In order to investigate the seismic behavior of a cavern and the surrounding rock, earthquake observations have been carried out in the rock tunnel started July, 1983. The characteristics of the earthquake motion in rock, deforming behavior of the cavern, and the relation between the strain of the caver and the particle velocity of the surrounding rock were clarified.
Afin d'etudier le comportement seismique d'une caverne et des roches avoisinantes, des observations de secousses telluriques ont ete effectuees à l'interieur d'un tunnel de roche commence en juil1et 1983. Les caracteristiques des deplacements telluriques à l'interieur des roches, le comportement de deformation de la caverne et le rapport entre les contraintes subies par la caverne et la vitesse de particule des roches avoisinantes ont ete mesures.
Zur Untersuchung des seismischen Verhaltens der Kaverne und des umgebenden Felsens wurden seit Juli 1983 Erdbeben-Beobachtungen im Felstunnel durchgefuehrt. Die Charakteristika der Erdbebenbewegung im Felsen, das Verformungsverhalten der Kaverne und die Beziehung zwischen der Beanspruchung der Kaverne und der Partikelgeschwindigkeit des umgebenden Felsens wurden geklart.
In recent years, new type of structures such as underground nuclear power stations, storage caverns for high level radioactive nuclear waste disposal, and oil storage caverns have been planned.
In Japan, for these structures which are to be constructed in seismic region, reliable seismic design should be achieved based on the observed data. However, a quantitative method of evaluating seismic stability is yet to be established, and from this viewpoint, the seismic designs providing high reliability will be required in the future utilization of rock cavern.
Therefore, in order to clarify the seismic behavior of a cavern and surrounding rock based on the observed data, we, in cooperation with JNR, carried out earthquake observations in Shin-Usami Tunnel of JNR's Itoh Line started July, 1983.
Shin-Usami Tunnel is a single tracked railway tunnel having a 3000 m of overall length. Its internal cross section has a circular form with inner diameter of 6 m, and the lining concrete is 30 cm in thickness. The observation section is a 100 m section located at approx. 1500 m from each entrance, and the depth from top of the mountain is approx. 260 m. The observation section is composed mainly of Alternated Basalt, and the velocity of the S wave Vs is 1.1 to 1.6 km/sec.
The earthquake observaiton is carried out using 8 accelerometers including one at the Entrance, 10 strain gauges set on the lining concrete, and 6 strain gauges set in the rock. Fig.1 and Fig.2 show the layout of measuring instruments for the earthquake observation. The accelerometers used are the servo type with measurable range of frequency 0.1 to 30 Hz and the minimum resolution 0.01 gal. The strain gauges are the differential trans type having the measurable range of frequency 0.1 to 30 Hz and the minimum resolut10n 0.03x10−6. The measuring unit is set so as to be triggered when anyone of the 3 components of the A-6 accelerometer installed 40 m below the tunnel's bottom receives 0.3 gal.
Fig.3 shows the position of epicenters of the earthquakes observed. As the examples of the observation records, the acceleration waveforms in rock are shown in Fig.4.
(Figure in full paper)
To clarify the properties of earthquake motion in rock, the principal axes of observed earthquakes were studied. In calculating the principal axes of earthquake motion: 1 principal axis I ••• the principal axes varying in time domain (t), where the cross power spectrum at the time (t) becomes the maximum, medium and minimum, and 2 principal axes II ••• the fixed principal axes, along which the total energy becomes the maximum, medium and minimum. are used out of the methods using the cross power spectrum proposed by Hoshiya [1].
Fig.5 shows the direction in horizontal plane (X-Y plane) and the direction in vertical plane (Z-X, Y plane) of the fixed principal axes of earthquake motion in rock A-4.
Conventionally, there have been reports on the direction of the fixed principal axes in horizontal plane of earthquake motion, one insisting that the maximum principal axis and the direction of epicenter show a favorable correspondence[2], and one. insisting that the intermediate principal axis and the direction of epicenter show a favorable correspondence[1],[3].
