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Updated ASCE Manhole Guidelines Focus On Issues, Rehab Methods
"Brick commonly used through the 1930s was labor intensive and it required a great number of the bricks to build the structure," Nelson says. "Concrete materials were a significant evolution as the manhole could be built in lifts or segments, utilizing precast materials. Today, another factor driving change results from attacks of nature in the forms of water infiltration and hydrogen sulfide (H2S). H2S issues have driven the industry to look for new construction materials which would be infiltration and corrosion resistant as well as allowing for structures to be built in lifts. Manholes are now being created from inert materials, for example fiberglass and polyethylene."
Corrosion severely compromises the structural integrity of both brick and concrete components, costing millions of dollars annually for repairs.H2S is a naturally occurring process that exists in a dissolved state within wastewater.
"When released into a gaseous state, it comes in contact with the moist surfaces of a manhole wall and as concentration levels rise, bacteria colonies proliferate, forming an extremely corrosive slime layer that can rapidly cause weakening and decomposition of even the most massive concrete and steel structures," Nelson says. "Newer materials such as modified cementitious and epoxy coatings have proven themselves to be resistant to this attack while maintaining their integrity."
Excessive I&I is another serious problem for wastewater collection and treatment systems, and Nelson points out that the hydraulic effects of these extraneous flows are particularly important because they utilize valuable collection and treatment system capacity that is needed for urban growth.
Flows of untreated wastewater from I&I can result in public health economic issues, and environmental issues which are deterrents to the overall objective of protecting the nation's water resources. Prolonged leakage can create voids outside a manhole structure with the potential of removing of soil resulting in the loss of lateral soil support which can create structural issues. The revised manual covers all these and other issues and provides technical guidance for implementing a successful manhole inspection and rehabilitation program.
Nelson cites two essential objectives in managing manhole assets: