2. Gravel and forest road components

2.1. General

This chapter will provide a summary of the components and assets of gravel roads and forest roads in order to provide an understanding of the terminology used in these types of roads.

2.2. Road geometry

2.2.1. Gravel roads

The geometry of gravel road varies a great deal over different countries due to the history of the roads and the local topography. Originally gravel roads were simply single track roads with passing places but currently most gravel roads in Nordic countries have been widened so that the passing places are no longer needed. That is why the width of public gravel roads can vary roughly from 5.5 m to 6.5 m depending on the design speed. Similarly gravel road crossfall can vary from 3% to 7%, but the recommended crossfall is usually 5%. During wintertime in areas with icy roads crossfall should be slightly less (3-4%). Inner slope inclination can vary 1:2-1:4. Ditch depths are normally 0.4-0.6 m or deeper.

In tight curves the width of the gravel road needs to be made wider to cater for long trucks. If the road is narrow passing places should be made at appropriate intervals. One important issue in gravel road maintenance is their widening with the time and this will be discussed in a later section. In general, wide gravel roads always pose problems with road performance and maintenance.

Most public gravel roads carry heavy trucks and as a consequence their vertical and horizontal geometry need to be similar to those of paved low volume roads. Most gravel roads however have not actually been designed to meet these requirements. For example many gravel roads do not have road cuts and are normally built to avoid cutting into small bedrock hills. Normally the design speed on gravel roads is 80 km/h but it can much be lower especially in village areas where houses or other buildings can be very close to the road.

A public gravel road in Sweden.

The vertical geometry on public gravel roads is normally designed so that the gradient of the road, especially on climbing hills, is not higher than 10%. If the gradient is higher than this warning signs should be provided. Steep gradients can be especially problematic during wintertime when the road surface is covered with snow or ice.

2.2.2. Forest roads

Most forest roads are one lane roads with passing places. According to the guidelines of the particular country the width of the lane can vary from 3.4 m to 4.2 m. Cross slope should be 5% to ensure water does not lie on the surface. Inner slopes can be 1:2 and ditches are normally only built on sections with wet subgrades. Backdrains are also used in some countries particularly on side sloping ground.

Due to the narrow nature of forest roads, and their use by long and heavy trucks, forest roads are mainly designed to be as straight as possible. Where curves are necessary, the design speed normally defines the curve radius. The width of a forest road depends then on the horizontal and vertical geometry of the road. In ROADEX countries there are certain rules of thumb for forest road geometry.

  • After a curve, there should be a straight section of at least the length of two trucks
  • When approaching a hill used by loaded timber trucks the road surface should be as smooth as possible to allow trucks to gain sufficient speed to climb the hill
  • The maximum vertical gradient for loaded trucks should be 10%, and unloaded trucks 12%
  • A curve over a hill should be a continuous curve, i.e. continuous left or continuous right, to avoid difficulties with the tracking of towed trailers
  • All junctions, especially those on to main roads, should be designed wide enough for long trucks and any trees cut back to ensure visibility
A forest road in Scotland.

2.3. Structures

2.3.1. Gravel roads

The structure of a public gravel road is normally composed of a) a wearing course, b) a base course and c) a variety of substructures above the subgrade. From these, the wearing course and base course generally follow the same standards across countries. Substructures on the other hand can vary from country to country. In many countries gravel roads are composed mainly of only a wearing course and base course above the subgrade.

The wearing course is normally made from crushed or screened material and its design thickness varies normally from 50-100 mm. The wearing course, sometimes called the “pavement” of the gravel road, is the most challenging structure for gravel road design and its materials and specifications are described in Chapter 3.

The substructure of a gravel road, if any, is normally composed of natural gravel or sand or natural fill material. Blasted rock has also been used. Geotextiles can be utilised to prevent the subgrade soil mixing with the road structure.

It should never be assumed however that a gravel road structure thickness will be even across the road cross section. In most cases the road, and structure, will be as a result of an evolution arising from truck loading, frost action and permanent deformation due to seasonal changes.

2.3.2. Forest roads

The road structure of a forest gravel road normally has three components, a) a wearing course, b) the road structure and c) substructure. The wearing course is usually 5-10 cm thick and can have a coarser grading than a public gravel road as forest gravel roads seldom have problems with dusting.

Forest road structures beneath wearing courses can vary. Normally there is a base course made of screened natural gravel or crushed gravel or crushed rock and beneath that locally found materials. Earlier substructures were made from the material excavated from the ditches after the top organic soil had been removed. More recently, especially on weak soils, blasted rock embankments have also been used on gravel roads. Geotextiles are also often used to separate the road structure from the subgrade soil.

2.4. Drainage components

2.4.1. Gravel roads and forest roads

There is a general slogan among road engineers: “the three most important issues in sustainable road condition management are drainage, drainage and drainage”.

Gravel and forest road drainage systems are composed of the same elements of any other road, and the primary purpose of the drainage system is to remove the water from the road and its surroundings. As documented in the ROADEX drainage elearning lessons, the road drainage system consists of two parts: dewatering and drainage. “Dewatering” means the removal of rainwater from the surface of the road. Dewatering covers the collection and transport of water from the surface and structure of the road so that there will be no ponds on the road or in the ditches. “Drainage” on the other hand covers all the different infrastructural elements to keep the road structure dry.

Dewatering consists of the following elements: 1) crossfall of sufficient size to remove water from the surface of the road, 2) freely draining road shoulders that should not block water flowing to the ditches, and 3) impermeable road surface materials that prevent water from infiltrating into the road structure.

A drainage system consists normally of the following elements: 1) side ditches, 2) outlet ditches, 3) back drains, 4) culverts, 5) inner and outer slopes, 6) road structures such as filter courses, and 7) underdrains.

As with paved roads the success or otherwise of a gravel and forest road drainage system is dependent on its “weakest link”. This means that if any of its elements is out of order, the whole system will not operate as planned and the road will be damaged. More information about the drainage on gravel roads and forest roads are provided in chapter 6.

2.5. Bridges

2.5.1. Gravel roads

In general, a drainage structure is called a bridge when it has a span of >2m as it crosses an obstacle, e.g. rivers, creeks or bays, and also deep valleys. Shorter structures are generally classified as culverts.

Bridges on public gravel roads are generally built to the same standards as paved roads where the maximum allowed truck weights are the same. A lower bearing capacity class can be applied to a gravel road, e.g. in Sweden and Norway, for a weak bridge.

One special feature bridges on public gravel roads is that the road is normally paved both sides of the bridges for distance of 20-200 m. This is done to “clean” passing vehicle tyres of wearing course materials containing fine particles and dust binding chlorides, thus keeping bridge deck clean and minimizing corrosion and other problems on the bridge slab.

Iceland paves the approaches to bridges to prevent potholes forming next to the bridge deck. These could pose safety hazards for vehicles crossing the bridge. Another reason is for ploughing in winter. Experience has shown that it is difficult to get the right compaction and transverse slope immediately next to the bridge and any settlements can cause problems in ploughing, even damage to the bridge itself.

A special case among in the ROADEX partner countries is Iceland which has many gravel roads crossing rivers without a bridge.

2.5.2. Forest roads

The quality of bridges on forest roads can vary from old and weak wood bridges up to high quality concrete or steel girder bridges that are capable of carrying the loads of the new heavy timber truck types. Because the bridges are expensive structures, they are often rebuilt only when are no other options. On many occasions forest road bridges have been built to be as short

2.6. Other assets

Other gravel and forest road assets are traffic signs, road barriers, distance poles, guard rails and in the wintertime ploughing sticks and snow poles.