Seismic loads on the foundations of hydraulic structures during strong earthquakes

Abstract

It is known that the degree of reliability of structures in the area of hydraulic engineering under conditions of construction in special regions with an increased hazard, for example, in earthquake hazard regions, is being investigated in the most different areas: in the area of mathematical and physical modeling, with a prObabilistic approach, method of calculating structures on the basis of accelerograms of known strong earthquakes (El Centro, Hyde Park, Olympia, Koyna, etc.), by the method of calculation based on specially constructed analog accelerograms, etc.It is completely obvious that of greatest interest is an analysis of real seismic records and especially of strong earthquakes recorded in the immediate vicinity of a hydraulic structure. Such records, in addition to primary practical interest, have also great methodological value. A comparison of the results of all indirect methods of establishing the character of the seismic load on a hydraulic structure with the data ofreal strong earthquakes presenting a direct danger will make it possible to develop a method of converting from the data of all other methods of evaluating the earthquake hazard to its real evaluation.Unfortunately, in world practice there is an insignificant number of records of earthquakes obtained on rock foundations in regions of large dams. This attests to the value of the data being used.In addition to the aforesaid, the examined data is of interest in the area of investigating the effects of a reservoir on a change in the seismic regime of the region of the hydro development. It has presently been established on the basis of the data of various authors that a reservoir can promote the occurrence of earthquakes in regions previously considered to be aseismic [i, 2], intensification of seismic activity in seismically active region [I, 2], and weakening of seismic activity that existed earlier [3,4]. The ambiguous results existing in this area indicate the complex role of a reservoir in the seismic regime and make it necessary to conduct appropriate investigations in each particular region.The earthquakes examined in this article occurred on January 10 and 13, 1975 six months after the start of filling of the reservoir of the Chirkey hydrostation.The region up to May 14, 1970, when an earthquake with an intensity of 8 according to the scale of the Institute of Physics of the Earth, Academy of Sciences of the USSR, occurred here, was considered an intensity 7 region. After the indicated earthquake and studies by a special commission that followed, a seismic activity of intensity 8 was established for the region.The 1975 earthquakes were recorded in two mutually perpendicular directions -horizontal N-S and vertical z. The APT-I accelerometers and S-5-S velocimeters, which recorded respectively the accelerations and velocities of the seismic oscillations, were used as the detectors. The natural frequencies of the pendulums for the S-5-S are fl = 0.2 Hz and for the APT-I f2 = 1500 Hz. The frequency band in which a uniform increase of ...