Jökull - 01.12.1973, Blaðsíða 56

obtain F = 1000N or 500N on each o£ the
cutting tools. Assuming conservatively the cutt-
ing area to be a = 0.5 • 10-4 m2 the pressure
on the cutting area is 100 bar. The resulting
lowering of the pressure melting point can be
found from the Clausius-Clapeyron equation
which gives the slope of the equilibrium curves
between phases in a p-T diagram:
dp L
dT T (V2 — Vi)
Inserting for vi and V2 values for specific
volumes of respectively ice and water, and for
T the pressure melting point of ice at 1 atm
(273 °K) and for L the value of the specific
latent heat of melting of ice (334 J/g) the
pressure difference Ap=100 bar lowers the
pressure melting point by AT = 0.7 °C. A tem-
perature gradient of the order of magnitude
AT/Ax = 100 °C/m results (see Fig. 1) and given
a thermal conductivity of K = 0.6 102 Wm-1
deg-1 for steel, the density of heat flow rate
becomes ifj = — KAT/Ax = 0.6 104 W/m2. The
heat flow rate dQ/dt = a • ij/ = 0.3 J/s through
the cross section a is maintained by freezing of
water at the rate dm/dt = (dQ/dt)/L = 0.9 10~3
g/s. Freezing of ice chips on the subcooled
bit and the lower end of the core barrel might
even be expected to be still more effective than
freezing of water. A combined effect of freezing
of water and ice chips seems to be a plausible
explanation of why drilling was observed to
stop in few minutes.
The problem of preventing freezing on the
cutting tools is a delicate task. The solution,
known from the arctic drilling, to pour an anti-
freezing mixture to the bottom of the hole
brought positive results in the present drilling.
In the light of the freezing process described
above other solutions might be proposed. First
of all using drilling tools with low thermal
conductivity or to insulate the most sensitive
parts of them. Freezing which takes place above
the cutting tools is harmless and circulation of
water diminishes such freezing. Further one
could try to reduce the temperature gradient
over the bit by decreasing the pressure at the
cutting area by a suitable choice of speed of
revolution and the form and number of cutting
bits. By increasing the speed of revolution some
problems could arise in drilling through the
tephra layers which are a common feature of
Icelandic glaciers.
An idea worth consideration is to change
completely the form of the cutting bits e. g. to
try an U-shaped bit commonly used for cutting
rubber. It might be possible to design a U-
shaped bit where refrozen ice is constantly re-
moved during the drilling.
Experiments along the lines drawn in this
note might still improve effectively the drilling
technique in temperate glaciers and funds
should be made available for continued work.
ACKN OWLEDGEMENTS
The author is indebted to the constructor of
the drill Mr. Karl Benjaminsson for many discus-
sions on the topics of this note, and to prof.
Thorbjörn Sigurgeirsson for reading the manu-
script.
REFEREN CES
Árnason, B., H. Björnsson, and P. Theódórs-
son. 1974: Mechanical drill for deep coring
in temperate ice. Journal of Glaciology, 13,
133-139.
54
JÖKULL 23. ÁR