PAVING: Drought crumbles pavement on Mississippi highways

Nov. 29, 2016

Moisture absorption of nearby trees along with shrinking soil is causing major damage to highways in the state

The drought in Mississippi this year is not only affecting recreational activities such as bonfires and camping, but is also affecting the state’s highways, according to the Mississippi Department of Transportation (MDOT).

“Our roads have suffered the effects of extreme loss of rainfall throughout Mississippi,” said Commissioner Tom King, Southern Transportation District. “Our biggest problem is large cracks in the asphalt that are becoming a hazard for the traveling public.”

Mississippi has large deposits of at least three different types of expansive clay soils, including Yazoo clay, which are spread over the state. These soils challenge MDOT during road construction and maintenance because of how they change with weather conditions.

For example, when Yazoo clay becomes wet, it may expand and occupy up to 400%  more space than when it is at normal moisture level. When soil loses moisture, it shrinks. Shrinking creates a void between the soil and asphalt or layer of earth above it. As soil continues to shrink,the void becomes larger and the weight of the pavement and vehicles causes the roadway to sink and crack, creating “slides” and “drop offs.” 

In the mid-1900s, county roads were given to the state, creating a paved network of farm to market routes; many of the Mississippi highways used today. As a result, right-of-way is narrow and lined by trees.

“As the trees reach further for water during extreme drought, the close proximity of trees to the roadway combined with expansive clay soils accelerate the shrinkage of the underlying roadbed materials,” said MDOT Executive Director Melinda McGrath.

A fully-grown tree will absorb 100 gallons or more of water from the ground on a hot day. During drought conditions, trees located in close proximity to the road draw all the moisture out of the soil beneath the roadbed. This, coupled with already shrinking soil, will cause major damage to the highway.

"With limited resources, it’s not an option to obtain more right-of-way along state-maintained highways to minimize the effects on existing routes,” McGrath said.

When the drought ends, MDOT will assess the overall needs and extent of the damage before any roads are properly repaired.

"We already know it will take more than simply placing a lift of asphalt to repair most highways," McGrath said. "Severe weather events such as droughts and ice storms prematurely age pavements forcing MDOT to alter the three-year paving schedule to repair damages."

MDOT applies strategies in the design phase of a project to account for extreme weather conditions the project may encounter once construction is complete. Current strategies include the following:

  • specifying embankment materials that limit the amount of clay;
  • undercutting and replacing problematic clay material just beneath the pavement structure;
  • treating the soil at the bottom of the pavement structure with lime or cement;
  • providing wide shoulders to support the edge of the pavement structure; and
  • providing gentle side slopes that are less prone to sliding.

These techniques along with construction quality control reduce the likelihood the pavement will crack and settle in times of drought.

“Droughts are beyond MDOT’s control and unpreventable,” said Commissioner Mike Tagert, Northern Transportation District. “Even though we can’t predict or stop a drought, we do take necessary, preventative measures to protect our highway system and keep the traveling public safe.”

“Unfortunately, we can’t control Mother Nature,” said Commissioner Dick Hall, Central District and chair, Mississippi Transportation Commission. “Our top priority is the safety of the traveling public in Mississippi. I’m proud of our maintenance crews and engineers for the commitment they have shown and continue to show to keeping our roads safe during this drought. Their efforts are protecting lives each and every day.”

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