OCR Text |
Show 2 no choice but to seek another and safer location. There sometimes is economic justification for accepting a limited amount of risk for buildings or other installation, especially mining or other enterprises where a definite and limited building life can be projected. Such risks can logically be taken only if their size can be reasonably estimated. This report presents methods for making such estimates, or encounter probability. Large, infrequent avalanches, like large, infrequent river floods, are the product of unforseeable weather and climate. For purposes of statistical analysis they are considered to occur at random even though there may be some evidence for their association with short- term ( in the geologic sense) climate cycles. The average time between a number of such random events is called the return interval. For the kind of avalanches under discussion, typical return intervals might be 25, 50 or 100 years. Most installations are designed for a useful estimated life which depends on such factors as economics, construction materials and rate of obsolescence. When such an installation is exposed for its estimated life to the threat of damage from an infrequent avalanche of a given return interval, there is a definite encounter probability which describes the chance that the avalanche will damage the installation during its estimated life. Table 1 and 2 enumerate these encounter probabilities for the given values of return interval and estimated life. They are derived from a paper by Borgman ( 1) which treats the subject of geophysical risks in; considerable depth. The design engineer seeking a more sophisticated treatment is referred to this paper. In using these tables, it is important to consider the restrictions imposed on their construction. First, and in general, statistics treat the relations between numbers or groups of numbers. These relations may . or:! may: nhot: describe |