They hide in plain sight.

But don’t let their stealth nature mislead you.

They are vitally important.

A culvert is a structure typically constructed of reinforced concrete or corrugated metal pipe, that allows water to avoid an obstacle. In most cases the water is diverted under a roadway or railroad, whether it be an active stream or a waterway that is utilized for seasonal irrigation or runoff.

In its simplest terms, if you don’t have a bridge – you have a culvert.

More often than not, you can note two distinct things about culverts:

• it is not uncommon for them to have little or no water passing through them
• and unless they have been installed very recently, the ones constructed from corrugated metal have corroded, rusted and deteriorated to a point where they need to be replaced or rehabilitated.

Many of the concrete pipe culverts have experienced cracking and separation of joints, allowing the water to erode the structural integrity of the system, also forcing rejuvenation.

Sadly, the urgency to address rehabilitation or replacement of a culvert is minimized when the culvert isn’t active at all times.

In many cases the culvert is like an insurance policy, or an airbag in a car: you don’t realize how much you appreciate it until you need it. If it is not working properly when it is put into action, it can result in flooding, habitat damage, sink holes, property loss, road washouts and even loss of life.

A 2005 study by the California Division of maintenance revealed that nearly 40 percent of the states’ 200,000 culverts under state highways needed maintenance or rehabilitation. A culvert inventory was performed by the Oregon Department of Transportation in 2021 and revealed that over 10,000 culverts connected to the state and federal roadway system are in a condition that will soon require repair or replacement to remain safe.

Similar studies can be uncovered across North America, as many of the culverts that are in place were installed before wider roads were built and vehicular traffic volumes were much lower. And now they present a risk because they cannot perform as efficiently as they need to.

When a culvert has deteriorated, replacement is time-consuming, labor-intensive and expensive. Excavation expenses, road closures, environmental impact, and the risk that it will only lead to the same process regenerating in 15-20 years when the new culvert fails, begs for a new solution.

So – what is the answer?

Even though we appreciate The Friends of Distinction hit song Grazing in the Grass, now is not the time to dig it.

There are numerous advances in trenchless technology that offer minimally invasive solutions to culvert repair and rehabilitation. There are four primary methods that are the current clubhouse leaders.

Cured-in-place pipe (CIPP) is a method that runs a felt lining into a failing culvert. A resin in the felt fabric is then attached to the interior walls of the pipe with a curing element. While CIPP can improve flow efficiency and can be executed with reduced manpower, it requires careful monitoring during the curing process and limits maintenance and cleaning options. There are also concerns about the harm that has resulted from chemicals used in the process.

Pipe bursting, as the name suggests, forces entry into the existing pipe material, breaking it and pushing it into the soil while pulling and installing a new pipe in its place. The process offers the advantage of having a new pipe, with the opportunity to upsize to a larger diameter if the soil conditions allow. The replacement pipe is typically high-density polyethylene (HDPE), which possesses a high flow rate, corrosion resistance, flexibility, leak-free properties and a long-term solution with a stress-life of 50 to 100 years. However, while pipe bursting is useful in certain situations, it is not as common in culverts due to its limitations of working only on smaller diameter pipe and shorter distances. Additionally, replacing corrugated metal with pipe bursting can be difficult, and the process may potentially damage nearby utilities. Any failure during the process can also be expensive to correct

A third option for repairing culverts is fold-and-form pipe lining. This method involves inserting a custom pre-fabricated liner typically made of polyethylene (PE) or polyvinyl chloride (PVC) into the existing pipe and expanding using heat and pressure to adhere to the interior of the pipe. One of the advantages is that it keeps the interior diameter of the existing pipe the same, and can handle bends in the route, and eliminates the need for additional grouting due to zero annular space. Like HDPE it also provides an environmentally safe option. However, it can only be used for smaller diameter pipes up to 36 inches, and like the two preceding examples, it can require more equipment than the final alternative.

While fold-and-form pipe lining and cured-in-place pipe adequately address many of the problems that are presented by a failing culvert, it is important to be aware that they may only serve as a temporary solution. If voids around the pipes are present, while the issues with the pipe itself have been addressed, the voids remain and leave the potential for continued deterioration or collapse of the roadway above the culvert.

That brings us to the fourth no-dig solution for culvert rehabilitation – slip lining. A new smaller diameter pipe is placed into the host pipe and the annular space between the pipes is sealed and grouted. Slip lining is a cost-effective solution that uses HDPE pipe which offers numerous advantages, installation is relatively simple comparatively, and overall offers a long-term solution. While the method requires a reduction in diameter with the new pipe, the smooth interior walls of HDPE, and the improved flow rate can compensate for the inequity.

Continuous slip lining is performed by pulling the entire pipe is through the culvert. With materials like Snap-Tite and Spirolite manufactured by ISCO, slip lining is further simplified with segmented HDPE pipe that have male/female or bell and spigot joints that can be literally snapped together without the need for fusion or welding, and fed through the existing pipe one piece at a time. In addition, Spirolite is available in diameters up to 120 inches. Both Snap-Tite and Spirolite address the voids that can lead to potential problems with the roadway.

It can be noted that none of the properly installed culverts in need of repair or replacement were initially constructed of HDPE, but three of the solutions utilize a form of polyethylene and two of them most commonly lean on HDPE as the long-term solution with its numerous built-in advantages.

Depending on your variables; i.e. diameter and distance of culvert, flow volume, staging area, environmental concerns, existing pipe conditions, budget and timeline, any one of or all four no dig options may be favorable to your application.

Regardless of the choice, the reduction in time and cost, environmental impact, and inconvenience by utilizing a no-dig rehabilitation solution as opposed to full replacement, makes it an easy decision. Not to mention the fact that replacing a failing culvert with its equal is illogical.

Federal funding has addressed infrastructure, but has previously focused on bridges and roadways, which have a higher profile. With the sharp increase of rapidly deteriorating culverts in need of repair and rehabilitation, state and federal governments have responded, including an initiative that is pouring $1 billion over five years in federal grants to specifically address failing culverts.

As financial resources are put in place, and low-impact solutions abound, the time has come for action to be taken to replace, repair and rehabilitate these hidden hazards as a preventative measure that can potentially avoid costly, if not catastrophic results.

That – we can dig.