I just started work on a piledriving project where the GT of Record provided final tip elevations, based on 16 indicator piles, for the Contractor’s use in casting the design lengths. With the diagram he included a disclaimer that the site is variable, and that many of the piles might be different lengths in order to satisfy the acceptance criteria. The GT, accompanying the final design information, recommends that the Contractor hire another geotechnical engineer to monitor each production pile and make appropriate length adjustments, and also a structural engineer to design splices. Without intending, I’m sure, the Owner’s engineer has converted the piledriving part of our project from conventional design-bid-build to a contract that relies on the Observational Method.
The Observational Method is extraordinarily effective on challenging sites, but also can be costly to implement. We use it explicitly on dams and tunnels. Ralph Peck’s 1969 Rankine lecture is, in my opinion, the best summary of the method’s origins, the rigor with which it is to be applied, and it’s significant advantages on projects with terribly difficult geotechnical conditions. Like all Rankine lectures, the paper is worth a read. You can get your own copy here:
I link to the full catalogue; scroll down to 1969. Consider browsing a bit while you’re on the site. And yes, the photo that accompanies this post is my preferred headshot of Karl Terzaghi, not the author. Read Prof. Peck’s paper to understand why.
In my practice I find it helpful to recognize when the design team’s RFI responses induct elements of the Observational method into our project. When they do, it’s time to evaluate whether or not the character of project has changed fundamentally from the project described in the bid documents. A conventionally procured project offers certainty in exchange for hard bid pricing. Other tendering formats are appropriate for projects where final design will be developed as conditions emerge, as is the case with the Observational Method.
It’s necessary to understand the Method in order to recognize unintentional implementations. Here’s a clip from Prof. Peck’s lecture, for those of you with just passing curiosity.
REVIEW OF METHOD
In brief, the complete application of the method embodies the following ingredients.
- Exploration sufficient to establish at least the general nature, pattern and properties of the deposits, but not necessarily in detail.
- Assessment of the most probable conditions and the most unfavourable conceivable deviations from these conditions. In this assessment geology often plays a major role.
- Establishment of the design based on a working hypothesis of behaviour anticipated under the most probable conditions.
- Selection of quantities to be observed as construction proceeds and calculation of their anticipated values on the basis of the working hypothesis.
- Calculation of values of the same quantities under the most unfavourable conditions compatible with the available data concerning the subsurface conditions.
- 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.
- Measurement of quantities to be observed and evaluation of actual conditions.
- Modification of design to suit actual conditions.
The degree to which all these steps can be followed depends on the nature and complexity of the work.
I am particularly fond of that last statement. “It depends.” In the 50 years since those words were penned by one of the giants in our field, consulting geotechnical engineers have yet to address the fact that, really, the right course of action depends on the nature an complexity of the construction. There is always a way forward, of course, eVen in the most difficult conditions. The level of effort, though, depends on the nature and complexity of the work.