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: are discontinuities on which identifiable shear displacement has taken place. They may be recognized by the relative displacement of the rock on the opposite sides of the fault plane.

Bedding planes divide sedimentary rocks into beds or strata. They represent interruptions in the course of deposition of the rock mass.

are generally highly persistent features, although

sediments laid down rapidly from heavily laden wind or water currents may contain cross or discordant bedding. Bedding planes may contain parting material of different grain size from the sediments forming the rock mass.

are the most common and generally the most geotechnically

significant discontinuities in rocks. Joints are breaks of geological origin along which there has been no visible relative displacement. A group of parallel or sub-parallel joints is called a joint set, & joint sets intersect to form a joint system. Joints may be open, filled or healed.

1- Orientation: The attitude of a discontinuity in space. It is described by the dip direction (azimuth) & dip of the line of steepest declination in the plane of the discontinuity.

2- Spacing: The perpendicular distance between adjacent discontinuities. It normally refers to the mean or modal spacing of a set of discontinuities.

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3- Persistence: The discontinuity trace length as observed in an exposure. It may give a crude measure of the areal extent or penetration length of a discontinuity. Termination in solid rock or against other discontinuities reduces the persistence.

Persistent discontinuity is a continuous plane in a soil or rock mass. Shear displacement takes place if the shear stress along the discontinuity plane exceeds the shear strength of the discontinuity plane.

Non-persistent discontinuity ends in intact soil or rock. Before movement of the material on both sides of a non-persistent discontinuity is possible, the discontinuity has to extend & break through intact material. As intact material has virtually always far higher shear strength than the discontinuity, a non-persistent discontinuity will have larger shear strength than a persistent discontinuity.

Abutting discontinuity

Abutting discontinuities might continue at the other side of the intersecting discontinuity, however, with a displacement to give so-called stepped planes. Shear displacement along the discontinuity can take place if the shear

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strength along the discontinuity plane is exceeded, & the blocks of material against which the discontinuity abuts can move or break.

4- Roughness: The inherent surface roughness and waviness relative to the mean plane of a discontinuity. Both roughness and waviness contribute to the shear strength. Large scale waviness may also alter the dip locally.

5- Wall strength: The equivalent compressive strength of the adjacent rock walls of a discontinuity. It may be lower than rock block strength due to weathering or alteration of the walls. It is an important component of shear strength if rock walls are in contact.

6- Aperture: The perpendicular distance between adjacent rock walls of a discontinuity, in which the intervening space is air or water filled.

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7- Filling: The material that separates the adjacent rock walls of a discontinuity and that is usually weaker than the parent rock. Typical filling materials are sand, silt, clay, breccia, gouge, mylonite. It also includes thin mineral coatings and healed discontinuities such as quartz and calcite veins. 56 Engineering properties of rocks .

8- Seepage: The water flow and free moisture visible in individual discontinuities or in the rock mass as a whole.

9- Number of Sets: The number of discontinuity sets comprising the intersecting discontinuity system. The rock mass may be further divided by individual discontinuities.

10-Block Size: The rock block dimensions resulting from the mutual orientation of intersecting discontinuity sets, and resulting from the spacing of the individual sets. Individual discontinuities may further influence the block size and shape.

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When the rock layer encountered during drilling operation rock coring may be necessary. For coring of rock a core barrel is attached to a drilling road.

Depth of recovery should be properly recorded for the further evaluation in the laboratory and saples are placed in a sample box.

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Based on the length of rock core recovered the following quantities may be calculated for a general evaluation of rock quality.

For RQD determination, the International Society for Rock Mechanics (ISRM) recommends a core size of at least NX (size 54.7 mm) drilled with double-tube core barrel using a diamond bit. Artificial fractures can be identified by close fitting of cores and unstained surfaces.

All the artificial fractures should be ignored while counting the core length for RQD. A slow rate of drilling will also give better RQD. The correct procedure for measuring RQD is shown in Fig. 4.6.

Recovery Ratio: Length of core recovered / Theo. Length of rock cored

Rock Quality Designation (RQD) = length of recovered pieces equal to or larger than 101,6mm / Theo. Length of rock cored

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Based on the length of rock core recovered the following quantities may be calculated for a general evaluation of rock quality.

Recovery Ratio

A recovery ratio of one will indicate the presence of intact rock A recovery ratio of 0.5 or less indicates highly fractured rocks

Rock Quality Designation (RQD)

General relationship was developed between RQD and quality of rock by Deere (1963). This is given in table below

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