On the basis of approx. 20 experiments of tectonic stress measurements in large underground excavations and active faults the suitability of employing the acoustic emission method and a miniaturization of the overcoring method as well as the effects of tectonics, topography and faults are examined. Moreover, the connection between the geodetically ascertained shear strain rate in Japan and the range of historically observed earthquakes over the past 1 200 years in the surroundings of underground power stations is discussed.
Cette communication examine l'efficacite de la methode d'emission acoustique, la miniaturisation de la methode de surcarottage et les influences tectoniques et topographiques en se basant sur les donnees acquises à partir d'une vingtaine de cas de mesures de la geocontrainte en rapport avec des fouilles en sous-sol, près de failles actives. De plus, on donne le taux de progression de la contrainte de cisaillement geodesique ainsi que l'influence des circonstances regionales de failles actives au cours des 1200 annees passees relativement aux tremblements de terre historiques. Enfin, on examine les mesures et les observations dynamiques effectuees aux alentours d'une centrale electrique souterraine lors d'un tremblement de terre.
Auf der Grundlage von etwa 20 tektonischen Spannungsmessungen in Bezug auf große unterirdische Hohlraume und aktive Verwerfungen wurden Betrachtungen in Bezug auf die Verwendbarkeit der akustischen Emissionsmethode. Verkleinerung der "Overcoring"-Methode, tektonische Einfluesse, Einfluß der Topographie und der Erdschichten usw. angestellt. Weiterhin wurde der Zusammenhang zwischen der geodatisch festgestellten Dehnungsrate in Japan, dem Bereichszustand der geschichtlichen Erdbeben ueber 1 200 Jahre, aktiven Verwerfungsgruppen usw. diskutiert. Abschließend werden Messungen bei Erdbeben in der Umgebung unterirdischer Kraftwerke und dynamische Betrachtungen angestellt.
Japan Islands are characterized by tectonic strain obtained by Geodetic survey 2),6) in Fig. 17) Measured overcoring stresses in Table 1 are principally affected by overburden, but they might be affected more or less by other rock mechanics factors:
Effect of Topography is characterized by the direction of maximum compression σ1 toward the summit.
Effect-of Geological Structure is featured by the direction of σ1 toward dip or strike of fault, fractured zone or strata.
Effect of Tectonics might often be recognized as rather large ratios σH max/σv.
The maximum ratio were around 2.4 ~ 2.6.
Multiplicated Effect is of course met as illustrated in Fig. 2, obtained at No.12 in Chugoku district. There is recognized σ1 toward the summit and σ3 is normal to the steep slope. There is recognized a coincidence between the direction of σ1and strike of fault in Fig. 3, too. In addition to these features, the ratio σH max/σv is rather large and overcoring feature is almost consistent to the independent survey by geodetic method.
Tectonic Effect is illustrated representatively by Fig. 4.
3)This shows the coincidence between measured σ1 direction and joint strike. Measured direction of σH max N 87° E is almost consistent to the other measurement N88° E and N89° W in Mining tunnel 4) (EL. 1220–1550) located at 65 km far from the site. This measured direction is also confirmed with the another seismic report 5) which has analysed the compressive direction deduced from the earthquake wave analysis of the shallow earthquakes with depth 20 km, 1975–1976 in Fig. 4. 6) Stress ratios σH max/σv were plotted in Fig. 5.for three groupes based on Table 1 7) It should be noticed that result of overcoring boring are affected by the depth of boring, change of rididity of rock heterogeneity of rock or change of P wave velocity, as shown in Fig. 6.
It might be interested in possible zone of maximum shear strain which might be mobilized at most. Rough scope of this resolution is illustrated in Fig. 1.8) Based on this illustration, geodetic strike shear along the Median Tectonic Line is shown in. Fig. 7. This procedure will be performed to another smaller ambiguious active faults.
Quantitative observation by Means of accelerometers and dynamic strain meters has been continued at Shiroyama Underground Power Station 9) since 1976 as shown in Fig. 8. 1) It is easily recognized that the underground siting is seismically feasible than the on ground siting as shown in Fig. 9. And also it is confirmed that the top of mountain is not so earthquake compared with foot of mountain. Accelarations around the cavern were approximately about 1/2 in horizontal directions and about 2/3 in vertical direction compared with top of mountain or ground surface as summarized in Fig. 10. Dynamic analysis will be discussed in the near future. Fig. 11 shows a representative records o correlation among the recorded acceleration, its integrated velocity and displacement and also recorded dynamic strain on the rock wall of the cavern.
