Risk-Based Probabilistic Approach to Site Selection

All sites are exposed to geologic processes which operate at intensities potentially damaging to certain classes and qualities of construction. Many sites are exposed to a significant number of processes which are independent of each other (e.g., earthquake shaking and expansive foundation materials), while others are exposed to processes which may or may not be dependent on each other (e.g., earthquake shaking and rockfall). Some site uses are regulated to require avoidance of sites with particular hazards. Many other site uses are regulated to a limited extent or even virtually unregulated. In these cases, site developers have the opportunity to do the minimum required by local ordinance, while individuals interested in their financial investment have the opportunity to compare different sites in like terms and make informed development decisions. Geologic processes operating at a site can be described in terms of intensity or magnitude and frequency of occurrence. Beyond some threshold intensity, normal geologic processes have the potential to cause damage to property and/or injury to people. By examining the geologic setting of a site and evidence of geologic processes which have been active in the recent past, engineering geologists can, in many cases, estimate the frequencies of different process intensities. Civil or structural engineers must assist in the correlation of process intensity to structural damage, and ultimately, to dollar value of potential loss which is the way risk should be described. Potential injuries, however, are considerably more difficult to deal with than dollar-value losses. Once magnitude-frequency relationships have been defined for the various geologic processes, probabilities of their occurrence at damaging intensities or higher (exceedance probabilities) can be calculated for time periods of interest, such as the design life or a longer financial period. Occasionally, probabilities must be assigned on the basis of subjective “expert” opinions. A probabilistic approach permits risk resulting from geologic hazards to be expressed in terms of dollar values and exceedance probabilities. Once all potentially hazardous processes are described with magnitude-frequency relationships, the aggregated risk at a site can be expressed in terms which permit systematic uniform assessment of the site conditions. This description of risk can be used to compare different sites and aid in making intelligent investment decisions. The probabilistic approach to site description is relatively new and very few investors recognize its potential value. Some ordinances, by virtue of legislating “acceptable” risk, imply that processes not regulated do not occur at damaging intensities. The uninformed investor may truly believe that “if it is built to code, it must be safe” even if it is in the flood plain or on the fault scarp.