Structural Evaluation of No-Dig Manhole Rehabilitation Technologies

Manholes are called windows to the sewer system as they are the most visible point in identifying the condition of underground infrastructure. In the United States alone, the number of manholes is estimated to be from 12 to 20 million. It is estimated that 20 percent of the manholes are at least 50 years old and another 25 percent are 30 to 50 years old. Additionally, more than 3 million manholes are believed to have serious structural decays and are in need of immediate rehabilitation or replacement.

Considering the multi-billion dollar market of manhole rehabilitation, it is not surprising that there are already numerous no-dig materials and methods available for manhole rehabilitation. This wide variety in manhole rehabilitation methods has its pros and cons. While utilities might benefit from many options considering the fierce competition among many system providers, determining the most feasible and economical rehabilitation material and method often imposes a challenge for design engineers, utility owners, municipalities and decision-makers.

To date, there has been a number of research studies on pipeline rehabilitation, focusing on the materials and methods used for pipeline rehabilitation from structural, hydraulic, and economic perspectives. Thereby, there is wealth of experience and tools for engineers to evaluate their options while selecting the material and technology to rehabilitate sanitary sewer pipes. Manholes, on the other hand, although a vital part of sanitary sewer systems and the main source of inflow (rainwater entry into wastewater collection systems) are often overlooked and there are very limited studies.

WERF Project

Last July, the Water Environment Research Foundation (WERF) entered into a contract with Benton and Associates of Jacksonville, Ill., and Center for Underground Infrastructure Research and Education (CUIRE) at the University of Texas at Arlington to evaluate structural capabilities of no-dig manhole rehabilitation methods and materials. Manhole rehabilitation materials include cement mortar (with added chemicals to prevent hydrogen sulfide induced corrosion), polymeric (polyurethanes, epoxies, polyureas and their mixtures), fiberglass, epoxy coated cement, coal tar epoxy, polymeric sheets (i.e., high density polyethylene), steel inserts, rubber inserts (used for chimney rehabilitation), elastic polyurethane (for chimney restoration), cured-in-place composites (similar to CIPP used for pipeline rehabilitation). Each of these materials and methods has its pros and cons, with a number of unknowns in terms of the structural support they can provide to a deteriorated manhole as a rehabilitated manhole is a system with the following parameters that play a role on the overall durability and life cycle:

• Residual strength of the manhole.
• Mechanical properties of the lining material.
• Adhesion between the lining and substrate (manhole component).
• Magnitude and type of loads exerted on the manhole.
• Durability of manhole material against environmental effects (particularly to hydrogen sulfide induced corrosion.
• Surrounding soil properties and groundwater.

Industry Participation

A half-day workshop was held at the WEFTEC12 in New Orleans, with the objective of obtaining as much contribution as possible from the participating industry professionals from wastewater utilities and rehabilitation material manufacturers/vendors by implementing small and large group discussions. The workshop hosted 30 attendees from manhole rehabilitation material manufacturers, wastewater utility representatives from larger urbanized cities, key project team members, and U.S. EPA. Although, opinions of the participants varied with respect to the most critical knowledge gaps in manhole rehabilitation, the workshop met its objectives and significantly contributed to the project scope and methods to be implemented..

Research Deliverables

The ultimate deliverable of the WERF research is a manhole rehabilitation manual that will help utilities and consulting engineers in implementing manhole rehabilitation programs. The manual will be educational in terms of giving information on the available products, their properties (particularly structural capabilities) and practical on how to approach manhole rehabilitation and how to make decisions on whether to leave as is, rehabilitate, or replace a manhole.

A Research Profile was created and published on the WERF website (Strategic Asset Management section). Co-principal investigators (Co-PIs), Dr. Firat Sever and myself are responsible for preparation and dissemination of all the reports and other communication pertaining to the proposed project. The Co-PIs have also been providing quarterly updates to the WERF Project Manager (i.e., completion of an interim report). A technical paper pertaining to this project is scheduled to be presented at the ASCE Pipeline Conference 2013 in Fort Worth, Texas. Two papers will be published in the WERF Journals Progress and Laterals. Findings of the research will also be presented in a WERF Webinar. Additionally, it is anticipated that this project will constitute parts of several Master’s theses and another manuscript will be submitted for publication in a peer reviewed journal

Dr. Mohammad Najafi, P.E., Ph.D., F. ASCE is CUIRE director and a member of the Department of Civil Engineering, The University of Texas at Arlington.