Abstract

Many rural county road networks were created at a time when funding was greater and rural populations were often larger than they are today. Eventually, surface treatments such as chip seals or thin asphalt were applied to many of these gravel roads to provide them with an all-weather surface. These treated surfaces were also desirable because conventional gravel roads are dusty, often develop washboarding quickly, and have high rates of gravel loss—which result in unsafe and uncomfortable conditions and greater damage to vehicles and crops. Today funding to maintain these low=volume roads has dried up, and this has led to the frequent development of deep potholes that create dangerous vehicle- and freight-damaging conditions. And while some road networks can be abandoned, most of these roads are still needed to
support the economic needs of tax-paying residents, by serving agriculture, forestry, and recreation area access.
A solution to this problem, called unpaving using engineered gravel roads, has been developed in South Africa and has been implemented by UCPRC/CCPIC researchers in several counties in California. Unpaving
involves pulverizing the existing surface of a gravel road and any granular base layers below it, and importing additional granular material as needed. The grindings and any additional granular material are checked in
the laboratory with simple and inexpensive tests to determine the amount of additional fines or clay material that needs to be added—typically less than five percent by total weight of aggregate—to ensure that the now unsealed wearing course will be tightly bound and not susceptible to
washboarding or excessive dust. The supplementary gravel and fines are spread on top of the existing road, mixed in place with a recycler (note that recycling depth can often be adjusted to incorporate a small amount of the subgrade material if it is suitable as an alternative to trucking in
fines), then shaped with a grader and compacted to finish up with a four to five percent cross-slope. A chemical treatment (stabilizer or dust palliative/fines preserver) can be applied during mixing or after the road has been compacted to seal the surface, which will increase the time
between grader maintenance work and lower the rate of gravel and fines loss. The resulting compacted surface layer creates the engineered gravel road that will be much smoother than the old distressed surface treatment/asphalt road, and if the correct grading and clay content is achieved and the road is correctly shaped, the surface will effectively shed water to prevent ponding and the formation of potholes.