In-Situ Deep-Sea Monitoring of Cement Mortar Specimen at a Depth of 3515 m and Changes in Mechanical Properties after Exposure to Deep Sea Condition

The results of the first-ever in-situ monitoring of a large mortar specimen at a depth of 3515 m in the Nankai Trough are presented in this study targeted at creating a technology platform for in-situ monitoring and evaluation of cement-based materials at the seabed to realize deep-sea infrastructures. We successfully monitored in situ the development of strain and hydraulic pressure in the specimen. In addition, the short-term behavior of the specimen can be explained by hydraulic confinement and stress relaxation due to water infiltration. Some contraction strain remained in the specimen even after approximately an exposure to the deep sea condition for one year, causing microstructural damage. The pore entry volume was enhanced toward the center of the specimen, and a decrease in compressive strength and Young’s modulus were observed in the specimen after exposure due to the microstructural damage. Further improvement of the in-situ measurements is required to ensure the waterproofing and pressure resistance of the strain and pressure gauges.