Corrugated metal pipes (CMPs) are some of the most commonly used materials for underground stormwater infrastructure. They go largely unnoticed, but are necessary to support the daily functions of our lives by properly moving water to avoid issues like water ponding, standing water on roadways, and localized flooding.
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CMPs are used for various underground infrastructure projects such as culverts, stormwater detention/retention systems, storm sewers, small bridge replacements, aeration pipes, material conduits, and more.
CMPs can require frequent maintenance and repairs if they aren’t given adequate protection through the use of things like structural polymeric linings. Keep reading to learn about CMPs, 3 common issues they face, and how to fix them!
A corrugated metal pipe (CMP) is a pipe—used for culverts and other underground infrastructure—that has a distinct set of parallel grooves and ridges.
As mentioned, they’re most commonly used for drainage culverts, storm sewers, material conduits, and other underground stormwater conveyance infrastructure pieces.
CMPs are commonly manufactured out of aluminum and steel, with different metallic coatings and alloys designed for different applications.
Aluminum alloy CMPs are designed to resist the effects of corrosion and abrasion underground. The surface of the core is coated with a higher-grade aluminum alloy.
Aluminized steel CMPs are manufactured by hot-dipping steel CMPs in commercially pure aluminum. These pipes can be effective at combating corrosion, in particular for culverts and drainage systems that involve acidic water.
Galvanized steel pipes are manufactured using a process called—you guessed it—galvanizing. Galvanizing involves applying a zinc coating to the pipe, thus protecting the base layer of steel from corrosion. Typically less effective than aluminized steel, the zinc helps protect from corrosion by acting as a barrier between water and the steel, reducing the effects of rusting and deterioration for the short term.
Asphalt-coated galvanized steel pipes are similar to galvanized steel pipes, but they’re coated on the interior and exterior with asphalt to delay corrosion and deterioration for a longer period. The asphalt, in combination with the zinc coating, provides additional abrasion resistance and helps to prevent water, debris, and abrasive run-off from reaching the base layer of steel.
Polymeric coatings and structural linings are emerging as great options in the industry to help CMPs stay sealed and strong. Polymeric coatings protect against abrasion from acids, salts, alkalis, and other materials that threaten CMPS in culverts and sewers.
Corrugated metal pipe joints are the areas/pieces that connect different adjacent sections of pipe. These areas can be a source of problems if they are opened due to misalignment. In these scenarios, backfill material can erode into the pipe, causing structural deterioration and eventually failure if left untreated.
To fix open joints, a process called joint sealing is carried out. It’s as simple as it sounds—the joint is sealed off to prevent unwanted material from entering. Polymeric linings are great for joint sealing, providing structural rigidity against soil intrusion and hydrostatic pressure
Voids are unwanted gaps/spaces between the CMP and the soil. They’re addressed by using polyurethane grouting to fill the void and using polymeric linings to seal/strengthen the CMP.
The invert of a corrugated metal pipe is the lowest point within the pipe that water flows across.
Since this is where the water makes the most contact with the pipe, it’s especially susceptible to abrasion during peak stormwater events and corrosion over time if left unprotected. Deteriorated CMP inverts are likely the most common and frustrating corrugated metal pipe problems that underground infrastructure professionals face. If the pipe doesn’t have a long-lasting, abrasion-resistant corrosive protective agent like a polymeric lining, it can rust through.
Once the invert rusts through, other serious problems can occur like voids or even total structural failure.
While there may be numerous approaches available in the market, many professionals are starting to see the advantages of trenchless pipe repair.
Trenchless pipe repair means fixing underground pipes from the inside. These projects can be carried out to completion within a few working days, far shorter than digging from the surface and ripping up roadways to get to the pipes.
Polymeric linings—as mentioned in the Corrugated Metal Pipe problems section before—are a great trenchless solution for CMP repair and rehabilitation.
One of the best products any trenchless pipe repair professional can use is SprayWall®, a polymeric lining that not only seals CMPs but strengthens them as well.
In fact, SprayWall® is scientifically proven to increase the load-carrying capacity of corrugated metal pipes by up to 800%.
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Last year, SprayWall® was chosen by the University of Texas at Arlington for a study aimed at creating testing and application standards for spray applied pipe rehabilitation liners in multiple Department of Transportation markets.
A key finding from the report was the fact that SprayWall® was strong enough to be considered structurally capable to perform as a new pipe inside of the host pipe.
SprayWall® lasts over 50 years and cures completely within 72 hours of application. It begins to gel within 8 seconds and is tack-free after two minutes.
This means trenchless repair crews who use SprayWall® will have completed an entire project in the time a crew would need to dig up and bury what they’re working on.
Overall, SprayWall® helps make CMP repair faster and more effective, with results that last for decades.
Underground infrastructure—supported by things like CMPs—is vital to our society, so it’s important that we keep it protected.
Trenchless repair methods for corrugated metal pipes are proving to be the superior solutions when applicable, as they save communities time and money through long-lasting solutions and minimal disruptions. Whether the issue is corrosion leading to maintenance issues, or deteriorating inverts posing a threat for structural failure, trenchless solutions like SprayWall® can solve the problem.
SprayWall® is minimally invasive and saves engineers, contractors, and communities their time and money.
Have questions about SprayWall® or want to learn more about applying to become a Sprayroq™ Certified Partner?
For the majority of applications, corrugated metal pipe (CMP) is the “go to” material for subsurface infiltration. This in turn, leads to a lot of design questions. Below is a list of six questions and answers we received from a recent webinar on designing CMP detention systems.
1. What are the design considerations for construction beneath buildings and material sorting areas?
Generally, detention systems are designed to be under some sort of flexible pavement / rigid pavement, and Contech prefers not to install them under buildings. However, if a system is to be built under a building, all live load and dead loading will have to be accounted for. This may increase the gage and backfill material of the system. Special attention must be given to the zone of influence of the foundation, and access for maintenance must also be addressed. These projects should be evaluated on a project by project basis.
2. Can you use CMP detention systems in high water table environments? How do you counter buoyancy in high water table environments?
Yes, CMP detention systems can be used in high water table environments. Tiedowns and special backfill may be recommended to counteract buoyancy issues.
3. Is pipe cover normally measured from bottom of asphalt section or to bottom of entire pavement section (i.e. bottom of rock subbase).
CMP pipe cover is measured from the bottom of flexible pavement (i.e.: asphalt) or the top of rigid pavement (i.e.: concrete.)
4. What is best, a single manifold system or a double manifold system?
It depends on site conditions and what is needed. Each has their benefits, but having a single manifold is more cost effective as it reduces the amount of fabrication needed and can make installation easier, by not having the concern of having the pipe barrels to match up perfectly on both manifolds.
5. What is the preference for the location of risers - on the edge of the storage pipe or in the center and why?
The only limitation on riser placement is that they cannot be placed on welded seams. Other than that the riser locations are up to the EOR.
6. Can CMP detention systems be used in coastal environments?
CMP detention systems can be made with different materials and coatings to accommodate different environments and service life requirements. Galvanized steel, Aluminized Steel Type 2, aluminum, asphalt coated and polymer coated options are available for different applications. For coastal environments we usually recommend CMP pipe made from aluminum because of the potential of salt water and/or more harsh soils. Giving the geotechnical report to your local representative can help you choose the right one for your specific project.
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