Terzaghi and Peck

It is probably the writings and work of Terzaghi in the early and middle parts of the last century that have illustrated the importance of this 'observational method' more than any other. The term appears to have been coined by Terzaghi and Peck in 1948 (p. 494): it became 'observational procedure', in Terzaghi and Peck, 1967 (p.294). In his Rankine lecture, Peck (1969) made the following comments on the observational method. He said, "If the governing phenomena are complex, or are not yet appreciated, the engineer may measure the wrong quantities altogether and may come to dangerously incorrect conclusions." This is where the observational method is particularly useful. Peck briefly gives the application of the method as follows:

  1. Exploration sufficient to establish at least the general nature, pattern and properties of the deposits, but not necessarily in detail.
  2. Assessment of the most probable conditions and the most unfavourable conceivable deviations from these conditions. In this assessment geology often plays a major rôle.
  3. Establishment of the design based on a working hypothesis of behaviour anticipated under the most probable conditions.
  4. Selection of quantities to be observed as construction proceeds and calculation of their anticipated values on the basis of the working hypothesis.
  5. Calculation of values of the same quantities under the most unfavourable conditions compatible with the available data concerning the subsurface conditions.
  6. Selection in advance of a course of action or modification of design for every foreseeable significant deviation of the observational findings from those predicted on the basis of the working hypothesis.
  7. Measurement of quantities to be observed and evaluation of actual conditions.
  8. Modification of design to suit actual conditions."

Peck makes the point that, for the method to be applicable, the character of the project must be such that it can be altered during construction. He also distinguishes between situations where the use of the OM is envisaged from the inception of the project, and those where the method is introduced part way through a project as offering almost the only hope of success. In the latter situation, there may be contractual problems to overcome.