This paper describes stability assessment of an underground cavern for hydro-electric power plant in an integrated manner. The approaches covered in this study are fracture investigation, monitoring of deformation and properties of the rock mass and numerical stability analysis.
Ce document decrit une manière integree d'evaluation de la stabilite d'une caverne souterraine pour une centrale hydro-electrique. Les methodes que l'on a employe au cours de l'etude comprennent l'examen des fractures, la surveillance des deformations et des proprietes de la masse rocheuse et 1'analyse numerique de la stabilite.
In diesem Papier wird die Stabilitatsbestimmung einer Untergrund-Kaverne fuer ein Wasserkraftwerk in integrierter Weise beschrieben. Die in dieser Studie unternommene Vorgehensweisen sind Bruch-Untersuchungen, Überwachung von Deformationen, Eigenschaften von Felsmassen und numerische Stabilitatsanalyse.
The structural stability of the underground cavern should be assessed in an integrated manner which combines all of the information available, to the full. This calls for close cooperation between the various engineers involved in the project. In this paper we describe how best to cope with the problem inherent in the stability assessment of a certain underground cavern. The cavern was excavated in the granite rock mass for the construction of an underground power plant. The cross section of the cavern is bullet shaped in appearance as illustrated in Figure 1. The rock making up the site is fairly good, as its uniaxial compression strength is approximately 1100 kgf/cm2. But fractures (joint) were likely to affect the structural performance of the cavern. For excavation the New Austrian Tunneling Method was employed. The task team on the contraction side was organized in order to perform safe and economical excavation work. This was made up of engineering geologists, rock mechanics engineers and numerical analysts, etc.
Fracture characterization is one of the prerequisites in evaluating the cavern stability and in estimating the safety during excavation work. From a practical viewpoint, the fracture investigation method is required to possess the following capabilities; One is that it should be economical as well as practical. The other is that it should provide quantitative results that can be used directly as input data for numerical stability analysis. In conventional methods, it seems hard to characterize the fracture distribution quantitatively, because firstly they mainly depend upon personal judgments of engineers and secondly limited number of sampling is inevitable. In order to solve these problems, we have developed a new investigation and evaluation method that meets the above two requirements. We describe features of the method below. In the investigation stage, we take photographs of excavated rock surfaces they are mutually perpendicular. Next, we pick out fractures by applying the image processing technique, and we can get fracture maps projected onto arbitrary three orthogonal rock surfaces. Due to the lack of space, we have only described resultant equations. Detailed description of the method is presented by Kusabuka [4]. Nk is the average number that fractures intersect a scan line with k r t h direction. Here let us define the direction of fracture trace line as shown in Figure 2. Then, we get the following equations by considering the geometric relation between the fracture and the excavated surface.
It seems crucial as well as useful to monitor changes in properties and deformation of the surrounding rock mass during the cavern excavation in evaluating the stability of the cavern. Measurements of seismic wave velocities, hydraulic conductivity and deformations of the surrounding rock mass were performed in every excavation stage as shown in Figure 3[5]. These measurement data are expected to provide us with useful information in the sense that they are measured before excavation as well as during excavation. The features of each measurement system are,
Seismic Wave Measurements: Using high frequency (10–5KHz) of wave in order to have accurate results.
Lugeon Tests: Employing low injection pressures (0.5–2.0 kgf/cm2) so that the side wall of the cavern might not be damaged by the test itself. Also, measuring very low flow rate, as results of low pressure, by means of specially developed high accuracy of devices.
Relative displacement Measurements: Installing the extensometer from outside the cavern (the access tunnel) so that we could measure their initial values free of excavation effect. In addition, some systematic deformation measurements were carried out[6], as also illustrated in Figure 3.
It is necessary to characterize fracture systems in the rock mass quantitatively, for the numerical stability analysis in which the effects of fractures are considered. Fracture maps obtained through image processing are shown in Figure 4. By applying Eq.(l) and Eq.(2) to these fracture maps, we got the density functions, also shown in the figure.
