The Delft Sand, Clay & Rock Cutting Model, 2019a

The Delft Sand, Clay & Rock Cutting Model.
2
UCS
m
m s 1
2 UCS
min
m 4 m s
UCS
min
(8-26)
Figure 8-11 and Figure 8-12 show the Hoek & Brown failure criterion for the top of the Mohr circles (A) and for
the real failure condition (B). Although still based on UTS or BTS and UCS and not on two tests with shear failure,
the resulting failure curve seems more realistic, which seems logic since it is based on many experiments. The
Mohr circles for UTS, BTS and UCS are determined for UCS=100 MPa, UTS=BTS=15 MPa. The transition
brittle-ductile according to Mogi (1966) is at a normal stress of 150 MPa.
Taking an average internal friction angle from a normal stress of zero to a normal stress of 240 MPa gives φ=27.1º.
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