岩石力学50原则

1、The 50 Principles of Rock Mechanics for Rock EngineeringRock is a natural material: its properties cannot be specified as with a fabr icated material; the properties have to be measured on site. The geological historyof a rock mass will determine several key characteristics. These include the inhom

2、ogeneity (different properties at different locations), the anisotropy (different properti es in different directions), the presence and mechanical characteristics of the disco ntinuities (pre-existing fractures), and the hydraulic properties.The nature of the intact rock will depend on its geologic

3、al type and the de gree of weathering to which it has been subjected. Rock mechanics started with d etailed studies of the deformation and failure of intact rock. The engineering propert ies of intact rock are functions of the rock microstructure which in turn is a functio n of the geological format

4、ion and history.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 conducte d with axial strain as the independent variable (the controlled variable) and axial str ess as the dependent variable (the

5、 measured variable). The complete stress-strain curve represents the structural collapse of the rock microstructure from initial loadin g to complete disintegration. The most widely-used characteristics of the complete stress-strain curve are the modulus (measured at 50% maximum stress) and the c om

6、pressive strength ( the maximum stress sustained).The complete stress-strain curve for intact rock depends on the specimen geometry and loading conditions of the test and the environmental conditions. In fa ct, neither the compressive strength nor the tensile strength is a material property-because

7、they both depend on the specimen geometry and the loading conditions of the test. A material property does not depend on these factors.When a confining pressure is also applied during a compression test on in tact rock, the rock will exhibit either brittle or ductile behavior. In brittle behavior, t

8、h e stress decreases after the compressive strength has been reached. In ductile be havior, the stress continues to increase. The confining pressure associated with thebrittle-ductile transition is, for example, 0 MPa for rock salt, 20-100 MPa for limest one, and more than 100 MPa for sandstone and

9、granite.The most widely used failure criteria for intact rock are the Mohr-Coulomb, Griffith, and Hock-Brown failure criteria. The Mohr-Coulomb criterion considers thecohesion and angle of friction associated with shear failure. The Griffith criterion co nsiders the energy required by a propagating

10、crack in terms of an initial crack leng th. The Hoek-Brown criterion is an empirical criterion using two parameters which c an be estimated from the rock description. Many other failure criteria for intact rock have been developed.In most cases, the properties and engineering behaviour of rock masse

11、s ar e governed by the discontinuities. The discontinuities are any breaks in the mechan ical rock continuum - which can occur at a variety of scales, from faults to beddingplanes to joints to fissures and micro-fissures. The most important discontinuities f or engineering are faults or other shear

12、features, but joints - which have been crea ted by normal tensile stresses - can be very significant as well. Discontinuities hav e little or zero tensile strength.Ten main characteristics are used to describe discontinuities: spacing; orien tation; persistence; roughness; aperture; number of sets;

13、block size; filling; wall stre ngth; and seepage.The most widely used parameter to describe discontinuity occurrence is theRock Quality Designation (RQD). This is the percentage of pieces in a borebole c ore or lengths along a seanline that are greater than l0 mm or 4 inches. The RQDcan be related t

14、o the discontinuity frequency if the nature of the discontinuity spac ing histogram is known.Because discontinuities tend to occur in sets (of parallel or sub-parallel di scontinuities), the discontinuity frequency value is different along lines in different di rections through a rock mass. It follo

15、ws that the RQD will also be different in differ ent directions through the rock mass.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 are formed, and th

16、e hydraulic connectivity of the discontinuity network. The roughness, aperture and filling of the discontinuities are also important for the mechanical and hydrological characteristics.The main mechanical properties of a discontinuity for engineering are the stiffness and strength. The stiffness sho

17、uld be considered as the normal stiffness a nd the two shear stiffnesses; the strength is specified by the shear strength, i.e. th e angle of friction (remembering that the discontinuity has essentially no tensile stre ngth, and is also assumed to have little or no cohesion). The angle of friction i

18、s acomplex combination of the basic friction angle, the strength of the asperites and the discontinuity roughness.A rock mass will contain a pre-existing natural stress state, the in situ str ess, which is caused by geological processes, mainly tectonic. The quantity stressis not a scalar or vector

19、quantity but a tensor quantity which has to be characteri zed by six independent valuesusually 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 four main methods of me

20、asuring the in situ stress: the flat jack,hydraulic fracturing, the USBM overcoring torpedo, and the CSIRO overcoring gau ge. The CSIRO gauge is the most reliable and hydraulic fracturing is the only met hod, that can be used a significant distance from man-access. It is the rule rather than the exc

21、eption that the maximum horizontal stress component is greater than the vertical stress component. Because discontinuities have a significant effect on t he local principal stress magnitudes and directions, measured stresses are expecte d to vary at the project location.Water can be present in the p

22、ores of the intact rock and in the discontin uities. The water pressure is subtracted from the normal stress components of the stress tensor to give effective stresses.Strain is a tensor quantity like stress. Assuming the rock is behaving elas tically, the six components of the strain tensor can be

23、related to the six component s of the stress tensor by the elastic compliance matrix. For isotropy, two elastic co nstants are needed: Youngs modulus and Poissons ratio. For transverse isotropy, f ive elastic constants are needed: two Youngs moduli, two Poissons ratios and a s hear modulus. For orth

24、otropy, nine elastic constants are required: three Youngs mo duli, three Poissons ratios and three shear moduli. For complete anisotropy, the 2 1 independent constants of the elastic compliance matrix are required.The ideal rock mass is a CHILE material: Continuous, Homogeneous, Iso tropic, and Line

25、arly Elastic. The actual rock mass is a DIANE material: Discontinuo us, Inhomogeneous, Anisotropic, and Not Elastic. Rock masses are discontinuous because they contain discontinuities. They are inhomogeneous and anisotropic bec ause they are composed of different geological strata and different disc

26、ontinuity geo metries at different locations and which have different properties in different directio ns. They are not elastic because the energy given to the rock mass during deform ation cannot generally be recovered completely.The deformability of a rock mass results from deformation of both the

27、 int act rock and the discontinuities. Because the intact rock can be anisotropic and be cause 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.The strength of a rock mass w

28、ill depend on whether failure occurs throug h the intact rock or along one or more discontinuities.The ease with which water flows through a rock mass is expressed by th e permeability. Like stress and strain, permeability is a second order tensor with si x independent components - usually character

29、ized by the magnitudes and directionsof the principal permeabilities. The permeability of fractured rock masses can vary greatly.The Representative Elemental Volume (REV) is an important concept for t he permeability of rock mass. In a rock mass sample, the number of discontinuitiespresent is a func

30、tion of the sample size, stabilizing in average properties when th e sample size is large enough. The REV is the rock mass sample size below whic h the permeability can vary significantly and at and above which the permeability is essentially constant. This REV concept also applies to all properties

31、 governed wh olly or partly by the discontinuities.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 different rock engineering objectives

32、, even though the testing techniques themselves can be standardized, the re can be no standardized site investigation.23.Because the REV size is generally of the order of tens of meters, it is g enerally not possible to conduct meaningful tests directly on the rock mass. Tests are conducted on the i

33、ntact rock and the discontinuities separately and their signific ance for the rock mass properties evaluated. The main organizations publishing test methods are the ISRM and the ASTM.The concepts of accuracy, bias, precision and resolution are useful when considering rock tests on intact rock and di

34、scontinuities. Accuracy is when the corr ect answer is obtained on the average, the bias is the difference between the sam ple mean and the actual mean, precision is when the results are closely spaced (whether they are accurate or not), and resolution is the number of decimal placesto which the val

35、ue is obtained.One of the most popular methods of combining the intact rock and disco ntinuity properties for assessing rock mass properties is through the use of rock m ass classification schemes. The two most popular schemes are the Rock Mass Rat ing (RMR) developed by Bieniawski and the Q rating

36、developed by Barton. The R MR system uses one property of the intact rock, three properties of the discontinuit ies, the ground water conditions and the orientation of the discontinuities relative tothe engineered structure. The Q system uses four properties of the discontinuities, the water flow an

37、d the stress condition. These systems have been successfully a nd widely used in practice to design tunnel supports and to estimate rock mass pr operties.It is very important in rock engineering design to establish the objectives of the project and the objectives of the supporting analysis. Once the

38、 objectives ha ve been established, the physical variables and their interactions can be establishe d using the Rock Engineering Systems approach. The main variables are listed alo ng the leading diagonal of an interaction matrix with the interaction between each pair of variables established for ea

39、ch position in the matrix. This defines the rock e ngineering system and, from the matrix, an audit of the information required for de sign, the variables that are most significant, the critical mechanisms and hence the optimal form of site investigation, numerical codes and the hazards that may ari

40、se can all be established.When rock is excavated for an engineering project, it is necessary to bre ak the rock being removed and avoid breaking the remaining rock. The created ro ck surface (e.g. the slope face or tunnel surface) is thus a critical interface betwee n the excavation and support obje

41、ctives.The excavation process consists of changing the in sim block size distrib ution to the fragment size distribution after excavation.There are only two main methods of excavation. One is blasting in which large mounts of energy are applied to the rock in seconds with quiescent periodsof several

42、 hours in between. The other is by mechanized excavation where a muc h smaller level of energy is continuously input to the rock (except when the machi ne is not operating).Optimizing rock breakage by explosives consists of optimizing the separat e effects of the explosives stress wave and gas press

43、ure and their interactions wit h the free face. Optimizing meehanised excavation consists of optimizing the transf er of energy from the tunnel boring machine cutters to the rock. This involves mec hanical engineering considerations, the configuration of the cutters, steering the ma chine, reducing

44、vibrations, minimizing down time etc.After the rock is excavated the stability of the resultant rock surfaces mus t considered. The three primary effects of excavation are as follows.Displacements occur because rock resistance has been removed.There are no normal and shear stresses on an unsupported

45、 excavation sur face and hence it becomes a principal stress plane-involving a change of the preexisting stress field.At the boundary of the excavation open to the atmosphere, water pressure is reduced to atmospheric pressure causing the excavation to act as a sink with water flowing into it.To stab

46、ilise an excavation, either no support, rock reinforcement (i.e. rock bolts) or rock support (e.g. a cast concrete roof) may be necessary. The reinforcement strategy is to bolt the rock blocks together so that they behave more like a r ock continuum. The support strategy is to maintain the rock disp

47、lacements to tolera ble levels.The ground response curve, in which the support pressure is plotted agai nst the boundary displacement, is a useful conceptual framework for considering th e stability requirements in continuous and discontinuous rock, and to illustrate the e ffect of rock damage that

48、might be caused by the excavation process.A method of rapidly assessing the potential of an excavation to initiate sli p on discontinuities or laminations in rock is thej theory in which the direction of t he stress at the excavation surface is considered in relation to the orientation of th e disco

49、ntinuities and their angle of friction.Slope instability can be caused by failure occurring through weak intact ro ck or along pre-existing discontinuities in harder rock. This indicates four main type s of rock slope instability: circular slip; plane sliding; wedge sliding; and toppling. Agreat dea

50、l can be achieved quickly in assessing the potential for instability by usin g simple solutions for circular slip potential and considering the slope and discontin uity dip and dip directions for failure initiated along pre-existing discontinuities.The same applies for foundations where consideratio

51、ns of, for example, th e Boussinesq solution for the point load of a half space (and how this might be m odified by anisotropic rock and slip on pre-existing discontinuities), are helpful.When designing surface slopes, there should always be an initial kinemati c analysis of slope instability, i.e.

52、given the geometry of the slope and the disconti nuities, is it physically possible for the rock to slide? Because the discontinuities te nd to occur in sets, there always needs to be consideration of designing the excav ation in harmony with the rock structure. For example, a surface excavation whi

53、ch i s circular in plan never provides optimal protection; an excavation which is ellipticalin plan is always better.Because of uncertainty in the input data in such design considerations, it is helpful to conduct a sensitivity analysis using probalisfic methods, fuzzy maths et c.Certain directions

54、and cross-sectional shapes of tunnels and other undergr ound excavations will always be better than others - because the discontinuities donot occur at random orientations. Preliminary considerations using methods such as hemispherical projection and block theory to identify the ustable blocks are m

55、ost helpful.The excavation must also be designed against failure induced by stresses. The process of excavation not only makes the excavation surface a principal stress plane, it also increases the stress component in one direction and decrease it in another. It is necessary to assess the regions of

56、 high stress and whether failure will occur, and the consequences of such failure. Simple considerations of the stres s can be most helpful in this regard. The load in the region of an excavation is co nserved before and after excavation so the redistribution of the load can be consid ered, and solu

57、tions for circular and elliptical excavations can be used to estimates stress concentrations.Sophisticated numerical codes are now available for assessing the effect of excavation on block movements, stresses and water flow within rock masses. M ost of the trends predicted by these programmes can be

58、 established from the princ iples already presented. Therefore, these programmes come into their own when sp ecific values are required, high speed sensitivity studies are needed, etc. These nu merical codes are a revolution in rock mechanics analysis capability.A useful concept when dealing with cl

59、osely spaced excavations is the zo ne of influence. The disturbance caused to the rock mass by excavation is evaluat ed and then the engineering zone of influence is studied, i.e. the volumetric extentof the rock where the stress for example is affected by more than say 5% of its original value. The

60、 zone can be estimated via the simple methods already mention ed and is of help not only in deciding how far away from each other proximate ex cavations should be but also the best sequence of excavation.The design life of the project is important because the rock mechanisms are time dependent. An o