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

The Delft Sand, Clay & Rock Cutting Model.
if h
if h
h then use h
b,m b b,m
h then use h
b,m b b
(3-25)
3.5. The Tear Type and Chip Type.
The Tear Type of cutting process has a failure mechanism based on tensile failure. For such a failure mechanism
to occur it is required that negative stresses may occur. In sand this is not the case, because in sand the failure lines
according to the Mohr-Coulomb criterion will pass through the origin as is shown in Figure 2-46 and Figure 2-47.
For the failure lines not to pass through the origin it is required that the soil has a certain cohesion or shear strength
like with clay and rock. In clay and rock, normally, the inertial forces and the gravity can be neglected and also
the water pore pressures do not play a role. Only with hyperbaric rock cutting the water pore pressures will play a
role, but there the Tear Type will not occur. This implies that for the Tear Type and Chip Type a soil with
cohesion and adhesion and internal and external friction will be considered.
Figure 3-10: The Tear Type cutting mechanism
in clay.
Figure 3-11: The Chip Type cutting mechanism
in rock.
If clay or rock is considered, the following condition can be derived with respect to tensile rupture:
With the relations for the cohesive force C, the adhesive force A and the adhesion/cohesion ratio r (the ac ratio r):
s ch
C
sin
i
w
s a hb
w
A
sin
a
h
r= c h
b
i
(3-26)
(3-27)
(3-28)
The horizontal Fh and vertical Fv cutting forces can be determined according to:
F c h w
h s i
F c h w
s
i
sin
cos
sin
sin
sin
cos
r cos
sin
sin
cos
cos
r cos
sin
sin
(3-29)
(3-30)
The shear angle is determined in the case where the horizontal cutting force h F is at a minimum, based on the
minimum energy principle.
Page 82 of 454 TOC Copyright © Dr.ir. S.A. Miedema