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Spring Weight Restrictions

Why Do We Need Spring Weight Restrictions?

Spring weight restrictions are necessary to help protect highways from damage which can cause untimely and expensive delays for the transportation industry, and the public who rely on the road network. In order to minimize damage to road surfaces, axle weights must be reduced during the spring thaw period. Damaged road surfaces cause delays and extra costs for highway maintenance and vehicle wear and tear.

How Roads Are Constructed

How a road performs under truck traffic is a function of the way that the road was built and the materials used. Roads are built by preparing and shaping a subgrade from the locally available soil and adding base layers of high-strength granular material such as sand or gravel. Typically, local soils may contain a high proportion of silt, which results in lower strength. Major highways, are constructed with a substantial thickness of high strength granular material in the road’s base layers. These roads have a greater ability to support the wheel loads imposed by trucks. However, many older roads were constructed on soils containing high amounts of silt, which when wet, is much weaker than gravel or sand. These roads contain little or no granular base material and as a result, are more susceptible to damage during the spring period.

What Happens to the Road?

Under freezing conditions silty soils attract water. The resulting high water content can cause the road to lose 50% to 70% of its strength during spring thaw and consequently the road surface is more likely to be damaged by heavy traffic. When freezing temperatures occur, a number of events take place beneath the road surface, and the basic structure of the soil changes.

Freezing

As fall approaches, the road freezes from the surface downwards. A negative pressure develops at the advancing frost line. This draws water up from below, increasing the moisture content of the soil.

Thawing

During spring, as temperatures warm, the road thaws from the surface first, moving downwards. As the frost line moves downward, the moisture is left behind, trapped directly below the road surface. This trapped moisture has no place to move since it cannot escape through the frozen soil or the sealed road surface. The relatively rigid road surface on top of soil containing trapped moisture has been compared to a sheet of glass on a waterbed, due to the high potential for damage from heavy traffic loads.

After Thaw

As the frost line moves further downward, the trapped water is finally able to escape, and the microscopic structure of the soil reverts to its original state and the soil strength increases.

When to Impose Weight Restrictions

Weight restrictions are based on the thaw rates of the road structure measured by thermistors located throughout some highway systems, and the observed condition of the road surface. (A thermistor is a type of thermometer that is capable of measuring temperatures at various depths in the road structure.) As the frost line moves deeper below the road surface, different weight restrictions are recommended until the frost line is deep enough for moisture to escape and the soil to regain its strength:

  • 100% restrictions are imposed when ambient temperatures are such that surface thawing is present and roadway surfaces show signs of distress.
  • On non-structural highways, 75% load restrictions are recommended when indicatations are that the thaw is 0.3 m below road surface.
  • Removal of load restrictions below 100% is recommended when indicatations are that the thaw is 1.0 m below the road surface.
  • Removal of all load restrictions is recommended when indications are that the thaw is 1.4 m below the road surface.
Full-strength structural roads such as major highways are typically not restricted below 100% of legal axle weights.

Spring weight restrictions are not arbitrarily imposed, but rather are set based on actual scientific information and observed road conditions. Your cooperation is requested to ensure that people and goods can move safely without damaging the road surface.

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