Gravel road

A gravel road in Asikkala, Finland

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. Gravel roads 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, and other Commonwealth countries, they may be known as metal roads. 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.

Characteristics

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 compacted earth or other material, sometimes macadamised, covered with one or more different layers of gravel. Graders are used to "blade" the road's surface (pass frequently to mix and distribute the gravel) to produce a more extreme camber compared to a paved road to aid drainage, to produce an A-shaped surface to the road called a "crown", as well as to construct drainage ditches and embankments in low-lying areas. Cellular confinement systems can be used to prevent the washboarding effect.

Construction of a gravel road begins with the base or subgrade layer. The expected road traffic volume and the average daily truck passage must be considered during the design process as they will influence the thickness of this layer, along with the balances of gravel and fines. Geotextile fabric may be laid to improve the stability of the subgrade layer. When geotextile fabric is used, a gravel layer with a minimum thickness of 6" (15 cm) is suggested to ensure the fabric remains unexposed. Road construction guidelines suggest that the crown in the road surface begins at the center point in the road, and does not exceed a 4% gradation from the center to the edge of the roadway.

The surface layer is constructed atop the subgrade layer. The amount of precipitation is taken into consideration for the selection of gravel size distribution. The surface layer will follow the crown established by the subgrade layer. Scarification of the subgrade layer prior to application of the surface gravel layer can be performed to increase the mixing and adherence between layers. Construction of the road surface is done gradually through multiple applications of layers of gravel, with compaction prior to the addition of the following layer. During reparation of a damaged road, ensuring that any washboarding, rutting, potholes, and erosion is adequately removed will minimize future need for reparation. Windrowing can be performed along the edges of roads in dry climates to allow easy access to gravel material for small repairs.

Materials

The gravel used consists of varying amount of crushed stone, sand, and fines. Fines are silt or clay particles smaller than .075 millimetres (0.0030 in), which can act as a binder. Crushed stone, also called road metal, is used because gravel with fractured faces will stay in place better than rounded river pebbles. A good gravel for a gravel road will have a higher percentage of fines than gravel used as a subbase for a paved road. This often causes problems if a gravel road is paved without adding sand and gravel sized stone to dilute the percentage of fines.

A gravel road is quite different from a 'gravel drive', popular as private driveways in the United Kingdom. This uses clean gravel consisting of uniform, rounded stones and small pebbles.

Laterite and murram roads

In Africa and parts of Asia and South America, 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. It 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.

Maintenance

Maintenance of a gravel road in Denmark

Gravel roads require much more frequent maintenance than paved roads, especially after wet periods and when accommodating 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.

Segments of gravel roads on grades also rut easily as a result of flowing water. When grading or building the road, waterbars are used to direct water off the road. As an alternative method, humps can be formed in the gravel along the road to impede water flow, thereby reducing rutting.

Another problem with gravel roads is washboarding — the formation of corrugations across the surface at right angles to the direction of travel. Narrow-spaced washboarding can develop on gravel roads due to inconsistent moisture levels in the gravel, poor quality gravel, and vehicular stress to the road. Washboarding can also occur when graders exceed recommended speeds during the construction or maintenance phase causing the blade to bounce on the surface creating a pattern of widely-spaced corrugations. Corrugations from washboarding can become severe enough to cause vibration in vehicles so that bolts loosen or cracks form in components. Proper grading is needed to remove the corrugations, and reconstruction with careful choice of good quality gravel can help prevent them reforming. Additionally, installing a cellular confinement system will prevent the washboard-like corrugations from occurring.

Gravel roads are often found in cold climates because they are less vulnerable to freeze / thaw damage than asphalt roads. The inferior surface of gravel is not an issue if the road is covered by snow and ice for extended periods.

Dust control

Dust control is routine practice on gravel roads in order to reduce the need for frequent maintenance, mitigate health concerns, and to prevent dust-related damage to roadside vegetation. Some common dust-suppression techniques are the application of a chloride solution (calcium chloride, magnesium chloride, sodium chloride), the application of a resin compound, or the incorporation of natural clay into the gravel mixture during the construction phase.

Calcium chloride as a dust suppressant

Calcium chloride provides dust suppression through its hygroscopic properties, allowing moisture to be drawn in and retained by the compound. Calcium chloride can be applied in either dry (pellet or flake) or wet (dissolved pre-prepared solution) form. Successful applications can be effective for up to three years, depending on the weather and traffic conditions for the roadway.

Dry application of this type of dust suppressant is begun by first preparing the road surface through grader passes, moving the top 5–8 cm of gravel creating windrows on the edges of the road. Calcium chloride is then applied to the road surface, and the road is then sprayed with water until the compound is dissolved. A grader "blades" the surface in numerous passes to ensure a uniform distribution of the compound. Compaction and the forming of the road surface is then performed to finalize the process.

Wet application begins by spraying the road surface with a 30% concentration solution of calcium chloride. After the solution is applied, the top 5–8 cm of gravel is mixed through numerous passes of a grader. The road is then formed and compacted.

Driving

Although well-constructed and graded gravel roads are suitable for speeds of up to 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 at high speed
• dust thrown up from a passing vehicle reducing visibility
• 'washboard' corrugations cause loss of control or damage to vehicles due to excessive vibration. These are most often found near intersections as stopping or braking causes them to form or otherwise if heavy farm or other equipment often uses these roads.
• skidding on mud after rain
• vehicle fishtailing as a result of ruts in the surface of the gravel. Often found on frequently traveled roads.
• 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.
• Excess dust permeates door-opening rubber moulding, breaking the seal.
• Lost binder in the form of road dust, when mixed with rain, will wear away the painted surfaces of vehicles.
• Many gravel roads are only one lane wide or slightly larger, thus requiring special attention when driving at higher speeds.

Related types

Resource road

According to the British Columbia Ministry of Forests, resource roads are typically "one- or two-lane gravel roads built for industrial purposes to access natural resources in remote areas". They may be used by industrial vehicles or the general public, and as a link to rural communities. Driving on resource roads can be hazardous for many reasons, including limited visibility, unusual road geometry, and the presence of wildlife. Disused resource roads can pose a danger to both drivers and passersby, due to the danger of landslides forming on unstable, poorly-drained ground.

Forest service road

Main article: Fire trail

A forest service road is a type of rudimentary access road, built by private companies, or government entities such as the United States 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 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.

Logging road

Logging road near Eséka in Cameroon

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 tradeoff 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 and so it makes sense to save construction costs with a narrow, winding, unpaved road that adds to the time (and haul costs) of the 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 that the practice is regulated, on non-highway roads with heavy logging traffic may be "radio-controlled", meaning that a CB radio on board any vehicle on the road is advised for safety reasons.

Image gallery

• 
  Gravel road at the Saviselkä village in Kärsämäki, Finland
• 
  Logging railroad converted to logging truck use in northwest Oregon
• 
  Gravel road south of Coober Pedy
• 
  Gravel road in Iran
• 
  A gravel road between the villages of Tamsa and Laguja, Estonia
• 
  A close-up of a gravel road in Terre Haute, Indiana
• 
  Gravel road in Namibia

Unpaved roads length by country

Country	Length of unpaved roads (km)
Afghanistan	29,800
Albania	10,980
Algeria	26,050
American Samoa	No data
Andorra	No data
Angola	46,080
Anguilla	93
Antigua and Barbuda	784
Argentina	161,962
Armenia	No data
Australia	727,645
Austria	No data
Azerbaijan	26,153
Bahamas, The	1,080
Bahrain	730
Bangladesh	19,248
Barbados	No data
Belarus	11,741
Belgium	33,498
Belize	2,382
Benin	14,600
Bermuda	447
Bhutan	7,603
Bolivia	80,776
Bosnia and Herzegovina	3,500
Botswana	No data
Brazil	1,368,166
British Indian Ocean Territory	No data
British Virgin Islands	No data
Brunei	417
Bulgaria	277
Burkina Faso	No data
Burma	No data
Burundi	10,822
Cabo Verde	418
Cambodia	41,102
Cameroon	47,242
Canada	626,700
Cayman Islands	No data
Central African Republic	18,893
Chad	No data
Chile	59,645
China	531,000
Christmas Island	110
Cocos (Keeling) Islands	12
Colombia	No data
Comoros	207
Congo, Democratic Republic of the	150,703
Congo, Republic of the	15,788
Cook Islands	287
Costa Rica	28,885
Cote d'Ivoire	75,494
Croatia	No data
Cuba	31,038
Curacao	No data
Cyprus	8,564
Czechia	No data
Denmark	No data
Djibouti	1,686
Dominica	750
Dominican Republic	9,833
Ecuador	37,198
Egypt	10,688
El Salvador	2,565
Equatorial Guinea	No data
Eritrea	3,136
Estonia	47,985
Eswatini	2,516
Ethiopia	96,060
European Union	No data
Falkland Islands (Islas Malvinas)	390
Faroe Islands	No data
Fiji	1,754
Finland	350,000
France	No data
French Polynesia	855
Gabon	8,073
Gambia, The	3,029
Gaza Strip	No data
Georgia	No data
Germany	No data
Ghana	95,728
Gibraltar	No data
Greece	75,603
Greenland	No data
Grenada	225
Guam	No data
Guatemala	10,132
Guinea	40,006
Guinea-Bissau	2,490
Guyana	7,380
Haiti	3,498
Honduras	11,375
Hong Kong	No data
Hungary	126,514
Iceland	8,108
India	No data
Indonesia	213,505
Iran	38,500
Iraq	No data
Ireland	No data
Isle of Man	No data
Israel	No data
Italy	No data
Jamaica	5,973
Japan	225,937
Jersey	No data
Jordan	No data
Kazakhstan	10,278
Kenya	147,032
Kiribati	No data
Korea, North	24,830
Korea, South	7,633
Kosovo	91
Kuwait	No data
Kyrgyzstan	No data
Laos	34,171
Latvia	55,233
Lebanon	No data
Lesotho	4,871
Liberia	9,943
Libya	42,810
Liechtenstein	No data
Lithuania	11,869
Luxembourg	No data
Macau	No data
Macedonia	4,549
Madagascar	31,373
Malawi	11,378
Malaysia	28,234
Maldives	No data
Mali	16,952
Malta	392
Marshall Islands	No data
Mauritania	8,265
Mauritius	49
Mexico	240,116
Micronesia, Federated States of	204
Moldova	517
Monaco	No data
Mongolia	44,449
Montenegro	621
Montserrat	No data
Morocco	17,279
Mozambique	23,718
Namibia	37,751
Nauru	6
Nepal	16,100
Netherlands	No data
New Caledonia	No data
New Zealand	32,400
Nicaragua	20,551
Niger	15,037
Nigeria	164,220
Niue	No data
Norfolk Island	27
Northern Mariana Islands	No data
Norway	18,116
Oman	30,545
Pakistan	78,879
Panama	8,786
Papua New Guinea	6,349
Paraguay	27,199
Peru	No data
Philippines	155,294
Poland	129,000
Portugal	11,606
Puerto Rico	No data
Qatar	No data
Romania	34,312
Russia	355,666
Rwanda	3,493
Saint Helena, Ascension, and Tristan da Cunha	30
Saint Kitts and Nevis	220
Saint Lucia	363
Saint Pierre and Miquelon	37
Saint Vincent and the Grenadines	249
Samoa	95
San Marino	No data
Sao Tome and Principe	102
Saudi Arabia	173,843
Senegal	10,539
Serbia	16,248
Seychelles	12
Sierra Leone	10,396
Singapore	No data
Sint Maarten	No data
Slovakia	No data
Slovenia	No data
Solomon Islands	1,356
Somalia	19,492
South Africa	588,062
South Sudan	No data
Spain	No data
Sri Lanka	97,116
Sudan	7,580
Suriname	3,174
Sweden	433,034
Switzerland	No data
Syria	6,813
Taiwan	396
Tajikistan	No data
Tanzania	79,380
Thailand	No data
Timor-Leste	3,440
Togo	9,205
Tonga	496
Trinidad and Tobago	4,068
Tunisia	4,662
Turkey	33,486
Turkmenistan	11,015
Turks and Caicos Islands	97
Tuvalu	No data
Uganda	16,287
Ukraine	3,599
United Arab Emirates	No data
United Kingdom	No data
United States	2,281,895
Uruguay	69,989
Uzbekistan	10,985
Vanuatu	814
Venezuela	No data
Vietnam	47,130
Virgin Islands	No data
West Bank	No data
World	No data
Yemen	65,100
Zambia	31,051
Zimbabwe	78,786
Total	11,705,250

See also

• Country lane
• Dirt road
• Erosion
• Forest Highway
• Forest railway
• Forest track
• Logging
• Roads
• Road surface
• Track bed
• Trail

External links

• Skorseth, Ken; Selim, Ali A. (November 2000), Gravel Roads Maintenance and Design Manual (PDF), FHWA, South Dakota Local Transportation Assistance Program, archived from the original (PDF) on 2017-01-31, retrieved 2014-12-28
• Environmentally Sensitive Maintenance Practices for Dirt and Gravel Roads U.S. Forest Service