When paving crews mobilize for an overlay project, attention focuses on paving operations, mix temperatures, compaction, and joint quality. Yet one of the most overlooked components sits right beneath the surface. Manholes, catch basins, and utility access structures present a recurring challenge for asphalt contractors. If these structures are not raised to grade before overlay, the result is uneven pavement that deteriorates faster and forces expensive remedial work. Asphalt Pavement Engineering Mix Design Construction Methods Rehabilitation must account for every detail of the finished surface, and manhole adjustments are a critical detail separating professional results from costly callbacks.
Adjustable manhole risers have emerged as the most efficient and cost-effective solution for raising utility structures to grade after repaving. A 2004 Kansas City study demonstrated the impact of proper elevation. Among manholes raised to grade before overlay, only 11 percent showed pavement distress. Among those left uneven, more than 35 percent exhibited bad pavement conditions. Smoother pavement lasts longer.
The Problem With Ignoring Manholes During Overlay Projects
Many paving contractors choose to pave directly over manholes during an overlay rather than taking the time to raise them to grade. This decision is often driven by schedule pressure or a desire to maintain paving momentum. However, the long-term consequences of this approach far outweigh the temporary convenience.
Pavement Damage Around Uneven Structures
When a manhole sits below the finished pavement surface, several mechanisms combine to accelerate deterioration:
- Traffic impact loads concentrate at the edge of the depression, creating a repeated pounding action that cracks the asphalt around the manhole frame
- Water pools in the low spot around the sunken manhole, infiltrating the pavement structure and weakening the base course
- Freeze-thaw cycles in colder climates exploit these cracks and water pockets, rapidly expanding damage each winter
- Snowplow blades catch on protruding or depressed manhole frames, causing sudden impact loads and potential equipment damage
- Vehicle tires striking uneven manhole covers create noise complaints and ride quality concerns from residents and business owners
The Hidden Cost of Deferred Manhole Work
Contractors who pave over manholes with the intention of returning later to raise them face substantially higher costs. Frank Turner of FJ Turner Company states unequivocally that returning to jackhammer through fresh asphalt is four to five times more expensive than using a riser during the initial paving operation. The delayed approach requires mobilizing a separate crew and equipment, cutting through the newly placed asphalt, removing debris, and reconstructing the supporting structure around the manhole before backfilling and repaving the patch.
The cost difference is not theoretical. A Washington state wastewater district analyzed three years of maintenance records comparing their traditional method to an adjustable riser approach. Under the old method, crews covered manholes during paving, then returned to jackhammer, excavate, and raise the frame using precast grade rings. The total materials and labor cost was $500 per manhole. After switching to adjustable manhole risers for three years, the same district calculated a cost of just $95 per manhole. That represents an 81 percent reduction in cost while achieving a superior finished product.
Types of Manhole Risers and Their Applications
Manhole risers are available in several designs, each suited to different site conditions, budget considerations, and contractor preferences. Understanding the options allows paving professionals to select the right product for each project.
Solid Drop-In Risers
Solid cast iron and steel drop-in risers rely on gravity and the bond created by the surrounding asphalt to hold the product in place. These traditional risers are heavy and durable, making them suitable for high-traffic roadways where impact resistance is paramount. The installation process involves placing the riser directly into the manhole frame after cleaning the seat, then allowing the asphalt overlay to lock it in position. Steel versions offer a lighter alternative while maintaining similar performance characteristics.
Expandable and Adjustable Risers
Modern expandable or adjustable risers are manufactured from engineered plastics, Styrofoam, or rubber compounds. These products incorporate a mechanical expansion mechanism that locks the riser firmly inside the existing manhole frame without relying on asphalt bonding. Alan Siebenthaler of Hamilton Kent notes that manhole risers help absorb the impact of traffic hitting the top of the manhole frame, and without them manholes end up too low, causing damage to vehicles, cyclists, and snowplows.
Adjustable risers are available in 1/4-inch height increments up to 5 3/4 inches, allowing precise grade matching to the overlay thickness. This is valuable when overlays vary in thickness across a project.
Comparative Analysis of Manhole Riser Types
| Riser Type | Material | Locking Method | Height Range | Best Application |
|---|---|---|---|---|
| Cast iron drop-in | Cast iron | Gravity and asphalt bond | 1/2 to 2 inches | High-traffic roads, heavy truck routes |
| Steel drop-in | Steel | Gravity and asphalt bond | 1/2 to 2 inches | Standard roadways, lighter alternative to cast iron |
| Adjustable plastic | Engineered polymer | Mechanical expansion | 1/4 inch to 5 3/4 inches | Variable overlay thickness, residential streets |
| Adjustable rubber | Elastomeric compound | Compression fit | 1/4 inch to 3 inches | Freeze-thaw zones, flexible pavement systems |
| Adjustable Styrofoam | Expanded polystyrene | Mechanical expansion | 1/2 inch to 4 inches | Light-duty paving, temporary elevations |
Jason Downing of American Highway Products emphasizes that manhole risers eliminate the need to come back after the road has been resurfaced to adjust utilities to the new road surface. When evaluated against the total project timeline, the upfront investment in proper risers pays for itself many times over through reduced labor, eliminated remobilization, and extended pavement life around each structure.
Installation Procedures and Best Practices
Proper installation of adjustable manhole risers is straightforward and requires no specialized training. The procedure can be completed in minutes per manhole once the paving crew understands the basic steps.
Step-by-Step Installation Process
- Remove the manhole lid using appropriate lifting equipment. Manhole covers can weigh between 600 and 800 pounds, requiring mechanical assistance or specialized lifters such as the Allegro Magnetic Lid Lifter, which attaches magnetically and allows one worker to wheel the cover out of the work area safely.
- Clean the seat and sidewalls of the existing manhole frame thoroughly. All debris, loose mortar, asphalt residue, and accumulated sediment must be removed to ensure a clean bearing surface for the riser.
- Place the adjustable riser inside the frame, centered on the cleaned seat. Verify that the riser is properly aligned with the frame geometry before expanding.
- Expand the riser using a screwdriver or the manufacturers supplied adjustment tool. Tighten until the riser contacts the frame walls firmly, typically to approximately 60 pounds of pressure. The mechanical lock ensures the riser remains in position during paving operations.
- Replace the manhole lid on top of the installed riser. Verify that the lid sits flush and level with the surrounding frame and that there is no rocking or movement.
- Proceed with paving operations. The paver and rollers can work directly over the adjusted manhole without concern for damaging the structure or creating an uneven surface.
Safety Considerations During Installation
Working around open manholes presents significant safety hazards that must be managed through proper procedures:
- Always barricade the work area around open manholes to prevent workers or equipment from accidentally stepping into the opening
- Use mechanical lifting aids for heavy manhole covers rather than relying on manual lifting, which causes back injuries across the industry
- Ensure adequate lighting if manhole work extends into low-light conditions or nighttime paving operations
- Inspect the manhole chimney and cone section for structural integrity before beginning work
- Coordinate with utility owners when the manhole contains energized or pressurized infrastructure
Alternative Approaches and Specialized Equipment
While adjustable risers represent the preferred solution for most overlay projects, certain situations require alternative methods. Understanding the full range of available approaches helps contractors make informed decisions for every project condition.
Manhole Saws for Remedial Work
When manholes have already been paved over and must be located and raised after the fact, specialized manhole saws provide an efficient solution. The Coneqtec HS-57 Manhole Saw was designed specifically for cutting out and repairing multiple manholes in minimal time. Originally developed to support preparation for the Olympics in China, where hundreds of manholes needed to be repaired before the games, the HS-57 mounts to a skid steer loader and cuts a clean circle around the buried manhole frame. The Series II HS-57 offers deeper cutting capacity for thicker overlays and features a re-engineered arm that maneuvers more easily around the manhole area.
Excavator-Mounted Repair Systems
For major manhole rehabilitation, systems such as The Six Shooter from Mr. Manhole attach to an excavator and can complete the repair in about one hour. Trip Davis of Mr. Manhole explains that the system removes the manhole frame and lid down to the cone level, cuts and removes the surrounding road material, then rebuilds the structure level to the exact road slope and height. This approach is ideal when the manhole chimney or cone section requires replacement rather than simple elevation.
Cost Comparison of Manhole Adjustment Methods
| Method | Cost Per Manhole | Equipment Required | Pavement Disturbance | Typical Duration |
|---|---|---|---|---|
| Adjustable riser during overlay | $95 | Hand tools, lid lifter | None | 10-15 minutes |
| Cover and return later | $500 | Jackhammer, excavator, compactor | Extensive | 1-2 hours plus remobilization |
| Manhole saw remedial cutout | $200-$350 | Skid steer with manhole saw | Moderate | 30-45 minutes |
| Full frame replacement | $800-$1,200 | Excavator, saw, compaction equipment | Major | 2-4 hours |
Contractors seeking to improve their pavement preservation programs should also explore related best practices in pavement maintenance. Refined Tar Based Pavement Sealers What Pavement Professionals need to understand about material specifications can complement a comprehensive maintenance strategy. Similarly, Maximizing Value At Pavement Maintenance Trade Shows Lessons from industry events can help contractors stay current with evolving techniques. For projects requiring more intensive pavement rehabilitation, Full Depth Reclamation Fdr Reducing Labor Materials and trucking costs offers an alternative approach to restoring failed pavement structures.
Conclusion
Manhole risers represent one of the most cost-effective investments a paving contractor can make in overlay project quality. The evidence from municipal studies, manufacturer data, and contractor experience converges on a clear conclusion: raising manholes to grade before paving produces better pavement performance at lower overall cost than the alternative of paving over and fixing later.
The 81 percent cost reduction documented by the Washington wastewater district tells a compelling story. At $95 per manhole using adjustable risers compared to $500 per manhole using traditional methods, the savings multiply quickly across a municipal overlay project containing dozens of manholes. Properly graded manholes contribute to smoother pavement that lasts longer and satisfies owner expectations.
Paving contractors who incorporate manhole riser installation into their standard overlay procedures gain a competitive advantage. Projects run more efficiently without return visits, customer satisfaction improves when surfaces are uniformly smooth, and extended pavement life means fewer warranty claims. For any contractor serious about delivering quality asphalt pavements, adjustable manhole risers are an essential component of professional paving practice.