ABSTRACT: Laboratory tests were conducted in this study in order to examine the changes in aperture of an open single fracture in a granite core over time, under confining pressures of 1-3 MPa and temperatures of 20°C and 60°C. During the experiment, the fracture aperture was monitored with X-ray CT. For a quantitative evaluation of the fracture aperture, the CT images were processed with image co-registration by the Scale-Invariant Feature Transform (SIFT) method and fracture surface extraction using the Canny edge detection algorithm. Based on the fracture surface geometry data extracted from the CT images, the aperture and contact ratio were calculated and compared. The effect of the confining pressure and temperature on the aperture and contact ratio was not clear from the results. However, the trend of the fluctuation in the aperture and contact ratio over time was found to be consistent with the trend of the fluctuation in permeability.
The aperture of an open fracture can change over time. It largely depends on the properties of the rock minerals and the geometry of the fracture surface asperities, and is a function of pressure and chemical dissolution, precipitation, and/or mechanical deformation. Although the importance of the thermo-hydro-mechanical-chemical (THMC) effects on the aperture of a fracture is widely recognized, few experimental studies have been conducted that directly observe the evolution of the fracture pore space geometry and the resulting changes in permeability.
The primary objective of this study is to visualize the evolution of the rock fracture aperture and asperities due to long-term pressure loading and temperature using the X-ray CT (Computed Tomography) technique. X-ray CT is one of the few experimental techniques for visualizing the inner structure of rock samples in a non-invasive and non-destructive manner. Many researchers have employed X-ray CT in the rock engineering field to visualize the heterogeneous micro-structure of rocks and micro-crack propagation (Verhelst et al. 1995, Sugawara 1997), to visualize the fluid flow of sedimentary rocks (Sato et al. 2002), to measure the tracer diffusion and migration into the rock matrix and fractures (Nakashima et al. 2004, Sato et al. 2007), and to visualize the fluid flow within deformed rock (Hirono et al. 2003). X-ray CT has also been used to measure fracture apertures and to detect contact areas (Sato et al. 2007, Yoshino et al. 2003, Sato et al. 2004, Re and Scavia 1999, Nakashima et al. 2010).
ABSTRACT: In measuring rock fracture apertures from X-ray CT images, a binarization method has generally been used to separate the air void parts (fractures) from the rock parts. However, reasonable thresholding is often an essential problem in this method, especially for heterogeneous rock materials. This paper suggests a new method for detecting fracture surfaces from CT images using an edge detection algorithm that is known well in the field of image processing as Canny edge detection. Applying this edge detection method to the CT images of a single granite fracture, 15 mm in diameter and 30 mm in length, this study successfully measures the fracture surface geometry, the aperture distribution, and the contact ratio.
Taking precise measurements of the geometric characteristics of rock fractures, such as the elevation distribution of fracture walls, the distribution of apertures, and the contacting asperities within the fractures, is essential because of their significant influence on the mechanical and hydrological behaviors of rock fractures. On a laboratory scale, the elevation distribution of fracture walls can be measured with micrometer accuracy using a system that combines a laser displacement sensor and a high-precision automatic positioning stage (e.g., ).
However, the distribution of apertures and the contacting conditions within a fracture are rather difficult to measure experimentally. In previous researches, various techniques have been proposed, such as 1) the surface topography approach, in which the topography of a pair of fracture surfaces is measured separately by a laser beam profiler, while the aperture is computed indirectly as the distance between the two fracture surfaces [2, 3]; 2) the injection approach, in which the specimen containing the fracture is cut into slices after some resin has been injected, and the aperture is measured as the thickness of the injected resin [4, 5]; and 3) the casting approach, in which replicas of the fracture apertures are made by casting .
ABSTRACT: It is important to consider the influence of temperature when evaluating the frictional behavior of a single rock joint since it may change due to the geochemical processes occurring at the asperity contacts such as mineral dissolution. It is thought that the chemical processes may be accelerated under thermal conditions. In this study, the direct shear tests under a slide-hold-slide process, using artificial rock (mortar), granite and andesite with a single natural rough joint, have been performed at 20 and 60 °C. Through the experiments results, the shear strength healings can be evaluated. Moreover, the log-linear equation and the rate- and state-dependent friction law are applied to the experimental results. In particular, the cutoff time of rate- and state-dependent friction law shows the response with the difference of the thermal conditions
The fluctuation of rock fracture friction has been recognized as one of the major causes in fault mechanical behavior. In previous studies, Dieterich (1979) and Ruina (1983) proposed a famous empirical rate- and state- dependent friction (RSF) law. This law could well describe the variation of rock friction as observed in the slide-hold-slide direct shear test. Since, then a large number of researches related to the earthquake’s occurrences were conducted based on this friction law [e.g. Stuart and Tullis, 1995, Marone 1998]. The authors has been carried out the Slide-Hold-Slide(SHS) type direct shear tests of rock fracture including natural joint surface roughness under relative low confining (Kishida et al., 2011; Kawaguchi et al., 2009). Kishida et al. (2011) have confirmed that the shear strength recovery during short-time holding may be attributed to a purely mechanical process, like creep deformation at the contacted asperities, while the shear strength recovery during long-time holding is affected by both mechanical and chemical processes. Kishida et al. (2011) and Kawaguchi et al. (2009) were conducted under room temperature, and then, the influence of various thermal conditions was not considered.
In this research work, direct shear tests have been carried out under two different temperature conditions following a SHS process in a residual state employing mortar replica specimen, granite and andesite with a single joint surface roughness. Then, the RSF law has been applied to the experimental results and the influence in temperature has been discussed.
We designed the wave energy conversion system which consists of water chambers array aligned along the wave propagation direction and the float-type wave energy converters, each of which is installed in the chamber and utilizes the gentle up/down motion of the water in the chamber. This system aims to match conditions required for practical use, i.e., durability against wave load, workability in setting and maintenance, high performance of energy gain, and reduction of total cost. Calculation is made along our previous mathematical model, but consideration on the load resistance connected to the generator is added for the information to practical use. Also the effect of the total length of the systems set on the time history of the total energy is considered more strictly.
A rising demand for energy coupled with the problem of environmental pollution has led to investigations into potential of new energy resources. Wave energy is one of the most dependable and predictable sources of renewable energy available which is free from the variations present in wind or solar energy(Takahashi, S. (1993); Malm O., and Reiten A.(1985); Evans, DV. (1982)). Various mechanisms for extracting wave energy have been developed but not fully realized due to structural strength and economic problems.
For the practical use of wave energy, all the following factors should be satisfied at some level: durability of the device, workability (without difficulty in installation, maintenance and repair), high performance of energy gain, and low cost. The durability of the device includes those of both the external structure and the power converting portion of the device. It can be said with certainty that the lack of fulfilment of the above mentioned conditions is the main reason that the wave power conversion technology has not reached a commercially generating stage.
In order to meet these conditions, the first author et.al.(2013) designed the system which consists of water chambers array aligned along the wave propagation direction and the float-type wave energy converters each of which is installed in the chamber and utilizes the gentle up/down motion of the water in the chamber. In this system, neither the wall(s) of the chambers nor the energy conversion device(s) are exposed to the impulsive load due to water wave. Also since this system is profitable when set along the jetty or along a long floating body, installation and maintenance are done without difficulty and the cost is reduced. Waves near the jetty or a loosely moored long floating body will propagate toward the length of these structures. Therefore, an array of water chambers set along the jetty or a long floating structure is profitable in the sense that the outer wall is never exposed to severe wave loads.
Interferometric Synthetic Aperture Radar (InSAR) is a technique for observing the topographic features and displacements of the Earth’s surface by employing SAR satellites. Useful applications of InSAR include measuring the topographic height of the Earth’s surface and providing the Digital Elevation Model (DEM). Many scientific observations and investigations require topographical information from DEM, for example, rock mechanics and mining projects, infrastructures and environmental planning, etc. Although the DEM obtained by InSAR can usually cover huge areas of over several hundred square kilometers, it also has the potential to cover smaller areas of only a few square kilometers. In this research, the InSAR technique is applied to generate the DEM of a limestone quarry.
InSAR generates DEM using at least two SAR images which are taken at different times. The existing SRTM or ASTER DEM is used during the co-registration process in InSAR. Taking interferograms using SAR images, DEM is produced under the assumption that there is no displacement of the ground during the SAR observational period. The method is applied here to generate a current DEM of a small area, namely, a limestone quarry with steep slopes. The contour line of DEM obtained by InSAR is compared with those obtained by the existing SRTM and ASTER DEM. It is found that the final DEM obtained by InSAR can produce an appropriate elevation.
The Digital Elevation Model (DEM) provides very important information about the shape (slope and aspect) of a surface. Many scientific observations and investigations require topographical information from DEM, for example, rock mechanics and mining projects, infrastructures and environmental planning, etc. There are many methods/techniques to generate DEM, such as leveling, optical remote sensing, and radar (Yu et al., 2011). The leveling method can provide a very high level of accuracy of DEM, but it will require much time and high labor costs if applied for wide area mapping. Optical remote sensing or photogrammetry can usually provide a moderate level of accuracy of DEM, but if the area of interest is covered by clouds, this technique will be useless. On the other hand, one technique, which uses InSAR (Interferometric Synthetic Aperture Radar), is very useful for generating DEM. It can cover large areas, but is usually applied over a few hundred square kilometers. Since InSAR employs microwave wavelengths, it is independent of weather conditions and can collect data both day and night.
GPS is now being used widely for monitoring rock displacements, and it has been a useful tool for various rock engineering projects. The most important technical issue for the practical use of GPS in monitoring displacements is to perform real-time and precise monitoring even under adverse observation conditions, i.e., steep slopes, the existence of trees and walls above/around the sensors, bad weather conditions, etc. The authors and their colleagues have developed a precise real-time displacement monitoring system using GPS and have established a method of data processing which can automatically reduce the errors caused by meteorological factors and obstructions above the antennas.
In this research, the GPS displacement monitoring system is applied to monitor the displacements of an unstable steep slope for the safe management of a national road in Japan over the long term. Three-dimensional displacement monitoring results are shown.
Monitoring rock displacements is important to assessing the stability of rock slopes. The Global Positioning System (GPS) can continuously measure three-dimensional displacements over extensive areas. The “ISRM Suggested Method for Monitoring Rock Displacements Using the Global Positioning System” was proposed (Shimizu et al., 2014) as technology which can be used by anyone.
In this research, the GPS displacement monitoring system developed by the authors is applied to assess the stability of an unstable steep slope along a national road in Japan. Since local slope failures have occurred in the slope several times over the last 20 years, displacement monitoring has been conducted by borehole inclinometers and surface extensometers. Some of the instruments, however, have occasionally not worked due to large deformations, and it has become difficult to perform the monitoring continuously. In order to overcome such trouble, the GPS monitoring system has been applied (Furuyama et al., 2014). In this paper, the results of three-dimensional displacement monitoring using GPS are shown, and the applicability of this system for assessing slope stability is discussed.
Land subsidence is a critical issue to be addressed for large cities located near the sea. The monitoring of land subsidence is vital for predicting and managing the disasters that might occur. Many methods have been established to conduct this work, such as using geotechnical monitoring instruments and applying artificial satellite technologies. Those methods can provide highly accurate measurements for small areas. However, it would be expensive and ineffective to apply them to extensive areas. Hence, a monitoring method, that is economical to conduct, can be applied quickly and continuously, and can provide accurate measurements over large areas, is needed. Multi-temporal Differential Interferometry Synthetic Aperture Radar (MT-DInSAR), such as the Small Baseline Subset (SBAS), is a powerful technique for meeting the above demands. And, since the lifespans of current SAR satellites are commonly designed to be around 5-7 years, continuous monitoring for longer periods by the MT-DInSAR technique is important. To deal with these types of issues, a new method is required that can utilize the data from multiple (different) satellites.
In this study, a method for long-term land subsidence monitoring by MT-DInSAR, using multi-sensor data sets, is presented. Firstly, the SBAS method is performed for each time series SAR data set. Secondly, the hyperbolic fitting method is applied to estimate real values from the results of each data set. Finally, the hyperbolic curve is used to connect the results of the unlinked time series data sets. To verify this method, the land subsidence in Semarang City, Indonesia is taken as an example case.
Interferometry Synthetic Aperture Radar
DInSAR is an invaluable tool for observing land surface deformation over vast areas with the high accuracy of centimeter and high-spatial resolution of 3-30 m after spatial averaging and geocoding. Moreover, DInSAR does not require the installation of any devices on the ground, and it has been widely used for detecting horizontal and vertical displacements of the land surface (Hanssen, 2002).
ABSTRACT: The Particle Flow Code (PFC) is a useful tool for simulating the failure behavior of hard rocks. However, one of the major drawbacks of the modeling is the unrealistically low ratios of simulated compressive strength to tensile strength for hard rock specimens. This means that the straightforward adoption of circular particles cannot fully reproduce the brittle failure of hard rocks. Instead, complex-shaped grain structures should be adopted for the model. The goal of this paper is to clarify the relationship between the irregularity of the clump configuration and the failure behavior of the simulation model. PFC simulations of unconfined compressive tests and Brazilian tensile tests were carried out using a triplet particle clump model, with clumps whose three same-radius particles partially overlap each other. From the simulation results, it was found that both compressive and tensile strength decrease with the increasing irregularity of the clump shape. It is inferred from the number of contact points and the growth of micro-cracks that irregular-shaped clumps have more freedom of movement, in relation to their adjacent clumps, and that this brings about the rapid growth of micro-cracks and lower compressive/tensile strength.
The Particle Flow Code (PFC), a numerical simulation code based on the distinct element method (DEM), is a useful tool for simulating the failure behavior of hard rocks. However, one of the major drawbacks of the PFC modeling is the unrealistically low ratios of simulated compressive strength to tensile strength for hard rock specimens. This means that the straightforward adoption of circular particles cannot fully reproduce the brittle failure of hard rocks. Therefore, complexshaped grain structures should be adopted for the model. To overcome this limitation, Cho et al. (2007) introduced clumped-particle geometry. It allowed us to reproduce correct strength ratios (e.g., Funatsu et al. 2008). On the other hand, no clear criterion has been found for determining the size and the configuration of the clumps (Nakashima et al. 2013, 2015).
The goal of this paper is to clarify the relationship between the irregularity of the clump configuration and the failure behavior of the simulation model. Simulations of unconfined compressive tests and Brazilian tensile tests were carried out. The effect of the surface roughness of the clumps was examined by varying the degree of the overlapping of the particles in the clumps. Based on the results, the effect of clump roughness on the compressive and tensile strength of the rocks will be discussed.
2 OUTLINE OF PFC SIMULATION WITH CLUMPED PARTICLE MODEL AND TRIPLET PARTICLE CLUMPS
2.1 Clumped particle model
In the PFC simulation, objects are modeled as an assembly of rigid balls (two-dimensional disks). The balls are connected by normal and shear springs at the contact points and by micro-bonding (parallel bonds), as shown in Figure 1. The clumped particle model is illustrated in Figure 2. A clump is a group of particles that is rigidly connected and that behaves as a single element. Each clump is unbreakable, and adjacent clumps are connected to each other by contact springs and micro-bonding, like the ball-ball connection in the non-clumped model. The clumped particle model enables the reproduction of a large ratio of compressive strength to tensile strength, characteristic of hard rocks.
ABSTRACT: Modern satellite technologies, i.e., GPS (Global Positioning System) and SAR (Synthetic Aperture Radar), have begun to be used for monitoring deformation over extensive areas in the field of Rock Engineering. SAR is an attractive tool which does not require any devices on the ground. However, the improvement of its monitoring accuracy is a key issue for practical applications. In this paper, a simple multi-temporal analysis is proposed for this purpose. In order to verify the procedure, the monitoring of the subsidence in a city in Indonesia is shown as an example. The results are then compared to displacements measured by GPS to confirm the validity. A map of the subsidence over the large area has been generated, and it is clearly seen that the trends in subsidence depend on the ground conditions.
Monitoring is an important task for assessing the stability of structures and for confirming the validity of the design. It is also important for predicting risks and managing safe operations. Many methods have been established to conduct this task, such as geotechnical monitoring instruments, survey methods and artificial satellite technologies. GPS is one of the useful methods for continuously monitoring displacements over an extensive area with high accuracy (Shimizu et al. 2014). However, GPS is only capable of measuring the displacement of points which have been installed with a sensor. Thus, if monitoring is to be conducted over a large area, such as a city, a mountain, a coastal region, etc., a huge number of GPS sensors will be required.
On the other hand, SAR is powerful technology for mapping the Earth’s topography. In particular, Differential Interferometry SAR (DInSAR) is a useful technique for observing the deformation of the ground surface (Hanssen 2002). DInSAR has already been applied to monitor the ground subsidence in large areas, the slope deformation in mines and landslide behavior, etc. (Raucoules et al. 2007, Hartwig et al. 2013, Akbarimehr et al. 2103), but the procedure still needs to be improved in order to obtain reliable results.
In this paper, a simple procedure is proposed to obtain accurate results for the long-term monitoring of subsidence. It is a method of the multi-temporal analysis of DInSAR, which consists of many pairs of selected SAR data with a short period and a small perpendicular baseline. The method is applied to monitor the subsidence over the city of Semarang in Indonesia. The results are then compared with displacements measured by GPS to confirm the validity of the method. A map of the subsidence over the large area has been generated, and it is clearly seen that the trends in subsidence depend on the ground conditions. Moreover, the hyperbolic method is also applied to the results of Multi-Temporal DInSAR in order to smooth the data and to predict future subsidence.
Iwata, N. (Chuden Engineering Consultants Co. Ltd.) | Adachi, K. (Chuden Engineering Consultants Co. Ltd.) | Takahashi, Y. (Chuden Engineering Consultants Co. Ltd.) | Aydan, Ö. (University of the Ryukyus) | Tokashiki, N. (University of the Ryukyus) | Miura, F. (Yamaguchi University)
ABSTRACT: Since the 1999 Chi-chi earthquake and the 1999 Kocaeli earthquake damaged many important structures due to surface rupture, as well as a strong motion by the fault rupture, displacement and inclination in ground surface become the significant issues. In this study, we conducted fault rupture simulation using two and three dimensional finite element method for the 2014 Kamishiro Fault Nagano prefecture earthquake, which is a thrust fault type earthquake with an observed surface rupture of 9 km in length, and confirmed the applicability of numerical method and conditions, such as initial stress distribution, modeling of the fault plane and constitutive law, by comparing results with the observed ground motions and displacement. As a result, the acceleration response could not be simulated due to mesh size and constitutive law of fault plane. The displacement of 2D-FEM becomes larger than the actual displacement while the 3D-FEM yields results, which are in good agreement with the actual displacement.
Since the 1999 Chi-chi earthquake and the 1999 Kocaeli earthquake damaged many important structures due to surface rupture, as well as a strong motion by the fault rupture, displacement and inclination in ground surface become the significant issues. Generally strong motions are estimated by Green’s function method and fault displacement is estimated based on geological surveys. However an earthquake occurs by rupture of earthquake source fault. When the displacement is large, it will come up to the ground surface as a surface rupture. Therefore, ideal analytical model is able to simulate a fault rupture process and estimate displacement and strong motion at the same time. Fault rupture simulations by Finite Difference Method (FDM), Finite Element Method (FEM) and Boundary Element Method (BEM) are carried out. However, they have not become practical as analytical results greatly vary according to initial stress conditions and modeling of fault rupture.
In this study, we conducted fault rupture simulation using two and three dimensional finite element method (2D-FEM, 3D-FEM) for the 2014 Kamishiro Fault Nagano prefecture earthquake (Mw 6.3), which is a thrust fault type earthquake with an observed surface rupture of 9 km in length. The analytical method and modeling of fault is dynamic response analysis considering fault rupture process proposed by Toki & Miura (1985) and Toki & Sawada (1988). We confirmed the applicability of numerical method and conditions, such as initial stress distribution, modeling of the fault plane and constitutive law, by comparison with the observed surface ground motions and displacement.