Page 113 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 7 Tunneling Hazards 83

ENVIRONMENTAL HAZARDS DUE TO TOXIC OR EXPLOSIVE
GASES AND GEOTHERMAL GRADIENT

There are serious environmental hazards due to toxic or explosive gases while tunneling in
the argillaceous rocks. Sometimes methane gas is emitted by blasted shales. Improper ven-
tilation also increases concentration of toxic gases like carbon monoxide, carbon dioxide,
hydrogen sulfide, and sulfur dioxide, so additional ventilation capacity is required. If there
is methane gas emission, permissible electrical equipment may be used. Attention should
be given to the physical properties of the gases, as some gases tend to collect either in high
or low pockets in a tunnel complex. Table 7.7 summarizes the properties of the previously
mentioned gases found in tunnels (Mathews, 1996). Monitoring of gases and oxygen
should be carried out near the face of a tunnel where blast fumes and gas emissions are
heaviest. Oxygen must be maintained at a level of 20% or greater. Dust inside the tunnel
should also be controlled to reduce health hazards; therefore, the wet drilling method is
recommended for both blast holes and bolt holes.

As rock engineers go deeper and deeper, they will have to face high temperatures.
The temperature may increase at a rate of about 30C per kilometer. This is in addition
to the average ground temperature, which is equal to the average temperature in a year.
The temperature inside a 1400 m deep NJPC tunnel in the Himalayas, in India, was more
than 45C. The efficiency of workers in such a high temperature was reduced drastically.
They worked for two to three hours, frequently bathing in buckets of ice-filled water. If
possible, cool fresh air should be used for ventilation to maintain a working temperature
of around 30C at the tunnel face.

CONCLUDING REMARKS

Rock has extraordinary geological occurrence (EGO) problems. Enormous time and
money are lost due to unforeseen tunneling hazards, particularly in the Himalayas and
other young mountain chains. Generally, if a shear zone or a weak zone is not seen within
200 m in the lower Himalayas, it means that it has been missed. Thus, geological uncer-
tainties may be managed by adopting a strategy of tunnel construction that copes with
most tunneling conditions. A hazard foreseen is a hazard controlled. Therefore, it is de-
sirable to use safe and effective tunneling methodology based on detailed engineering
geological explorations before and during the tunnel construction. The modern trend
of insuring the tunneling machine and the losses due to delays because of unexpected
geological and geohydrological conditions takes care of the contractor’s interests.

REFERENCES

Barla, G. (1995). Squeezing rocks in tunnels. ISRM News Journal, 2(3 & 4), 44–49.
Barton, N., Lien, R., & Lunde, J. (1974). Engineering classification of rock masses for the design of tunnel

support. In Rock mechanics (Vol. 6, pp. 189–236). New York: Springer-Verlag.
Bhasin, R., & Grimstad, E. (1996). The use of stress-strength relationship in the assessment of tunnel

stability. In Proceedings of the Recent Advances in Tunnelling Technology (pp. 183–196). New Delhi,
India: CSMRS.
Bieniawski, Z. T. (1973). Engineering classification of jointed rock masses. Transactions of the South
African Institution of Civil Engineers, 15(12), 335–344.
Cantieni, L., & Anagnostou, G. (2009). The interaction between yielding supports and squeezing ground.
Tunnelling and Underground Space Technology, 24, 309–322.