Engineering Geology

methane toxic, it also is combustible and highly explosive when 5–15% is mixed with air.
Carbon dioxide, CO 2 , and carbon monoxide, CO, are both toxic. The former is heavier than
air and hangs about the floor of an excavation. Carbon monoxide is slightly lighter than air
and as with carbon dioxide and methane, is found in Coal Measures strata. Carbon dioxide
also may be associated with volcanic deposits and limestones. Hydrogen sulphide, H 2 S, is
heavier than air and is highly toxic. It also is explosive when mixed with air. The gas may be
generated by the decay of organic substances or by volcanic activity. Hydrogen sulphide
may be absorbed by water that then becomes injurious as far as concrete is concerned.
Sulphur dioxide, SO 2 , is a colourless pungent asphyxiating gas that dissolves readily in water
to form sulphuric acid. It usually is associated with volcanic emanations, or it may be formed
by the breakdown of pyrite.
Temperatures in Tunnels
Temperatures in tunnels are not usually of concern unless the tunnel is more than 170 m
below the surface. When rock is exposed by excavation, the amount of heat liberated
depends on the virgin rock temperature, VRT; the thermal properties of the rock; the length
of time of exposure; the area, size and shape of exposed rock; the wetness of rock; the air
flow rate; the dry bulb temperature; and humidity of the air.
In deep tunnels, high temperatures can make work more difficult. Indeed, high temperatures
and rock pressures place limits on the depth of tunnelling. The moisture content of the air
in tunnels is always high and, in saturated air, the efficiency of labour declines when the
temperature exceeds 25∞C, dropping to almost zero when the temperature reaches 35∞C.
Conditions can be improved by increased ventilation, by water spraying or by using refrigerated
air. Air refrigeration is essential when the virgin rock temperature exceeds 40∞C.
The rate of increase in rock temperature with depth depends on the geothermal gradient that,
in turn, is inversely proportional to the thermal conductivity, k, of the material involved:
0.05
Geothermal gradient = (approximately) ∞
k
Chapter 9
-
Cm 1
(9.3)
Although the geothermal gradient varies with locality, according to rock type and structure,
on average it increases at a rate of 1∞C per 30–35 m depth. In geologically stable areas,
the mean gradient is 1∞C for every 60–80 m, whereas in volcanic districts, it may be as much
as 1∞C for every 10–15 m depth. The geothermal gradient under mountains is larger than
under plains; in the case of valleys, the situation is reversed.
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