Computer Aided Planning for Tunnel Construction

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.
Keywords: Tunnelling projects; Geological strata variation; Rock mass classification; Q-system; Project management
INTRODUCTION
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.
The Q-system is a system of rock mass classification1. This
system is essentially used in determining the support system
requirement for tunnelling under the prevalent geological
condition. The parameters of the Q-system are representative
of the geological conditions of the project. These parameters
have been used in predicting the problems likely to be faced in
tunnelling.
Manual calculation for Q-system is very tedious and time consuming.
This is significant considering the fact that the calculations
have to be done for every drill and blast cycle of the
tunnel construction, which may run into thousands for each
project. Thus a computer tool has been developed to aid in the
process which will save both time and effort.
H I Bidaiah is with International Metro Civil Contractors, 8 Jantar Mantar
Road, near Connanght Place, New Delhi 110 001; Dr K Varghese is with
Building Technology and Construction Management Division, Department
of Civil Engineering, Indian Institute of Technology, Madras, Chennai
600 036 while A N Mazumdar is with L&T-ECC Construction Division,
Mount Poonamallee Road, Manapakkam, PO Box 979, Chennai 600 089.
This paper was received on August 13, 2001. Written discussion on the paper
will be entertained till July 31, 2003.
Tunnels are typically constructed either using the drill and
blast method or by using tunnel boring machines. The
tunnelling operation is a repetitive process wherein the same
activities are repeated in each successive cycle. Also, it is
common practice to open up more than one face of a tunnel
for excavation, so that the project can be expedited. Thus, the
various activities fight for the same resource. This makes
resource allocation an important issue to be considered in
tunneling operations. This also influences the project
schedule.
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
o Rock Quality Designation;
o Number of joint sets;
o Roughness of the most unfavourable joint or
discontinuity;
o Degree of alteration or filling along the weakest joint;
o Water inflow; and
o Stress condition.