Computer Aided Planning for Tunnel Construction

H I Bidaiah, Associate Member Dr K Varghese, Non-member A N Mazumdar, Non-member

Tunnels are subterranean conveyance systems. Historically tunnelling projects have suffered significant cost and time overrun on account of variation in geological formation. Variation in strata may lead to various problems like tunnel caving in, water seepage etc.
An attempt has been made to extend the Q-system of rock mass classification for reliable prediction of these tunnelling problems. For the purpose tunnelling problems have been identified and classified. A computer tool has been developed to aid the process.
Also simulation as an aid for time planning while tunnelling under such varying geological conditions has been investigated. Reliable estimates of tunnelling problems and effect of geological variations on the project schedule will help the project management in informed decision making as regards issues like resource requirement. This paper describes the compilation and classification of tunnelling problems, matching problems to the Q-system parameters and the simulation model developed.
Tunnels are used in the transportation sector for conveyance of highway and railway traffic. They also serve as a means of conveyance of water in irrigation and hydroelectric power projects. During construction of tunnels variation of strata is often encountered. This poses various problems in tunnelling.
Also, the decision regarding the support system required for various strata is critical to ensure trouble free construction. As the geological formation for each tunnel project is usually very different and all scientific studies are carried on a project specific basis, tunnelling in a new formation remains an art than a science wherein the designers rely on intuition and previous experience to effectively solve problems.
As mentioned earlier one of the major uncertainties faced during tunnelling is that of variation of strata. Initial geological investigation gives a fair idea of the type of rock that might be encountered. But uncertainty prevails and makes time planning all the more difficult. Simulation can be used as an effective tool to aid in planning as regards issues like resource requirements while working under varying geological conditions.

Q-SYSTEM

The Q-system1 of rock mass classification was developed in Norway in 1974 by Barton, Lien, and Lunde all of the Norwegian Geotechnical Institute. The Q-system is based on a numerical assessment of the rock mass quality using six different parameters
These six parameters are grouped into three quotients to give the overall rock mass quality Q as:
where RQD, Rock Quality Designation; J n , joint set number; J r , joint roughness number; J a , joint alteration number; J w , joint water reduction number; and SRF, stress reduction factor.
The rock quality can range from Q = 0.001 to Q = 1000 on a logarithmic rock mass quality scale.
The Q-value is related to tunnel support requirements by defining the equivalent dimensions of the excavation. This equivalent dimension, which is a function of both the size and the purpose of the excavation, is obtained by dividing the span, diameter, or the wall height of the excavation by a quantity called the Excavation Support Ratio (ESR).
Thus
The relationship between the index Q and the equivalent dimension of an excavation determines the appropriate support measures. The use of Q-system in its present form is limited to the determination of support system requirement for tunnelling through the existing geological conditions. The input parameters of the Q-system are based on these geological conditions.
Apart from the support system requirement, a reliable estimate of the tunnelling problems will help the tunnelling engineer in planning. The Q-system in its present form does not predict the problems that might be encountered while tunnelling under the given geological conditions. However, the Q-system can be easily extended for prediction of tunnelling problems, as the input parameters are based on the existing geological conditions. An initial step in the direction would be to identify and compile the various tunnelling problems. Then these problems can be mapped to the input parameters of the Q-system to enable reliable prediction of tunnelling problems.

References
1. Bieniawski Z. T. 'Engineering Rock Mass Classification.' John Wiley and Sons, New York, 1989.
2. Peduzzi A. 'Tunnel and Shaft Sealing Methods using Polyvinyl Chloride Sheet.' ASCE Journal of Construction Division, vol 102, no CO1, 1976, p 111.
3. Stella C, Ceppi G. and Appolonia E. D. 'Temporary Tunnel Support using Jet-grouted Cylinders.' ASCE Journal of Construction Engineering and Management, vol 116, no 1, 1990, p 35.
4. Jethwa J. L. and Dhar B. B. 'Tunnelling under Squeezing Ground Condition.' Conference on Recent Advances in Tunnelling Technology, New Delhi, 1996, p 209.
5. Madan M. M. 'Brief History of Tunnel Construction at Loktak Hydroelectric Project.' Publication No 217, Central Board of Irrigation and Power, New Delhi, 1990.
6. Kutty M. R. 'Rock Reinforcement and Support.' Proceedings of 6th National Symposium on Rock Mechanics, 1992, p 239.
7. Madan M. M. 'Various Aspects of Ventilation System for Underground Works - Experiences and Useful Suggestions.' Tunnelling Asia 2000, New Delhi, 2000, p 351.
8. http/:www.strobos.ce.vt.edu (visited on April 10, 2001).
9. Touran A and T Asai. 'Simulation of Tunnelling Operations.' ASCE Journal of Construction Engineering and Management, vol 113, no 4, 1987, p 554.
10. Ahuja H. N. and Nandakumar V. 'Simulation Model to Forecast Project Completion Time.' ASCE Journal of Construction Engineering and Management, vol 111, no 4, 1985, p 325