Compaction making the road stronger

Question: This summer I put a chip seal on a road that previously was gravel. I make it a practice to post such roads for a 20 ton load limit, all year around. A group of agriculturists came to the board meeting to ask that the load limit be removed. The claimed that their trucks, including some 18-wheelers, would compact the road and make it stronger. The Board wasn’t so sure about this. Is there any chance that the trucks will improve the roads?

Answer: In short, you asked whether truck traffic on such a road would compact it, and make it stronger. We doubt it. Since you find you have to post load limits, this implies that the pressure on the road from the truck tires must be close to the bearing strength of the subsurface materials.

If the road is properly constructed, it will be able to support legal truck loads for a reasonable number of years. If you have to post load limits, then we suspect the roads probably need to be much stronger.

Roads that have been surfaced with a chip seal (sometimes called oil and stone) derive their strength from the gravel base course which is immediately beneath the surface. The surfacing provides abrasion resistance and (where it is properly crowned) it waterproofs the base, but it does not provide structural strength directly.

The bearing strength of the materials in the road changes from season to season, and usually the road is weakest during the spring season. At that time of the year heavy loads cause the road surface to deflect the most. Somewhat like bending a paper clip back and forth in your hands, large deflections cause the road surface to crack after a few cycles. If the subsurface materials are strong, the surface layer does not deflect very much under the loads, and thus the rate of deterioration is slow.

The granular materials used as a base must be clean in order to be strong. Clean gravels should not contain more than five or eight percent silt and clay. When they get wet, the fine-grained materials cause the base course to become weak, and thus it will deform (i.e., from ruts, and crack, and eventually develop potholes) under heavy truck traffic. It would take many automobiles to do the same amount of road damage as one truck, but the eventual outcome, a failed road, is still the same.

A properly constructed single-layer stone and oil surface treatment, placed on a clean granular base, can reasonably be expected to remain in good condition for about five years or more in Western New York State. If you are experiencing a road life of this duration, then you can conclude that the truck loads are not damaging your roads excessively. However, if the surface treatments are breaking up after only one or two years, you can conclude that your roads are not strong enough to support the heavy truck loads. You may need to consider strengthening the roads or posting load limits on such roads.

Load limits are effective only where they are honored by the road users. If you post a road, but it is not enforced, then if the road fails prematurely you have no way of knowing whether the load limits were set too high, or whether they were being ignored by road users. Also, local traffic can still use the road for deliveries or to access a field.

A better approach is to build the roads to hold the loads, the legal loads, that is. In doing so you do not disrupt the commercial traffic which needs to use the roads, and you can expect your road maintenance problems to be minimized. Certainly, you should use an adequate base coarse depth as needed to spread the wheel loads properly over the native subgrade soils. It may be necessary to place a separation fabric between the native soils and the clean base stone to prevent the fines in the native soil to infiltrate into the clean stone and cause it to fail. Ask your peers for local sources of good quality gravel.

Good drainage is vital for long-lasting roads. If the base course and subgrade materials do not get too wet, they will remain much stronger. Thus you must be sure the roadway and shoulders are crowned, so the surface water will run into the ditches. The ditches and culverts must be clean and functional. And occasionally, it may be necessary to put perforated pipe underdrains beneath the road in wet areas, to get the water out of the road.