At times our city streets look like a patchwork of potholes, trench cuts and repairs – all signs of a long life of hard work. Some of these also point to the unseen problems and hazards that lie beneath the pavement.
Cycling to work is one of my favorite parts of living in Portland. Engineering work is interesting and engaging, but like many modern professions it involves a good amount of sitting at a computer, and it is nice to be able to start and end each day with some exercise. I also feel like regular riding helps me as an engineer to better understand the needs of drivers, cyclists and pedestrians, and the effects of pavement conditions and different levels of pavement maintenance.
Each evening, my commute home takes me along Concord Street, a quiet bicycle boulevard through North Portland’s Overlook neighborhood. It’s often getting dark at this point in my trip, making it more difficult to see potholes and other potential ground level hazards. Hitting one of these isn’t serious, but a cyclist has to stay aware and be prepared to recover from a random jarring bump.
For months one particular pothole seemed to surprise me every time I passed. Each day I tried to remember the cross streets and location of the pothole, and each day I seemed to hit it head on. The situation was even more frustrating because this bump wasn’t really that big and should have been easy to avoid. It was less a pothole and more a dimple really. Instead of being broken or missing, the pavement here was depressed about 2 or 3 inches within a 6 or 8 inch diameter circle.
A dimple is a strange shape to see pressed into the pavement, even in a place that sees a pretty wide range of pavement distresses. Being an engineer, I would inevitably spend the next mile or so after hitting the dimple wondering what could have left such a weird shape in the ground. Construction equipment? A meteor? A bowling ball falling off a passing moving van?
And then, one day, there was a sinkhole. This video was from about a month after it first appeared.
The hole shown here is about 8 inches in diameter through the pavement – about the size of the preceding dimple – and widens to roughly twice that below ground. The cut extends down and around the adjacent manhole for about 5 or 6 feet. The axis of the hole is on a slight angle, but there is a fair amount of loose dirt on the low side, which could mean that the walls of the collapse are below the surface that we see here.
So where did this hole come from? It is a strange thing for something as strong and reliable as pavement to suddenly be swallowed up by the earth, and the question is (understandably) asked whenever a sinkhole appears. Believe it or not, even the biggest sinkholes are usually caused by very small, almost unnoticeable problems occurring over long periods of time. The overwhelming majority of these problems involve two things: rats and leaky sewers.
Our urban sewer systems are essentially massive underground tunnel networks and they make perfect highways for rodents to move around the city. Once in the sewer, a rat would exit through a break in the pipes – of which there are many more than we would like to believe. Digging upward, he would eventually reach a point where the roadway subgrade is too hard for him to continue, would flatten his trajectory and would head for softer ground, usually ending up in a landscaped area.
This rat burrow is now an open conduit for surface and ground water to enter the sewer system, much like our catch basins and pipes except that this conduit isn’t planned or lined. Over time, flowing water will wash soil into the sewer, which will in turn carry it away. The pavement above the hole will grow weaker and weaker as it loses support until it is entirely relying on its own material strength to stay in place. The hole will of course continue to grow and then, one day when either the hole has grown too big or a heavy load is placed above it, there will be a sinkhole.
While ratholes are a major cause of this type of problem, I don’t think they are the cause of this specific instance of the problem. First off, the hole is bigger and more wandering than might be expected from a rat. Second, as you can see on the map below, the manhole next to our sinkhole isn’t part of some long continuous pipe network. In fact, it isn’t a manhole at all. It’s actually a drywell, disposing of stormwater runoff collected from nearby local streets.
This map comes from the engineer’s best friend portlandmaps.com – a database of a wide range of GIS information for Portland and its outlying areas. The red dot on the map is the approximate location of the sinkhole. You can see that in this location water flows from the intersection into one of four catch basins, then into a manhole and through a series of two drywells.
I don’t know much more about the system than what you see here, but I think this is enough information to suspect that if a rat wanted to move from one end of the system to the other it might just cross the street. However, even if rats aren’t to blame for this sinkhole, the description of how it may have formed is basically the same. Somehow water starts to enter a leaky sewer, bringing dirt with it. Slowly over time the leak creates a hole until, one day, there is a sinkhole.
In this case the sinkhole borders a drywell. Drywells basically act as local disposal points for stormwater, which is directed into a well and then infiltrates into the ground. Systems like this one work well and are used throughout Portland to keep stormwater from overloading our ancient combined sewer system, in turn keeping sewage from flowing into the Willamette River. These drywells and the streets that surround then, however, are quite old themselves.
The City of Portland requires that all new conveyance pipes be designed for a 100 year service life. That span is shortened to 50 years for manhole structures. It is unlikely then that this drywell has “failed.” More likely is the possibility that groundwater, in addition to the intended surface water, is entering the drywell. The walls of a drywell are perforated and water is intended to move from inside the well to outside. Reversing the flow direction usually results in a small amount of dirt washing into the well each time there was a storm, creating a void under the pavement and eventually a sinkhole.
So, we have a suspected cause for the sinkhole! What does that mean, though? The hole has still been sitting in the street for nearly two months with a single traffic barricade and a paint circle the only signs that the City knows it exists. My suspicion is that this repair may be complicated and that the City is having difficulty working it into their already overburdened maintenance budgets and schedules. Still, there is now a hole in the road and it will probably keep growing, especially as we start to see more rain.
Also, thinking back to the original dimple, I am pretty sure that I have seen similar marks on the pavement. Could these be hidden sinkholes waiting to surface? Could there be a way to use pavement distress to easily and proactively identify possible sinkholes like this one before they result in a potentially dangerous collapse? Does this process already in exist?
I plan to explore all of these questions in future posts. I am keeping a close eye on this particular sinkhole since I ride past it every day, and will track any developments here on the blog. In the meantime, I wish you safe travels. Watch out for those odd dips in the pavement. They might not be anything, but sometimes who knows.
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