Canadian Consulting Engineer

Manholes Rethought

The National Research Council's Institute for Research in Construction (IRC) has developed an innovation that will prevent heaving and cracking of pavement around manholes.Frost damage around manholes...

March 1, 2000   By Nicole Baer and J.F. Gallagher

The National Research Council’s Institute for Research in Construction (IRC) has developed an innovation that will prevent heaving and cracking of pavement around manholes.

Frost damage around manholes is a common sight in many parts of the world, from the northern United States to Canada to Europe and northern Asia. When one considers the number of municipalities plagued by frost damage and the number of manholes in each, the cost of the problem can readily be seen.

IRC’s solution, developed by researcher Otto Svec, P.Eng., is so simple that one could say it has been staring us in the face all these years. The simplicity lies in harnessing and using the natural warmth of the water flowing through storm and sanitary sewers. The technique promotes convection, which causes warm air to rise upwards in the manhole.

Near the surface, the warmth is encouraged to radiate horizontally outwards where it can protect the soil around the manhole from frost. The pavement immediately above suffers less stress, and thus will not heave.

Temperature differences between the bottom and the top of a manhole can reach 20C. Rather than let this warm air dissipate to no effect, it makes sense to use it to solve the frost problem.

Harnessing natural warmth

Natural warmth is harnessed by changing the design of the manhole and altering the surrounding soil. Conventional concrete rings near the top of the manhole are replaced with a ring ventilated with eight narrow slots. The holes in the manhole cover are plugged and the steel manhole cover is insulated from below, causing the warmer air to flow out through the slots.

Modifying the ground beyond the slots enables the warm air to dissipate beneath the pavement one or two metres in all directions. This is accomplished by placing a layer of large, clean, crushed stones around the manhole. The air flowing through the spaces between the stones is capable of transferring the warmth farther outwards than is possible with the more common soils containing clay.

The outward spread of the warmer air is enhanced by the use of a “radiator bag” placed around the crushed stone. This bag consists of two geotextile blankets, spread atop and beneath the stone layer, which together work to keep the warm air where it’s needed — close to the pavement surface.

A galvanized steel mesh, wrapped around the cylindrical manhole structure during installation, prevents the crushed stones from dropping through the slots in the cement ring.

Proof in the field

With any proposed innovation such as this, municipalities and utility companies are naturally concerned about performance and cost. To prove the new manhole design concept in a real-life setting, IRC carried out a field test with engineers in the city of Nepean, next door to Ottawa. Wayne Newell, general manager of design and construction in Nepean’s Public Works Department, paved the way for the test in the winter of 1998-99.

Nepean’s enthusiasm for new technology was amply rewarded. Good results flowed along with the warm air rising in the test manhole. Despite fluctuating temperatures during the winter, de-icing salt applied to city streets, and wear and tear from vehicles, the IRC innovation worked like a charm. At winter’s end, the pavement around the test manhole remained flat and free of cracks. This was in sharp contrast to pavement around standard manholes, some of which heaved by five centimetres.

The news on cost is also good: the slotted cement rings can easily be manufactured, and the entire installation procedure costs little more than conventional manhole construction. Indications are that by extending the life of pavement by just one year, municipalities can recoup their investment. The protected pavement is expected to last as long as pavement where frost is not a concern.

Widespread adoption of the innovation could mean great potential for savings because of the sharply lower incidence of road repairs. Secondary benefits include less traffic disruption, savings in energy and environmental advantages.

IRC has applied for an international patent on its innovation. Meanwhile, research will turn to making the technology as cost-effective as possible by developing a heat bag of smaller diameter and thickness, especially for road rehabilitation projects.CCE

For information on the new manhole technology, contact Dr. Otto J. Svec, Ph.D, P.Eng., senior research officer in the Urban Infrastructure Rehabilitation Program of the National Research Council’s Institute for Research in Construction. E-mail address: otto.svec@nrc.ca.

Nicole Baer is an Ottawa-based freelance writer. Jim Gallagher is manager of publication services, Institute for Research in Construction, National Research Council of Canada.

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detects rollover by measuring the lateral forces (using the anti-lock braking system electronic control unit and accelerometers) and individual wheel speeds (using sensors). Field studies in British Columbia have shown these systems give a return on investment of 2:1 within five to 10 years.

Road safety is something that should be prevalent and considered in all business decisions, especially those made by governments. We expect governments to provide leadership to promote a safe highway system. They should ensure, for example, that the enormous competitive pressures in the trucking industry resulting from just-in-time inventory and globalization do not compromise public safety.

The cost of traffic collisions is rising both in human and economic terms. As a result, highway safety is starting to become an issue of worldwide concern, something like the growth of environmental awareness in the early 1990s. Such rising public interest demands that road authorities and consulting engineers face the challenge of improving safety on our roads. CCE

Said Easa is professor at Lakehead University and author of two chapters on “Geometric Design” and “Automated Highways,” published, respectively, in Civil Engineering Handbook, CRC Press, 1995 and the Encyclopaedia of Electrical and Electronics Engineering, John Wiley, 1999. He is also editor of Urban Planning and Development Applications of GIS, just published by the American Society of Civil Engineers.


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