Springtime Road Restrictions
Several Canadian provinces and most northern states, including Alaska, place special springtime restrictions on the weight of trucks allowed on surfaced roadways. Since, in some instances, the restriction permits only 50% of normally-allowed gross vehicle weight, it seriously curtails economic trucking during the breakup period.
The reason for limiting truck loads, and for further restricting them during breakup, is that heavy axle loadings can cause road pavement to flex to the point where the pavement cracks. At first, the cracks may not be visible because they mostly start at the bottom of the pavement and break upwards toward the top. Once the cracks start, they can propagate quickly and lead to breaking up of the entire road surface.
Road builders and maintenance personnel have devised ways to measure pavement deflection under the load of a rolling wheel. One method, the Benklemen Beam test, involves direct measurement of the deflection when a wheeled axle of specified weight is moved along beside a straightedge placed on the road under test. Measured deflections in the road surface amount to a few hundredths of an inch where a standard test weight, 18,000 pounds, is placed on the axle. (Except when special permits are issued, the maximum legal load on a single axle in Alaska is 20,000 pounds; on a tandem, it's 34,000 pounds).
Like a piece of wire, road pavement can be flexed very slightly a great number of times before it breaks. But also like the wire, the pavement will break rather quickly if the bending is sharp and large.
The flexure a road surface undergoes directly depends upon the load applied. The lifetime of the road surface depends upon the degree of flexing and how many times it is flexed. Consequently, a road surface that would last 20 or 30 years if traveled only by many thousand passenger cars each day might fail in only a few years if subjected to the passage of a far less number of extremely heavy trucks.
Enough is known about road surface failure when different loadings are applied that engineers can calculate loading effects and the maintenance costs created by these effects. For example, it has been determined that the road maintenance cost of one 18,000-pound axle rolling along a surfaced Alaskan road for one mile is about six cents. A legally loaded 18-wheeler truck-trailer grossing 88,000 pounds that rolls one mile costs the state about 18 cents. For that same cost to the state, approximately 16,000 small cars weighing 4,000 pounds each can travel the same distance (one mile). This is an extreme example, but it illustrates the remarkable dependence a road surface's lifetime has upon the weight of the vehicles that travel it.
At the time of spring breakup, heavy loads cause more road flexing than normal because high moisture content in the roadbed during thaw lowers the strength of the subsurface. Tests in the Fairbanks area show that the flexing is greatest when the spring thaw reaches to depths of two to four feet. The increased flexing then will lead to premature road failure unless special load restrictions are put in effect.
Avoiding unnecessary application of load restrictions while still making sure that they are in effect when needed is a tricky problem. The issue is nicely described by engineer Billy Conner in a document "Rational Seasonal Load Restrictions and Overload Permits" available from the Research Section, Alaska Department of Transportation and Public Facilities, 2301 Peger Road, Fairbanks, Alaska, 99701.
