Fiberglass manholes are among the most useful solutions for flow rate measurement in piped flows that are below grade, and these manholes come in many different forms to make the job as easy as possible. Whatever form you need, fiberglass is the material that will see you through with its moldability and reliable construction rated for strength and durability.
To have a fiberglass manhole work properly, however, it needs to be installed to specific standards. Fortunately, the process is easy once you know the steps. Plus, the fiberglass construction of these manholes makes the process much easier compared with other materials. Learn how to install fiberglass manholes, and discover the benefits a proper installation can bring to the entire process.
Types of Manholes
Before you understand the installation process, it’s important to grasp the various types of fiberglass manholes that are available. For the most part, you’ll have seven options available, and they each have something special to offer. Here are the basic configurations to be aware of.
- Packaged Metering Manholes
A packaged metering manhole is one that includes a measurement device in its design such as a flume or weir. It’s watertight and a singular piece that’s molded with fiberglass making it resistant to corrosion. Plus, they work well with open and closed pipe flows. Because they can be used for various purposes, they additionally feature several different top styles for use in an array of environments based on traffic.
- Grinder Manholes
If you need to do any sewage grinding, nothing works quite like a grinder manhole does. These manhole structures feature a custom channel on which you can mount your grinder with the strength and customizability you need to fit your unique flow channel conditions.
- Energy Absorbing Manholes
An energy-absorbing manhole is one that is designed to ensure that the approach flow is tranquil enough to work properly with a weir or flume device. In order for a water measurement device to function properly, the approach flow needs to be subcritical, then accelerated to a critical or supercritical velocity. With an energy-absorbing manhole, the design ensures that the approach flow is properly conditioned to allow for accurate measurements.
- Laser Manholes
Laser manholes are high-tech manholes that provide an easy drop-in-place metering solution in a superstructure that’s resistant to corrosion, watertight, and long-lasting. They utilize lasers to monitor flow, which is particularly useful as a noncontact kind of flow monitoring technology. With noncontact solutions, you won’t need as much maintenance to keep your measurements accurate.
- Flow Splitting Manholes
Flow splitting manholes work to direct and distribute the flow in a particular way with bends, drops and channels to help you keep the flow direction under control. They are particularly useful when you’re trying to split the flow stream. Keep in mind, however, that these only work for non-full-pipe flows and come in a variety of configurations like J-bends, interior drops and Y-channels.
- Magnetic Flow Meter Manholes
A magnetic flow meter manhole makes use of a noncontact kind of flow rate measurement that works in open channel systems. This technology uses pressure in the effluent to measure flow rate using high-end flanges crafted from Hetron 922 resin. Because it’s a non-contact method of measurement, you won’t have to worry about maintenance as much.
- Wet Well Manholes
With wet well manholes, you can enjoy a flow metering solution designed for wastewater and sanitation. These wells offer two separate and unique features including storage and screening. During the screening function, solids and debris are taken out of the flow. As for storage, these wells are often designed to be able to store wastewater for pumping purposes.
Installing the Manhole
With the different types of manholes in mind, it’s time to learn the installation process. Fortunately, the process is relatively straightforward, and given that fiberglass is about a tenth of the weight of concrete, it’s much easier using these kinds of manholes.
- Excavate the Installation Site
The first step is to excavate the installation site. Keep in mind that the excavation should be enough to not only support the manhole but a working area for the crew as well. It’s always more efficient to excavate too much, leaving plenty of room to work, than to have to install in a place that’s too confined. Remember that the excavation’s size isn’t solely a matter of convenience. It’s a matter of safety as well because the crew will need to have enough space to handle the installation process properly. Of course, you’ll need to make sure you’re compliant with any construction codes on the federal, state, and local levels.
- Pour the Concrete Slab
While the manhole itself is made from fiberglass, you’ll still need a concrete foundation to install it properly. You may want to pour it off-site, then lower it into the excavated site, but you can always pour it on-site as well. It all depends on what would work for your needs the best. Typically, it’s best to pour it off-site if you want to minimize the flow line’s downtime as much as possible. To ensure that your slab is going to get the job done properly, it needs to be level and smooth. That means it should be level from front to back and side to side. The troweled surface shouldn’t have any debris or aggregate exposed to ensure that the installation process works properly and you get accurate measurements.
- Set the Manhole
The next step is setting the manhole in place. Fortunately, this is fairly easy considering the fiberglass construction of manholes. They’re constructed in a singular connected piece unless they’re particularly large, so you can often lower it all at once. It’s absolutely essential that you make sure the slab is free of debris at this point. You should’ve already done so in the previous step, but it’s important to make doubly sure here before lowering the manhole down. Part of the manhole will be the foam support bed that rests at the bottom. Mounting legs should have these foam pads too. While the foam pad is technically optional, it’s typically best to make use of it. Just keep in mind that you may have to remove it during the installation process to ensure that the manhole is level with the concrete pad. When moving a fiberglass manhole, it’s important to only use fabric slings like nylon. Cables or chains can cause damage and can make the manhole difficult to control upon its descent. Similarly, you’ll want to use slow and smooth movements when lowering it that should be planned out in advance. The last thing you want is for the manhole to overturn on the way down.
Once the manhole is in place, it’s time to secure it to the concrete pad. This is typically done with anchors. While the wedge style is the most common, you’ll find other options. Just try to avoid anchor bolts that are preset, because they don’t offer much flexibility if you ever need to adjust the manhole.
- Connect the Piping
The piping needs to be connected to the manhole once it’s in place to ensure that the flow channel travels through the manhole and the measurement device at its bottom properly. Quite a few different connection types are available including flanges, caulking collars, and pipe stubs. Pipe stubs are the most common, and they’re typically complemented by flexible couplings to seal the connection easily, making it watertight. If conditions prohibit the use of flexible couplings and pipe stubs, flat-faced flanges may be better. These utilize rigid connections rather than flexible couplings.
You may need to use caulking collars, which are pipe stubs of enormous size. Given the size difference between these and the line piping, the space between the pipes will have to be filled with oakum, which is a watertight material. Just remember that flow monitoring manholes don’t typically work well with this connection type because it often doesn’t allow the flow to be uniform as it approaches the measurement device.
- Backfill the Excavation Site
Once the manhole is in place and the piping is properly connected, it’s time to backfill the excavation site. Make sure it’s free from any debris or materials bigger than an inch in size that can compromise the integrity of the site, and start filling in the area. Backfill materials are typically pea gravel or sand, though several other materials can work as well.To begin the backfill process, it’s typically good to start underneath the piping connections and around anything else outside the main manhole tube. With these small spaces taken care of first, it’s easier to ensure that the rest of the backfill progresses uniformly up to the ground level. You’ll want to make sure the backfill is applied evenly with no more than a foot between the highest point and lowest point at any point during the process.
Manhole Top Styles
After the manhole itself is installed, you’ll need to make sure you’ve got the right kind of top. There are three different options available, and they all have unique benefits. You can find several different materials used for tops including cast iron, ductile iron, fiberglass, plastic, and reinforced concrete depending on the top style used. Take a look at the central top options for manholes.
- Dome Tops
The dome top is what you’ll want to finish off your manhole with if you’re installing in an area that doesn’t get much traffic. Keep in mind that traffic here refers to both foot traffic and vehicle traffic. When this kind of top is installed, the manhole tends to protrude from the ground by about a foot, so it’s an obvious obstacle in high-traffic areas. The benefits these offers are numerous in that they are lockable and provide more egress than any other kind of top apart from having no top at all. They’re equipped with a locking lift arm and various diameter dimensions and offer a stainless steel piano hinge for convenience.
- Aluminum Hatches
With an aluminum hatch, you’ll have a manhole that’s best suited for areas that get a lot of foot traffic. These tops sit flush with the ground, so there’s no change in elevation or obstacles to worry about as people walk around the manhole area. Sidewalks and pathways are the most common locations for manholes with these kinds of covers. These tops come in many different shapes and sizes with several diameters available along with materials other than aluminum such as steel. You may also want to opt for custom engraving, multiple locks, or spring assists to make things easier for operators. Plus, these manhole tops come in watertight and non-watertight variations.
- H-20 Highway Tops
As the name suggests, H-20 highway tops are manhole tops that are rated for use in areas that get a lot of vehicular traffic. These areas include roads, highways, and parking lots. To ensure their strength, highway tops are made out of incredibly durable materials and conform to AASHTO H-20 axial loading standards. You’ll also find them equipped with grade wings for additional support. With these kinds of tops, it’s important that they be able to withstand a lot of weight coming from vehicles passing overhead. Still, they make work for operators easy by including pull-up ladder assists and variable opening sizes.
Fiberglass Manholes From Tracom
At Tracom, we’re happy to offer everything you need for a quality fiberglass manhole including custom designs and assistance with installation. All of our manholes are manufactured to ANSI/ASTM D-3753-81 specifications, so you can always count on quality.
Prefabricated manholes make the installation process easy and can help ensure that you’re getting the accurate measurements you need. Plus, our fiberglass manholes feature a variety of other benefits such as interior lighting, motorized vents, instrument mounting plates, additional utility taps, NPT coupling for wiring and tubing access, and more. Contact our team today to find the best manhole for your unique flow channel conditions.