1、岩石力学50原则The 50 Principles of Rock Mechanics for Rock Engineering1. Rock is a natural material: its properties cannot be specified as with a fabricated material; the properties have to be measured on site. The geological history of a rock mass will determine several key characteristics. These include
2、 the inhomogeneity (different properties at different locations), the anisotropy (different properties in different directions), the presence and mechanical characteristics of the discontinuities (pre-existing fractures), and the hydraulic properties.2. The nature of the intact rock will depend on i
3、ts geological type and the degree of weathering to which it has been subjected. Rock mechanics started with detailed studies of the deformation and failure of intact rock. The engineering properties of intact rock are functions of the rock microstructure which in turn is a function of the geological
4、 formation and history.3. Using a stiff or servo-controlled testing machine, the complete stress-strain curve for intact rock in uniaxial compression can be obtained. The test is conducted with axial strain as the independent variable (the controlled variable) and axial stress as the dependent varia
5、ble (the measured variable). The complete stress-strain curve represents the structural collapse of the rock microstructure from initial loading to complete disintegration. The most widely-used characteristics of the complete stress-strain curve are the modulus (measured at 50% maximum stress) and t
6、he compressive strength ( the maximum stress sustained).4. The complete stress-strain curve for intact rock depends on the specimen geometry and loading conditions of the test and the environmental conditions. In fact, neither the compressive strength nor the tensile strength is a material property
7、because they both depend on the specimen geometry and the loading conditions of the test. A material property does not depend on these factors.5. When a confining pressure is also applied during a compression test on intact rock, the rock will exhibit either brittle or ductile behavior. In brittle b
8、ehavior, the stress decreases after the compressive strength has been reached. In ductile behavior, the stress continues to increase. The confining pressure associated with the brittle-ductile transition is, for example, 0 MPa for rock salt, 20-100 MPa for limestone, and more than 100 MPa for sandst
9、one and granite.6. The most widely used failure criteria for intact rock are the Mohr-Coulomb, Griffith, and Hock-Brown failure criteria. The Mohr-Coulomb criterion considers the cohesion and angle of friction associated with shear failure. The Griffith criterion considers the energy required by a p
10、ropagating crack in terms of an initial crack length. The Hoek-Brown criterion is an empirical criterion using two parameters which can be estimated from the rock description. Many other failure criteria for intact rock have been developed.7. In most cases, the properties and engineering behaviour o
11、f rock masses are governed by the discontinuities. The discontinuities are any breaks in the mechanical rock continuum - which can occur at a variety of scales, from faults to bedding planes to joints to fissures and micro-fissures. The most important discontinuities for engineering are faults or ot
12、her shear features, but joints - which have been created by normal tensile stresses - can be very significant as well. Discontinuities have little or zero tensile strength.8. Ten main characteristics are used to describe discontinuities: spacing; orientation; persistence; roughness; aperture; number
13、 of sets; block size; filling; wall strength; and seepage.9. The most widely used parameter to describe discontinuity occurrence is the Rock Quality Designation (RQD). This is the percentage of pieces in a borebole core or lengths along a seanline that are greater than l0 mm or 4 inches. The RQD can
14、 be related to the discontinuity frequency if the nature of the discontinuity spacing histogram is known.10. Because discontinuities tend to occur in sets (of parallel or sub-parallel discontinuities), the discontinuity frequency value is different along lines in different directions through a rock
15、mass. It follows that the RQD will also be different in different directions through the rock mass.11. The persistence, or extent of a discontinuity, is an important characteristic for many engineering characteristics, such as the modulus of deformation of the rock, the degree to which rock blocks a
16、re formed, and the hydraulic connectivity of the discontinuity network. The roughness, aperture and filling of the discontinuities are also important for the mechanical and hydrological characteristics.12. The main mechanical properties of a discontinuity for engineering are the stiffness and streng
17、th. The stiffness should be considered as the normal stiffness and the two shear stiffnesses; the strength is specified by the shear strength, i.e. the angle of friction (remembering that the discontinuity has essentially no tensile strength, and is also assumed to have little or no cohesion). The a
18、ngle of friction is a complex combination of the basic friction angle, the strength of the asperites and the discontinuity roughness.13. A rock mass will contain a pre-existing natural stress state, the in situ stress, which is caused by geological processes, mainly tectonic. The quantity stress is
19、not a scalar or vector quantity but a tensor quantity which has to be characterized by six independent values-usually the magnitudes and directions of the three principal stresses. These rock stresses are mainly caused by tectonic activity but old, residual stresses can also be present.14. There are
20、 four main methods of measuring the in situ stress: the flat jack, hydraulic fracturing, the USBM overcoring torpedo, and the CSIRO overcoring gauge. The CSIRO gauge is the most reliable and hydraulic fracturing is the only method, that can be used a significant distance from man-access. It is the r
21、ule rather than the exception that the maximum horizontal stress component is greater than the vertical stress component. Because discontinuities have a significant effect on the local principal stress magnitudes and directions, measured stresses are expected to vary at the project location.15. Wate
22、r can be present in the pores of the intact rock and in the discontinuities. The water pressure is subtracted from the normal stress components of the stress tensor to give effective stresses.16. Strain is a tensor quantity like stress. Assuming the rock is behaving elastically, the six components o
23、f the strain tensor can be related to the six components of the stress tensor by the elastic compliance matrix. For isotropy, two elastic constants are needed: Youngs modulus and Poissons ratio. For transverse isotropy, five elastic constants are needed: two Youngs moduli, two Poissons ratios and a
24、shear modulus. For orthotropy, nine elastic constants are required: three Youngs moduli, three Poissons ratios and three shear moduli. For complete anisotropy, the 21 independent constants of the elastic compliance matrix are required.17. The ideal rock mass is a CHILE material: Continuous, Homogene
25、ous, Isotropic, and Linearly Elastic. The actual rock mass is a DIANE material: Discontinuous, Inhomogeneous, Anisotropic, and Not Elastic. Rock masses are discontinuous because they contain discontinuities. They are inhomogeneous and anisotropic because they are composed of different geological str
26、ata and different discontinuity geometries at different locations and which have different properties in different directions. They are not elastic because the energy given to the rock mass during deformation cannot generally be recovered completely.18. The deformability of a rock mass results from
27、deformation of both the intact rock and the discontinuities. Because the intact rock can be anisotropic and because the discontinuities occur in sets causing the discontinuity contributions to be anisotropic, the deformation modulus of the rock mass will be different in different directions.19. The
28、strength of a rock mass will depend on whether failure occurs through the intact rock or along one or more discontinuities.20. The ease with which water flows through a rock mass is expressed by the permeability. Like stress and strain, permeability is a second order tensor with six independent comp
29、onents - usually characterized by the magnitudes and directions of the principal permeabilities. The permeability of fractured rock masses can vary greatly.21. The Representative Elemental Volume (REV) is an important concept for the permeability of rock mass. In a rock mass sample, the number of di
30、scontinuities present is a function of the sample size, stabilizing in average properties when the sample size is large enough. The REV is the rock mass sample size below which the permeability can vary significantly and at and above which the permeability is essentially constant. This REV concept a
31、lso applies to all properties governed wholly or partly by the discontinuities.22. In order to establish the properties of rocks, testing techniques are used. These testing techniques can be standardized. However, different properties are required for different projects. Because there are many diffe
32、rent rock engineering objectives, even though the testing techniques themselves can be standardized, there can be no standardized site investigation.23. Because the REV size is generally of the order of tens of meters, it is generally not possible to conduct meaningful tests directly on the rock mass. Tests are conducted on the intact rock and the discontinuities separately and their significance for the rock
