The "Top 12 Inches" Problem
If you inspect your system during a heavy rainstorm, you will likely notice a pattern: the deepest part of the manhole is often dry, but water is pouring in right at the top, just below the lid.
This is the Chimney Section (or Grade Ring area)—the adjustment space between the heavy cast iron frame and the structural concrete cone.
Why does this area fail first?
- Traffic Loading: Every time a truck hits the manhole lid, that impact shockwave travels down into the chimney rings. Rigid mortar cannot handle this vibration; it cracks and crumbles.
- Freeze-Thaw: Located just below the pavement, this zone freezes and thaws with the weather, causing the ground to heave and separate the rings.
- Road Salt: In cold climates, chlorides seep down and eat the mortar.
The Result: Studies show that up to 40-50% of all manhole infiltration enters through this small vertical section.
The Fix: Mechanical Chimney Seals
Stop using mortar to fix a movement problem.
Slapping more hydraulic cement on a cracking chimney is a temporary patch. It will crack again as soon as the ground moves.
The permanent solution is a Mechanical Internal Chimney Seal.
This is typically a flexible, high-grade rubber sleeve (EPDM or similar) that spans the gap between the iron frame and the concrete cone. It is locked in place not by glue, but by mechanical expansion bands (stainless steel).
How It Works:
- The Bridge: The rubber sleeve bridges the joints of the grade rings.
- The Lock: A stainless steel band is expanded (using a hydraulic tool or torque wrench) against the iron frame.
- The Anchor: A second band is expanded against the concrete cone below.
- The Flex: When traffic hits the lid or the ground heaves, the rubber stretches and compresses. The seal remains watertight because it moves with the structure.
Installation: The "Lunch Break" Repair
One of the biggest advantages of mechanical seals is speed. A two-man crew can often install a seal in 20 to 45 minutes without entering the manhole (using long-handled tools) or with shallow entry.
The Process:
- Clean: Wire brush the frame and cone to remove loose debris.
- Measure: Verify the vertical height needed to span the damaged rings.
- Position: Drop the sleeve in place.
- Expand: Torque the bands to the manufacturer's spec (often 4,000 lbs of force).
- Done: No cure time. The road can be opened immediately.
| Feature | Mortar/Parging | Mechanical Seal |
|---|---|---|
| Material | Cementitious (Rigid) | EPDM Rubber (Flexible) |
| Vibration Proof | ❌ No (Cracks easily) | ✅ Yes (Absorbs shock) |
| Lifespan | 1–5 Years | 25–50 Years |
| Installation Time | 30 mins + cure | 30 mins (no cure) |
| Cost | $ Low | $$ Medium |
| Skill Level | Low (Masonry) | Medium (Tooling) |
The Verdict
If your manholes are located in roadways, parking lots, or sidewalks subject to movement, rigid mortar is a waste of budget. It is a band-aid.
For a "One and Done" solution that eliminates I&I from the grade rings, specify Mechanical Internal Frame Seals. They are the only technology designed to handle the dynamic H-20 traffic loads that punish your infrastructure daily.
Looking for Suppliers?
Find manufacturers of mechanical seals and certified installers in our directory.