Browsing by Author "Kodama, J"

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item: Conference-Full-text
Effect of fracture stiffness in a fault damage zone on seismic source parameters of induced fault-slip
(Division of Sustainable Resources Engineering, Hokkaido University, Japan, 2024) Gang, M; Sainoki, A; Kodama, J; Iresha, H; Elakneswaran, Y; Dassanayake, A; Jayawardena, C
It is well recognized that inherent stress concentration within a fault damage zone may lead to induced fault-slip, resulting in severe damage to underground facilities. Previous research suggests that the intensity of fault-slip is influenced not only by the mechanical properties of the fault core but also by the stiffness of the surrounding rock mass, implying that fracture stiffness could be an important factor that needs to be studied. Therefore, in this study, the effect of the fracture stiffness on seismic source parameters of induced fault-slip is investigated using a mine-wide scale heterogeneous continuum model. The model is constructed based on a discrete fracture network within a fault damage zone, utilizing the crack tensor theory and boundary traction method. The fault core is simulated as a discontinuous plane with interface elements at the center of the model, and fault-slip is induced by gradually reducing the effective normal stress on the fault plane. Seismic source parameters are computed and analyzed under various fracture stiffness conditions. Seismically radiated energy is defined as the work done by the stress perturbation across a closed surface at a distance from the earthquake source, while seismic moment is calculated using the moment tensor of a seismic source in an anisotropic medium. This study investigates increasing fracture stiffness while maintaining a normal-to-shear stiffness ratio of three. Dynamic analysis results reveal a notable impact of fracture stiffness on seismically radiated energy and seismic moment, both of which decrease significantly with increasing fracture stiffness. These findings imply the importance of considering fracture stiffness for more accurate estimation of seismically radiated energy and seismic moment.
item: Conference-Full-text
Fundamental study of the influences of discontinuities on rock slope displacement due to excavation
(Division of Sustainable Resources Engineering, Hokkaido University, Japan, 2024) Kubo, T; Zhang, C; Sainoki, A; Kodama, J; Iresha, H; Elakneswaran, Y; Dassanayake, A; Jayawardena, C
In the Higashikagoe limestone quarry, discontinuities are thought to be one of the factors contributing to the deformation of the rock slope. In this study, we investigated the impact of discontinuities and dip angles on slope deformation through numerical simulations, developing four models: a model without any discontinuity and three models with discontinuities of different dip angles. Results showed that discontinuities significantly influence deformation, with forward movement observed in models with discontinuities, aligning with field measurements. This suggests that discontinuities are critical to the deformation of the rock slope in the Higashishikagoe quarry.
item: Conference-Full-text
Numerical analysis of effects of clay on a cut rock slope deformation at an open-pit limestone mine, Japan
(Department of Earth Resources Engineering, University of Moratuwa, 2021-12) Amagu, AC; Zhang, C; Kodama, J; Iwasaki, H; Ikegami, S; Sainoki, A; Fukuda, D; Fujii, Y; Dissanayake, DMDOK; Jayawardena, CL
Rock slope instability is one of the major challenges of rock engineering projects, including open-pit mining. In this regard, rock slope deformation due to excavation, change in temperature, and influence of rainfall and snowfall have been previously investigated to understand characteristics and causes of slope deformation observed at an open-pit limestone quarry in Japan. The results only revealed characteristics of the deformation as forward and downward displacement of the cut rock slope, but its causes were not clarified. To deduce the causes of the rock slope deformation, we employed the 2-dimensional finite element method (2-D FEM) to investigate the deterioration effect of clay found at the footwall of the rock slope in terms of reduction in Young's modulus of the clay based on experimental results. Firstly, change in distances was analysed from displacement data measured by the automated polar system (APS) over five years, which decreases gradually with time. Secondly, the simulation results were discussed and then compared with the measured displacement data, which shows similar tendencies at the middle and top of the rock slope revealing maximum displacement at the middle of the rock slope. Conclusively, deterioration of clay at the footwall of the rock slope is one of the possible causes of the deformation in the quarry.
item: Article-Full-text
Tangent modulus method – An original method to measure in-situ rock stress
(Elsevier, 2018) Fujii, Y; Makasi, M; Kodama, J; Fukuda, D; Goto, K; Kumakura, S; Kanaoka, M; Dassanayake, ABN
This paper proposed Tangent Modulus Method (TMM) which is an improved oriented core method to determine in-situ rock stresses. In this approach, the cylindrical specimens prepared along different directions from thick core samples were uniaxially compressed twice to a given stress level. The stress value of the bending point in the first loading cycle of the stress-tangent modulus curve is considered as the normal component of the in-situ rock stress along the drilled direction of the specimen. Four types of rocks from soft porous tuff and sandstone to hard crystalline granite was investigated to evaluate the potential of this method. The effects of changes in strain rate, temperature, water content, confining and pore pressure, and stresses larger than the preload on the stress value of the bending point were experimentally investigated on preload specimens to investigate their influence on TMM. Comparison of the stress measurement results by TMM and an overcoring method at AK tunnel in Hokkaido, Japan was also performed to validate the TMM.
item: Article-Full-text
A Three-dimensional constitutive model for rocks based on a strain-dependent elastic modulus
(The Geological Society of America, 2022) Li, Z; Fujii, Y; Kodama, J; Fukuda, D; Dassanayake, ABN; Sinkala, P
Our research group previously proposed a simple two-dimensional (2D) constitutive model for rocks to simulate not only the axial stress–axial strain relationship, but also the axial stress–lateral strain relationship, with few complicated equations. However, the failure envelope that it predicted was linear, and it did not consider the effect of the intermediate principal stress (⁠σ2 ⁠). In the present study, the authors modify this simple 2D model to have a convex failure criterion. Then, the model is extended to a simple three-dimensional (3D) model that well approximates true triaxial stress–strain curves for real rocks under specific values of σ2 and σ3 and uses only four parameters. However, the predicted peak stress–σ2 relationship is linear. Finally, a modified 3D model was developed, which exhibited the true triaxial convex failure criterion. The equations in this model are simpler than the conventional true triaxial failure criteria. The proposed models can be implemented with a finite element method to improve the design of rock structures.
item: Article-Full-text
Water drainage from Kushiro Coal Mine decreased on the day of all M ≥ 7.5 earthquakes and increased thereafter
(Nature Publishing Group, 2018) Fujii, Y; Ichihara, Y; Matsumoto, H; Kodama, J; Fukuda, D; Dassanayake, ABN
The amount of water drainage from Kushiro Coal Mine in Hokkaido, Japan decreased on the day of all M ≥ 7.5 earthquakes with epicenters within 300 km of the mine during the monitoring period and increased after these earthquakes. This is a valuable finding which would give us a clue to understand pre- and post-seismic rockmass behaviors and contribute for progress in earthquake prediction in future.