Uncertainties in the tunnel projects

All phases of the tunnel project are influenced by numerous uncertainties. These can be categorized into two groups:

 Usual uncertainties in the course of tunnel design, construction and operation

 Occurrence of extraordinary events (failures) causing significant unplanned changes of the expected project development

Both types of uncertainties influence the stakeholders’ objectives, which can be expressed using performance parameters such as costs, time, quality etc. Examples of the uncertainties and performance parameters, in division to the project phases, are given in Table 2.3.

Table 2.3: Examples of uncertainties and the influenced performance parameters in the tunnel projects

Planning phase Construction phase Operation phase

Usual uncertainties Usual uncertainties Usual uncertainties - quality of planning team - geological + hydrological cond. - number of vehicles - quality of designer - performance of the technology - quality of maintenance - geotechnical survey - quality of organization and works - durability of materials - tendering - prices of materials, labour…

Extraordinary events Extraordinary events Extraordinary events - Strong public aversion - Tunnel collapse or flooding - Fire

- Rejection of financing - Unpredicted existing structures - Vehicle accident - Legislative obstructions - Extensive deformations - Tunnel collapse Performance parameters Performance parameters Performance parameters - Land acquisition time and costs - Construction costs - Income/availability - Design cost, time and quality - Construction time - M&O costs

- Time for acquisition of regulatory - Quality - Environmental impacts

Distinguishing between the two types of uncertainties is necessary, because the principal divergence of their nature requires different approaches to their analysis. It is further evident that the usual uncertainties influence the occurrence of extraordinary events. For example, unpredicted geological conditions and poor quality of construction management is likely to lead to a tunnel collapse. These dependences must therefore be considered in the quantitative risk analysis.

For the modelling purposes it is further convenient to distinguish between the aleatory and epistemic uncertainties:

 Aleatory uncertainty is the natural, intrinsic randomness in the analysed system, which cannot be reduced.

 The epistemic uncertainty is the uncertainty resulting from incomplete knowledge of the system and it can be reduced when additional information is available.

To give examples, the geotechnical parameters (e.g. number and orientation of discontinuities, compressive strength) can include both types of uncertainties. The aleatory uncertainty corresponds to the spatial randomness of these parameters (the discontinuities are not equally distributed in space). The epistemic uncertainty results from the fact that we are not able to describe the randomness with certainty because we only have limited knowledge about the geology (e.g. from local boreholes). A thorough discussion on aleatory and epistemic uncertainty is available in Der Kiureghian and Ditlevsen (2009). The authors claim that, in principle, distinguishing between the two types of uncertainties depends on the view and intentions of the modeller, i.e. on the judgment, whether or not the uncertainty can be reduced in later phases of the analysis.

2.8 Summary

Chapter 2 introduces the reader into the topic of tunnel project planning and management. While the remaining part of the thesis only deals with modelling of tunnel construction, Sections 2.1, 2.6 and 2.7 put the construction phase into the context of project’s life cycle. It is shown that the time and costs of construction are not the only criteria for making decisions, but certainly very important ones. Additionally, it is demonstrated that analysing the uncertainties and risk is crucial for identifying optimal solutions in all phases of the project. A categorization of uncertainties influencing the tunnel projects is brought in this chapter as well, since it is important for their proper analysis and modelling.

Sections 2.2 - 2.5 briefly discuss the tunnel construction itself. The commonly used tunnelling technologies are introduced in Section 2.2 with particular attention to the conventional tunnelling, which is used in application examples later in this thesis. Because the geological conditions and their proper description is essential for the tunnel construction and for the prediction of the construction time and costs, Section 2.3 gives a summary of the widely used geotechnical classification systems. It shows that the description of geotechnical conditions is highly site- specific. Therefore, the transfer of experiences between different projects is not a straightforward task and it cannot be easily automated. This issue will be discussed later in relation to the data analysis in Chapter 7. Section 2.4 briefly explains, how the construction time and costs are estimated in different phases of the project. At present, the deterministic estimates are used in the majority of cases. However, the tunnelling community recently recognised the limitations of the deterministic approach and more attempts are made to quantify the uncertainties and risks (see Section 3.2). Finally, Section 2.5 discusses the possibility of occurrence of extraordinary events (failures) during the tunnel construction. These events represent a high risk for the tunnelling

procedure, as is shown in several studies collecting information on past events. Modelling of these events and their impact is discussed also in remaining part of the thesis.

Approaches and methods for risk analysis are presented in this chapter. As was introduced in Section 2.6, the risk is defined as “effect of uncertainty on the objectives”. The uncertainties influencing the tunnel project were briefly discussed in Section 2.7; it is distinguished between the usual uncertainties and extraordinary events. The objectives of a tunnel construction (measurable performance parameters) are as follows:

 Completion of the construction on time

 Completion of the construction within the budget  Fulfilment of the technical requirements

 Ensuring safety during the construction

 Minimization of impact on operation of adjacent structures  Minimization of damage to third party property

 Avoidance of negative reaction of media and public

In the following, the text mostly focuses on effect of uncertainty on the construction time, partly also on the construction costs, the other performance parameters are not explicitly considered in this thesis.

Note that the risk is not a universal quantity. The objectives of individual stakeholders can differ and they also develop during the project. Additionally, the perception of the consequences of not meeting the objectives is also individual. Therefore, the risk must always be analysed with regard to the context and objectives.

The state-of-the-art of tunnel construction risk analysis is summarized in this chapter: Section 3.1 is focused on the qualitative method; Section 3.2 provides an overview of approaches to quantitative risk analysis. The purpose and limitations of the qualitative and quantitative approaches are discussed. Section 3.3 introduces selected methods and models for analysis of uncertainty and risk, which are utilized later in this thesis.