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The interaction between the wellbore and pre-existing fractures
PROPERTIES:TIME considerably larger strengths than the rest in the system. As the fraction of these bonds is raised from zero, the fragmentsize distribution transforms smoothly from exponential forms into a power law. (from Author) 967098
A hierarchical model of rough rock joints based on micromechanics J.-J. Dong & Y.-W. Pan, International Journal of Rock Mechanics & Mining Sciences, 33(2), 1996, pp 111-123. In this model, the global behavior of a rough joint depends on the microfeatures of the contact planes on the joint. The contact mechanics on contact planes controls the mechanical behavior of the joint via a homogenization process. Also, the complex mechanical behavior of a joint is associated with simple microfeatures of the joint, including the frictional properties and the structure of contact planes. To account for scale dependency of joint roughness, a hierarchical representation of a joint profile is proposed in the form of a multilevel-asperity model. The constitutive relation of the multilevel-asperity model is derived through a recursive homogenization process. Major deformation mechanisms of joint asperities, i.e. interlocking, wearing, shearing-off, sliding, separation and degradation, are taken into account. (from Authors) 967099 The application of fracture mechanics to crack propagation in dry soil P. D. Hallett, A. R. Dexter & J. P. K. Seville, European Journal of Soil Science, 46(4), 1995, pp 591-599. Discs moulded from sand and kaolinite pastes, into which artificial tension cracks were introduced, were fractured using the indirect tension technique. The basic linear elastic fracture mechanics (LEFM) approach was found inadequate. Fracture mechanics have been extended to dry soil by adding a crack length increment, AC, to the imposed flaw size, c, to account for energy dissipation by processes other than the creation of new surfaces. This increment seems to represent the effective size of a ‘process zone’ ahead of the crack tip in which energy is dissipated by micro-cracking and internal friction. Further experiments with two collinear cracks showed that they behaved effectively as one for separation distances less than about AC. (from Authors) 967100 The interaction between the wellbore and pre-existing fractures C. Atkinson & M. Thiercelin, International Journal of Fracture, 73(3), 1995, pp 183-200. Down-hole imaging techniques have revealed that natural fractures can be reactivated in the vicinity of a wellbore. Fracture reactivation, however, has never been analysed in detail. This work aims to quantify the influence of various controlling parameters on fracture instability. This work shows that fracture reactivation due to near wellbore effects is likely to occur although the fracture movement may not be easily detected. However the occurrence of this mechanism is quite sensitive to the fracture location with respect to the wellbore and to the fracture orientation. This work has also revealed the possibility of two length scales in fracture instability: the first length scale is related to the interaction of the wellbore with the fracture and the second one is related to fracture instability with respect to the far-field state of stress. (from Authors) 967101 Acoustic emissions monitoring with triaxial double sonde method at Hijiori HDR test site K. Tezuka & M. Miyairi, Geothermal Science & Technoiogy, 5(1-2), 1995, pp 3-20.
The paper proposes a method to monitor acoustic emissions (AE) with a triaxial double sonde svstem in a sinale borehole. This method, which incorporates both the triaxial hodogram method and the travel time inversion method, is an advanta-
DEPENDENCE
3038
geous way to monitor AE events indexes by artificial well stimulation such as hydraulic fracturing, especially when the number of observation wells is restricted. A real time AE monitoring system has been developed on the basis of this method. The performance of the method and system were confirmed as the results of a hydraulic fracturing experiment in 1992. The AE source locations estimated provided us with profitable information on the created fracture system and with an idea about how to determine the drilling target of the production well for the host dry rock project. (from Authors) 967102 Characterization of subsurface era triaxial doublet analysis and focal H. Moriya & H. Niitsuma, Geothe ogy, 5(1-2), 1995, pp 123-146.
The paper proposes a procedure to characterize the dynamic behavior of subsurface crack systems on the basis of triaxial doublet analysis, which estimates precise relative source location of microseismic doublets in the d~w~~o~~ 3-component measurement. A doublet is a pair or a group of microseismicity with very similar waveforms origination times, and is thought to express stress release on the same or similar structural plane in a crack system. The concept and procedure of triaxial doublet analysis is explained using the cross-spectrum and spectral matrix, and it is shown that this analysis provides information on the structural plane related to a subsurface focal mechanism of microseismicity is ana a moment tensor analysis and grid test. An understanding of both the structural plane and the moment tensor of the events allows an estimation of whether the st~~tural plane of a crack system behaves as a single crack or multiple cracks. (from Authors) 967103 A fracture-mechanics-based tunnel linings
design
etho
r SFRC
& H. Horii, Tunneiling & ~~~e~g~~~n~ Space Technology, 1l(l), 1996, pp 39-43.
P. Nanakorn
The current design provision on the estimation of the load carrying capacity of steel-fiber-reinforced concrete (SFRC) tunnel lining is briefly introduced. The design method is based on the concept of fracture mechanics of concrete. In this study, the validity of the current design method is verified, and the possible improvements of the design method are proposed. It is shown that the crack length at the maximum load depends on the specimen size and the fiber property. The source of the size effect which appears between a small specimen for the material test and the actual lining is the dependence of the crack length at the peak on the specimen size. (Authors)
Tim?
depen
967104 Soil tensile strength as affected bulk density
ater ~~nteni and
D. Blazejczak, R. Horn & J. Pytka, ~~?e~~~~~o~~~ Agrophy-
sits, 9(3), 1995, pp 179-188.
Investigates the effect of soil water conditions and soil compaction on the age-hardening process of loamy sand and silty loamy sand in relation to the tensile strength. Soil samples from Getmany (loamy sand) and Poland (silty loamy sand) were moulded at water contents compacted up to 1.35, 1.45, 1.55 g/ems. time was 10 days. With increasing time the soils became stronger at the same water content. The higher the initial water content the less pronounced was the strength increase with time. Furthermore, increase in bulk density resulted in higher values of tensile strength. Two di~erent mechanisms of age-hardening could be identified. (from A~t~ors~
A hierarchical model of rough rock joints based on micromechanics J.-J. Dong & Y.-W. Pan, International Journal of Rock Mechanics & Mining Sciences, 33(2), 1996, pp 111-123. In this model, the global behavior of a rough joint depends on the microfeatures of the contact planes on the joint. The contact mechanics on contact planes controls the mechanical behavior of the joint via a homogenization process. Also, the complex mechanical behavior of a joint is associated with simple microfeatures of the joint, including the frictional properties and the structure of contact planes. To account for scale dependency of joint roughness, a hierarchical representation of a joint profile is proposed in the form of a multilevel-asperity model. The constitutive relation of the multilevel-asperity model is derived through a recursive homogenization process. Major deformation mechanisms of joint asperities, i.e. interlocking, wearing, shearing-off, sliding, separation and degradation, are taken into account. (from Authors) 967099 The application of fracture mechanics to crack propagation in dry soil P. D. Hallett, A. R. Dexter & J. P. K. Seville, European Journal of Soil Science, 46(4), 1995, pp 591-599. Discs moulded from sand and kaolinite pastes, into which artificial tension cracks were introduced, were fractured using the indirect tension technique. The basic linear elastic fracture mechanics (LEFM) approach was found inadequate. Fracture mechanics have been extended to dry soil by adding a crack length increment, AC, to the imposed flaw size, c, to account for energy dissipation by processes other than the creation of new surfaces. This increment seems to represent the effective size of a ‘process zone’ ahead of the crack tip in which energy is dissipated by micro-cracking and internal friction. Further experiments with two collinear cracks showed that they behaved effectively as one for separation distances less than about AC. (from Authors) 967100 The interaction between the wellbore and pre-existing fractures C. Atkinson & M. Thiercelin, International Journal of Fracture, 73(3), 1995, pp 183-200. Down-hole imaging techniques have revealed that natural fractures can be reactivated in the vicinity of a wellbore. Fracture reactivation, however, has never been analysed in detail. This work aims to quantify the influence of various controlling parameters on fracture instability. This work shows that fracture reactivation due to near wellbore effects is likely to occur although the fracture movement may not be easily detected. However the occurrence of this mechanism is quite sensitive to the fracture location with respect to the wellbore and to the fracture orientation. This work has also revealed the possibility of two length scales in fracture instability: the first length scale is related to the interaction of the wellbore with the fracture and the second one is related to fracture instability with respect to the far-field state of stress. (from Authors) 967101 Acoustic emissions monitoring with triaxial double sonde method at Hijiori HDR test site K. Tezuka & M. Miyairi, Geothermal Science & Technoiogy, 5(1-2), 1995, pp 3-20.
The paper proposes a method to monitor acoustic emissions (AE) with a triaxial double sonde svstem in a sinale borehole. This method, which incorporates both the triaxial hodogram method and the travel time inversion method, is an advanta-
DEPENDENCE
3038
geous way to monitor AE events indexes by artificial well stimulation such as hydraulic fracturing, especially when the number of observation wells is restricted. A real time AE monitoring system has been developed on the basis of this method. The performance of the method and system were confirmed as the results of a hydraulic fracturing experiment in 1992. The AE source locations estimated provided us with profitable information on the created fracture system and with an idea about how to determine the drilling target of the production well for the host dry rock project. (from Authors) 967102 Characterization of subsurface era triaxial doublet analysis and focal H. Moriya & H. Niitsuma, Geothe ogy, 5(1-2), 1995, pp 123-146.
The paper proposes a procedure to characterize the dynamic behavior of subsurface crack systems on the basis of triaxial doublet analysis, which estimates precise relative source location of microseismic doublets in the d~w~~o~~ 3-component measurement. A doublet is a pair or a group of microseismicity with very similar waveforms origination times, and is thought to express stress release on the same or similar structural plane in a crack system. The concept and procedure of triaxial doublet analysis is explained using the cross-spectrum and spectral matrix, and it is shown that this analysis provides information on the structural plane related to a subsurface focal mechanism of microseismicity is ana a moment tensor analysis and grid test. An understanding of both the structural plane and the moment tensor of the events allows an estimation of whether the st~~tural plane of a crack system behaves as a single crack or multiple cracks. (from Authors) 967103 A fracture-mechanics-based tunnel linings
design
etho
r SFRC
& H. Horii, Tunneiling & ~~~e~g~~~n~ Space Technology, 1l(l), 1996, pp 39-43.
P. Nanakorn
The current design provision on the estimation of the load carrying capacity of steel-fiber-reinforced concrete (SFRC) tunnel lining is briefly introduced. The design method is based on the concept of fracture mechanics of concrete. In this study, the validity of the current design method is verified, and the possible improvements of the design method are proposed. It is shown that the crack length at the maximum load depends on the specimen size and the fiber property. The source of the size effect which appears between a small specimen for the material test and the actual lining is the dependence of the crack length at the peak on the specimen size. (Authors)
Tim?
depen
967104 Soil tensile strength as affected bulk density
ater ~~nteni and
D. Blazejczak, R. Horn & J. Pytka, ~~?e~~~~~o~~~ Agrophy-
sits, 9(3), 1995, pp 179-188.
Investigates the effect of soil water conditions and soil compaction on the age-hardening process of loamy sand and silty loamy sand in relation to the tensile strength. Soil samples from Getmany (loamy sand) and Poland (silty loamy sand) were moulded at water contents compacted up to 1.35, 1.45, 1.55 g/ems. time was 10 days. With increasing time the soils became stronger at the same water content. The higher the initial water content the less pronounced was the strength increase with time. Furthermore, increase in bulk density resulted in higher values of tensile strength. Two di~erent mechanisms of age-hardening could be identified. (from A~t~ors~