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This study presents a novel fiber Bragg grating (FBG) extensometer system designed for the high-precision, continuous monitoring of ground displacement. Utilizing cost-effective 3D-printing technology, the device is engineered to provide accurate strain measurement in field applications while mitigating temperature cross-sensitivity. The sensor design involves embedding five individual FBGs within a thermoplastic polyurethane (TPU) substrate, a material selected for its superior flexibility and durability. These sensors are distributed at equal intervals along a horizontal PVC pipe structure and tested within a soil-filled chamber to simulate in-situ conditions. Experimental results demonstrate the system's effectiveness in detecting lateral ground movement, exhibiting an average responsivity of 0.0158 nm/cm and a high linearity of 99%. Furthermore, the integration of a specific reference FBG successfully provides temperature compensation for the entire array. Consequently, the proposed extensometer system proves to be a reliable and effective solution for continuous geostructural health monitoring.