Engineering:Observational method (geotechnics)

In geotechnical engineering, during the construction of earth structures (dams and tunnels, for example) the observational method is a continuous, managed and integrated process of design, construction control, monitoring and review enabling appropriate, previously-defined modifications to be incorporated during (or after) construction. All these aspects must be demonstrably robust. The objective is to achieve greater overall economy, without compromising safety.

Overview

The observational method was proposed by Karl Terzaghi and discussed in a paper by Ralph B. Peck (1969) in an effort to reduce the costs during construction incurred by designing earth structures based on the most-unfavorable assumptions (in other words, geological conditions, soil engineering properties and so on). Instead, the design is based on the most-probable conditions rather than the most-unfavorable. Gaps in the available information are filled by observations: geotechnical-instrumentation measurements (for example, inclinometers and piezometers) and geotechnical site investigation (for example, borehole drilling and a CPT). These observations aid in assessing the behavior of the structure during construction, which can then be modified in accordance with the findings. The method may be described as "learn-as-you-go".

The observational method may be described as follows:

• Exploration sufficient to establish the general nature, pattern and properties of the deposits (not necessarily in detail)
• Assessment of the most probable conditions, and the most unfavorable conceivable deviations from these conditions. Geology plays a major role.
• Creating the design, based on a working hypothesis of behavior anticipated under the most-probable conditions
• Selection of quantities to be observed as construction proceeds, and calculation of their anticipated values based on the working hypothesis
• Calculation of values of the same quantities under the most unfavorable conditions compatible with the available data concerning subsurface conditions
• Selection (in advance) of a course of action or design modification for every foreseeable significant deviation of the observational findings from those predicted based on the working hypothesis
• Measurement of quantities to be observed and evaluation of actual conditions
• Design modification in accordance with actual conditions

The observational method is suitable for construction which has already begun when an unexpected development occurs, or when a failure or accident threatens or has already occurred. The method is not suitable for projects whose design cannot be altered during construction.

The most serious blunder in applying the observational method is failing to select (in advance) an appropriate course of action for all foreseeable deviations (disclosed by observation) from those assumed in the design. The engineer must devise solutions to all problems which could arise under the least-favorable conditions. If he or she cannot solve these hypothetical problems (even if the probability of their occurrence is very low), he or she must revert to a design based on the least-favorable conditions.

References

• Nicholson, D, Tse, C and Penny, C. (1999). The Observational Method in ground engineering – principles and applications. Report 185, CIRIA, London.
• Peck, R.B (1969). Advantages and limitations of the observational method in applied soil mechanics, Geotechnique, 19, No. 2, pp. 171–187.
• Terzaghi, K., Peck, R.B, Mesri, G., (1996). Soil mechanics in engineering practice, Third Edition, John Wiley & Sons, p. 34.