Gravel road
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A gravel road is a type of unpaved road surfaced with gravel that has been brought to the site from a quarry or stream bed. They are common in less-developed nations, and also in the rural areas of developed nations such as Canada and the United States. In New Zealand, they are known as 'metal roads'.[1] They may be referred to as dirt roads in common speech but that term is used more for unimproved roads with no surface material added. If well constructed and maintained, a gravel road is an all-weather road.
The gravel used consists of irregular stones mixed with a varying amount of sand, silt, and clay which can act as a binder. A gravel road is quite different from a 'gravel drive', popular as private driveways in the United Kingdom, which uses clean gravel consisting of uniform rounded stones and small pebbles.
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[edit] Characteristics
[edit] Construction
Compared to sealed roads which require large machinery to work and pour concrete or to lay and smooth a bitumen-based surface, gravel roads are easy and cheap to build. However, compared to dirt roads, all-weather gravel highways are quite expensive to build, as they require front loaders, dump trucks, graders and roadrollers to provide a base course of hard-packed earth or other material, sometimes macadamised, covered with one or more different layers of gravel. Graders are also used to produce a more extreme camber compared to a paved road to aid drainage, as well as construct drainage ditches and embankments in low-lying areas.
[edit] Laterite and murram roads
In Africa and parts of Asia, laterite soils are used to build dirt roads. However laterite, called murram in East Africa, varies considerably in the proportion of stones (which are usually very small) to earth and sand, and ranges from a hard gravel to a softer earth embedded with small stones. Not all laterite and murram roads are therefore strictly gravel roads. Laterite and murram which contains a significant proportion of clay becomes very slippery when wet, and in the rainy season it may be difficult even for four-wheel drive vehicles to avoid slipping off very cambered roads into the drainage ditches at the side of the road. As it dries out such laterite can become very hard, like sun-dried bricks.
[edit] Maintenance
Gravel roads require much more frequent maintenance than paved roads, especially after wet periods and when faced with increased traffic. Wheel motion shoves material to the outside (as well as in-between travelled lanes), leading to rutting, reduced water-runoff and eventual road destruction if unchecked. As long as the process is interrupted early enough, simple re-grading is sufficient, with material being pushed back into shape.
Another problem with gravel roads is washboarding — the formation of corrugations across the surface at right angles to the direction of travel. They can become severe enough to cause vibration in vehicles so that bolts loosen or cracks form in components. Grading removes the corrugations, and reconstruction with careful choice of good quality gravel can help prevent them re-forming.
Gravel roads are often found in cold climates because they are less vulnerable to freeze / thaw damage than asphalt roads and also because the inferior surface of gravel is not an issue if the road is covered by snow and ice for extended periods.
[edit] Driving
Although well-constructed and graded gravel roads are suitable for speeds of 100 km/h (60 mph), driving on them requires far more attention to variations of the surface and it is easier to lose control than on a paved road. In addition to potholes, ruts and loose stony or sandy ridges at the edges or in the middle of the road, problems associated with driving on gravel roads include:
- sharper and larger stones cutting and puncturing tires, or being thrown up by the wheels and damaging the underside, especially puncturing the fuel tank of unmodified cars
- stones skipping up hitting the car body, lights or windshields when two vehicles pass
- dust thrown up from a passing vehicle reducing visibility
- 'washboard' corrugations cause loss of control or damage to vehicles
- skidding on mud after rain
- in higher rainfall areas, the increased camber required to drain water, and open drainage ditches at the sides of the road, often cause vehicles with a high centre of gravity, such as trucks and off-road vehicles, to overturn if they do not keep close to the crown of the road.
[edit] Related types
[edit] Forest service road
A 'Forest Service Road' is a type of rudimentary access road, built by the Forest Service to access remote undeveloped areas. These roads are built mainly for the purposes of the logging industry and forest management workers, although in some cases they are also used for backcountry recreation access.
Networks of tributary roads branch off from a trunk FSR. Roads are usually named after a regional district, and branches have an alphanumeric designation.
Typically, a large high-clearance four wheel drive vehicle is required to travel effectively on a road, especially where large potholes and/or waterbars are present. Switchbacks are employed to make the road passable through steep terrain.
These roads rapidly fall into disrepair and quickly become impassable. Remnants of old roads can exist for decades. They are eventually erased by washout, erosion, and ecological succession.
[edit] Logging roads
Logging roads are constructed to provide access to the forest for logging and other forest management operations. They are commonly narrow, winding, and unpaved, but main haul roads can be widened, straightened or paved if traffic volume warrants it.
The choice of road design standards is a trade off between construction costs and haul costs (which the road is designed to reduce). A road that serves only a few stands will be used by relatively few trucks over its lifetime, so it makes sense to save construction costs with a narrow, winding, unpaved road that adds to the time (and haul costs) of these few trips. A main haul road serving a large area however will be used by many trucks each day, and each trip will be shorter (saving time and money) if the road is straighter and wider, with a smoother surface.
Logging trucks are generally given right of way. In areas where this practice is regulated (or is supposed to be) non-highway roads with heavy logging traffic may be "radio-controlled", which is to say a CB radio on board any vehicle on the road is advised for safety reasons.
Logging roads are often the major source of sediment from logging operations, which can continue long after operations are completed in the area. Construction of these roads, especially on steep slopes, can increase the risk of erosion and landslides which can increase downstream sedimentation. The decommissioning of these roads involves the restoring of natural habitat, which can be quite expensive, usually as much as it originally cost to construct the road.
The cost to the public (in public forests) of such road-building varies with each jurisdiction and the type of logging licence. Although many roads are justified to the public as providing access for recreational and other non-logging users, after their use to log extraction is over they are often quickly "decommissioned" to reduce their environmental impacts, and become relatively useless to other vehicular users. Amortizing the cost of these roads over and estimated use of 500 years has provided an accounting arrangement by which only 1/500 of the cost of the road is charged against the timber whereas the road builder is granted credit for the entire cost of the road. The builder can use the excess credits to bid on additional US Government Timber Sales. Mountainbikers and hikers and others still can access these roads, but they are not maintained.
[edit] References
- ^ Kiwi - Words & Phrases (from a private website)
[edit] See also
[edit] External links
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