For a comprehensive understanding of the Nankai Trough seismogenic zone, the Integrated Ocean Drilling Program/International Ocean Discovery Program (IODP) conducted a deep ocean drilling project referred to as the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) from 2007 to 2019 off the Kii Peninsula, Japan. To investigate the subduction zone’s physical properties and reveal the sediments’ compressive strength, we carried out consolidated–undrained triaxial compression tests with pore water pressure measurements on NanTroSEIZE core samples. The core samples used were retrieved from depths of approximately 400 m below the seafloor at sites C0006 and C0007 with ∼4000 m water depths. The rocks are Pliocene siltstones with a porosity of ∼50%, classified as sedimentary soft rocks. As a result of the triaxial compression tests, the cohesion c_cu and internal frictional angle φ_cu derived from the total stress analyses ranged in 1.8–1.9 MPa and 15–17°, respectively. In addition, the cohesion c′ and internal frictional angle φ′ determined by the effective stress analyses were within 1.6–2.2 MPa and 18–28°, respectively. All the specimens tested caused brittle failure and formed clear shear fractures. In the case of these brittle failures, we found that post-failure specimen area correction when calculating differential stress in post-failure might cause a nonnegligible underestimation of the differential stresses. Therefore, we suggest that the area correction for undrained triaxial compressive tests of soft rocks should be conducted until reaching their peak strength if brittle failure occurs.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 71 (2022) 251–258. The abstract and caption of Fig. 6 are slightly modified.