Follow up to "Nasty Culverts"

As I write this segment, my road has been closed for about a week with another week to go in order to replace a deteriorated cross culvert. There are a large number of culverts near the end of their design lives throughout New England. Below is a frequent scene on many back roads. Although it appears like the culvert is relatively

stable, with each storm event runoff flows through some of the openings along the side of the pipe and  erodes the soil away from the outside of the pipe.

A circular cross section is an engineering marvel offering culverts a great deal of strength. The force of a heavy vehicle passing over the top is redirected along the sides of the pipe into the soil below. The key is that this mechanism requires soil along the outside of the pipe to keep the pipe from compressing in the vertical plain while expanding in the horizontal plain. To demonstrate for yourself, stamp on a soda can. Note that in one direction it is now flat while in the other it is now wider. That is called pancaking in the culvert world. Long before pancaking normally occurs, soil that is eroded from the side of the pipe leaves voids that lead to settlement and sink holes.

This is a sinkhole in Connecticut that occurred on the side of a state road. At first glance it appears to be a roadside washout, but careful inspection revealed minimal runoff at this location and the road and bituminous curb were actually undercut before they collapsed. The culvert below this location had the tell-tale holes at waterline and a small sandbar inside the pipe downstream the holes created by the eroded material flowing back into the culvert.

The more dangerous of the sinkholes occurs directly under the pavement. The cohesiveness of the pavement allows the area below the pavement to be hollowed out considerably before a noticeable failure occurs. This innocuous little hole in the pavement was in the shoulder of a  New England interstate. Although barely large enough to drop a bowling ball through, this hole required nearly twenty-five cubic feet of material to fill the void. When driving roads not every pavement deformation is a sinkhole. Most are just good old pot holes more common in our environment, but when the culverts show significant deterioration look for these additional signs of a more pressing problem. Also look for the situation below. This is a sink hole that was probably mis-identified as a harmless pot hole and patched cosmetically without ever addressing what is likely to be a much larger void below the pavement.

The sinkhole in the interstate pavement above led me to the Colorado School of Mines for a short course on Microtunneling which has since expanded to a fascination of the different methodologies of trenchless construction of culverts. Traditional methodologies are still effective depending on the depth of fill over the culvert and the density of the traffic traveling over the top. As methods such as pipe ramming, pipe bursting, and Microtunneling become more accepted in the east options for pipe replacement become more numerous.

Before leaving common problems with our culvert infrastructure, pipe separations should also be included. In this case the pipe was constructed in sections and spliced together with bands instead on an actual bolted connection. Luckily this separation was in a flattened slope not actually under a road. The road embankment had a similar depression as the one above from Connecticut.

 The earlier article touched upon the condition of our culvert infrastructure, then went on to talk about the ramifications the deficient culverts had on the water quality and animal passage. A newer form of infrastructure are culverts specifically designed for animal passage, sometimes even without any accommodations for runoff. To the left is a black and white photo of a fox passing through what is sometimes affectionately known as a "critter crossing". Other culverts are designed to provide runoff in a lower channel and an adjacent shelf for animal crossing all housed inside a box culvert. During high flow the runoff can utilize both the lower and upper channels.

It will be interesting to see the final New England culvert condition report. My understanding is that many of the culvert replacements would fall under state betterment funding of the type that was significantly cut with the sunsetting of the registration fee this year in New Hampshire. This places an additional burden on the State Agencies as they add the cost of maintaining culvert infrastructure to other underfunded programs. Adding water quality and animal passage including fish, amphibians, reptiles, and mammals to the discussion of culvert maintenance, more cost effective measures such as slip lining (rehabilitation) may need to be abandoned in favor of full reconstruction to meet all of the identified challenges. With replacements significantly more expensive than rehabilitation this will further strain the balance between the need and the available funding.