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Yuse Lajiminmuhip: Hello and welcome to the AGRU America podcast. My name is Yuse Lajiminmuhip, and joining me today is Michael Rublaitus, who has been using geosynthetics toward environmental protection applications for over 10 years. In this podcast, we will discuss the infiltration problem and how to eliminate it in precast manholes using a combination of HDPE liner technologies.
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Yuse Lajiminmuhip: Hello, Mike, and welcome again. Let’s talk about the infiltration problem as it relates to manholes. Exactly how big is this issue?
Michael Rublaitus: Well, almost every major utility has a manhole infiltration problem somewhere in their collection system. Some of the more common causes of infiltration in the manhole structures are sealant failures between the precast components of the manhole, deterioration of the concrete from gas in the system, and leaks in the chimney. Most of the seals between the different components of a manhole are a low-grade rubber or a mastic-type of sealant. Over time, these materials can either become brittle and crack with age or can it can deteriorate from gas attacking them. In deep manholes, these seals may not be able to sustain the constant head pressure from the groundwater over an extended period of time. A structure may have also experienced some sort of differential settlement that cause cracks in the manhole or gases attack the concrete and weaken the structure to the point where it leaks, creating a leak path for infiltration to occur. Leaks in the chimney commonly occur as the mortar between the bricks loosen or cracks.
Yuse Lajiminmuhip: Can you explain other types of components of a manhole?
Michael Rublaitus: Sure. Almost all standard manholes today are made at a precast concrete plant. First you have the base, which is at the bottom of the manhole, and that is where the pipe comes into the manhole. The base is where the inverts or flow channels for the sewage are. On top of the base, it’s the risers. The risers stack on top of each other, and the number of risers you have depends on the depth of the manhole. The deeper the manhole, the more risers you have. The risers have a gasket or some sort of mastic material sealing them to the base. They also have the same type of seal between each riser section. The next component is the cone. The cone transitions the diameter of the risers down to the diameter of the manhole ring. The chimney sits on top and completes the structure, bringing it to the finished elevation where the manhole lid is installed. The chimney is considered to be the weakest point of the structure. Over time, road traffic and freeze-thaw cycles can cause the mortar to crack in brick chimneys. In systems that use grade rings rather than bricks, the seals between the rings can fail over time, and the grade rings themselves can become cracked as well.
Yuse Lajiminmuhip: Oh, that’s very interesting. So where does the infiltration entering the structure come from?
Michael Rublaitus: The water infiltrating the manhole comes from two main sources. The first source is groundwater. If the structure is in the water table, it will have a constant flow of infiltration from a crack or void in the structure. The second and most overlooked source is rainwater. When a rain event occurs, you generally see most of the rainwater entering through the chimney of the manhole. This is especially true in aging systems where brick was used to form the chimney. As I mentioned previously, the mortar between the brick often cracks, creating a leak path for infiltration from rainfall. The chimney is rarely in the water table, so it usually only sees infiltration during rain events. Manholes are usually inspected when the weather is good. With no rain event going on during inspection, there are usually no noticeable leaks, so the manhole is often thought to be leak-free when it actually may allow significant amounts of infiltration into the collection system during rain events. The infiltration through the chimney is one of the main reasons you see a spike in water entering the treatment plants during significant rain events.
Yuse Lajiminmuhip: Exactly how much does a leaking manhole structure cost a municipality?
Michael Rublaitus: That’s a good question, and it’s a question that most municipalities struggle to answer. I built a simple cost model that can easily be adapted to any municipality to calculate the treatment cost from a leaky structure. All you need to know is the combined annual operating costs for all the water treatment plants and the total capacity for those plants. Let’s take a look at Miami-Dade for example. If you look at their published budget, in 2017 their water treatment costs were $92,347,000. The gallons per day that they treated, or their total capacity, was 300 million gallons per day, so if we multiple that out and annualize it, they’re treating 109,500,000,000 gallons a year. Now, if we divide those numbers into each other, we come up with a simple cost of 0.0008 per gallon of treated water. Now, if we make some assumptions and we look at a manhole that’s leaking 10 gallons per minute, we can multiply that out. We see that that’s 600 gallons per hour, 14,400 gallons per day, and weekly that’s 100,800 gallons coming in of infiltration per week. Again, if we annualize that over a year period, that’s 5,256,000 gallons of water coming into the system. If we do some simple math and divide that by the treatment cost of 0.0008, we come up with a number of $4,432.66, and that’s the annual cost of the infiltration into a manhole at 10 gallons per minute. If we were to have a thousand structures leaking that 10 gallons per minute, our annual cost would $4,432,656.
Yuse Lajiminmuhip: Well, that’s quite a bit of money. Obviously this is a problem that municipalities have to deal with, and how do they usually do this?
Michael Rublaitus: In the past, many municipalities have tried different types of spray coatings, mostly the epoxy type of spray coatings, and they’ve had limited success.
Yuse Lajiminmuhip: Now you say they have limited success. What are some of the problems that they face?
Michael Rublaitus: To start with, epoxies are rated by how well they adhere to the surface that they are sprayed onto. They are tested by peeling the epoxy off the surface and seeing how well it sticks, so let’s think about that for a second. There is nothing inside a manhole or any other part of a sewage collection system that could possibly grab and peel a coating off the concrete from inside the structure during normal operating conditions. That makes the test irrelevant to the real world operating conditions of a sewage collection system.
Yuse Lajiminmuhip: Yeah, that makes sense. So what actually causes a spray liner to fail?
Michael Rublaitus: Well, the main cause of failure is back pressure. It is not the internal forces inside a manhole that cause a coating or liner to fail. It is the external forces outside the structure. You simply can’t have infiltration without some kind of back pressure forcing the water into the system. As you go deeper into the groundwater, the back pressure on the coating increases, making it more difficult, if not impossible, for a spray type of liner to stay on the concrete. The second main source of failure for a spray coating is cracking. For example, the chimneys in a manhole are susceptible to different environmental conditions, including compressive loads like heavy truck driving over the manhole, freeze-thaw cycles or differential settlement. Many epoxies have a maximum expansion rate of around 4%. That means the coating is likely to crack with the structure or come off completely if the structure cracks or expands. Once this happens, there’s a leak path in the structure and infiltration begins.
Yuse Lajiminmuhip: Well, so what are some of the options available in the market to design and install such a manhole?
Michael Rublaitus: Well, AGRU makes an embedded HDPE or high-density polyethylene liner that’s concrete protection that is cast into the base, the risers, and the cone of the structure, and that product is called Sure-Grip. One side of the liner is smooth and the other side of the liner has anchors, sometimes called studs. The smooth side is placed against the forms or molds at the precast plant and the anchors face the area on the form where the concrete will be poured. As the concrete cures around the anchors, it mechanically bonds the liner into the concrete, making it a permanent part of the concrete structure. Since the liner is mechanically and permanently cast into the concrete, it does not rely on a surface bond like spray coatings do. It’s a mechanical bond that allows the AGRU Sure-Grip liner to handle the high amounts of back pressure, therefore allowing the manhole to go deep into the water table with no risk of infiltration. Since the material is HDPE, it can be welded to other premolded HDPE products designed to eliminate infiltration through areas like the chimney, for example. The end result is a manhole that is infiltration-free with no opportunity for leaks.
Yuse Lajiminmuhip: Other than the elimination of infiltration, what are some of the other advantages of this kind of system?
Michael Rublaitus: The Sure-Grip liner has been used for over 25 years, so it will also protect manholes and lift stations from H2S gas corrosion into the concrete.
Yuse Lajiminmuhip: And what kind of service life does this product have and what kind of maintenance does it require?
Michael Rublaitus: Well, the intended design life of a Sure-Grip liner is the life of the structure under normal operating conditions. Unlike spray coatings, the product does not come off over time, nor does it need to be reapplied. In a normally operating system, there are no reoccurring maintenance costs. If someone were to damage the liner for some reason, it can easily be repaired in the field. Since the liner is mechanically a part of the concrete, it does not come off over time and does not need to be reapplied.
Yuse Lajiminmuhip: Okay, and are there other applications for the product outside of manholes.
Michael Rublaitus: Actually, there are. There’s quite a few. Some of the more common applications for Sure-Grip concrete protection including corrosion and infiltration protection for lift stations. Sure-Grip also offers corrosion protection for above-ground concrete applications in water treatment plants, digesters, aquaculture farms, chemical plants, power plants, leachate collection systems, and industrial wastewater collection systems.
Yuse Lajiminmuhip: That’s quite a few applications. Is this a new product to the market?
Michael Rublaitus: No. The product has been in use in Europe and North America for over 25 years.
Yuse Lajiminmuhip: That’s great to hear. Well, thank you so much for joining us today, Mike, and I look forward to our next podcast together.
Michael Rublaitus: Okay, thank you.
Yuse Lajiminmuhip: We hope you’ve enjoyed this AGRU America podcast. If you’re interested in learning more about designing a system to eliminate water infiltration using Sure-Grip, then consider reaching out to one of our representatives today. When you choose AGRU, you partner with a world leader in geosynthetics. Products like AGRU Sure-Grip have a rich history spanning decades of successful installations across multiple continents. Beyond manufacturing high quality products, AGRU is committed to excellence in customer service and quality control. With product representatives in every region, a dedicated quality assurance team, and detailed job specifications for each product, we’re prepared to support you from concept to production. Are you ready to eliminate infiltration? Visit us online at agruamerica.com for more information.