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Recent work by University of Manitoba civil engineering researchers will improve the longevity of roads in that province, reduce maintenance and improve driver safety.

In 2018, Professor Ahmed Shalaby completed a study of the mechanical and drainage performance of unbound granular material — “gravel” to non-engineers — as it is used as a road base.

Gravel is an essential ingredient of roads, because it forms the major portion of their pavement structure.

Moisture that gets into a road’s pavement or base can lead to what engineers call “surface distress.”

Shalaby says pavements with moisture-saturated sub-layers can have their service life reduced by up to 10 times compared to pavement with sub-layers that are well drained.

“Drainage in Manitoba and Saskatchewan can be a problem because of the flat terrain,” said Shalaby. “Roads can retain moisture for weeks or even months following a significant rain event instead of draining it off in two or three days.”

The retained moisture weakens the road structure, causing it to be more vulnerable to damage, such as ruts or cracks that are caused by the passage of heavy trucks.

“The faster we can drain the moisture, the better we can protect the road and prolong its service life,” said Shalaby.

The university researchers worked with Manitoba Infrastructure (MI), a provincial government department, on the project.

MI assigned a curve realignment project on an asphalt road in southwestern Manitoba to evaluate the performance of two kinds of gravel base.

The conventional base material that MI has been using is dense but tends to retain moisture and become damaged quickly, while the new materials have larger pores.

The research showed that larger, coarser gravel improves drainage and reduces the sensitivity of the pavement to seasonal moisture variation.

Although it is slightly more expensive, the additional cost is justified, Shalaby says. Because the better gravel is stronger, less of it is required.

“The performance improvement will reduce the required layer thicknesses, eliminate premature failures, and accelerate the recovery of layer stiffness during spring thaw periods,” said Shalaby.

Tara Liske, MI’s director of materials engineering, says the ministry has already incorporated the changes recommended by the University of Manitoba research into its current tenders and specifications on an interim basis, “until we have more data on field performance and contractors [get comfortable] with producing the material.”

Liske says the new gravel specs will benefit MI in a number of ways.

Because the base material is stiffer, the thickness of the pavement layer can be reduced.

“We also expect the new drainable materials will increase the service life of our pavement structures, because of reduced deterioration due to lower water infiltration,” Liske said.

Shalaby’s colleague, research associate Leonnie Kavanagh, recently completed a study of culvert installations in rural Manitoba.

In partnership with MI, Kavanagh’s research evaluated alternatives for cost-effective culvert installation techniques that mitigated road roughness caused by bumps and dips at culvert locations.

Inadequate compaction, erosion of the back-fill or supporting materials, differential frost heave in back-fill and surrounding materials often leads to road roughness at culverts, says Kavanagh

“It can adversely affect ride quality and create potentially unsafe conditions for drivers,” she said.

The study looked at innovative culvert installation techniques over a 10-kilometre section of a provincial highway in a rural area north of Winnipeg.

It tested a number of different culvert installation techniques, such as using differing backfill materials; rigid insulation below the culvert to protect the frost-susceptible clay soils from freezing; and geosynthetic reinforcement installed between layers of the clay backfill to reinforce the weaker clay soil and increase its bearing capacity.

Temperature sensors and data loggers were installed at each test site to collect continuous temperature profiles in the vicinity of the culverts and to monitor frost depths.

After two years of field monitoring and analysis, the results showed that the culvert with the granular backfill and the one with the reinforced clay backfill scored highest in reducing surface elevation changes caused by excessive settlement and differential frost heaving.

The recommended best practices include using granular backfill at all culverts; applying geosynthetic reinforcement to clay backfills where granular backfill is not available, or the cost of hauling and disposing of granular material is prohibitive; and forgoing the insulation of granular back-fill at culverts where it is used, since the insulation had little impact on performance.

“Based on the results of the study, Manitoba Infrastructure is going to be changing the specs for the granular back-fill that is used in culverts,” said Kavanagh.

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