ABSTRACT
The concepts of risk and uncertainty are familiar to geotechnical engineers. Since predictions are not perfect there is always the possibility of failure or unsatisfactory performance. Structures are designed so that the risk of failure is acceptably small. The two principle components of geotechnical design are the “calculated risk” (Casagrande 1965) and the observational method. The first is described by Casagrande as (1) “the use of imperfect knowledge to estimate the possible ranges for all pertinent quantities that enter into a solution…” and (2) “the decision on an appropriate margin of safety, or degree of risk taking into consideration …losses that would result from failure”. Where the consequence of failure is large and a conservative design is expensive, Terzaghi proposed the use of “the observational method”, which is described by Peck (1969) as, "Base the design on whatever information can be secured. Make a detailed inventory of all the possible differences between reality and the assumptions. Then, compute on the basis of original assumptions, various quantities that can be measured in the field. …On the basis of the results of such measurements, gradually close the gaps in knowledge and, if necessary, modify the design during construction." These concepts can be matched with well known relations in reliability and decision making as illustrated in Figure 1 and 2.
