Compared to paved roads, gravel and forest road have a much greater variation in maintenance technologies and machinery starting from totally manual tools to almost fully automated machines. This chapter provides a short summary of the techniques and machinery used on gravel and forest road operations.
8.2. Graders, road drag graders, levelling drags and underblades
Graders, road drag graders, levelling drags and underblades have the same main task in gravel road maintenance: to even out and shape the road surface to a form that is smooth and safe to drive. The difference between the types is their efficiency and variety of applications. Previously graders dominated in gravel roads maintenance but, due to their price in the market, more and more tractor pulled road drag graders are now being used. Underblades can also be seen doing this work in the ROADEX area. Light levelling drags have on the other hand become rare in many ROADEX countries.
8.2.1. Grading, blading and dragging
Graders have been the most efficient machinery especially in wearing course maintenance and in winter maintenance because of their weight and strength. Modern graders do not have problems with grading depth, even though the gravel road surface can be dry and hard. Forming the correct cross slope is not an issue with these graders.
Basic grading should move the material from the edge of the road towards the centre and thereafter maintain or change to the correct crossfall with each successive pass. At the same time, all surface defects should be removed. The number of passes needed will be governed by the depth of irregularities and width of the road. The stacked material on the centreline is thereafter spread beyond the centreline, and then spread back again across the surface to the correct crossfall.
One of the key issues with graders is the blade angle which can be changed on whether the grader is doing cutting, blading or light surface maintenance. For instance, loose corrugations (washboarding) consisting of parallel crests of loose and fine sandy material can be removed by blading. But fixed corrugations always require cutting.
If the main problem is potholes, minor potholes may be corrected with the use of a grader fixed with tipped blades. Where potholing is severe, the surface may require scarifying, remixing and reshaping. The grader blade should have an appropriate cutting angle to ensure enough mixing of the material, and cutting should always be done deeper than the bottom of the pothole.
Grading is generally classified into three classes: light grading, medium grading and heavy grading. Heavy grading should normally be done in moist conditions. Heavy grading is usually made to a depth of 100 mm, or at least to the depth of any potholes, but not to the depth of the base course level. Compaction should always be carried out after heavy grading. A medium grading shapes the road from a flat surface to the correct crossfall and crown shape. This is done by using existing material. Light grading, that can also be called dragging, is made to smooth the road surface and ensure free water flow.
There are mainly four problems that can arise in the grading of gravel and forest roads in the ROADEX area: a) the grading or blading has not been made deep enough, b) the grading or blading speed is too high leading to corrugation, c) the grading and blading is done in such a way that moves material to the roadside which leads to road widening, and d) wearing course material is cut from convex “hill tops” and is deposited into the next concave section leading to thick wearing course layers, and further to permanent deformation and spring thaw weakening problems.
The second problem is grading speed which is often too high leading to the grader bouncing, especially with tractor pulled road drag graders. The results of this are ripples on the road surface that lead to dynamic loading of vehicles and to corrugations. On moist road sections, corrugations lead to potholes, and in frost areas to surface thaw weakening damage. These problems can be prevented by a lower grading speed and lower tyre pressures and/or using a heavier and more powerful grader. With tractor pulled drag graders, one working solution has been to place a heavy weight in the front bucket to preventing bouncing of the pulling tractor.
The third common mistake with grading is that material is moved to the roadside during the grading. This can be fixed by moving the road material from the road shoulder back to the road surface.
If road has already widened modern graders can be used to lift the material from the road shoulder back to the road surface and narrow the road to the original shape. If the material from the road shoulder has big stones or turf edges, they can and should be removed from the road surface. This can be done for instance by using a special sieving bucket.
The fourth problem is that the grading is done using a fixed cutting or blading depth which moves material to the concave spots in the road. This can be avoided by changing slightly the cutting or blading depth depending on whether the location is on a convex hill or a concave dip.
Compaction of the wearing course material after grading is vital as an uncompacted wearing course is not dense enough to assure rainwater flow to the roadside. An uncompacted wearing course will also lead to Mode 0 rutting. As written earlier in this eLearning package, salt is a good substance to aid compaction.
20-30 years ago good compaction was a bigger problem, but currently most of the modern graders and tractor pulled road drags have a set of wheels at the rear that can compact the wearing course surface. If contractors do not have this type of system, compaction should be made with a separate compactor. In earlier times this was done by driving a truck along different lines on the road, but in many cases the final result was questionable.
8.4. Timber truck technologies in forest road maintenance
A new trend in forest road maintenance is the development of supplementary techniques or gadgets that can be connected to the timber trucks used in road maintenance. One example is the underblade that can be used in the winter maintenance of forest roads, and also in light blading of the forest road surface.
Another new technique successfully tested in Finland is ‘rut filling’. The technique can be used before or after timber haulage to even out and strengthen road sections suffering Mode 2 rutting. The benefit of the system is that it does not require additional maintenance machinery to be brought on to the site, and timber trucks can quickly fix the problems that they have created.