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

Rock Cutting: Hyperbaric Conditions.
Chapter 9: Rock Cutting: Hyperbaric Conditions.
9.1. Introduction.
For rock cutting in dredging and mining under hyperbaric conditions not much is known yet. The data available
are from drilling experiments under very high pressures (Zijsling (1987), Kaitkay and Lei (2005) and Rafatian et
al. (2009)). The main difference between dredging and mining applications on one side and drilling experiments
on the other side is that in dredging and mining the thickness of the layer cut is relatively big, like 5-10 cm, while
in drilling the process is more like scraping with a thickness less than a mm. From the drilling experiments it is
known that under high pressures there is a transition from a brittle-shear cutting process to a ductile-flow cutting
process. Figure 9-2 and Figure 9-3 from Rafatian et al. (2009) show clearly that with increasing confining pressure,
first the specific energy Esp increases with a steep curve, which is the transition brittle-ductile, after which the
curve for ductile failure is reached which is less steep. The transition is completed at 690 kPa-1100 kPa, matching
a water depth of 69-110 m.
The Carthage Marble has a UCS value of about 100 MPa and the Indiana Limestone a UCS value of 48 MPa. The
cutter had a blade angle α of 110º. Figure 9-29 shows the specific energy (according to the theory as developed in
this chapter) as a function of the UCS value and the confining pressure (water depth). For the Carthage Marble a
specific energy of about 400 MPa is found under atmospheric conditions for the ductile cutting process. For the
brittle shear process 25%-50% of this value should be chosen, matching Figure 9-2 at 0 MPa. For a water depth of
65 m, matching 0.65 MPa the graph gives about 500 MPa specific energy, which is a bit lower than the
measurements. For the Indiana Limestone a specific energy of about 200 MPa is found under atmospheric
conditions for the ductile cutting process. Also here, for the brittle shear process, 25%-50% of this value should
be chosen, matching Figure 9-3 at 0 MPa confining pressure. For a water depth of 65 m, matching 0.65 MPa the
graph gives about 280 MPa specific energy, which is a bit lower than the measurements.
For deep sea mining applications this is still shallow water. Both graphs show an increase of the Esp by a factor
2-2.5 during the transition brittle-shear to ductile-flow, which matches a reduction factor of 0.25-0.5 for the average
versus the maximum cutting forces as mentioned before. Figure 9-22 and Figure 9-23 show the results of Zijsling
(1987) in Mancos Shale and Figure 9-1 shows the results of Kaitkay & Lei (2005) in Carthage Marble.
The experiments of Kaitkay & Lei (2005) also show that the transition from brittle-shear to ductile-flow takes
place in the first few hundreds of meters of water depth (from 0 to about 2.5 MPa). They also show a multiplication
factor of about 3 during this transition. The experiments of Zijsling (1987) are not really suitable for determining
the transition brittle-shear to ductile-flow because there are only measurements at 0 MPa and about 10 MPa, so
they do not show when the transition is completed, but they do show the increase in forces and Esp.
5000
Cutting Forces in Carthage Marble
Fh, Fv (N)
4000
3000
2000
1000
0
0 4 8 12 16 20 24 28 32 36 40
Pressure (MPa)
Fh - 105 deg. Fv - 105 deg. Ductile Fh - 115 deg. Fv - 115 deg. Ductile
Figure 9-1: Variations of average cutting forces with hydrostatic pressure, Kaitkay & Lei (2005).
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