# Classification of Discontinuities.ppt

Mar. 29, 2023

### Classification of Discontinuities.ppt

1. Classification of Discontinuities Two bases for classification of discontinuities in a rock mass : Genetic geological classification Classification based on mechanical characteristics
2. Geological Genetic Classification  Specifies the nature of the discontinuity in describing a rock mass. It is essential to distinguish between a joint and a fault, and between a joint and a bedding plane only partly satisfactory for engineering purposes
3. Table 10 Genetic classification of discontinuities Rock group Syngenetic Epigenetic Igneous Cooling joints Flow surfaces Plutonic contacts Dykes, Sills and veins Faults Tectonic joints Sheeting Sedimentary Bedding planes Internal laminations Cross bedding Shrinkage joints Unconformities Faults Tectonic joints Cleavages Bedding plane shears Sheeting Metamorphic Foliations, Metamorphic layering, joints Faults, Tectonic joints, Sheeting
4. Mechanical Classification of Discontinuities
5. Analysis of Fracture Systems Mode of Analysis Geometrically in the first instance This depends on: Firstly, certain recognition of origin of fractures Secondly, the system being homogeneous  i.e. that the pattern, at least statistically, is uniform for the mass under investigation. If this condition is not satisfied, the mass is divided into units or sub-areas each having a uniform fracture pattern
6. Geometric Analysis of Fracture Systems Use statistical analysis to: a) establish the average orientation of a set b) determine the sets which constitute a system
7. Applicable Methods a) The fracture rosettes  Only strikes are considered.  Method does not take account of the dip of the fracture, hence, does not include horizontal fractures. (Horizontal fractures equally important as dipping fractures when determining the mechanical characteristics of a rock mass)
8. An Example of Fracture Rose
9. b) Equal Area Analysis Poles of fractures are plotted on an equal area net bringing out the concentrations by counting and contouring (Fig 12). (This method takes account of both strike and dip and also includes horizontal fractures in the analysis).
10. Example of equal area diagram of poles of fractures
11. Description of Persistence Description Modal trace length Very low persistence <1 m Low persistence 1 – 3 m Medium persistence 3 – 10 m High persistence 10 – 20 m Very high persistence > 20m
12. Stress Analysis • Qualitative analysis of stresses in rock mass is performed using the equal area method • Fig. 13 shows the ideal fracture system resulting from a system of stresses. • The fractures intersect in σ2, the intermediate principal stress; σ1, the maximum principal stress is the acute bisector of the shear fractures, normal to σ2 and lying in the σ1 σ2 plane; and σ3 is normal to the σ1 σ2 plane
13. σ2 σ1 σ3 σ1 σ2 σ3
14. Permeability of the rock mass  Flow of water through the intact rock material under the influence of hydraulic pressure referred to as Primary permeability .  Many rock materials are impervious.  Presence of fractures, other discontinuities and solution cavities impart secondary permeability to the rock mass.
15. Secondary permeability is determined by the Lugeon or packer test. The test is conducted in a bore hole in which a packer or a double packer is used to seal off a section of the rock mass Water is pumped under pressure into the test section and allowed to saturate the rock The loss of water over a specified period is then determined.
16. Packer Test
17. Test usually carried out over a test section 1m long under a pressure of 1 MPa. A loss of 1lmin-1 represents 1 Lugeon (~ a coefficient of permeability of 10-4 cm s-2). Secondary permeability depends on: a) Spacing of discontinuities b) Continuity c) Infilling d) Openness e) degree of interconnection of the discontinuities in the rock mass.
18. Seismic Velocity in the Rock Mass Seismic Velocity is the velocity of propagation of pressure waves through a rock mass. Seismic velocity is a function of a multitude of rock properties including: a) Density b) Porosity c) Composition d) Cementation e) Degree of fracturing.
19. E. G: Variations in the degree of fracturing or degree of weathering can be correlated with seismic velocity. 1) The effects of discontinuities in a rock mass can be established by comparing the velocity (VF) of compressional wave in the rock mass with the sonic velocity (VL) of intact rock measured on a core in the laboratory.
20. PUNDIT (Portable Ultrasonic Non-Destructive Tester)
21. • For a fresh rock mass with only a few, tight discontinuities, VF / VL is close to unity. • With an increase in the degree of fracturing, VF / VL is less than 1.
22. Rock Quality Designation Rock Quality Designation (RQD) is a fracture index used in many rock classification systems
23. Determination of RQD
24. Standard terms to describe the quality of rock mass based on RQD RQD (%) Description of rock quality 0-25 25-50 50-75 75-90 90-100 Very poor Poor Fair Good Excellent
25. Mechanical classification of rock mass
26. Objectives of rock mass classifications • Identify the most significant parameters influencing the behaviour of a rock mass. • Divide a particular rock mass formulation into groups of similar behaviour – rock mass classes of varying quality. • Provide a basis of understanding the characteristics of each rock mass class
27. Objectives cont’d • Relate the experience of rock conditions at one site to the conditions and experience encountered at others • Derive quantitative data and guidelines for engineering design • Provide common basis for communication between engineers and geologists
28. Benefits Improving the quality of site investigations by calling for the minimum input data as classification parameters. Providing quantitative information for design purposes. Enabling better engineering judgement and more effective communication on a project.
29. NOTE: No all-purpose mechanical or engineering classification of rock mass has been developed due to: a) the number of parameters involved b) the diversity of interests of the engineer.
30. The South African Geomechanics Classification (SAGC) Rock mass is divided into a number of units each having a characteristic set of properties (e.g. uniform fracture pattern and fracture spacing)
31. Parameters determined for each unit • Uniaxial compressive strength • RQD • Weathering characteristics • Joint and other fracture spacing • Fracture openness, continuity and infilling • Orientation of fractures • Groundwater conditions
32. • Based on these criteria, five classes are established. • Relative indices are awarded the various parameters (see Table 12). • The parameters are considered to be of unequal importance hence, each is assigned a weighted minimal value called a rating. • The rock quality is defined by the sum of all of the values of the various parameters
33. APPLICABILTY Applicable to rock slopes and foundations even though it was primarily designed for underground mining. DEFICIENCIES  Does not account for anisotropy or elasticity which are important considerations in determining the stress distribution around openings.  Does not take account of the tensile strength of the rock material –an important parameter in mining operations.
34. Geomechanical classification of jointed rock masses
35. Rock Structure Rating Developed for use in mining. Rock Structure Rating considers three basic parameters namely: 1) Rock type, folding and discontinuities 2) Joint spacing and orientation 3) Water inflow and joint openings. Each parameter is assigned a value and the rock structure rating RSR is defined by the sum of (1), (2) and (3). •
36. Rock Mass Quality • The Rock Mass Quality system due to Barton et al (1974) was designed for underground excavation. It considers six parameters as follows: i) RQD ii) joint set number Jn iii) joint roughness number Jr iv) joint alteration number Ja v) joint water reduction factor Jw vi) stress reduction factor, SRF.
37. Concluding Remarks The SAGC system is of more general applicability than any of the above. It is simple and partly quantitative. All the above systems are applicable to hard, brittle, jointed rock masses but would certainly give conservative results if applied to other rocks