15.1 Sewer Rehabilitation, Trenchless Renewal and New-Pipe Acceptance Testing

From Inspection to Decision

CCTV inspection coded to NASSCO PACP gives each pipe segment structural and operation-and-maintenance grades from 1 (minor) to 5 (failure imminent or occurring). Utilities combine those grades with the consequence of failure to rank work. Consequence factors include pipe depth and size, traffic, nearby waterways and critical customers. Typical outcomes:

Trenchless Rehabilitation Methods

Cured-in-place pipe (CIPP). A felt or fiberglass tube saturated with resin (polyester, vinyl ester or epoxy) is inverted into the host pipe with water or air pressure, or pulled in. It is then cured with hot water, steam or UV light, creating a tight-fitting new pipe inside the old one.

• Cleaning and CCTV beforehand are critical; roots and protruding taps must be removed.
• After curing, a robotic cutter reinstates service laterals from inside the main.
• Some resins release styrene during curing. Manage odors and vapors in buildings, and watch confined-space atmospheres.

Pipe bursting. A bursting head is pulled or pushed through the old pipe, fracturing it outward, while a new pipe (usually fusion-welded HDPE) follows behind. It can increase diameter and is good for brittle clay or cast iron. Watch for nearby utilities and for heave or settlement near shallow lines.

Sliplining. A smaller new pipe is inserted into the host and the annular space is grouted. It is simple, but it loses some diameter.

Fold-and-form and close-fit liners. These are thermoplastic pipes folded for insertion, then expanded with heat and pressure.

Spot repairs and grouting. Short sectional liners fix single defects. Chemical grout (acrylamide, acrylate or urethane) is injected at leaking joints using a packer and sealed after an air test of the joint. Grouting controls infiltration but does not add structural strength.

Lateral rehabilitation. Lateral-main connection liners (top hats) and lateral lining seal one of the largest infiltration sources. Private laterals often need a cost-sharing or ordinance-based program.

Manhole Rehabilitation

• Chimney seals (internal or external rubber sleeves) stop infiltration between the frame and cone.
• Inflow dishes under lids and raised frames keep street runoff out.
• Cementitious or epoxy liners restore walls attacked by hydrogen sulfide corrosion.
• Grout leaking pipe penetrations and rebuild benches and inverts.

Cross-Bore Safety

A cross bore happens when a gas line or other utility installed by horizontal directional drilling passes through a sewer main or lateral. A root cutter or rodder can rupture the gas line, causing explosions inside buildings. Before cutting a blockage, especially in a lateral, check CCTV records and known cross-bore locations. If a sewer backup is near a newer gas service installed by boring, stop and call the gas utility first.

Acceptance Testing of New Sewers and Manholes

Safety during air testing: never stand in front of plugs while pressurized, and use pressure relief devices. A plug blown out at a few psi on a large pipe carries enormous force.

Measuring Results

I/I programs should measure flow before and after rehabilitation, using temporary flow meters and rainfall data, to show results. Rehabilitating public mains alone often removes less infiltration than expected, because water migrates to unrehabilitated laterals. That is why whole-basin approaches including laterals often perform best.

Worked Example: Adjusting an Air Test for Groundwater

Groundwater stands 4.6 feet above the top of a new sewer.

• Backpressure = 4.6 ft × 0.433 psi/ft ≈ 2.0 psi
• Instead of timing the drop from 3.5 to 2.5 psig, time the drop from 5.5 to 4.5 psig.

Without the adjustment, groundwater pushing in through a leaking joint would offset the air leaking out and hide the defect. Check the project specification for any maximum test pressure, and plug and brace laterals before the test. Plugs under pressure can blow out with enough force to injure workers, so nobody enters a manhole while a line is pressurized.