Earth Crust Engineering
To make a continuous progress of human beings, new technologies to develop mineral and energy resources would be required considering the preservation of global environment and to utilize and maintain underground space. In our lab, by integrating rock fracture mechanics, rock friction dynamics, rock hydraulics, seismology, and so on, we intended to contribute to the development of new technologies to overcome various difficult conditions.
Academic Staff
Eiichi FUKUYAMA
Professor (Graduate School of Engineering)
Research Topics
Large scale rock friction experiments and fault rupture simulation along complicated fault system have been conducted based on rock fracture mechanics and dynamics of rock friction to evaluate the strength of Earth's crust and stress applied there, whose information had been considered to be difficult to evaluate. Large scale friction apparatus is a very unique testing machine in the world. Using this apparatus, scale dependency of friction on the sliding surface size and detailed rupture front propagating along the fault have been studied. In addition, using boundary integral equation method, dynamic rupture propagation along complicated fault system can be achieved, which provides us with an information on the strength of the fault as well as the applied stress to the fault system. These may lead us to understand the evaluation of crustal rock strength.
Contacts
Room 355, Bldg. C1, Katsura Campus, 615-8540, Kyoto, Japan
TEL: +81-75-383-3209
E-mail: fukuyama.eiichi.3x
kyoto-u.ac.jp
Yoshitaka NARA
Associate Professor (Graduate School of Engineering)
Research Topics
When designing sub-surface structures in a rock mass, it is necessary to consider their long-term stability. For this purpose, understanding brittle deformation and fracturing in rock is essential. In order to contribute ensuring the long-term stability of structures in a rock mass, I am conducting studies related to rock mechanics and fracture mechanics. Specifically, I am investigating influences of environmental conditions (e.g., temperature, humidity, and water) on physical and mechanical properties of rock.
Contacts
Room 356, Bldg. C1, Katsura Campus, 615-8540, Kyoto, Japan
TEL: +81-75-383-3210
E-mail: nara.yoshitaka.2nkyoto-u.ac.jp
Nana YOSHIMITSU
Assistant Professor (Graduate School of Engineering)
Research Topics
Waveform analysis of micro fractures observed during rock compression experiments and induced earthquakes accompanied with resource mining unveil the stress and heterogeneity condition of the seismogenic environment. Broadband records observed in a laboratory will make it possible to apply similar analysis technique with natural seismic events to laboratory events. Interaction between laboratory scale analysis and natural scale analysis lead a universal understanding of fracture process in a crust.
Contacts
Room 354, Bldg. C1, Katsura Campus, 615-8540, Kyoto, Japan
TEL: +81-75-383-3211
E-mail: yoshimitsu.nana.6ikyoto-u.ac.jp
Research Topics
Experimental and numerical study on rock friction using large-scale biaxial shear friction apparatus
Rocks have many cracks at deep depth in the crust and the propagation of cracks results in unstable rupture of basement rocks (i.e., earthquakes). Because of high confining pressure at depth, the existence of cracks does not always lead to the failure of the rocks in the crust, but the friction controls the propagation of the rupture on the crack.
In our lab, we are doing large scale friction experiments using meter-scale rock blocks (Fig. 1). From these experiments, we obtained spatial heterogeneity in friction on the fault surfaces, which could be a main cause in scale dependence of friction. In addition, since the fault length is long enough, we are able to measure the details of rupture propagation and compare them with those predicted by the theory. These information serve significantly for the construction of dynamic friction constitutive law, which is quite useful to model and predict the rupture propagation in rocks.
Moreover, we are investing the relation between the lab friction and that in the field using numerical simulation approach (Fig. 2). Especially, the fault surface geometry significantly affects the propagation of rupture because of heterogeneous distribution of stress on the fault. Whether crack propagates as rupture or not depends on the balance between the friction and stress on the fault and can be evaluated through the numerical simulation approach.
Figure 1. Large-scale biaxial rock friction apparatus (owned by National Research Institute for Earth Science and Disaster Resilience).
Figure 2. Numerical simulation results for the 1995 Kobe earthquake using boundary integral equation method. Fault geometry, stress field around the fault and friction on the fault were properly taken into account.
Influence of surrounding environment on fracturing in rock
It is essential to ensure the long-term integrity of a rock mass surrounding structures, such as the repository of radioactive wastes, the cavern to store liquified petroleum gas or natural gas, or the underground power plant. For the construction of these structures in the underground, the evaluation of long-term strength of rock is required. Therefore, it is important to investigate the time-dependent deformation and fracturing in rock under various environmental conditions.
We have investigated slow crack growth (subcritical crack growth) in rock using a fracture mechanics test called "Double Torsion method". Specifically, the influence of surrounding environment on the crack velocity has been investigated. Fig. 3 shows the photo and the schematic illustration of the apparatus for Double Torsion test.
For our investigations, the influence of the environmental condition on the change of the crack velocity. It has been clarified that the crack velocity in rock in water is much higher than that in air. If the relative humidity increases, the crack velocity in rock increases in air. The crack velocity decreased when the concentration of calcium ion in water is high. It is concluded that the influence of the surrounding environment on the crack velocity in rock must be considered carefully to ensure the long-term integrity of a rock mass.
Figure 3. Apparatus for Double Torsion test
Estimation of the stress and heterogeneity condition around rupture zone using induced earthquakes and rock fracture experiment
In addition to naturally occurring earthquakes, it is known that the stress change accompanied with mining and natural resource extraction can induce earthquakes. This kind of place has an advantage that we can observe seismic cycle in relatively shorter time scale than natural earthquake, thus it helps to observe crustal stress and heterogeneous state prior to a large seismic event.
Laboratory rock fracture experiment enable us to observe the occurrence of micro fracture and rupture process under the more controlled environment. By artificially transmitting elastic wave through a rock sample which diameter is a few centimeters, we can estimate the increase and connection of the micro cracks. Besides, we are trying to understand the natural and induced earthquake process by using small fractures in a sample that called AE (Acoustic Emission), as if they are micro earthquakes.
The amplitude and the velocity of the transmitted wave through a sample decrease when we press the sample under the tri-axial condition that simulates pressure conditions several kilometers underground. Fig. 4 overlap the hypocenter location of AE which occur during the loading with an X-ray CT image. Overlapped AE hypocenter and fracture in a sample indicate the connecting cracks generate the rupture plane.
AE observation reveal that the amount of released stress is similar between AE and natural earthquakes (Fig. 5). These analyses are achieved with multi-channel measurements with broadband sensor under tri-axial condition.
Figure 4. X-ray CT image of the rock sample after the compression experiment with AE hypocenters
Figure 5. Source characteristics of AE events with natural events