An important concern in the design of an underground cavity is to define the optimum solution among different alternatives (1). The present work emphasizes the development of evaluative criteria to judge the performance of various design solutions with respect to the depth, shape of the opening and the cavity wall reinforcement. As an example eight different alternatives are considered for a preliminary design of a 50 meter span underground power plant as shown in Table (1). Two geometric configurations are retained, namely the horseshoe and the circular shape. Also two systems are adopted for the stability of the opening, a simple concrete liner and a combined system of a concrete liner and rock bolts. Also two depths are considered at 75 and 125 meters respectively. The necessary quantification of the evaluative criteria is provided by means of four computational units related to the finite element uncertainty analysis. These computational units include (a) the effect of the underground water flow taking into consideration the water table variation, (b) the effect of the excavation considering the average displacements read on extensometers installed in the vicinity of the opening, (c) the dynamic analysis performed using a model approach, and (d) the stability of the liner.
From the technical point of view, the performance of a given engineering project can be described by a number of parameters generally defined as attributes or criteria. In the case of the performance of underground openings three categories of technical attributes can be recognized, Table (2). The first category comprises the attributes obtained from an initial site investigation of rock media in opposition with the second and third categories which contain the attributes related to the excavation and the earthquake phenomenon and their effects on the cavity system.
