Geomechanical Musings

We’re very pleased to be supporting Weeks Marine on a dock-building project in Corpus Christi.  I don’t have a photo pass, so I can’t share with you pictures of the work.  It’s a fantastic site, though. I took this photo looking away from the site at the start of our shift this past weekend and thought it was worth sharing.

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:

https://www.britishgeotech.org/prizes/rankine-lecture

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.

 

One benefit of working with friends is the non-work time we spend together, like sharing meals.  Longtime collaborator and good friend Steve Dickenson joined us in San Francisco for a project meeting last week.  While the meeting achieved the intended outcome, the real highlight of the day was the excellent bahn mi sandwich shop that we found on Eddy Street.  L&G Vietnamese Sandwich is a no-nonsense, locally owned, quick service shop. Lunch for the three of us cost less than $20, which surprised me considering we were in the middle of San Francisco.

The bread had the exact right texture, the jalapeno slice on top added the right amount of bite, and the rest of the ingredients were super fresh. There’s no seating in the shop, but there’s plenty of great public space in the neighborhood.  Here’s their website in case any of you guys need to pre-order before your next meeting in the Civic Center neighborhood:  http://orderlandgvietnamesesandwich.com/

The Atlas Geotechnical blog doesn’t really focus on restaurant reviews, but the unique experiences we collect in the course of our work-related travel are a thoroughly satisfying part of our careers. From Seattle seafood to a great hole-in-the-wall yakitori place on Oahu, from proper Kansas City bar-b-que to amazing camp meals in Athabasca, we share a lot of great meals with good friends as a direct result of our unique engineering practice.  Sharing great bahn mi with a good friend in San Francisco is another great memory from a 30-year engineering practice that just keeps getting more and more interesting.

Seattle imposes a local requirement for verification testing of shoring tiebacks in addition to proof and performance testing. Because the required test load is 2.0 times the design load, and our design safety factor is 1.5, the test is almost certain to fail the anchor on the soil/grout interface.  At least, if our design achieves the target conservatism, the verification test ought to fail the soil/bond interface; otherwise our strength estimate is low and our design is pointlessly conservative.

One charming aspect of such high test loads is that verification anchors need additional strands in order to safely transfer the test load down to the bond zone. (We prefer to test a typical bond length with extra strands rather than use a typical strand count and shorten the bond length.) that means that the test anchors look really robust. The anchor below is only a 180-kip anchor, but it has 9 strands because it’s going to be tested to 360 kips.

Another fun aspect of testing sacrificial anchors is that they need to be installed in between soldier piles so they don’t take up the pocket for a production anchor. That means that we get to use a cool reaction frame for the test. Setting the frame is an extra step, but I think it makes the test setup look super old-school.

Really, though, the point of this post is just to share the photo that Wes sent down from the jobsite. The test results, to be honest, were disappointing.  The setup is really clean and efficient, though. We’ve already installed a similar anchor and then post-grouted it looking for higher capacity. I expect that later today we’ll have a similar photo of a great looking verification test and also proof of the high strength we used in our design.

Is it just us, or does it seem like earthquake activity has been particularly high this summer? Even while I’m writing to talk about a fascinating photograph of slope failures in Hokkaido, but also this morning was another major event near Fiji.

For those of you just interested in the photo, here’s a quick link:

Mudslides in a wide range by magnitude 6.7 earthquake(Atsuma, Hokkaido, Japan) from CatastrophicFailure

Here’s the link to an article about the Hokkaido event and also, I believe, the photo’s source:

http://the-japan-news.com/news/article/0004713745

None of those failure are on the scale of the Oso landslide in Washington, but the cumulative volume is quite considerable. Thankfully the terrain is so steep that the area is sparsely populated.

So far this summer damage has been generally modest despite some of the earthquakes being quite large. Let’s just hope that it continues that way.