1. CHAPTER THREE DISCONTINUITIES

2. Definition and types of discontinuity
***
Definition and types of discontinuity
Discontinuity in rocks represents planes of weakness across which the rock material is structurally separated
any separation in the rock continuum having effectively zero tensile strength
The most common discontinuities in rocks are joint and bedding planes

3. Types of discontinuities
***
Type
Definition
Fracture
A general name given for any separation or break in the rock mass
Joint
A fracture along which no displacement or movement occurred between the adjacent block of rocks
Fault
A fracture along which movement occurred between the two adjacent blocks of rocks
Foliation planes
Series of parallel layers resulted from parallel orientation of minerals crystals during metamorphism
Bedding planes
Contact between sedimentary rock layers or strata
Cleavage
Stress fractures from folding

4. Vertical joints in rock mass

5. folded rocks syncline with cleavage

6. folded rock anticline with cleavage

7. Rock strata with bedding plane

8. faults in rocks

9. A rock mass contains (i) rock material, in the form of intact rock blocks of various sizes, and
(ii) rock discontinuities that cuts through the rock, in the forms of fractures, joints, faults, bedding planes, and dykes
Rock mass = Rock materials + Rock discontinuities

10. Rock mass components

11. In the engineering context discontinuities can be
***
In the engineering context discontinuities can be
the single most important factor governing the deformability, strength and permeability of the rock mass
persistent discontinuity could critically affect structures

12. Geometric properties of discontinuities
Geometrical properties of discontinuities include:
Discontinuity spacing and discontinuity frequency
Orientation
Persistence, size and shape of discontinuity
Roughness of discontinuity wall
Aperture of discontinuity
Discontinuity sets
Block size

13. Discontinuity spacing and frequency
the perpendicular distance between two discontinuities

14. Discontinuity frequency is the number of discontinuities per unit distance
discontinuity frequency,  = NIL m-1

15. Rock Quality Designation
Rock Quality Designation (RQD) is defined as the percentage of rock cores that have length equal or greater than 10 cm over the total drill core

16. RQD
the Rock Quality Designation (RQD) index is measured on borehole cores
RQD only represents the degree of fracturing of the rock mass
< 25 Very poor
25 – 50 Poor
50 – 75 Fair
75 – 90 Good
99 – 100 Excellent

17. Actual rock core recovered

18. Palmström (1982) suggested that, the RQD may be estimated from the number of discontinuities per unit volume:
RQD = Jv
where Jv is the sum of the number of joints per unit length for all joint (discontinuity) sets known as the volumetric joint count

19. RQD can be correlated to discontinuity frequency ():
For 6<<16 /m RQD can be approximated by:

20. Discontinuity orientation
Discontinuity orientation in spherical coordinates is defined by strike, dip angle and dip direction
Strike is the line of intersection of inclined plane with a horizontal plane
Strike direction is the angle between the North direction and the strike line

21. Dip angle is the maximum angle of inclination of discontinuity plane from the horizontal
Dip direction or azimuth is the direction of the horizontal projection of the line of dip measured clockwise from the north
Plunge is the dip of the line of intersection of two planes or the axis of a borehole or a tunnel

23. Attitude of beds

25. Map/Cross section

26. Dip direction and strike direction are always perpendicular
****
Dip direction and strike direction are always perpendicular
Strike and dip are always perpendicular to each other on a map
Dip Azimuth/Dip
210/35, 030/35
Strike/dip format
120/35SW

27. (a) Ordinary geological compass (b) An electronic geological compass

29. Representation of discontinuity in unit sphere
Orientation of a joint plane can be represented graphically using hemispherical projection method. The projection method is to represent a 3D plane by a 2D presentation.
Representation of discontinuity in unit sphere
****

30. For stereographic projection a unit reference sphere is used in which any discontinuity plane can be represented by one pole point on the surface of the lower hemisphere.
Definition of plane orientation in the unit sphere and stereo-net projection
****

33. Stereonet

35. stereonet

38. Planes
Measured with Strike and Dip

39. Plot the Planes
034/25NW 104/65S 177/33E 065/82NW

40. Lines Linear features Lineations are measured using plunge/ azimuth
Examples of lineations are slickensides on a fault surface, fold axes, mineral stretching lineation or ripple crests

42. Plot the lineations
032/20NW
102/65S
177/33E
065/82NW

43. Intersection of a Plane
Strike 012 degrees, Dip 60 degrees SE
Strike 107 degrees, Dip 41 degrees SSW
Lines
206°/46° and 273°/58°

47. Strike: the line of the horizontal on a plane
Strike: the line of the horizontal on a plane. Measured from north in degrees and recorded as three figures eg. 057 Dip: The maximum dip of a plane. Measured in degrees from the horizontal and recorded as two figures eg. 34. Perpendicular to the strike Also need dip direction to fully describe the plane eg SE E.g. 057/34SE

48. Rose Diagram

49. Joints are generally in sets, i.e., parallel joints
***
Discontinuity Sets
Discontinuity set is a group of discontinuities having the same orientation or which run parallel to each other.
Joints are generally in sets, i.e., parallel joints
As the number of joint sets increase the quality of the rock decreases

50. Rock Mass (a) with one joint set (b) with two joint sets

51. A fault plane with orientation 234/56 has been discovered during site investigation. Closer inspection shows that it has surface lineations which have a pitch of 78 degrees measured from the north set strke line . What is the trend and plunge of these lineations?

52. ISRM Suggested description of the rock mass quality based on the number of joint sets
Class Suggested Rock mass description

53. Block Size and Volumetric Joint Count
Joint spacing defines the size of rock blocks
When a rock mass contains more joint numbers, the joints have lower average spacing and smaller block size

54. RQD can be related to volumetric joint count Jv by:
****
Volumetric joint count (Jv) is the number of joints per unit cubic meter of volume of the rock
RQD can be related to volumetric joint count Jv by:
RQD = 115 – 3.3 Jv, for Jv between 4.5 and 30.
RQD = 100% for Jv < 4.5,RQD = 0% for Jv > 30.

55. ISRM suggested block size designation

56. Persistence, roughness and aperture of discontinuity
Persistence of discontinuity
Persistence of discontinuity is the lateral extent or length of a discontinuity plane
can be crudely quantified by observing the trace lengths of discontinuities on exposed surfaces

57. Persistence
the areal extent or length of a discontinuity

58. Discontinuity Surface Roughness and Matching
The word 'roughness' is used to denote the deviation of a discontinuity surface from perfect planarity
Joint surface roughness is a measure of surface unevenness and waviness relative to its mean plane
The roughness is characterized by large scale waviness (undulation) and small scale unevenness (irregularity) of a joint surface

59. ****
Joint Roughness Coefficient (JRC) is a quantitative measure of roughness, varying from 0 for the smooth flat surface to 20 for the very rough surface.
Joint Roughness Coefficient (JRC) method compares a profile of a discontinuity surface with standard roughness profiles

60. Joint roughness is affected by geometrical scale
***
Joint roughness is affected by geometrical scale
JRC20 is the profile for 20 cm and JRC100 for 100 cm
The value of JRC decreases with increasing size

61. Roughness profiles and corresponding JRC values (After
Roughness profiles and corresponding JRC values (After Barton and Choubey 1977).

62. ***
The geometrical roughness is naturally related to various mechanical and hydraulic properties of discontinuities.
On the purely geometrical side, it is possible to predict the amplitude of asperities from the JRC and profile length.
On the mechanical side, shear strength can be predicted via JRC and other properties.

63. estimating JRC from. measurements of surface
estimating JRC from measurements of surface roughness amplitude from a straight edge (Barton 1982).

64. Figure 3.21: Rock with joint surface in 3D

65. A joint surfaces in 3D, also noting
A joint surfaces in 3D, also noting change of linear profiles in directions.

66. Discontinuity (Joint) Matching
Joint surface can be in good contact and matched, or they can be poorly contacted and mismatched. A Joint Matching Coefficient (JMC) has been suggested
JMC is 1 for completely matched joint and two surfaces fully in contact
JMC is 0 for completely mismatched joint and two surfaces in contact at a few points only

67. Different degree of joint matching surfaces with
Different degree of joint matching surfaces with different values of Joint Matching Coefficient

68. Discontinuity (Joint) Aperture and Fillings
The perpendicular distance separating the adjacent rock walls is termed as aperture
Joint opening is either filled with air and water (open joint) or with infill materials (filled joint)
Open or filled joints with large apertures have low shear strength
Aperture also associates with flow and permeability

69. Different types of discontinuity related with
Different types of discontinuity related with aperture closer, opening and fillings.

70. Classification of discontinuity aperture

71. Filling is material in the rock discontinuities separating the adjacent rock surfaces.
In general, properties of the filling material affect shear strength, deformability and permeability of the discontinuities.

72. Schematic of geometrical properties of
Schematic of geometrical properties of discontinuities in rock (from Hudson, 1989)