The importance of geology and geologic information in tunneling operations is an accepted fact. However, in the pre-construction phases of a tunnel project it is difficult to justify further geologic investigation, as the consequences of such expenditures could not, until now, be evaluated. A solution to this problem is proposed. It is based on the Tunnel Cost Model, a computer model for estimating cost and time-to-completion of a tunnel project, developed at M. I.T. The solution uses basic decision analysis, subjective input and probabilistic tools to conduct a pre-posterior analysis. The analysis yields an expected monetary value for a particular exploration plan. This dollar value can then be compared to exploration cost to determine whether or not exploration should be performed. A case study is pre&ented to illustrate the proposed analysis.
La connaissance des conditions géologiques est particulièrement importante en construction souterraine. Il est cependant difficile de justifier une reconnaissance additioneile dans la phase précédant la construction car-jusqu' a présentil n'était pas possible de comparer le coiît d'une reconnaissance additionelle avec la réduction potentielle du coû*t de construction. Une solution "à ce problème est présente dans cet article. Celle solution se base sur le "Tunnel Cost Model", un modele simulateur dévelopé au M.I.T. qui permet d'estimer le coût et le temps de construction d'un tunnel. Des méthodes simples de théorie de décision et de théorie probabilistique sont employées dans une analyse pré-postérieure avec des données de caractère subjectif. Le résultat de cet analyse est une économie probable pour une reconnaissance géologique particulière. Cette valeur monétaire peut "être comparée avec le coût de la reconnaissance et^de cette manière il est possible de déterminer si la reconnaissance doit'être exécutée. Un cas réel est présente pour illustrer G analyse proposée.
Die Kenntnis der geologischen Verhältnisse ist im Tunnelbau besonders wichtig. Es ist jedoch schwierig zusätzliche geologische Erkundungen im Projektstadium zu rechtfertigen, da bis anhin die zusätzlichen Ausgaben für solche Erkundungen nicht mit möglichen Baukostenverminderungen verglichen werden konnten. Der vorliegende Artikel beschreibt eine Lösung dieses Problems. Die vorgeschlagene Methode beruht auf dem Tunnel Kost Model, einem am M. I.T. entwickelten Simulationsmodell, mit welchem Baukosten und Bauzeit eines Tunnels abgeschätzt werden können. Mittels einfacher Methoden der Entscheidungs-und Wahrscheinlichheitstheorie wird eine Analyse a preposteriori mit subjektiven Eingabewerten durchgeführt, welche einen zu erwartenden Geldwert für die betrachtete geologische Erkundung ergibt. Ein Vergleich zwischen dem zu erwartenden Geldwert und den Kosten der Erkundung ermöglicht dann zu entscheiden, ob die Erkundung durchzuführen ist. Zum Abschluss des Artikels wird die Methode an einem praktischen Beispiel angewandt.
'Geologic conditions are among the most important factors that deter- mine the cost and speed of tunneling and it is consequently essential to predict them." Klehn 1971
“Geologic conditions have a greater influence on the selection of tunneling method than any other single factor.” Deere, et al, 1969
It is commonly recognized that geology and geologic prediction are major factors in the design and construction of tunnels. With improper geologic information, errors may occur in the estimate of project time and cost, or in the selection of alignment location and construction technique. The result of such errors is generally the loss of time and money; sometimes the price is human life. Hence, the acquisition of geologic knowledge by exploration should be a primary requirement on any tunnel project. Exploration, however, is often an expensive proposition. It is also difficult to weigh the difference between the immediate cost of exploration and some undetermined future benefit in project cost and time. This is due to the fact that exploration cost and the benefit derived from it are not directly or simply related. To establish such a relationship, it would be first necessary to determine the value of geologic knowledge with regard to the particular tunnel project, and then in some manner relate exploration and geologic knowledge. A method will be presented that attempts to establish such a relationship, and hence makes it possible to evaluate exploration. To relate geologic conditions to tunnel construction cost and time, a computer model is used. The Tunnel Cost Model, or TCM, employed in the analysis has one outstanding feature compared to other existing models - it allows the uncertainty of geologic know- ledge to be incorporated in the assessment of project time and cost. With this model as a basis, it becomes possible to quantify the value of exploration. Using simple decision analysis and probabilistic tools, the uncertainties which exploration attempts to diminish can be evaluated. The initial steps in tunnel exploration and the basics of the TCM will be examined first, followed by a detailed description of the methodology to evaluate exploration. A case history is also presented.
