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Rock Mechanics.pdf - Mining and Blasting

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Figure A.4 Determining the dip direction<br />

<strong>and</strong> true dip of a plane.<br />

DETERMINATION OF DIP DIRECTION AND TRUE DIP<br />

the normals to two planes, then the construction may be used to determine the angle<br />

between the two planes.<br />

(1) Plot the projections of the two lines using the procedure described in section<br />

A.1. These points are marked A <strong>and</strong> B in Figure A.3a.<br />

(2) Rotate the tracing about the centre point until points A <strong>and</strong> B lie on the same<br />

great circle of the stereonet (Figure A.3b). The dip <strong>and</strong> dip direction of the plane<br />

which contains the two lines are measured from the stereonet as 60 ◦ <strong>and</strong> 200 ◦ ,<br />

respectively.<br />

(3) The angle between the lines is found to be 64 ◦ by counting the small circle<br />

divisions between A <strong>and</strong> B along the great circle (Figure A.3b).<br />

A.5 Determination of dip direction <strong>and</strong> true dip<br />

A common problem encountered in mapping geological features underground is the<br />

determination of the orientation of a feature from the orientations of the traces made<br />

by the intersection of the feature with the boundaries of an excavation. Figure A.4<br />

illustrates the steps involved in the determination of the true dip <strong>and</strong> dip direction<br />

of a discontinuity plane in a simple case. A square tunnel has vertical side walls<br />

which trend in the direction 140 ◦ to 320 ◦ . The apparent dip of a discontinuity where<br />

it intersects the side wall is 40 ◦ SE. The same discontinuity intersects the horizontal<br />

571

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