Needs the proper precast concrete manhole supplier? The purpose of a manhole is to provide safe and reliable access to underground drainage, sewer and/or utility systems, resist soil and groundwater pressure, and remain undamaged for decades without leaking, shifting or posing a danger to open. Otherwise, it goes unnoticed. But if it doesn’t, the issue is usually not the concept of the manhole but rather the supplier who constructed it.
This is a guide to assessing a precast concrete manhole supplier to ensure that your manhole will maintain its peace underground instead of creating issues during the first inspection.
Choosing a Supplier Depends on More Than Diameter and Depth
The buyers generally begin by comparing their internal diameter, section height and price per unit. That’s a good starting point, but it leaves out the reinforcement, sealing and finishing of the concrete to resist groundwater intrusion, soil loading, corrosive environments within a live drainage or sewer line, etc., the job of the supplier.
Manholes are exposed to a more severe environment than most underground structures. They must withstand external soil and groundwater pressure, react to the corrosive gases that can be generated by wastewater, and allow frequent access by workers for inspection and maintenance purposes. A supplier that views manholes as more than a concrete pipe with a cover is already thinking about all of this when designing. If you do not, you will get a delivery-day perfect looking unit and within a few years it will be leaking at the joints, begins to corrode internally or settles unevenly in the unit.
Understand Precisely What Kind of Manhole Is Required for Your Project
But before you begin comparing suppliers, you should make sure you know the type and quality of manhole that you need for your project, because not all precast suppliers offer the entire spectrum.
Sewer Manholes
Sewer manholes can be a source of entry to sanitary sewer lines, and are exposed to wastewater and the hydrogen sulfide gas that can be produced by the wastewater, which is very corrosive to standard concrete over time. Such linings or special mix designs have to be used for such environments because regular precast concrete deteriorates considerably when exposed to sewer gas.
Stormwater and Drainage Manholes
Sewer manholes are exposed to more chemicals than stormwater manholes and are typically more frequently accessed by workers.Sewer manholes are more often accessed and exposed to more chemicals than are stormwater manholes, but they are also subjected to higher surges of flow and sedimentation. These focus on base and channel design appropriate for smooth flow-through, as inadequate internal channeling can result in maintenance and sediment problems.
Utility Access Manholes
Cable, telecom or other service connections are brought into the manhole, but it does not contain a flow. Unlike drainage applications they focus more on the flow and corrosion aspects of cable routing/duct access within the pipe, rather than on clearance.
The Number of Junction and Inspection Chambers Was Reduced to Just One
Junction and inspection chambers are smaller and more numerous than main access manholes, and are located at pipe intersections or at intervals between pipe intersections. These rely on an accurate base channel formation to control multiple pipe connections without turbulence or blockage points.
Once you’ve identified the type of precast (and the type of exposure condition) your project requires, you can then narrow the list of potential suppliers to those that can supply you only the category for which they are experts, rather than just having it as one of many different types of products they offer.
Inquire About Their Mix Design and Corrosion Protection
A supplier who is serious about manhole manufacturing knows how to choose a mix design and when required a protective lining that will suit the environment which a manhole will be exposed to, especially when used for sewer applications, where hydrogen sulfide corrosion is a real long-term risk. This typically requires a minimum of a low permeability, sulfate-resistant cement-based mix, and an internal protective lining for more aggressive sewer conditions.
If you ask directly, the mix design and permeability class will be known, and if there is a protective lining, its availability and suitability for your application will be known, and what reinforcement cover protects the structure against long-term corrosion exposure. If a supplier responds with specifics which fit your exact application, they are speaking your language. Anyone who provides the same blend of ingredients, whether it’s for a sewer or a stormwater system needs to be dug up.
Discuss How They Seal Together and How They Keep Water Out
Infiltration/exfiltration issues usually occur at the joints between precast sections of a manhole. Discuss the type of jointing system between sections, gasketed or sealant, and test to ensure watertightness before or after assembly.
Some suppliers use rubber gasket joints as standard and provide reliable, tested watertightness, less dependent on the installation quality. Others use mortar joints that are very reliant on the application on site. Both can be satisfactory, but you need to know what you are getting and to what standard the system is designed for water tightness as infiltration and exfiltration are both problems.
A Look at Their Manufacturing Quality Control
On paper, two manholes may be identical but will perform differently in actual use due to the manner in which the manhole is cast. Inquire about section casting and the curing process and ask about any inspections that take place before the unit is dispatched, such as dimensional inspection of step irons, cover seat and base channel.
People don’t realize how critical base channel accuracy is. The turbulence, sedimentation and long-term maintenance problems caused by a poorly formed invert channel extend the life of a well-constructed manhole.Even a well constructed manhole can have a poorly formed invert channel, which can cause turbulence, sedimentation and maintenance issues throughout its useful life. Inquire in particular about the way the supplier oversees the formation of base and channel, because this is a common point of difference among manufacturers.
Request a Site-Specific Configuration, NOT a Standard Catalogue Unit
A competent supplier will design the manhole to your specific pipe sizes, invert levels and pipe angles, not assume you have to fit a manhole that is a standard size. This should incorporate base sections that are cast in the proper pipe entry position and level for your specific connections, the correct amount and arrangement of sections to accomplish the desired depth and placement of steps/iron to provide safe access at that depth.
Suppliers who are able to configure this type of product will detect site-specific issues before the unit is cast, such as mismatched invert levels, awkward connection angles and an inadequate depth allowance, rather than after delivery when the unit has not been properly configured to match the existing pipework in the ground. When a supplier provides only standard configurations, you may find yourself paying extra for adjustments to your site to accommodate these needs.
Discuss Installation and Bedding Requirements Precast Concrete Manhole Supplier
The depth of installation and ground conditions, as well as the use, will affect installation needs. A shallow utility or inspection chamber may only require a compacted granular bed, while deeper sewer or stormwater manholes may require installation of a suitable concrete base slab, compaction of the backfill, and consideration of groundwater conditions.
Inquire from your supplier about their recommended bedding and backfill, installation instructions, and if they have previously installed at a similar depth and ground conditions. If you discover mid-installation that you need to install a manhole with a greater depth for groundwater control that wasn’t planned, it’s a costly addition to your site.
Look at Their References and Track Record Precast Concrete Manhole Supplier
Mix specifications and jointing details will show you what they can deliver by a supplier. When you install structures, you’ll know what they’ve actually delivered. Request details of manholes provided for identical uses and exposures and what the results have been after years of service for watertightness, corrosion conditions or structural integrity.
A manhole, which has been left in the conditions it is supposed to operate, and after years is “cleanly” doing exactly what it was designed to, is a good indicator of the quality of the suppliers as much as anything else.
It Is Important to Be Aware of the Following Red Flags
The few warning signals that occur in less reliable suppliers of manholes are:
- No clear response when asked about corrosion protection for sewer use.
- Vague or generic answers for jointing and watertightness.
- Hesitation to tailor units to your connection and level requirements.
- A price that is significantly lower than other suppliers without an explanation of the difference.
While no single factor is enough to completely eliminate a supplier, 2 or 3 factors together should cause you to pause and inquire further before ordering.
Integrating and Applying the Four Strands Precast Concrete Manhole Supplier
The right precast concrete manhole supplier is the one who has a good understanding of the exposure and connection requirements of your project, and confirms that the supplier has designed for those conditions (with a suitable mix design and corrosion protection, reliable jointing, careful manufacturing and the actual site used as the basis for the precast manhole not just a standard catalogue unit).
If you get it right, your manhole is doing what it was designed to do—it sits quietly underground, bringing safe access and dependable service to the public for decades and not a second thought is needed.