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CHAPTER THREE DISCONTINUITIES

CHAPTER THREE DISCONTINUITIES. ***. 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

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CHAPTER THREE DISCONTINUITIES

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  1. CHAPTER THREE DISCONTINUITIES

  2. *** 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

  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 • 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, = NILm-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 = 115 - 3.3 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 • 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

  22. Attitude of beds

  23. Map/Cross section

  24. **** • 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

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

  26. **** • 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

  27. **** • 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

  28. Stereonet

  29. stereonet

  30. Planes Measured with Strike and Dip

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

  32. 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

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

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

  35. 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

  36. Rose Diagram

  37. *** 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

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

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