A tunnel design using numerical analysis relies on the selection of realistic parameters for success. The use of back analysis in tunnel design is important for the determination of parameters in numerical modeling, and for the understanding it brings to numerical modeling. However the process suffers from non-uniqueness, many different combinations of parameters can give an acceptable fit to observations. The technique of predictive analysis offers a solution to the problem of non-uniqueness. In this technique, the observations are matched to the parameter set which maximises or minimises a particular prediction outcome - an example being the maximum settlement above a tunnel. The advantages of using the technique of predictive analysis is demonstrated. Predictive analysis is useful in that the judgement of the predicted outcome is related to the match of the observation points and less emphasis is required on the judgement of parameter values in the rockmass.
The use of numerical modeling is now widespread in geotechnical engineering. The designer can examine new construction methods in ground for which no database of previous construction exists. Practically, the numerical model is limited by lack of knowledge or "true" ground and support properties. There are many ground models available to the modeler, starting with simple elasticity and ranging up to including plasticity or time dependent behavior. The results of the numerical method are very sensitive to input parameters and to the choice of ground model. A sensitivity analysis may be carried out on key input parameters to estimate the likely range of outcomes. Even so, the most important issue is the choice of values for the model input parameters. Nearly all models used in geotechnical analysis require a value of Young's modulus. There are numerous methods for estimating a value. These include: direct tests on pieces of intact core, correlations with RQD, RMR and Q values, use of seismic velocities, and the GSI method of Hoek et al (1995). Engineers usually make a number of estimates for Young's modulus, which are likely to vary from lowest to highest by as much as a factor of 20. The Judgement of experienced professionals in this process for the choice of applicable parameters is a very valuable ingredient, This paper demonstrates a technique for numerical modelling that removes the traditional focus on the model parameters and places it on physical observations and model predictions. Models an their parameters are simply a way to characterise a physical process using mathematics. Whilst it is desirable that the parameters have a physical interpretation this is not essential. Stresses, ground models and input parameters are philosophical concepts - deformation is the physical reality (Burland, 1967). Using the technique outlined in this paper, the designer's judgements are based on observations and model predictions of pilot construction or other large scale tests (eg displacements) rather than preconceived notions suitable values of input parameters.
The South East Transit Project is a $520 million project in Brisbane.
