERSON] [22] also integrated geosynthetics with strain gauges to detect strain in 1992. They used high-performance copper-nickel (Cu-Ni) strain gauges to measure the load and strain of a woven geotextile independently in prototype field centrifuge tests. Strain gauges were installed on both sides of a woven geotextile through two epoxy strips, and the results were compared with Instron and photographic measurements. However, the extension strain gauges were not as effective. Another strain-gauge-based smart geosynthetic was reported by [PERSON] and [PERSON] [23]. They installed 12 pairs of foil strain gauges in different directions along the centreline strand of an 870 \(\times\) 740 mm extruded polypropylene biaxial geogrid to investigate the stabilising force provided by a geogrid layer. The geogrid was placed in the body of a sloped fill loaded from a footing near the crest. The authors found that using only one strain gauge per location markedly reduced the accuracy of the tensile strain and geogrid force estimated based on the nominal stiffness, particularly at low loads, and noted that considerable caution is required when utilising such an approach. Indeed, the geogrid reinforcement was reliably instrumented by installing strain gauge pairs (i.e., on the bottom and top faces of the geogrid) at each location across the geogrid reinforcement, and the use of the average strain minimised the influence of the geogrid flexural strains. To investigate similitude conditions in modelling geosynthetic geogrid materials, [PERSON] and [PERSON] employed special strain gauges on low-elastic-modulus materials without reducing the gauge sensitivity [24]. Their strain gauges were 3 \(\times\) 2.3 mm and exhibited a 9.5 \(\times\) 4 mm backing and a nominal resistance of 120 \(\Omega\). A gauge factor (GF) of 2.13 and a strain limit of 3% were used. The geogrids were instrumented with tiny strain gauges adhered using strengthened epoxy adhesive to the base material. Their results were difficult to calibrate and had to be calibrated during several loading and unloading cycles until a reproducible response was achieved. In addition, the influence of the base epoxy adhesive materials on the geogrid tensile strength-strain behaviour must be considered.