Page 240 - Rock Mechanics For Underground Mining

EXCAVATION DESIGN IN MASSIVE ELASTIC ROCK

such that, during development of the second opening, no support problem or local
instability is produced in the existing opening.
5 The figure below shows a horizontal section through the midheight of a vertical,
lenticular orebody. The orebody is mined with long vertical blast holes in such a
way that a slot approximately elliptical in a horizontal section is extended along
the strike of the orebody. The field principal stresses in the horizontal plane are of
magnitudes 25 MPa and 10 MPa, and oriented as shown. For the case where the length
(L)/breadth (B) ratio of the mined excavation is 2.8, determine the magnitudes and
corresponding locations of the maximum and minimum boundary stresses. If the rock

mass tensile strength is zero, and compressive failure is defined by a Coulomb law
◦
with c = 30 MPa, = 30 , determine the likelihood of boundary failure. Assess the
consequences of the results for mining activity.

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