In advance of construction of structures such as dams, underground power stations, and nuclear power stations, properties of rock foundations must be known to examine stability of these structures. To know static properties of rock foundations, jack tests as well as rock shear tests and others have been performed in adits of sites. Dynamic properties for stability of a structure in an earthquake have usually been examined through seismic prospecting. The seismic prospecting, however, is performed at a very low stress level of about 0.01 to 0.1 kgf/cm2 and with a high frequency of approx. 500 Hz. A static contact stress on a ground beneath a reactor building of a nuclear power station is approx. 5 kgf/cm2, dynamic stress amplitude in an earthquake approx. ±3 kgf/cm2, and a predominant period 0.2 to 1 sec. In this case, the seismic prospecting is performed at a much lower stress level and with a much higher frequency than the above mentioned condition of stress in rock foundation in an earthquake. Thus the values of properties are considered to differ according to the stress level and frequency. To evaluate dynamic properties of rock foundations under conditions conforming to the stress and response frequency in earthquakes, an apparatus for cyclic loading tests has been developed. Measurement with this apparatus has been performed at 10 and several sites to obtain property values that have been used in the stability analysis for aseismic design. This report outlines the apparatus and deals with the obtained results.
As shown in Fig. 1 (a), the loading apparatus Consists of the loading plate with a diameter of 60 to 80 cm, the hydraulic vibrator (maximum capacity: 50 tons, maximum piston stroke: ±25 mm, frequency range: DC ~ 5 Hz), and the posts. As shown in Fig. 1 (b), loads are controlled with the hydraulic pressure unit (maximum pressure: 21D kgf/cm2, AC200V) and the load/frequency control unit. The load form is sinusoidal, and the frequency is 0 to 1 kHz. As shown in Fig. 1 (c),measurement is performed with displacement transducers (measurement accuracy: 1/100 mm) or gap sensors (measurement accuracy: 5/1000 mm) while recording is made with an oscillograph or a data recorder.
(Figure in full paper)
As to the intensity of loading, contact stress on rock foundation beneath a reactor building are taken into account as well as influences of load variations in an earth- Quake. The magnitude of a static load σs provided by the dead weight of the building onto the ground is approx. 5 to 6 kgf/cm2, and the dynamic load amplitude σd in an earthquake is mostly approx. 20 to 60% of the static load. Thus the loading pattern of the cyclic loading test is as shown in Fig. 2. Since the response predominant frequency is approx. 1 to 5 Hz in an actual 4 earthquake, kinds of the frequency are 8 cases of 0.01, 0.1, 0.5, 1, 2, 3, 4, and 5 Hz as the standards.
