Safety of Dams Flood and Earthquake Criteria (1985) / Chapter Skim
Currently Skimming:
Design Earthquake Estimates: Methods and Critique
Design Earthquake Estimates: Methods and Critique
Pages 61-73
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 61... ...
The sudden release of stress that is triggered by the failure generates stress waves that propagate in all directions and produce earthquake shaking at the surface of the ground. The stress failures that produce destructive shaking are initiated at depths of a few miles or a few tens of miles, and at these depths the weight of the superposed rock produces large compressive stresses and, as a result, only shearing stress failures can occur.
|
From page 62... ...
An additional difficulty in estimating the nature of ground shaking is that as the seismic waves travel away from the fault, they traverse heterogeneous earth and are affected by reflections and refractions at the heterogeneities. Therefore, the shaking at a point on the surface of the ground depends not only on the details of the source mechanism but also on the details of the travel path, neither of which are well known.
|
From page 63... ...
Such methods are also used, in some cases, to provi~le background information on seismic hazards of major dams. 63 DETERMINISTIC-STATISTICAL METHOD The cleterministic-statistical method requires certain basic information: earthquake magnitude, smallest distance from the fault or the earthquake source zone to the dam site, equations or curves relating magnitude and distance to peak ground acceleration, peak ground velocity and duration of strong ground shaking, and sometimes a site correction for the soil layer above the bedrock at the dam site.
|
From page 64... ...
for estimating the MCE. However, there is appreciable scatter in the data that are used to determine the fault rupture length versus magnitude relation, because of variations in some of the other physical characteristics of the earthquake source.
|
From page 65... ...
There are many proposed "attenuation relations," relations in the form of equations or curves that give an estimate of the strong ground motion if the magnitude, or epicentral intensity, and distance to the site are known. Examples for the western United States are shown in Figure 6-3.
|
From page 66... ...
ground motion for different soil conditions may be used, such as shown in Figure 6-5. When all the uncertainties that appear in this method of estimation of peak ground motion are combined, the mean plus one standard deviation value may be almost twice the mean value.
|
From page 67... ...
if; `2 ~ ·~ ed fan + 1 CJ `. ~ \ Median .\ \\ 2~.i\2 2\\ \ \\\ \N 3 10 30 100 CLOSEST HORIZONTAL DISTANCE FROM ZONE OF ENERGY RELEASE, km 300 FIGURE 6-4 Regression analysis of the peak accelerations recorded during the October 15, 1979, Imperial Valley earthquake.
|
From page 68... ...
Beyond this stage, the analysis proceeds as for the deterministic-statistical method, except that the distance from the earthquake source to the dam site is considered to be the smallest distance from any point in the seismotectonic province to the site, if the site lies outside the province. When the dam site lies within the seismotectonic province, the epicentral distance would become zero if the above criterion were applied, and the only attenuation of the strong ground motion would be due to the vertical travel path from the earthquake focus to the epicenter.
|
From page 69... ...
Similar to the deterministic-statistical and seismotectonic methods, the risk analysis method requires a knowledge of the location and extent of active faults or earthquake source zones, of the MCE associated with each, of the attenuation relations for peak ground acceleration and ground velocity, and of the site correction for soils and unconsolidated rock. In addition, the recurrence times for earthquakes of various magnitudes are required; oftentimes this relation is assumed to satisfy a simple mathematical form.
|
From page 70... ...
, and probabilistic methods principally was concerned with the estimation of the maximum credible ground acceleration and velocity and their uncertainties. Because permanent displacement or failure of an embankment dam due to earthquake shaking may be the result of incremental slope failures or the consequence of liquefaction of the soil material comprising or supporting the dam, and because those effects are influenced by the number of cycles of strong ground shaking, the time duration of the maximum credible ground motion also must be estimated.
|
From page 71... ...
Wherever possible, the selection of a time history is done on the basis of similarity of earthquake magnitude, distance to the site, and rock or soil conditions at the site. One or more time histories of ground acceleration representative of the maximum credible ground motion can be provided for analysis.
|
From page 72... ...
72 0.40 0.20 an on J o -O.40 3.5 3.o to 2 5 cow Lo J 2.0 1.5 J a: ° 1.0 0.5 o 0 2 4 6 8 10 12 14 16 TIME, s A,,.`,, ,1 ~ .,, ~ l \ \ . ~ ~ \ MEAN ROCK - MSD MOD I F I ED CASTA I C N69W -1 1 0 0.5 1.0 1.5 2.0 2.5 3.0 PER I OD, s FIGURE 6-6 Design accelerogram and spectrum for Camanche Dam.
|
From page 73... ...
The first five dams listed in Table 6-1 were all located in regions of relatively low historical seismicity. The possibility of reservoir-induced earthquakes should therefore be given consideration when setting design criteria for new high dams, particularly in regions of low historical seismicity where the seismotectonic province method indicates a low intensity of shaking.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.