8.1 Manhole, Tap & Lateral Rehabilitation and Exfiltration Control
Key Takeaways
- Manhole rehabilitation is specified by component: frame and cover, chimney and adjusting rings, cone, barrel, bench, and invert each have a distinct repair method.
- The chimney is the single largest inflow source in most manholes because adjusting rings shift under traffic load and open a direct path to the road base.
- A top hat is a T-shaped cured-in-place liner that seals the lateral connection and extends a short distance up the lateral.
- Exfiltration is sewage leaving the pipe into the surrounding soil; unlike infiltration it does not increase treatment flow, so it is invisible in plant records and must be found by testing.
8.1 Manhole, Tap & Lateral Rehabilitation and Exfiltration Control
Exam Focus: The Need-to-Know Criteria specify the rehabilitation task by component — manholes (e.g., collar, risers, cone, barrel, invert), taps (e.g., top hat, grouting, protruding laterals), lift station (e.g., wet wells, fittings, and piping) — and add eliminate infiltration, inflow, and exfiltration. Know which repair belongs to which component.
Manhole Anatomy and Component Repair
Repair specifications are written component by component, from the street surface down.
| Component | What it is | Common defect | Repair |
|---|---|---|---|
| Frame and cover | Cast iron ring and lid at grade | Loose, rocking, pick holes admitting inflow | Reset in non-shrink grout; solid or gasketed cover; internal insert dish |
| Collar | Concrete ring cast around the frame at the pavement | Cracked, allowing runoff into the chimney | Remove and recast; flexible sealant at the pavement interface |
| Chimney / adjusting rings | Short grade-adjustment rings between cone and frame | Shifted rings, open joints — the largest single inflow source | Internal or external chimney seal (rubber sleeve); replace rings |
| Cone | Tapered top section, eccentric or concentric | Cracks, corroded surface | Cementitious liner or spray-applied epoxy/polyurethane |
| Barrel / risers | Precast cylindrical sections forming the shaft | Leaking joints, offset sections, corroded brick | Chemical grouting from inside or outside; cured-in-place manhole liner |
| Bench | Sloped shelf around the channel | Eroded, holding solids | Rebuild with corrosion-resistant mortar |
| Invert | The flow channel through the floor | Eroded, mis-shaped, causing turbulence and deposition | Reform the channel to match pipe inverts, smooth and continuous |
Why the Chimney Dominates
The chimney sits directly under the pavement, where traffic load, freeze-thaw, and road settlement work the adjusting rings loose. That opens a path from the permeable road base — which drains a large area of pavement — straight into the manhole. Repairing the chimney is usually the highest-return inflow reduction available per dollar. A chimney seal is a flexible rubber sleeve bonded to the frame and to the cone with expansion bands, designed to accommodate continued movement rather than resist it. A rigid mortar patch simply cracks again.
Coatings and Liners
- Cementitious liners (spray-applied or hand-troweled) restore structure and profile, and calcium aluminate formulations resist biogenic acid far better than Portland cement.
- Spray-applied epoxy or polyurethane provides a chemically resistant, monolithic barrier, and is the standard treatment where H₂S has already attacked the concrete.
- Cured-in-place manhole liners insert a resin-saturated fabric tube and cure it against the wall, sealing the entire shaft.
Surface preparation controls the outcome for all of them. Active leaks must be stopped with hydraulic cement plugs and the surface abrasively cleaned to sound substrate before coating; coating over a wet, corroded, soft surface guarantees delamination.
Tap and Lateral Repair
A tap is the connection where a service lateral joins the main. It is a chronically leaky and chronically obstructive detail.
Protruding (intruding, hammer) taps. A lateral installed by breaking a hole in the main and shoving the pipe through leaves an end projecting into the flow. That projection snags rags and grease and starts blockages, and the broken annulus around it leaks. The correct repair is to mill the protrusion flush with the pipe wall using a robotic cutter, then seal the annulus. Blindly jetting a line with a protruding tap tends to shear the lateral off.
Leaking taps. Chemical grouting through a test-and-seal packer injects grout into the soil around the connection, sealing the annular leak path from outside the pipe wall.
Top hats. A top hat (or T-liner) is a T-shaped cured-in-place liner with a brim that seats against the main's inner wall and a sleeve that extends a short distance up the lateral. It seals the connection permanently and is the standard finish after a main is lined, since the robotic cutter that reinstates the lateral leaves a raw cut edge.
Lateral rehabilitation. Full-length lateral CIPP, pipe bursting of laterals, and cleanout installation address the reality that in many systems the majority of I/I originates on the private side.
Lift Station Structure Rehabilitation
Wet wells suffer the most aggressive H₂S exposure in the system because of turbulence and detention. Rehabilitation includes surface preparation and epoxy or polyurea coating of the concrete, replacement of corroded fittings and piping — especially ductile iron discharge piping and guide rails — replacement of anchor bolts and hardware with stainless steel, and restoration of the bench and fillets that keep solids moving toward the suction.
Exfiltration: The Defect Nobody Sees
Infiltration is groundwater entering the pipe. Inflow is surface water entering the pipe. Exfiltration is sewage leaving the pipe into the surrounding soil — and it is fundamentally different in one respect that makes it dangerous.
Infiltration and inflow announce themselves. They raise flow at the lift station and at the treatment plant, they show up in flow monitoring, and they cause surcharging and SSOs. Exfiltration produces none of those signals. Flow at the plant goes down, not up, so nothing in the operating record flags it.
Exfiltration occurs through the same defects as infiltration — open joints, cracks, failed tap connections — but under the opposite gradient: when the pipe's internal water level is above the surrounding groundwater table. A given defect can therefore infiltrate in a wet season and exfiltrate in a dry one.
Consequences: contamination of groundwater and nearby wells, soil saturation, loss of bedding support leading to voids and eventual pipe or pavement collapse, and regulatory liability under the Clean Water Act and state groundwater rules.
Detection requires deliberate testing, not routine monitoring:
- Low-pressure air testing of an isolated segment (see Section 8.5) — the standard acceptance test, equally valid on existing pipe.
- Hydrostatic exfiltration testing — plug the segment, fill to a specified head above the crown, and measure the loss rate over time.
- CCTV inspection during low-groundwater conditions, looking for defects that are not actively leaking inward.
- Dye testing into an isolated reach with monitoring of nearby wells, seeps, and drains.
A manhole inspection identifies a section of lateral pipe projecting approximately 4 inches into the main at the tap connection, catching rags and initiating repeat blockages. What is the correct repair?
Why is exfiltration substantially harder to detect than infiltration in an operating collection system?
A manhole inspection finds the largest single source of inflow to be the adjusting rings between the cone and the frame, which have shifted under traffic loading. Which repair is designed for this condition?