Abstract: Fibre optic sensors are used to monitor the integrity of a subsurface concrete structure such as a pile or diaphragm wall. A fibre optic sensor array (48) is attached to a reinforcement or framework assembly (20) for the subsurface concrete structure. Concrete is applied to surround the reinforcement or framework assembly (20) and fibre optic sensor array (48). The fibre optic sensor array (48) is then used to collect temperature data during hydration of the subsurface concrete structure. The temperature data is monitored in real time to determine differentials across the structure, indicative of a problem within the structure.