Observational Method Services

Observational Method is used to reduce the cost of uncertainties in Geotechnical Engineering projects without compromising safety. Brown (2007) stated that there are two general types of uncertainty:

      • parameter uncertainty: what we know we don’t know; and

      • conceptual uncertainty: what we don’t know that we don’t know.

McMahon (1985) outlined six types of uncertainty encountered in geotechnical engineering:

      • Risk of encountering an unknown geological condition (unknown structures or weak zones, unexpected presence of water, etc)

      • Risk of using the wrong geotechnical criteria (incorrect failure mechanism identified, inappropriate numerical modeling applied)

      • Risk of bias and / or variation in design parameter being greater than estimated (material properties variability underestimated, poor understanding of joint spacing and length)

      • Human error (poor quality data collected, poor sampling practices)

      • Design changes (poor planning requiring redesign, design changes made in the field without consultation)

      • Over Conservatism (assuming full excavation width wedges, applying discounting factors at all stages of the design)

Dunn (2013) outlines how uncertainties result in increased cost or missed opportunities:

      • Design

        • Poor understanding of geotechnical environment lack of data

        • Over-conservatism in design

        • Design methodology (e.g. empirical)

        • Rogue feature or events

      • Implementation

        • Design specifications and requirements not effectively communicated

        • Design specifications and requirements not understood

        • Poor QA /QC

The cost is associated with over-designing, under-designing, re-designing, delays, and failures (harm to people and property, lost or delayed production, damage or loss of equipment, remediation costs, insurance consequences, and public opinion and regulatory consequences).

Specially, during the construction of important earth structures such as dams and tunnels or when the ground conditions are so variable that an economic design cannot be achieved without an exhaustive and expensive field, laboratory, and modeling investigation (which itself makes the project uneconomic) the Observational Method is used. The Observational Method shall be used as an integrated process of the design, construction control, monitoring, and review, that enables previously-defined adjustments to be incorporated during or after construction, if needed. The method may be described as a learn-and-adjust-as-you-go procedure.

The Observational Method is also suitable for already in progress constructions when an unexpected behaviour is observed, or when a failure or accident threatens or has already occurred. The method is not suitable for projects where design cannot be altered during or after construction.

Peck (1969) discussed the Observational Method to reduce the costs during construction incurred by designing earth structures based on the most unfavorable geological conditions, soil engineering properties, construction conditions, and so on. In Observational Method, the design is based on the most probable conditions and not the most unfavorable. The uncertainties due to lack of information are balanced by observation of geotechnical instrumentation and site investigation data. These observations are used to evaluate the observed behaviour of the structure during construction and adjust the construction as needed.

The Observational Method can be described as follows:

      • Sufficient site investigation and laboratory testing 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 the most favorable conditions based on the geology and other project factors

      • Conducting 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 observed during construction

      • Preparing scenario based action plans in advance, including courses of action or design modifications for each and 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 most serious pitfall in using the Observational Method is failing to prepare (in advance) an appropriate course of action for every foreseeable deviation from those assumed in the design.


Alpha Adroit Engineering Ltd. provides Observational Method and advanced materials testing services for GeotechnicalEngineering,Environmental Engineering, Permafrost Engineering, and Forensic Geotechnical Engineering projects.


 Call us Toll Free: 1-844-4-ADROIT (1-844-4-237648) or please contact us here for more information.



Brown, E.T. 2007. Block Caving Geomechanics, Second edition, JKMRC, Brisbane, 696 p.

Dunn, M. 2013. The cost of uncertainty in geotechnical design and implementation. WAGCG / EAGCG, Adelaide, 7-8 Nov 2013. Accessed online at "http://www.srk.com/files/pdfs/cost_of_uncertainty_in_geotech_design_dunn_07112013_final.pdf" on May 21, 2015.

McMahon, B.K. 1985. Geotechnical design in the face of uncertainty. Austalian Geomechanics Society E.H. Davis Memorial Lecture.

Peck, R.B. 1969. Advantages and limitations of the Observational Method in applied soil mechanics. Geotechnique, 19 (2): 171-187.