Manhole Inspection and Rehabilitation

A manhole, alternatively referred to as a maintenance hole, an access point, a utility hole, an inspection chamber, or an access chamber, is an access point whose functions include facilitating inspections and maintenance, sewage ventilation, and the joining or changing of direction of a connected pipeline system. Development of materials and technology over the last 30 years have greatly reduced the need to replace manholes; however, estimating the age when a manhole becomes more cost-effective to replace than to rehabilitate is a task that must be considered on a local basis. The Manhole Assessment Certification Program provides standardization and consistency to the methods in which manhole conditions are identified, evaluated, and managed, providing direction for users of this book to begin the process of building an effective manhole rehabilitation program.

Public safety in terms of unauthorized or accidental access to manholes begins with the design of the manhole structure and its components, with foresight into the potential of hazards that could be presented through both human and natural events. Whereas public safety is a notable concern, especially during construction, this chapter focuses on worker safety. The inspection, repair, and rehabilitation of manholes necessitates a high level of worker awareness and preparedness. The chapter illustrates and discusses typical hazards associated with manholes, especially those related to sewer manholes, and the pre-entry, entry, and rescue procedures that should be incorporated into an existing safety program. Each of the hazards that may be encountered by personnel should be identified and evaluated to prepare and implement control measures and monitoring procedures. This chapter emphasizes the more common hazards and procedures.

Many manholes have significant defects that need to be defined and subsequently rehabilitated. The first step toward properly managing manhole assets is inspection to determine the condition of the manhole by identifying what defects need to be corrected. Consistent data can be realized project-to-project and inspector-to-inspector enabling recommendations that may range from simply updating a geographic information system and system mapping to grouting to stop active infiltration to rehabilitating with structural solutions or lining for corrosion protection. Manholes contain multiple components and features of the components, each serving a specific purpose and constructed from various material types. Structural degradation of components within a manhole does not necessarily mean structural failure of the manhole itself. Corrosion in manholes is rarely caused by chemicals injected directly into the sewer stream; however, MIC results in the structural degradation of concrete, cast, and ductile iron surfaces throughout the interior of a manhole and connecting pipes.

Characterization of Infiltration and Inflow (I&I) is useful during manhole assessment to estimate the pathway and rates of flow that may be cost-effectively removed through rehabilitation. The evaluation of the structural condition is important for maintenance planning, selection of appropriate rehabilitation method(s), and prioritizing manholes for rehabilitation. Assessment of I&I and structural condition is usually based on visual inspection versus physical measurement. Inflow is specific to each situation and should be estimated based on drainage area, depth of ponding, number and size of cover holes, and the condition of the cover and frame-bearing surface. Infiltration sources observed when sewer flows are monitored to evaluate the effects of storms and respond slower; the effects are more long-term when compared to inflow sources. In addition to rating flow from I&I, establishing a similar system that can be used to define structural and operational conditions is necessary for structural assessment.

Material and technology choices for rehabilitation are dependent on the specific reasons for the implementation of the project including the correction of structural deficiencies, addressing maintenance requirements, elimination of I&I, and prevention of future corrosion damage. Leaking manhole covers can be sealed by plugging vent or pick holes or through replacement with new watertight covers. Ground movement, thermal expansion and contraction of the surrounding pavement, frost heave, and traffic loadings can cause the seal between the frame and chimney to deteriorate or break, allowing surface water to enter the manhole. Ground movement and/or worn, failing, missing, or improperly installed joint gasket material may allow infiltration to occur through the wall joint. One method for sealing the wall joint is using mechanical joint seals. Cured-in-place manhole liners used for rehabilitation generally consist of a polyester needle punched felt or fiberglass reinforced “bag” saturated with a thermoset resin.

This chapter focuses on the examination of the cost effectiveness of rehabilitating deteriorated manholes as a component of the overall sewer system known to permit Infiltration and Inflow (I&I) to enter the system and substantially contribute to higher costs for overall system operation. It discusses the degradation of various components and lists current technologies and practices that can be employed to correct deficiencies and problems. A cost-effectiveness analysis should be performed considering rehabilitation, transportation, and treatment costs at the various levels of rehabilitation being considered. A detailed evaluation of manhole repairs can be performed once the cost-effective ratio is determined. Evaluation of alternative manhole rehabilitation methods should utilize life-cycle costing, which considers the initial rehabilitation cost and the estimated life of the repair. Life-cycle costing can be performed with present worth analysis. This analysis shows the current funds needed to pay for all costs during the planning period.

Full-depth rehabilitation of manholes involves applying a coating or liner to effect renewal of lost structural and/or hydraulic integrity and provide corrosion resistance intended to extend the service life of the asset. This chapter provides guidance as to the assessment and definition of the wall thickness of a coating or liner material that is required to resolve the existing problems and meet the expectations for extended service life. The rational engineering process for a coating or liner rehabilitation of a manhole must begin with an analysis of the current condition of the remaining wall section, verifying the stability of the soil envelope around the manhole structure, and determining the issues present that must be addressed in the design of the liner. Cementitious liners consist of Portland cement concrete mortars, micro silica cement mortars, calcium aluminate cement mortars, and geopolymer cement mortars.

Quality assurance and quality control are valuable practices implemented to help ensure that what solutions need to be done in rehabilitation projects are done. The procedures of quality management program included during execution start with manhole assessment, with the role of the inspecting contractor being to gather the necessary data for the designer to evaluate and select methods for rehabilitation. Field investigators should be educated to recognize and document defects and conditions common to manholes. An essential component of manhole rehabilitation specifications is the clear identification of performance expectations of the material and process referencing industry standards for test methods, installation practices, and materials. The purpose of inspection includes verification that the materials and construction processes being used are in compliance with project specifications.

This chapter presents closing thoughts on the key concepts covered in this book. The book is a compendium of updated information provided by industry experts to assist agencies and their engineers in the safe, effective, and economical inspection and rehabilitation of manholes. It provides useful considerations to help an engineer and agency make the best choices based on performance, cost effectiveness, and desired service life. This Manual of Practice is designed to educate users on how to assess current conditions and select applicable solutions. It also provides users with the necessary tools to provide the best design and management practices for effective, long-term solutions.

This back matter contains a glossary.