Compressive Strength of a Pliocene Sedimentary Soft Rock Retrieved from Nankai Trough Ocean Drillings

Abstract

For comprehensive understanding of Nankai Trough seismogenic zone, IODP (Integrated Ocean Drilling Program / International Ocean Discovery Program) has conducted a deep ocean drilling project referred to Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) from 2007 to 2019 in off the Kii peninsula. As an investigation of physical properties of the subduction zone, to reveal compressive strength of the sediments 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 around 400 meters below seafloor at site C0006 and C0007 with ~4000 m water depths. The rocks are the Pliocene siltstones with a porosity of ~50%, classified as sedimentary soft rocks. As results of the triaxial compression tests, the cohesion ccu and internal frictional angle ϕ_cu by 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’ 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 case of such brittle failures, we found that specimen area correction in post failure for calculating differential stress might cause a non-negligible underestimation to the differential stresses. Therefore, we suggest the area correction for undrained triaxial compressive tests objected to soft rocks should be conducted until their peak strength if brittle failure occurs.