manhole adjustments
Adjusting a manhole refers to the task of raising or lowering a casting or precast top from its existing elevation to line up in harmony with the adjacent grade, pavement, curb, etc. Other residual tasks might include grouting pipe penetrations inside the manhole, mortaring/grouting the manholes ‘chimney’ between the casting and structure top, and pouring cast-in-place concrete inverts. These tasks are often looked at separately by a contractor particularly because they are typically done after final surrounding grade elevations are achieved after the manhole structure below was placed. While the final grade may be able to be referenced during the initial manhole and casting installation, it is almost impossible to gauge exactly where the final surrounding elevations or grades will end up before they are actually finished.
Typically manholes are designed for anywhere from 1-6″ of precast grade rings or more which allow for the vertical buffer zone needed to assure the manhole casting can be placed right in line with the proposed grade surrounding. Initial rough placement is often set slightly lower than planned elevation because it is often easier to slightly raise a manhole casting (via brick/mortat, shimming, grade rings) than to have to lower it. Butyl sealant is often required between each grade ring as an additional measure to resist infiltration/exfiltration.
The extent and scope of the manhole adjustment depends solely on the scope of the work surrounding it. A common industry rule of thumb is that a manhole can only be adjusted with precast grade rings, brick, mortar, etc up to 12″ before the scope of work becomes more of a structure ‘reconstruction’ than an ‘adjustment’. In this case a precast riser or top piece may be more efficient than building up with brick/block. The idea behind this requirement is that anything more than 12″ of grade rings or cobbled brick and mortar is ultimately structurally unstable and will be more apt to infiltration and ex-filtration of water than a monolithic piece of precast concrete.
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After the manhole casting is essentially ‘shimmed’ into its final vertical and horizontal location to match the final grades (often by way of taught strings), it is often required that mortar be mixed and plastered on the inside of the structure from the bottom of the casting to the top of the manhole. This limits infiltration and exfiltration of drainage water and ground water and inhibits corrosion and damage over time. It is also common for the inside and/or outside of the casting flange/manhole joint be caulked to seal.
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Lastly, depending on whether the manhole is within an area with vehicle traffic, a ‘concrete collar’ may be required. This is typically the last task done when installing a manhole and is often lumped into the overall task of adjusting a manhole. A concrete collar is a cast in place concrete section formed and poured around the casting giving additional strength and reinforcement to the manhole from vehicle traffic. The concrete collar also helps resist the entire assembly from shifting or moving with vehicle traffic or freeze thaw conditions.
Manhole concrete collars are most commonly round, anywhere from 12-24″ or more inches wide, and extend several inches below the bottom of the casting, often several inches below the top of the manhole structure itself.
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Concrete collars are often poured with fast setting concrete so they may be opened to traffic quicker. Standard concrete will often reach sufficient strength for traffic in 7-14 days, ‘Moderate Set’ mix in 3-7 days, and ‘Fast Set’ 1-3 Days.
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There are two primary approaches to removing the round section of asphalt to make way for the concrete collar, chipping the material out by hand with a compressor and jackhammer, or utilizing a large coring attachment on a backhoe or skidsteer. Both methods are explained below.
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Jackhammer Removal for Concrete Collar
A common way to remove pavement for concrete collar placement is to employ an electric jackhammer powered by an air compressor. First the collar perimeter is laid out, often with chalk or paint by way of a string pulled from the center of the casting. Then the jackhammer spade is engaged along the perimeter line removing pavement. Then any aggregate base below is excavated by hand or with a backhoe bucket to make way for the specified collar thickness. The casting is adjusted to it’s proper elevation and then the collar is poured, finished, and protected for the curing period.
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Coring Removal for Concrete Collar
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A newer system/technology has been developed where a specialized large coring bit assembly is utilized on a backhoe, excavator, or skidsteer to core the pavement around manholes in preparation for adjustment and a concrete collar.
Several proprietary products exist but the premise is similar for each. The coring assembly is equipped with an outer shield to block flying dust and debris during coring. The machine cores down to the bottom of the pavement leaving a clean, smooth sawcut edge. The remaining piece or ‘cookie’ is removed from the road and from the drilling assembly.
Remaining aggregate base is excavated and the casting is adjusted for the concrete collar pour. Some proprietary coring systems also call for a piece of pipe (often 30″ +/- diameter) to be cut to size and placed between the manhole structure and casting frame bottom flange.
This piece of pipe is used as support and receives sealant at the structure/pipe and pipe/casting joint.
The following are the most common situations in which a manhole may need adjusted:
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New Pavement Surrounding Newly Placed Manhole
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This manhole adjustment scenario is similar to the above once the adjustment phase starts, but leading up to this the scope of work, things are a bit different. On a new road or reconstruction project when a new manhole is placed followed by new pavement, the frame and casting is often not set on the manhole until after the pavement goes down. Instead, workers typically place a metal plate on top of the manhole top’s opening to keep debris from entering the manhole, and to provide a detectable material for later locating. Then the aggregate base and pavement crew pass right over top of the manhole. When this approach is taken, it’s important to mark the locations of the manholes along the side of the road, allowing for them to be located easier during adjusting. The adjustment crew will then return, find the manholes, remove the pavement around them, place the castings, and pour the concrete collars.
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New Lawn Area Surrounding Manhole
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If a manhole is placed in a lawn or grassy area, it will typically not require a concrete collar. The only adjustment work typically necessary is placement of precast grade rings if needed and grouting/mortaring interior structure joints. This work is commonly done after final dirt grade is achieved surrounding the manhole.
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New Pavement Surrounding Existing Manhole
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This is a common scenario for asphalt overlaid or resurfaced roads. It is quite common for asphalt pavers to pass asphalt surface course material right over manhole castings. Typically manhole castings are even with or 1/4″below the adjacent pavement grade, which often leaves the castings buried or skimmed over slightly by the paver. Crews will commonly mark these manhole locations out beforehand so they can be found later, or locate them later with a metal detector. When the adjustment crew returns to perform the adjustment, a concrete collar is typically required to protect the manhole and casting from future movement, reduce pavement deterioration that would otherwise occur at the casting/asphalt joint, and provide added strength to the overall structure assembly as a whole.