July 2011, Vol. 66 No. 7
Features
Manhole Rehabilitation: Sometimes Misunderstood, Yet Key Infrastructure Solution
Editor’s Note: This is the first of three articles focusing on the current state of manholes, manhole assessment and manhole rehabilitation. Part two will be published in the August.
The nation’s failing sewer collection system infrastructure encompasses main pipelines, lateral sewers and manholes. There exist over 20 million manholes in the U.S. of which over four million are older than 50 years and over five million are 30 to 50 years old. The manhole is a key element of the collection system, as it provides access to the pipeline infrastructure for maintenance, inspection, evaluation, renovation and testing.
Manholes have been constructed since the 19th century from a variety of different materials including brick, concrete block, concrete, fiberglass and other materials. In addition to different materials, manholes have been configured in a variety of shapes, configurations and depth to meet specific location requirements.
A manhole, also referred to as a person hole, utility hole or maintenance hole, is the access point to an underground utility including sewer, telephone, electricity, storm drains and gas pipes.
Typically sealed by a cover or grate, a manhole is designed to prevent accidental or unauthorized access to the utility system. The interiors of a manhole can be constructed with metal or plastic steps or ladders installed in the inner side of the wall to allow access by maintenance personnel. As the main access point to the collection system, a manhole must be accessible from the ground surface, and must extend from the ground surface to the wastewater pipe connections.
A manhole must be structurally sound to resist earth and groundwater pressures at various depths and be capable of resisting damage as a result of mowing equipment and other machinery, withstand constant traffic in street installations, and must be resistant to a variety of weather conditions including ground frost heave in colder climates.
Perhaps the most important requirement of a manhole is that it must be watertight, not allowing the entrance of inflow from surface runoff or infiltration from groundwater into the sewer system. In addition, the interior of a manhole must also be resistant to powerful deterioration mechanisms such as erosion, abrasion, hydrogen sulfide induced corrosion and other corrosive chemicals that occur naturally or may be introduced into the system from businesses or industries.
Manhole assessment
With all the challenges facing collection systems, it is not surprising that many manholes have significant defects that need to be identified using standard coding. After coding the defects and evaluating conditions, the manhole may require rehabilitation to extend its life expectancy. The first step toward rehabilitation is to conduct a comprehensive assessment and determine the overall condition of the manhole by identifying what defects, if any, need to be rehabilitated. Based on the standard Pipeline Assessment Certification Program (PACP), NASSCO created the Manhole Assessment Certification Program (MACP) which now includes two levels of inspection for condition assessment.
A Level 1 inspection allows utility owners to identify basic assessment information to evaluate the general condition of a manhole and to gather enough information to determine if a comprehensive Level 2 inspection is required or appropriate. A Level 1 inspection can typically be completed without the use of any special equipment or manned entry into a manhole.
A Level 2 inspection allows the utility owner to gather detailed information to fully document all existing defects, determine the condition of a manhole and to provide specific information to recommend or specify corrective actions.
Manhole components
Unlike the main pipeline portion of the collection system, a typical manhole consists of many different components, each requiring condition assessment and identification of defects that affect the function of the overall manhole structure. These typically include:
• Cover – is used to control access from the ground surface into the manhole. The cover, typically made of cast iron, is replaced if defective;
• Frame – is used to provide support for the manhole cover and connect the cover to the body of the manhole. The frame, also cast iron, is typically replaced if defective;
• Chimney – can be described as a manhole component constructed of brick, concrete or polymer material that connects the frame to the cone of the manhole. In addition to reconstruction, a number of technologies are available to repair and rehabilitate the chimney area of the manhole;
• Cone – is a tapered, transition section frequently installed between the smaller diameter chimney section and the larger diameter wall for a certain depth of manhole. A number of coating, grouting and lining techniques are available to effectively repair and rehabilitate this portion of the manhole;
• Wall – is the vertical section installed from a cone to the bottom of a manhole. A number of coating, grouting and lining techniques are available to effectively repair and rehabilitate this portion of the manhole;
• Joint Seal – is a material or device to prevent intrusion of groundwater water at the joint between precast wall sections, or cone and wall sections. Cementitious, polymer and mechanical repair methods are available for long term rehabilitation;
• Manhole Step – can be metal, plastic or configured as a ladder. These devices can be replaced if defective or removed entirely as specifically required;
• Bench – is the bottom section of the manhole. Typically constructed of brick or mortar, if defective, can be replaced using plastic materials or rehabilitated using a variety of long term proven materials;
• Channel – is a shaped flow-way within the bench which directs sewer flow through the manhole. Usually constructed of cementitious or sewer pipe materials, it can be rehabilitated with grouts and many other available proven products;
• Pipe Connections – Connections are a continuation of the sewer pipe through the manhole wall and into the manhole. Typically the seal through the wall can contribute excessive flow and require rehabilitation;
• Pipe Seal – The pipe seal can be described as the material or device installed at the pipe and wall or whatever manhole component in which the pipe enters. These can be rehabilitated using a number of effective techniques; and
• Drop Inlet – Pipe Drop Inlets are special pipe systems that connect to the incoming pipe, turn 90 degrees downward to the bottom of the manhole, then turn horizontally into the manhole. These structures can be rehabilitated using available products.
Exploring the issue
As the nation’s collection system infrastructure continues to deteriorate at an accelerated rate, rehabilitation, renewal or reconstruction of the manhole component of the system become more urgent.
The nature and type of manhole defects must be evaluated to determine the best method of rehabilitation. Defects can occur in the form of structural, operation and maintenance, construction features and miscellaneous. Deterioration can be identified by a number of conditions including cracks and fractures, root intrusion, deteriorated mortar, loss of wall surface in concrete, water leakage through defects and failed joints, failed seals, poor pipe connections and more. The causes of deterioration can be from chemical or biological attack, poor maintenance, infiltration/inflow and in some cases, just old age.
Many rehabilitation technologies are available for rebuilding existing manhole structures without the need for excavation and replacement. These can include cementitious, polymer, cure-in-place, grouting, mechanical and more, for both internal and external coating, lining and sealing techniques.
The questions which arise frequently from industry professionals are: which system works best? And under what conditions and for how long? Most products and technology systems have verifiable histories of applications and longevity. Many products have been proven over time for rehabilitating specific defects, yet on occasion the installation does not perform as expected.
Questions are asked frequently about why a product has not performed as expected. Confidence in manhole rehabilitation technologies and products becomes questionable as perceived by the customer.
Was the wrong product installed or improperly applied? Was the structure prepared incorrectly for long-term product performance? What should be changed to ensure long-term performance? Are there recommended repair techniques that work? How should the new product be designed in a manhole? How is a product designed, using industry appropriate design equations? What does it mean when a product is termed structural versus non-structural? What testing should be performed to verify proper application? Should the level of applicator training be improved through advanced training, education and certification? Should third party inspection, by certified inspectors, become part of the technology or product application process?
These and many more questions will be answered by industry professional in parts two and three of this series in subsequent issues of Underground Construction magazine.
In August, the next article will focus on available manhole rehabilitation technologies and correct application and use. It will define the best practices for manhole rehabilitation including interviews with technology providers, engineers and municipalities to define what is necessary for manhole rehabilitation success.
In September, design approach for manhole rehabilitation will focus on what soil and groundwater forces act on an existing manhole and the proposed rehabilitation technology and what design approaches apply to manholes.
Editor’s Note:
References for this article include: Manhole Assessment Certification Program, copyright © NASSCO Version 6.0 June 2010 Rev 6-29-10; American Society of Civil Engineers (ASCE). (2009). “Manhole Inspection and Rehabilitation”; and ASCE Manuals and Reports on Engineering Practice No. 92, Second Edition.
FOR MORE INFORMATION:
NASSCO, (410) 486-3500, www.nassco.org
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