Manhole frames and covers must be replaced in areas where they are damaged, misaligned with street grade, or allow storm water to infiltrate the sewer system. The replacement process involves excavating a small pit around the manhole, which typically takes a couple of days to complete, including excavation, installation, backfilling, and resurfacing.
Internal Manhole Repairs
– Grouting pipe
Manhole grouting is an effective rehabilitation method designed to prevent runoff and groundwater from entering structurally sound manholes. Different grout formulations are selected based on local soil conditions and the materials of the manhole.
Preparation for Grouting
Before applying grout, the manhole is thoroughly cleaned to remove debris, solids, roots, and other obstructions. Once cleaned, grout is injected from inside the manhole using various techniques. This grout interacts with the surrounding soil and manhole material, forming a water-tight seal that fills voids.
Efficiency of Grouting
The grouting process can be completed within a few hours, requiring no disturbance to the ground or paved surfaces and minimal interruption of service.
– Manhole Lining
Lining is another rehabilitation technique used for manholes needing structural reinforcement or protection from groundwater and corrosive gases. The process begins with cleaning the manhole and removing loose materials. A custom-sized liner is then fitted, expanded, and pressed against the interior wall until it cures, creating a new hard surface. This lining process usually takes two to four hours and leaves no visible disturbance on the surface.
Manhole Replacement and Abandonment
– Replacement Process
Manholes are only replaced when all other rehabilitation options have been exhausted. The replacement involves completely removing the old manhole and installing a new one, which may require constructing new manholes for better sewer line maintenance.
Excavation Requirements
Excavation for a new manhole typically measures 10 feet by 10 feet and is deep enough to reach the existing sewer main. The entire process can take up to 10 days, including restoring the surface to its original condition. A sewer bypass is necessary during the replacement to ensure continued service.
– Abandonment of Manholes
When a manhole is no longer needed, it can be abandoned in place. This involves removing all components within a foot of the ground surface and backfilling the remaining parts with concrete or flowable fill. The abandonment process requires a shallow excavation and can be completed in a few days, with resurfacing occurring once the concrete has cured.
Grouting Techniques and Applications
– Injection Grouting
Injection grouting effectively eliminates inflow and infiltration (I&I) issues, restoring sewer capacity. This method is used for sealing mainlines, laterals, and manholes, and is often combined with other rehabilitation techniques for a comprehensive approach.
Safety Considerations
Safety is paramount in grouting projects. The Unified Safe Operating Practice Program (USOPP) Grout Safety Certification ensures the protection of all personnel involved, from field workers to engineers.
– Historical Context
Grouting began in the U.S. in the early 1960s and has since become a global standard for controlling infiltration in sewer systems.
Chemical Grouting Systems
Various chemical grouting systems are available to seal leaks and prevent infiltration and exfiltration in sewer systems. Most leaks in structurally sound sewers occur at pipe joints, manholes, service connections, and the initial segments of service laterals.
– Mechanism of Action
Chemical grouts do not merely fill cracks; they are forced through openings and bond with surrounding soil, forming a waterproof collar around leaking pipes and manholes. This gel collar adheres to the pipe’s outer surface and remains effective unless removed through excavation. Although the long-term durability of grouts is still being studied, samples examined after 30 years have shown them to be in excellent condition.
– Environmental Considerations
Grouts can dry out if soil humidity decreases for extended periods. However, when humidity returns, the grout absorbs moisture and reactivates. The flow of wastewater and surrounding soil humidity typically prevent significant grout shrinkage, though additives can be used to mitigate shrinkage in dry conditions.
Application of Grouting
– Grouting in Man-entry Pipelines
In man-entry pipelines, chemical grout is usually applied through manholes using handheld equipment. For smaller diameter pipes (24 inches or less), remote-controlled equipment is required. A test/seal packer is employed to pressure test joints and seal any failures.
– Equipment Overview
Basic equipment for grouting includes:
– Closed-circuit TV System: To locate the packer over the pipe joint.
– Test/Seal Packer: For isolating sections of pipe.
– Control Panel: To manage inflation, testing, and chemical injection.
– Hose and Reel System: For delivering air and chemicals.
– Chemical Tanks and Pumps: For grout preparation and application.
– Remote-Controlled Winches: For maneuvering packers.
– Types of Grout
There are four primary types of chemical grouts commonly used:
1.Acrylamide: A monomer that becomes stable and non-toxic after curing.
2.Gel Grouts: These have a low viscosity, allowing them to penetrate cracks quickly and form a gel/soil matrix that is impermeable to water.
3.Urethane Foam Grouts: Useful in sealing treatment plant structures and manholes, creating flexible seals in cracks and joints.
4.Other Prepolymers: Similar to acrylamide, these grouts also become inert and stable after curing.
Testing and Quality Control
– Barrel Test
To ensure the integrity of the grouting process, operators conduct a barrel test at the start of each work shift. This test simulates the conditions of a sewer pipe and verifies that the equipment can maintain adequate pressure.
– Pipe Wall Control Test
A structurally sound section of pipe is tested to establish the maximum performance of a good joint. If the packer cannot maintain the void pressure, it indicates that grout sealing will not be effective.
– Joint Testing and Sealing
After testing, the packer is positioned over the first joint or lateral connection. If a joint is leaking, sealing can begin immediately without prior testing. The packer must be centered correctly, and the sleeves inflated to create a seal.
– Monitoring Pressure Changes
During testing, the void pressure is monitored to determine if the joint is leaking. If the required test pressure cannot be achieved, the joint has failed the test and must be sealed.
Joint Sealing Process
If a joint passes the pressure test, it is recorded, and the packer is moved to the next joint. If it fails, the sealing process begins with pumping grout into the packer void. The two components of the grout must be mixed correctly to ensure proper curing.
– Verification of Sealing
Once a leak has been sealed, the packer is deflated, and the void pressure should return to zero. If it does not, further adjustments are needed, and the leak should be retested until it meets the specified criteria.
– Residual Material Management
Residual sealing materials that extend into the pipe can restrict flow and should be removed. Sealed joints must remain flush with the existing pipe surface to maintain optimal flow.
Conclusion
Manhole grouting and rehabilitation techniques are crucial for maintaining sewer system integrity and preventing water infiltration. By employing effective grouting methods and ensuring rigorous testing and quality control, utilities can enhance the performance and longevity of their sewer infrastructure.
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Reference
1.Manhole Frame and Cover Replacement (small excavation)
https://www.wsscwater.com/manhole
2.Grouting
https://www.nassco.org/grouting/
