Dam Design Field Work & Modeling
Last week we had a meeting with the State dam specialist to get him up to speed on one of my awesome projects! Sadly, I missed the meeting due to 800 other meetings. BUT! I got to preview the presentation and go to the pre-briefing.
Of the maaaaaaaaaaaaany duties I have this month, I spend the least amount of time on this project (~2 hours per week as the client-interfacing assistant project manager). However, it's certainly one of my favorite projects and is the most engineering-intensive ones. Even if it's mostly the senior engineers and the design centers that get to do the fun stuff.
We are rehabilitating this dam. It was designed for a 50 year lifespan and at about 60 years...it needs some work! You might recall that I've posted some pictures of this dam and another dam before.
On our initial visit, we visited both dams owned by the city. They serve basically the same purpose: capture water to use during times of drought. Water from this dam is used for drinking water for the city and can also be sold as irrigation water. They can also be used for recreation--water sports, fishing, boating! And the lakes make beautiful sights for lakefront properties ;)
Good morning from the site! This is our client and the technical engineers we flew in from our other offices to investigate the spillway. We've got the dam locked out/tagged out so that it is safe to walk on the spillway. No one can accidentally open a gate.
After our initial investigation we found that the rock anchors just weren't working anymore. We also had suspicions about the underdrains--holes in the spillway which relieve pressures beneath the slab. So we got our contract amended and went back to do a few more investigations.

Engineer's doing what they do best: standing around. JK. But we do a lot of watching other people work in the field so we can make sure it's done to design. We'll need to sign off on it for the state.
We used a little metal detector to figure out where the rock anchor would be and marked it off with spray paint for the contractor. All in an engineer's days work.
I spent a lot of that day spray painting each slab with a highly technical (not) numbering system. And then marking on my paper the assessment of expansion joints, contraction joints, cracks, spalling, etc. I walked around with our badass senior structural engineer from Seattle and he told me all about dam design and how those joints were supposed to age. We then walked around with the geotechnical engineer from Oregon and he talked about erosion underneath the spillway and how that was supposed to age. Over the past 60 years, the way dams are designed have changed quite a lot just by engineers seeing something not work and realizing it needs to be changed. We've also been able to optimize the design better. In fact, I'm working on another dam project which has been altered after the lessons learned from the Oroville Dam failure.
From the model, the green represents the degraded clay layer we found underneath the slab. Below that is siltstone (original rock) interbedded with sandstone. It doesn't make sense that someone would knowingly put a huge structure like this right into malleable soil. So our thinking is that it was all siltstone originally and when the overburden pressure was taken off to excavate for the dam placement, the siltstone degraded to clay. We don't know for sure because we don't have a whole lot in the way of original design documents.
In the model, we inserted the new rock anchors in a couple of different configurations to obtain a good Factor of Safety (FS). The system will have to withstand high swelling pressures of the clay. Because our geotechnical work found that it was mostly clay underneath, we now know why the underdrains aren't as effective as they should be. We will design deeper drains into more permeable zones underneath the slab.
Here's a rock anchor detail:
Basically with the model runs, we can optimize the anchor length and anchor spacing by using the model to look at the moments in the slab. And you thought you weren't going to use your statics class notes ever again!
We proposed 4 methods of rehab to the client and decided on one that was mid-ranged in price and mid-ranged in how long the solution would last. Then we completed some more modeling and realized that in order to optimize the design better, we'd need some more data. We ran the numbers on how much we'd save in construction cost to do a bit more engineering, and the client approved it! One of the models we extended was for the hydraulics analysis.
The hydraulic jump is when high velocity water in an open channel flows into a slower moving basin. The water in the basin increases in height converting kinetic energy into potential energy. A typical model run is to determine the height of water at 75% PMF (potential maximum flood--calculated off historical events).
We combed through the historical data from logbooks, had some of our global administration folks input the data digitally, performed some data analytics on it, and input it into the model. The blue line you are seeing is the height of water expected and its overlay onto the photograph. You can see the training wall isn't quite high enough to contain the hydraulic jump so a design decision is to increase the height of that wall. The stilling basin (basin at the the end of the spillway used to dissipate energy before it goes downstream) also isn't quite long enough.
Lots has changed in how we design things today. 60 years ago they didn't have a model they could plop the numbers into and figure out the exact length of the basin for the site's conditions.
Based on some not-quite-so-old drawings we have, there was a recent design to heighten the training walls but for some reason never got built. It took a little bit of back and forth (my job) to get survey data that a separate consultant performed on the wall. We used it to verify the exact height of the training wall--we wouldn't want to develop a design document saying to extend the vertical wall up if it's already been done!

Reading: Half a Yellow Sun by Chimamanda Adiche. I just started it and it's wrapped me up in its world already. I had started What the Dog Saw and Other Adventures by Malcolm Gladwell (who I love his other books) but it just didn't draw me in at the beginning as fast. So I put it on the to-read shelf for later.
Listening: Oathbringer!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Working: Wrapping up the monthly report for December to invoice a client, putting together a proposal for another project, coordinating submittal reviews for engineers on a reservoir, put in a review for a coworker whose doing a great job, responding to a well abandonment submittal for one of my designs, AND coordinating a monthly community of practice (CoP) call for the groundwater group. Wow, it's been a productive Sunday. And I have one more task to do before it's time to settle in with Zanzamittens and watch some Dr. Who. :P