Archive for the ‘Roads’ Category

I’ve spent a lot of posts explaining a bit about what engineers do, but let’s dive deep this time.  I’m going to tell you a little about how I spend my days on a project by project basis.  Today I’m going to start with a project that ties into the recent Metrocenter theme I’ve had going on.

After the Corps of Engineers finished raising and rebuilding the Metrocenter levee, they were confident it would stand up just fine to a 100 year storm.  However, someone quickly came to the conclusion that stopping at 100 year protection wasn’t enough given the potential for up to $4 billion.  If Katrina taught us anything, it’s that when a levee fails, it fails hard.  The levee itself is high enough to hold back a 500 year flood on the Cumberland (with the help of the reservoir system), but there are a few holes in the protection at some strategic points.

The primary hole in the flood coverage is actually a bridge on Interstate 65.  If you look at the old maps from my previous posts you can see a drainage stream of some kind and a rail line running under the bridge footprint.  Those may have been in place when the bridge was built in 1969, but these days there’s nothing running under the bridge except a buried gas line.  As best I can tell there may have been two railroad tracks servicing the Marquette Cement Yard, but they were removed when Metro bought the property in the mid 1990’s. Frankly, I’m not sure the tracks were there even then, because the ground under the bridge is at least 25 ft higher than the parking lot just to the north (where a rail line would go). 

The bridge is 20 ft above ground on the Metrocenter side

This is where I come into the picture.  The Corps and Metro asked the state department of transportation if they could build a small ridge under the bridge to keep water out.  It seems the river was backing up the low ground just south of the I-65 embankment and if the river got high enough it could pour through the bridge and into the low ground inside Metrocenter.  During the process of getting this project approved, the May 2010 flood happened and the Corps had to call out volunteers to lay sandbags under the bridge.  They tell me that water got up to the bottom layer of sandbags.

The low spot where water comes up from the Cumberland

My job as a hydraulic/bridge engineer was to look over their plans and make sure that the bridge wouldn’t be compromised and the project wouldn’t create a new flooding issue.  It was a fairly simple assessment.  The area under the bridge was already a high point, it just needed to be raised a little further, and the construction plan just consisted of bringing in soil and compacting it in the right spot.  With the rail line gone the bridge could be completely torn out and filled in if not for the traffic disruption it would cause on I-65.  So state approval was simple enough.

The major problem arose because the Interstate system is actually owned by the federal government.  States only manage them on behalf of the federal government, so this project required approval from the Federal Highway Administration.  They just happen to have a policy against using road embankments as a levee despite the fact that this one already is being used that way.  It took a lot of negotiation (the bureaucratic equivalent of slamming your hand in the car door) but the project was finally approved and construction is essentially complete.  Between this project and Metro’s efforts to replace the pump system Metrocenter is even more protected than it was during 2010.

The final product.

So there you have it.  One project in my life.  It started out quite interesting and ended up with fingers stuck in car doors, but it’s actually one of the simpler projects I’ve been involved with.  Mostly because someone else was doing all the design work.


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I’ve been to Memphis quite a few times over the last ten years or so, but on my last trip I discovered something new. We were cruising the I-240 loop around the south side of Memphis when I had to pull a double take (fortunately, I wasn’t driving). There’s a giant tub of ice cream on a pole out in front of the Klinke Brothers plant right next to I-240.

The view from the interstate

Of course I had to Google that when I got back to my computer. Deciding what search terms to use stretched my Google-Fu skills but I found some interesting info. Turns out 2012 happens to be the 25th anniversary of the giant tub of ice cream. It was originally built in 1987 by Klink Brothers as an advertisement for their Angel Food brand of ice cream owned and manufactured by Klink Brothers. I’ve never noticed Angel Food in Nashville, but apparently it was a big regional brand in the Memphis area at the time. In 2006 Klink Brothers discontinued the brand and licensed the trademark to an ice cream company in Arkansas, so they could focus on all the Baskin Robbins franchise stores they own. So they had to take down the sign and change it from Angel Food to Baskin Robbins ice cream.

Overhead shot for size perspective.

I gathered a few interesting facts about the ice cream sign from my internet surf (most of them from the Memphis Business Journal):

  • It cost $15,000 to erect back in 1987, but Klinke execs are confident it “paid for itself in the first six months”.
  • It’s 20 ft in diameter and hollow.
  • It could hold 24,000 gallons of ice cream if you filled it with its smaller cousin commonly sold at the grocery. Someone actually did the math.

Changing the branding from Angel Food to Baskin Robbins took three days and required a crane to take the tub off its pedestal so a crew could work on it at ground level.

I grabbed my photos from Google Maps but you can see better ones if you follow the links above. As a long time Nashvillian I’ve felt pretty good about our superiority over Memphis, but this giant ice cream tub calls all that into question. I may have to send this post to someone in the offices over at Nashville’s Purity dairy.

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You have to admire any piece of equipment that was inspired by a herd of sheep and now sells for more money than the average car.  

For scale, the drum on the front is approximately 5 ft in diameter.

That monstrosity is a sheep’s foot roller.  One of the more interesting looking pieces of construction equipment found sitting on stylish construction sites around the world.  The sheep’s foot roller is quite similar to its more mainstream cousin the smooth drum roller, but it has lots of little bumps or knobs that give it the memorable name.  Both the smooth drum and sheep’s foot rollers are used in highway construction for compacting the material you’re going to be driving on but the sheep’s foot is without a doubt more interesting, and perplexing, to look at.

Reminds me of one of my dog’s chew toys.

Pull behind Sheep’s Foot with independent drums. Courtesy of Flickr user palustris345.

 The idea for the sheep’s foot roller came about when an enterprising young contractor- to-be noticed the heavy compaction on trails commonly used by shepherds to get the flock out.  Sheep or other livestock were herded across ground that needed to be compacted, and the idea was mechanized into what you see in the accompanying pictures.

Variant roller for use in trenches. Courtesy of Wikipedia commons

Sheep’s foot rollers can be self-propelled, dragged behind other equipment, or as an attachment to multi-function equipment.  Some varieties also vibrate to increase compaction, and they come in varieties small enough to pull by hand.  The size and shape of the ‘foot’ attachments vary depending on the manufacturer and the intended use.  (If they have rectangular ‘feet’ they may also be called padded food.)  All the varieties give it a lot of versatility when it comes to compacting various kinds of soil.

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The bridge failure stats from the previous post were updated through 2006, but I originally started looking at the stats when I was in grad school in 2000, and the data itself was no more recent than 1991.  (True story, back then 42.3% of all failures were dinosaur related.)  When I got my hands on the 2006 update I thought it might be interesting to look for changes over time.

I was looking for a change in the distribution of the failure causes.  As the statistics from the last post point out, the majority of bridge failures are caused by hydraulic issues, but hydraulic factors have always been one of the least understood parts of the bridge design process.  Through the 1980’s it was mostly an approximate process.  That all changed due to two major bridge failures, one in New York state and one in Tennessee.  Lives were lost during both failures, and both drew a lot of attention nationally from the public and from engineers.  As a result, a lot of research has been done on bridge hydraulics and scour and state DOTs developed design standards in these areas.  So I was curious to see if that shifted the failure statistics away from hydraulics.

Nope.  Sure didn’t.  The statistics from the 2006 dataset were surprisingly consistent with the 1991 data.  Hydraulic failures went from 60% to 58% with the 2% drop being offset with a 2% rise in overload failures.  Since the 2006 data was merely an updated version of the 1991 data I decided to separate it into pre 1991 and post 1991 data sets.

So as one final check of the data, I separated the New York state data from the rest of the country.  The assumption being the New York data would be more comprehensive since New York State DOT was collecting the data.  The resulting analysis did show a significant shift.  Hydraulic failures dropped to 48% while collision failures went up to 26%.  I think this is because collision failures happen more often on smaller bridges and aren’t as widely reported in the national media.

I don’t think it’s a good idea to make any grand conclusions due to the nature of the data collection, but I suspect the analysis of the New York state failures shows the distribution between hydraulic and collission failure causes more accurately.  The most surprising part of the analysis was how consistent all the numbers were.  There was very little change from 1991 to 2006, and even breaking out just one state showed almost no change in any of the failure causes other than hydraulic and collision.  (And earthquake failures were not represented in the New York data.

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Over the weekend the local news stations were carrying a story about a bridge failure in Kentucky.  In light of that story, it seems like a good time to share some research I’ve been doing into bridge failure statistics.

It’s tough to find any statistics on bridge failures.  Surprisingly, there’s no national database on bridge failures.  The best I was able to find is an database that New York State DOT keeps on bridge failures.  It basically includes entries for whatever they have heard about either in the media or what was volunteered by other state DOTs.  It’s nowhere near comprehensive on the national level, but presumably it would be pretty accurate for New York since they would be well informed about their own state.

Before I share the statistics, there are a couple of things to keep in mind:

  1. This includes all bridges, not just the big ones that end up in the national media.  Most bridges are small structures  and not of interest to anyone outside the immediate area they are located in.
  2. Failure doesn’t necessarily mean going down in a spectacular cloud of dust.  A structure failure is anything that keeps it from being used in the way it was intended.  And now for the statistics…….

The biggest number of failures by far are caused by bridge hydraulics.  This means anything related to water and includes things like bridge scour, being clogged by ice or debris, approach road wash-outs, and just being pushed over by water.  Hydraulics is my specialty area and I’ll cover some of those in more depth in later posts.  The second greatest is collision.  I’m not entirely clear about this label, but I assume it means collisions by boats as well as collisions by trucks.  The overload and earthquake labels seem self-explanatory.  I assume fire refers to wooden bridges, but I do know of at least once instance  where a tanker truck blew up under an interstate bridge and melted the beams so badly the entire structure had to be replaced.  (It does occasionally happen outside of Michael Bey movies.)

I’m going to do a more in-depth analysis of the statistics in my next post so you folks who got your fill here can skip it.


Source: Wikipedia

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I looked at a lot of Google Map while I was working on the previous post about Briley Parkway.  I just wanted to share some additional thoughts that jumped out at me.

There is a pretty significant kink in the main road that looks like it was added in order to avoid the airport runway.  This caused the Briley interchange at Interstate 40 to be skewed on an angle making it more difficult to build.    Those angles make it difficult to move traffic and keep them at interstate level speeds.  I’ve always thought Briley was way too curvy for such a high speed route, and I suspect the leg through Donelson doesn’t meet a lot of the design guidelines about curves on a four lane or higher road.  It was much worse before a major overhaul was done around 2008.  Looking at the maps (and satellite imagery) makes it pretty obvious why it was so curvy.

When I moved to middle Tennessee in the mid-90’s the of Briley through Donelson and up to Gallatin Pike was extremely curvy and the lanes felt cramped and narrow.  It was more than a little dangerous if you drove at the average traffic speed.  I wasn’t a middle Tennessean when this part was built, but I suspect it was built in the 1960’s as an alternate to driving through downtown, and/or to get people to Opryland.  The curves and cramped feeling were a product of trying to thread a major highway through a dense residential area.  The designers obviously tried to veer around the denser populated areas and the cramped feeling was an effort to take up as little space is possible.  The version above is actually the rebuilt version which was constructed in the mid-00’s and some of its curves have been smoothed down and adjusted.

That’s just a brief peek into the world of highway layout.  There are an impressive number of other factors that also have to be juggled in order to get it just right.

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Dirtwork 101 – The Borrow Pit

SR-840 Ramp

SR-840 Ramp view from I-40

Say you’re driving west on Interstate 40 through Wilson County headed toward downtown Nashville. Just west of Lebanon you’ll see these big hills rising up out of nowhere right next to the road. The hills are paved on top and capped off with a bridge over the interstate because you’re at the I-40 interchange with SR-840.

Did you ever wonder where all that dirt to build the highway ramps comes from? No?

Well I’m going to tell you anyway, and pay attention because there is a pop quiz at the end.

It’s a pretty elementary question, but I don’t think it occurs to many people the highway business to think much about it. Personally, I find all these little details fascinating in a behind-the-scenes kind of way. It makes you appreciate day to day existence a little more if you know about the work and details that go into it.

Below is an overhead shot of the intersection. Those ramps that go over I-40 are have to get up to at least 20 ft above the level of traffic below, and you can see they’re pretty long. The source of all that dirt fill is also in the picture. See those oddly regular shaped ponds? Those are the borrow pits.

Overhead view of Interchange

Borrow pits are exactly what the name says they are if you’re willing to be a little liberal about the ‘borrow’ part. When you need fill, it’s sometimes cheapest to just dig a big hole you can ‘borrow’ it from. In middle Tennessee it often doesn’t actually require a pit because there is nearly always a hill somewhere that can stand to be a little shorter. If you do have to dig a pit you have the added benefit of ‘Hey, a pond!’ when the pit fills up with water.

Now that you have been introduced to the concept, I suspect you’ll be noticing them everywhere not unlike most of the trainee engineers who come through our office. I’ll give you two pieces of advice to guide you in your borrow pit spotting…….. (1) If that pond looks too regularly shaped to be natural, AND there is elevated bridge or highway nearby…. Borrow pit. (2) Your wife and other people who travel with you will get tired of you pointing them out a lot sooner than you would have thought.

And now for the pop quiz…… Below is an overhead view of a certain local airport. See if you can find the borrow pit they used to make those runways so flat and long. I’ll provide the answer in the comments if anyone needs it, but it should be pretty self-evident at this point.

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