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  • Writer's pictureSi Shen

Demystifying the observational method

The ‘observational method’ in a nutshell is a ‘design-as-you-go’ design approach that integrates data from on-site observations during actual construction.

Normally at the design stage, which is ahead of construction, an engineer has to make allowance for a large quantity of uncertainties, including:

  • Execution tolerance – usually smaller than maximum allowed in design

  • Structural behaviour, such as concrete strength – usually higher than minimum assumed in the design

  • Geology/ground conditions: such as ground stiffness, permeability, drained/undrained behaviour, unexpected voids in the ground

  • Site conditions, such as surcharge, de-watering, effects from ground treatment

The engineer has no other choice but to make reasonable and conservative assumptions around these simply because these can not be known until the works have been done. To be conservative, by Eurocode, the engineer is asked to choose a characteristic value of these, where statistically speaking only 5% of the situations turn out to be even worse than the chosen value. In Ultimate Limit State design, the use of factors pushes this probability further down (more conservative). Here you can see naturally the vast majority of cases should be a lot better than an engineer’s assumption. This begs the thought that if these uncertainties can be validated and eliminated through monitoring and observations during construction, a great amount of savings could be achieved. This is a core principle of the Observational Method. On the other hand, in case the actual situation turns out to be even worse, the observational method also provides a higher degree of safety, since the worsening trend can be detected early and the design adjusted accordingly.

On a philosophical level, the fundamental difference between the conventional partial factor design method and the observational method lies in the way they deal with uncertainties. The conventional method deals with uncertainties by making sufficient allowance for variations; the observational method deals with uncertainties by adapting the design to the actual situation observed.

The observational can be initiated either before or during the project

  • Planned ahead: either one single design could be chosen based on the ‘most probable’ scenario (statistically 50% is better and 50% is worse than this scenario), or ‘moderately conservative’ at the designer’s discretion, and revised up or down as construction goes; or multiple design solutions be defined upfront, and the most suitable is chosen based on observation.

  • Initiated during construction: this can be when you run into a problem and seek the best way out, or can be when you realise the conditions you are dealing with are far better than what was assumed in the original design.

The use of Observational Method has a few pre-requisites:

  • The analysis and re-analysis is usually designer’s work whereas instrumentation and monitoring is contractor’s work. A closely joined up team between the designer and the contractor with sound contractual relationship and trust is needed.

  • Relevant, frequent and accurate instrumentation and monitoring is a must. This is easier said than done

  • Fast cycle time (feedback loop) from observation to re-analysis to on-site execution

Do NOT use Observational Method in the following situations:

  • The behaviour of the structure or the ground is brittle (sudden failure)

  • The response time is insufficient to intercept a worsening trend when observed

  • Any worse-than-expected scenario causes irreversible damage

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